Recent Advancements in Bulk Metallic Glasses and Their … · 2018-03-16 · Recent Advancements in...

Recent Advancements in Bulk Metallic Glasses and Their Applications: A Review

Muhammad Mudasser Khan a, Ali Nemati a,b, Zia Ur Rahman a, Umair Hussain Shah a, Hassnain Asgar a,and Waseem Haider a

aSchool of Engineering and Technology, Central Michigan University, Mount Pleasant, Michigan, USA; bDepartment of Materials Science &Engineering, Sharif University of Technology, Tehran, Iran

ABSTRACTBulk metallic glasses (BMGs), that display extraordinary properties of high strength, corrosionresistance, polymer-like formability, and excellent magnetic properties, are emerging asmodern quintessential engineering materials. BMGs have garnered significant researchenthusiasm owing to their tremendous technological and scientific standing. In this article, therecent advancements in the field of BMGs and their applications are put in a nutshell. Novelstate-of-the-art production routes and nano/microimprinting strategies with salient featurescapable of circumventing the processing related complexities as well as accelerating moderndevelopments, are briefly summarized. Heterogeneous BMG composite systems that lead toincredible combination of otherwise conflicting properties are highlighted. Biocorrosionstudies and recent developments in the field of magnetic BMGs are presented owing to theirsignificance for prospective biomedical and magnetic applications, respectively. In the lastsection, the current status of BMGs applications in the field of catalysis, biomedical materials,structural materials, functional materials, microelectromechanical systems (MEMS), and micro/macro devices are summed up.

KEYWORDSBulk metallic glasses;combinatorial development;BMG composites;nanoimprinting;biocorrosion; catalysis

Table of Contents

1. Introduction ................................................................................................................................................................................................22. Structural models and GFA predictors ................................................................................................................................................33. Novel fabrication routes ...........................................................................................................................................................................7

3.1. Combinatorial development.............................................................................................................................................................73.2. Synthesis of single-component amorphous systems ...................................................................................................................73.3. Liquid and solid-state joining ..........................................................................................................................................................83.4. Selective laser melting ........................................................................................................................................................................93.5. Powder metallurgy (spark plasma sintering) ..............................................................................................................................11

4. Micro/nanofabrication of BMGs for MEMS and NEMS ...............................................................................................................115. Bulk metallic glass composites .............................................................................................................................................................13

5.1. In-situ composites ............................................................................................................................................................................145.2. Ex-situ composites............................................................................................................................................................................155.3. Nanolayered crystalline/amorphous composites .......................................................................................................................15

6. Biocorrosion ..............................................................................................................................................................................................167. Magnetic properties.................................................................................................................................................................................18

7.1. Soft and hard magnetic properties ................................................................................................................................................197.2. Magnetostriction...............................................................................................................................................................................197.3. Magnetocaloric effect.......................................................................................................................................................................207.4. Spin dynamics and heavy fermion behavior ...............................................................................................................................217.5. Novel magnetomechanical interactions .......................................................................................................................................21

CONTACT Waseem Haider [email protected] versions of one or more of the figures in the article can be found online at www.tandfonline.com/bsms.© 2017 Taylor & Francis Group, LLC

CRITICAL REVIEWS IN SOLID STATE AND MATERIALS SCIENCEShttps://doi.org/10.1080/10408436.2017.1358149

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7.6. Piezomagnetism................................................................................................................................................................................218. Recent applications of BMGs................................................................................................................................................................21

8.1. Applications in catalysis ..................................................................................................................................................................228.2. Biomedical applications...................................................................................................................................................................248.3. BMG-based micro and macro devices..........................................................................................................................................26

8.3.1. Fuel cell ...................................................................................................................................................................................268.3.2. Microcantilever......................................................................................................................................................................278.3.3. Microscanner .........................................................................................................................................................................288.3.4. Current sensor .......................................................................................................................................................................288.3.5. Cardiovascular stent .............................................................................................................................................................298.3.6. Wear-resistant gear...............................................................................................................................................................29

9. Current issues and outlook....................................................................................................................................................................29References...................................................................................................................................................................................................31

1. Introduction

Bulk metallic glasses (BMGs) are metallic alloys pro-duced via rapid cooling of liquid melt to suppress nucle-ation and growth of crystalline phases. They are typicallymulticomponent systems in which different species, pref-erably possessing appreciable atomic size differences,1

are added to perturb the crystallization of the liquid meltand vitrified using high cooling rates. Synthesizingmetallic glass (MG) is analogous to “racing against athermodynamic clock”2 that starts ticking when anattempt is made to undercool a liquid melt below itsmelting temperature Tm, prompting to expedite thesupercooling in order to escape nucleation. Owing to themetal’s inherent predisposition to crystallize, a consider-ably high cooling rate is required to freeze the disorderedliquid-like structure and, thereby, circumvent the nucle-ation of crystalline phases. On grounds of absence oflong-range order in metallic glasses, different modelshave been proposed based on complete randomness,3

short-range-order,4 and the most recent medium-range-order5,6 with the latter two being endorsed by a compel-ling experimental evidence using nanobeam electrondiffraction.7

Various processing routes have been deployed for thesynthesis of BMGs such as rapid cooling of liquidmelt,8–11 liquid splat-quenching,12 pulsed laser quench-ing,13,14 melt spinning,15,16 powder metallurgy,17–19 andmagnetron sputtering.20–24 The conventional processingmethods necessitate extremely high cooling rates tobypass crystallization which, consequently, make thedevelopment of BMGs very cumbersome and puts a cru-cial constraint on the size and geometry of samples. Thatbeing the case, the identification of multicomponentnovel BMGs having a desired combination of propertiesfor a specific application, out of a wide compositionalspace, inarguably, becomes a challenging task. One solu-tion, however, is a high-throughput novel strategy, calledthe combinatorial approach via co-sputtering,20,21 where

substantial compositional libraries are produced andcharacterized simultaneously. Such a powerful approachhas the potential to accelerate the discovery of BMGs forfunctional applications.21 Apart from multi-componentsystems with more than three elements, binary systemsof Cu-Zr,25–27 Au-Si,28,29 Zr-Pd,23 and Ni-Nb11,30,31 havebeen successfully developed.

In the pursuit of finding new BMGs, an importantparameter is the glass forming ability, GFA (defined asthe maximum completely amorphous thickness or diam-eter of any sample produced via quenching from the liq-uid state). From thermodynamic point of view, thechange in Gibbs-free energy, DG, between the two prom-inent states—crystalline solid and supercooled liquid—isalways positive in the latter, which indicates that suchsystems always have tendency to crystallize. Chemicaland topological differences in the selected speciesimpedes the formation of nuclei as well as raises the vis-cosity of the melt, both of which raises the GFA.32 Ingeneral, GFA varies inversely to the critical cooling rateand directly with the critical thickness of the sample.Other than this, GFA can be tuned using Lindsay Greer’sconfusion principle33 according to which higher numberof different-sized species in the liquid melt results in acompetition for a viable crystal structure and the resul-tant confusion increases the probability of the overallsystem to go into amorphous form. Predicting GFA isboth challenging as well as vitally important for BMGssince their widespread application is primarily restrictedby limited GFA. Compelling GFA predictors have beenproposed in several interesting studies based on bothexperimental parameters34–36 and computational techni-ques,37–39 with the latter having potential to expedite thediscovery of novel metallic glasses.

BMGs are relatively new class of advanced materialsand are finding applications in different fields.40–44 Theyexhibit some outstanding properties like superiorstrength,31,45–47 good corrosion resistance,48,49 excellent

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wear resistance,50,51 and high elasticity41,52 as comparedto their crystalline counterparts. Such exceptional prop-erties are attributed to the absence of crystal defects suchas grain boundaries that usually decreases the strength ofa material and makes them susceptible to intergranularcorrosion and stress-corrosion cracking. The corrosioninvestigations of different BMGs are very appeal-ing10,23,53–56 as they display a substantial lead in corro-sion resistance properties in comparison to conventionalcrystalline materials in different biological media andother aggressive solutions. The tantalizing combinationof properties makes BMGs the most suitable candidatesfor wide range of applications such as biomateri-als42,44,57–60 structural materials,50,61,62 magnetic materi-als,63–66 and catalysis.67–72

One of the most distinctive properties of BMGs is thatthey become significantly soft in the supercooled liquidregion offering great opportunities to be formed easilyinto intricate shapes.73–75 This softening behavior,referred to as the polymer-like formability,41 allows thepossibility of BMG micro/nanoimprinting73,74,76 that canbe exploited for high-performance applications. More-over, because of the ease of nanoimprinting, BMGsare tailor-made for applications necessitating high sur-face areas, such as catalysis, MEMS, and NEMS. In crys-talline materials, the size of nanopatterns is restricted bythe grain size, however, the isotropic nature of BMGsdown to atomic scale77 offers the possibility of extremelysmall nanometer-scale architectures.2 Designing tailoredBMG systems for applications like electrocatalysis canresult in massive improvements in numerous fields suchas energy storage, fuel cells,61 and engery conversion.78

As opposed to BMGs’ excellent attributes, they, how-ever, possess certain key shortcomings in their mechani-cal properties. The most crucial one is their limitedplasticity and brittle nature,9,79 which is mainly becauseof the absence of dislocations and crystal structure. Fail-ure in crystalline materials is a function of crystallinedefects such as grain boundaries, dislocations, their den-sity and distribution, etc. Their absence increases thestrength of a material, but does so at the cost of homoge-nous deformation, which is produced by stress redistri-bution in crystalline systems. The absence of suchdefects, therefore, leads to localized shearing, strain soft-ening, and catastrophic failure. Apart from brittleness,toughness, which is a useful mechanical property forstructural materials, varies in BMGs from system to sys-tem, ranging from mild to extremely tough BMGs,9,62

however tensile ductility has always been an upsettingparameter in this field.9,75 Additionally, the mechanicalproperties of BMGs during long-term service cannot berelied upon since these out-of-equilibrium materials arepredisposed to shift into the metastable equilibrium

state—a phenomenon called structural relaxationobserved in different glassy systems,80,81 which results ina concomitant alteration of the mechanical and thermo-dynamic characteristics.82 This behavior, although a det-riment to BMGs’ performance, however, offers onealternative to tailor their mechanical and physical prop-erties, as reported for instance, in case of improvedtensile ductility and triggering of shear-transformation-zones (STZ)83 and, therefore, can have implications indeciphering and solving the issue of brittleness.84,85 Therelaxation behavior—usually the secondary (b) relaxa-tion which is linked with structural heterogeneities inbulk metallic glasses86—is of paramount importance forthe deformation behavior and service reliability ofMGs.87 The grim challenges in mechanical properties, inparticular the susceptibility to catastrophic failure, never-theless, has stimulated researchers to develop BMG-based composites8,9,45,47,88,89 with excellent combinationof strength and plasticity.

Research in the field of BMGs has burgeoned over thepast several decades with substantial progress in theprocessing, characterization, and practical applicationsencompassing various fields. Despite several goodreviews on BMGs,1,41,42,44,62,90 there is a need for a com-prehensive summary of the most recent advancementsmade in this class of materials. The incredible propertiesof BMGs truly substantiate their prospects as lucrativeengineering materials for high performance applications.In the following paragraphs, the recent developments inBMGs, their structural models, novel synthesis techni-ques, composite BMG materials, nano/microimprinting,novel magnetic properties, biocorrosion resistance, andpotential applications in various fields are summarized.

2. Structural models and GFA predictors

Notwithstanding the research enthusiasm in metallicglasses for a variety of high-performance applications,many aspects of their local atomic structure has been aconundrum. Understanding their atomic structure, glassformation and, thereby, predicting GFA is both challeng-ing as well as vitally important since the widespreadapplications of BMGs are halted, chiefly, by their limitedGFA. In the following paragraphs, the different struc-tural models and smart GFA predictors are chronicled.

Various models have been proposed for metallicglasses over the past several decades. The absence ofsharp diffraction peaks, during X-ray and electron dif-fraction, signal the absence of periodic structures in thesedisordered materials. That being so, material scientistswere earlier induced to go in favor of models based oncomplete randomness. Among the random packingmodels, Bernal’s model3—frozen liquids with 3D “dense

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random packing” of hard spheres—is so far widely citedfor these disordered materials. However, this and othersimilar models, postulating that solute atoms are jammedinto the cavities produced by densely packed solventatoms, were later discarded upon the uncovering of sol-ute atoms larger than solvent atoms, for example inCu60Zr40 system.91 Bernal’s model, although good formonoatomic systems, fail to describe some binary sys-tems, in particular metal-metalloid glasses.6 On top ofthat, it fails to explain higher level organization observedin real, multicomponent glassy systems with low criticalcooling rates.1

In contrast to structural models based on completerandomness, later, however, a short-range order (SRO)was proposed, and the models were altered accordingly.In light of SRO, Gaskel presented a model based on theprinciple that local unit in glassy systems is well definedand identical to the corresponding crystalline systemhaving the same composition.4 This model, however, isnot endorsed in metal-metal based metallic glasses1 aswell as lack conclusive experimental evidence.6

The inadequacy of SRO models for determining theoverall structure of these disordered materials, therefore,led researchers to propose a higher-level organizationand thus, defining the overall structure of MGs extendingbeyond a short range, became all-important. In the questof a higher-level organization—the 3D arrangement ofthe local structural units beyond the nearest-neighbourshort-range order—is the medium range order (MRO)reported by Miracle.5 In this model “solute-centeredatomic clusters” are assumed as the local structural ele-ments that are idealized as spheres and via efficient pack-ing of these sphere-like clusters, with favored fcc/hcppackings, extended 3D structure is produced. The hcp/fcc packing offers physical basis for solute orderingbeyond the nearest-neighbor shells. The order, however,cannot transcend few clusters primarily because of thepresence of internal strains and topological frustration,that is maintained to interrupt the long-range order.According to this model, an extended structure by theoverlapping of clusters—via edge-, vertex-, or the pre-ferred face-sharing—is produced with no orientationalorder between clusters.

To further solve the mystery of 3D positioning andtopological mapping, Sheng et al.,6 instead of proposinga model a priori, used reverse Monte Carlo methods andab initio simulations for various binary structuresobtaining results that are in good agreement to experi-mental data. The results suggest that, as opposed to fcc/hcp cluster packing models, icosahedral five-fold packingis preferred for cluster connections in metallic glasses.Using Voronoi tessellation, Sheng found a range ofquasi-equivalent clusters, having different topology and

coordination number, that allow efficient filling of the3D space. In the study of different systems, the clusterpack with appreciable icosahedral medium-range orderas well as generate cavities for solute species. Each clusteris surrounded by around 12 neighboring clusters gener-ating a super icosahedra or fragment of 1.5 nm width.Besides, the ab initio calculations show that with increas-ing solute content a “string-like” connection is producedbetween the neighboring solutes, akin to a network, gen-erating a spectrum of atomic packing.

The disordered atomic structures of BMGs are exten-sively characterized using various diffraction and spec-troscopic techniques. However, they provide insufficient,average information, and a direct evidence of the localatomic order as proposed by different theoretical models,therefore, remains inconclusive, consequently arising theneed of characterization techniques capable of revealingmulti-dimensional structural information. In this regardin a study by Hirata et al.,7 a conclusive evidence of localatomic order, as a result of the direct observation of well-defined diffraction spots (see Figure 1), is found in aZ66.7Ni33.3 system using a sophisticated coherent nano-beam electron diffraction (NBED) with 3.6 A! diameter(full-width at half-maximum, FWHM). The soundagreement of experimental and simulated interatomic-spacing distribution justifies the interpretation of theexperimental patterns via ab initio molecular dynamicssimulation. Distinct diffraction patterns both from indi-vidual atomic clusters and their assemblies, that are theo-retically predicted as short- and medium-range order,are directly observed. Additionally, using an electronbeam of higher diameter 7.2 A!—that leads to diffractionfrom interconnected polyhedral clusters—a correlationbetween clusters is identified. “Interconnected super-clusters” and the corresponding NBED patterns obtainedvia simulation, are also in good agreement with experi-mental ones, indicative of the reliable modelled super-cluster (see Figure 1). This is a compelling structuralmodel for metallic glasses since it bolsters the previousprediction of metallic glasses exhibiting SRO and MRO.

The structural models of BMGs as discussed earlierare, fundamentally, not predictive in nature. Many pre-dictors for glass formation have been successfullyreported in different studies,34–36 but their commonshortcoming of being dependent on experimentalparameters have rendered them, close to, impractical.For instance, in a study by Cheney,34 GFA is evaluatedusing modeling tools that considers structural topologycritical for amorphization. Turnbull35 incorporates ther-modynamic factors such as cooling rate and reducedglass transition temperature while in a report by John-son et al.36 GFA hinges on fragility parameter, transi-tion temperature, and reduced glass transition

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temperature. Thus, they cannot lend a helping hand incase of novel metallic glasses not yet experimentallyinvestigated. For that reason, predicting the trend ofGFA from factors such as constituent elements, alloycompositions, and atomic structure—as opposed to thetrial-and-error methods based on experimental parame-ters—seems indispensable in order to discover vitalBMG compositions.

In the light of this, a smart GFA predictor, based onfirst-principles static and molecular dynamics simula-tions, is reported by Yu et al.37 using the concoction offormation enthalpies and atomic-scale defect structures.A simple expression has been proposed that accuratelypredicts GFA from mere alloy compositions. In thisstudy, atomic bonding defects are correlated with thekinetic factor involving atomic mobility in the super-cooled liquids. This convincing model, with predicteddata unambiguously verified through experimentalresults of a model Zr-Cu system, has implications in thedevelopment of BMGs with appreciable GFA for engi-neering applications.

Predictive capability for bulk metallic glasses is alsoattained using the interdependence of atomic structureand glass-forming ability. The structural features that arecrucial in metallic glasses to contend with crystallizationhas remained obscure and the efficient-cluster-packing

(ECP) models fail to predict systems that are favorablefor the formation of metallic glasses. While addressingsuch critical issues and bringing insight to the funda-mental question of predicting GFA from atomic struc-ture, is a seminal study by Laws et al.38 In this work, newfamily of defects—substitutional, super-substitutionaldefects, for instance—are detailed to impede glass forma-tion in binary systems, although less so in complex ter-nary, quaternary systems. Most importantly, this workincorporates the idea of, and strategies to accomplish,efficient packing around both solute and solvent atoms,necessary for glass formation. The model is validated bycomparing the predicted atom sizes and concentrationsto already reported ternary glass systems. Additionally,this work modifies the description of the earlier modelsand maintains that any atom satisfies the requirementsfor both solute site at cluster center and solvent site inthe cluster first shell. This work is established based onrelative atom sizes and concentrations, leaving out theimportance of chemical effects, since structures withchemically dissimilar atoms have substantially differentGFA.

Perim et al.39 proposed a descriptor based on theheuristics that structural and energetic “confusion” hin-ders the growth of crystalline phases. Based on the asser-tion of Lindsay Greer’s principle, confusion during

Figure 1. Schematic showing the characterization of MRO by NBED: (a) Two face-sharing <0 2 8 1> polyhedra with a common on-axisorientation for Bragg’s diffraction. (a’) The super-cluster displaying the face-sharing configuration. (b) Experimental NBED pattern includ-ing two sets of possible rectangle diffraction patterns. (c) Simulated NBED pattern obtained from the super-cluster shown in (a) with theon-axis electron incidence. (d) and (e) Simulated NBED patterns with (d) obtained from cluster A of the super-cluster while (e) obtainedfrom cluster B. (© NPG. Reprinted with permission from Hirata et al.7 Permission to reuse must be obtained from the rightsholder.)


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crystallization encourages glass formation. A prioriknowledge to quantify such confusion to facilitate glassformation, is not fully investigated. In this study, a modi-fied definition of confusion is proposed: the existence ofmultiple phases exhibiting similar energy and dissimilarstructures, lead to the formation of several distinct clus-ters that struggle to reach critical size required for crys-tallization and the ensuing competition, therefore,produces confusion that raise the chances of glass forma-tion. To aid glass formation, the formation of critical sizenuclei needs to be hampered via this confusion effect. Aheuristic descriptor, called the entropic factor—thatrelates to the configurational entropy of the systemand, in general, represents the frustration of the

crystallization—is proposed in this impressive work.Compositions with large entropic values are expected todisplay comparatively more disordered structures andcorrespondingly higher GFA. Sound agreement, betweenentropic factor and experimental results, is demon-strated, for instance, in the manifestation of Cu50Zr50 asthe best glass forming composition, which is predictedby the descriptor and confirmed by experimental results.Additionally, systems with high and low GFA, asobserved in experiments for Cu-Zr and Ni-Zr, respec-tively, are correctly predicted with correspondingly highand low entropic factors. In order to further enrich thequantitative model, more factors such as enthalpy prox-imity and structural similarities, that correlate with GFA,

Figure 2. Schematic of GFA descriptor spectra for different alloy systems: (a)–(f) Black line/solid red fill shows the predicted values; greenline/transparent green fill displays compositions that are experimentally reported; while the grey fill represents area under the threshold.(a) Cu-Zr, reported glass formers Cu50Zr50, Cu56Zr44, and Cu64Zr36 in different studies;92,93 (b) Ni-Zr, reported glass formers Ni1¡xZrx with0.35 < x < 0.45 and 0.60 < x < 0.63;94 (c) Cu-Hf, reported glass former Cu1¡xHfx with 0.35 < x < 0.60;95 (d) Au-Si, reported glass formerAu75Si25;

28 (e) Be-Ti, reported glass former Be1¡xTix with 0.59 < x < 0.63;96 (f) Ni-P, reported glass former Ni81P19.97 (g) Reported vs. pre-

dicted glass-forming concentrations for the 16 training systems, the red crosses indicate missed glass formers. (h) Statistical distribution ofthe maximum peak GFA value for 1,418 different binary alloys. Inset shows a close up of the same plot. Grey fill indicates area under thethreshold. (© Nature Publishing Group (NPG). Reprinted with permission from Perim et al.39 Permission to reuse must be obtained from therightsholder.)

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are added to the descriptor. Following this, the descriptoris confronted with experimental data and a threshold isset self-consistently (see Figure 2).

The data in AFLOW repository98,99 is screened usingthis descriptor, and new plausible compositions for glassformers are revealed, (see Figure 2). In short, a smartpredictor, based on structural and thermodynamic prop-erties of contending crystalline phases, is proposed andsubsequently validated by detailed nanocalorimetryexperiments of two model systems. This model, quickand computationally predictable without the need ofexperimental inputs, can accelerate the discoveries in thefield of BMGs as it unveils many glass formers not previ-ously reported.

3. Novel fabrication routes

The fabrication of bulk metallic glasses is, inarguably, avery demanding task since the suppression of crystallinephases demands very high cooling rates. Even thoughnumerous compositions have been developed that can bevitrified using slow cooling rates, still there are chancesof formation of crystalline domains while using largersample sizes. Other than this, BMGs are characterizedusing snail-paced trial-and-error methods. With multi-component systems, and wider compositional space, theconventional manufacturing routes are extremely time-and resource-consuming that have greatly impeded thedevelopment of novel bulk metallic glasses. To bypasssuch hindrances, novel fabrication strategies have beenreported possessing a high throughput nature as well ascapable of producing amorphous single-component sys-tems and appreciable size BMGs. The novel fabricationmethods are summarized in Table 1.

3.1. Combinatorial development

Combinatorial development is a truly remarkable state-of-the-art approach in which a co-sputtering setup withmultiple guns is operated to produce multi-componentamorphous thin films (see Figure 3). This strategy can beutilized to synthesize substantial compositional libraries

by changing certain processing parameters, for example,the angle and power of individual gun, angle, and rota-tion speed of substrate, gaseous environment, and com-position of each target. Furthermore, it allowssimultaneous characterization of the fabricated librariesto identify a specific system offering striking combina-tion of, otherwise unrelated, properties. For instance, Liuet al. used this novel approach to find a specific composi-tion in the Zr-Cu-Al-Ag system that offers an optimumcombination of glass forming ability and antibacterialproperties out of the vastly available compositionalspace.20 Such faster characterization is, otherwise,extremely difficult to carry out using the conventionalmethods. The same high-throughput strategy has beenutilized for Mg-Cu-Y system (Figure 3) in order touncover a compositional range that impart the highestthermoplastic formability.21 In this study, a library ofapproximately 3,000 alloy compositions is fabricated andcharacterized simultaneously via blow-forming. In a sim-ilar fashion, this classical approach is deployed for theidentification of parting line in a metallic glass systemfor dealloying purposes.78

3.2. Synthesis of single-component amorphoussystems

As per Innoue’s rule101 for BMGs, multiple elementalspecies are imperative to ease glass formation whichmeans that single-component systems are quite challeng-ing to vitrify. It is maintained that any liquid can bebrought into amorphous state if the cooling rate is madeuncommonly high.102 Surprisingly, even gases with com-plex molecular shapes103 are possible to vitrify. Zhonget al. detailed an ultra-high super quenching method tovitrify monoatomic liquids with cooling rates approach-ing 1014 K/s.100 In this method, two surfaces with nano-protrusions are brought into contact and melted by theapplication of a short square electric pulse for a veryshort duration of time (3.7 ns). The melting zone rapidlysolidifies into amorphous structure upon the discontinu-ation of electric pulse (see Figure 4). Such techniquesrely on extreme experimental conditions. To avoid the

Table 1. Novel fabrication routes used for BMGs and their distinctive features.

S No. Novel Fabrication Route Salient Feature/s Reference/s

1 Combinatorial development (via co-sputtering) High-throughput nature, ease of glass formation, high cooling rate,chemistry optimization

20,21,78

2 Ultrafast liquid quenching Incredibly high cooling rates, Vitrification of single-component systems 100

3 Solid-state amorphization Amorphization of single-component systems, avoidance of extremeexperimental conditions

32,104,105,106

4 Monoatomic liquid vitrification Vitrification of single-component system 103

5 Liquid-solid joining of BMGs Fabrication of BMG-based composites, counters the size-constraint 107

6 Selective laser melting Synthesis of complex geometries, high cooling and heating rate 108–113

7 Powder Metallurgy (Spark plasma sintering) No size constraint, synthesis of BMG-based composites 17–19,114,115


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strict parameters of glass formation, in fact tremendouslystrict for single component systems, a novel hubble-bub-ble approach is a noteworthy option that has been suc-cessful with different elements such as Fe, Co, and Niamorphous nanoparticles.57

Apart from the requisite of considerably high cool-ing rates, solid-state amorphization is another methodfor the formation of metallic glasses. Zirconium is suc-cessfully converted into fully amorphous state by theapplication of high static pressures and low tempera-tures followed by subsequent slow cooling.32 Otherexamples demonstrate amorphization of crystalline

copper nanolayers embedded in Cu-Zr glass,106 anddeformation-induced localized amorphization in nano-crystalline nickel.105 Moreover, since pressure candepress the melting point in many systems, it mightalso help in the vitrification of certain single-compo-nent liquids, such as in the glass formation of melt-quenched germanium.103

3.3. Liquid and solid-state joining

The most crucial limitation in the field of BMGs is thesize constraint, a real stumbling block hindering the

Figure 3. Combinatorial development of BMGs via co-sputtering: (a) D.C. magnetron co-sputtering system. (b) Compositional mappingusing EDX analysis and its correlation with the x, y coordinates of the Mg–Cu–Y compositional library. (c) X-ray diffraction mapping. (d)Back side and front side (main image and upper-right inset) of the compositional library with Si wafer as substrate. Lower-left inset:Deep reactive ion etching done to create membranes. (© Nature Publishing Group (NPG). Reprinted with permission from Ding et al.21

Permission to reuse must be obtained from the rightsholder.)

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commercial success of metallic glasses. BMGs are diffi-cult to fabricate with larger sizes as it gives rise tolower heat dissipation and less chances of glass forma-tion. To achieve appreciable size amorphous samplesfor industrial applications, BMGs have been success-fully welded via different liquid and solid-state joiningtechniques. The former includes laser beam welding116

and electron beam welding117 while the latter includesfriction,118,119 ultrasonic120 methods, thermoplasticdeformation,121 etc. In an elegant strategy designed byHuang,107 two BMG samples with different composi-tions are successfully joined using a novel liquid-solidjoining process with atomic-scale metallurgical

bonding having a transition layer of 50 microns, asshown in Figure 5. This strategy can be exploited todevelop large-size BMGs with combination of desiredproperties that can prove instrumental for advancedstructural applications.

3.4. Selective laser melting

Selective laser melting (SLM), an additive manufacturingtechnique, is another superior route to synthesize bulkmetallic glasses with good mechanical properties110,111

appreciable dimensions122 and complex geome-tries.113,122 This process is endowed with both high

Figure 4. Schematic of ultrafast liquid-quenching method: Nano-tips are brought into contact (a) and are melted by electric pulse (b).Rapid heat dissipation vitrifies the molten region (c). d, e, HR TEM images showing two contacting nano-tips of Ta (d), forming a TaMetallic Glass (e) after the application of electric pulse. The glass-crystalline interfaces are indicated by yellow dotted curves. j–l, FastFourier transformations confirming a fully vitrified region (g) bounded by two crystalline substrates viewed along the [100] (f) and [110](h) crystallographic orientations, respectively. (© NPG. Reprinted with permission from Zhong et al.100 Permission to reuse must beobtained from the rightsholder.)


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heating and cooling rates, exceeding those required forBMGs. The vital parameter of high temperature gra-dients, along with other controlling parameters112 suchas laser power density, spot size, and scan speed, can beoptimized to achieve completely amorphous systems.Among the aforementioned parameters, lower laserenergy density—as opposed to higher that usually intro-duce severe crystallization—can ensure completelyamorphous structures.108 This process, although

beneficial in many aspects, suffers from the crucial limi-tation of inception of crystallization that is induced bythermal cycling, and which is, almost, unavoidable dur-ing this incremental layer-by-layer technique. One of theparameters during SLM, the temperature variation rate,however, is shown to diminish the nucleation process,and this behavior, might in future, provide new strategiesto avoid crystallization.111 SLM may also induce severethermal stresses which consequently produces cracks in

Figure 5. TEM images of a novel liquid-solid joining of BMGs: The bright-field images of the interfaces (a) between region I and region II,and (b) between region II and region III, (c, d) the Fast Fourier Transformation (FFT) filtered HR-TEM image, with the white dashed linesdisplaying the boundary between regions I, II, and III, (e, f) the Selected Area Electron Diffraction (SAED) patterns of the crystals in regionI, (g, h) the SAED patterns from the columnar crystals in region II, and (i) the SAED pattern taken from region III. (© Nature PublishingGroup (NPG). Reprinted with permission from Huang et al.107 Permission to reuse must be obtained from the rightsholder.)

10 M. M. KHAN ET AL.

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the material. Nonetheless, a low-energy density re-scancan be exploited to diminish the stresses as well as stopcrack propagation.110

3.5. Powder metallurgy (spark plasma sintering)

Powder Metallurgy (PM) route can be used to avoid thecomplexities of size-constraint and cooling rate. In thismethod, amorphous powders are first produced viamechanical alloying, high-pressure gas atomization, orwater atomization and then densified using different tech-niques such as cold pressing, equal channel angular extru-sion, hot pressing, and the most efficient method, the sparkplasma sintering (SPS).114 SPS has the capability to sinterthe compacted material in the shortest possible times toensure that no crystallization commences anywhere duringsintering. The densification, an important aspect duringPM route, can be additionally improved by the introduc-tion of dynamic pressing and high stresses alongwith rapidheating. This strategy even guarantees the glass formationof marginal glass formers.123 Moreover, the SPS techniqueenables sintering at comparatively lower temperatures andshorter times which are the desired processing attributesfor BMG formation.115

4. Micro/nanofabrication of BMGs for MEMSand NEMS

BMGs exhibit significant softening in a specific range oftemperatures, making possible the fabrication of micro-patterns, nanopatterns, porous surfaces, and micro/nano

devices with high precision, smoothness, and productionrate. As opposed to crystalline materials, BMGs areexcellent candidates for high precision devices because oftheir remarkable mechanical properties combined withsurface smoothness resulting from the absence of grainsand other crystal defects. Nanoarchitectured metallicglasses with high surface area-volume ratio exhibitingsuperior mechanical properties along with isotropicnature down to nano-scale can greatly enhance perform-ances in the areas of detecting, sensing, MEMs, NEMs,and catalysis applications. Exhibiting thermoplasticdeformations analogous to plastics, metallic glasses areemerging as modern engineering materials. Their homo-geneous isotropic structure, characterized by superiorstrength, absence of size effect, high elastic limit, andpolymer-like-formability, make them ideal candidatesfor MEMs and NEMs.

Carousal oblique angle deposition (COAD) is an ele-gant method to fabricate nanopatterened metallic glasseswith high yield, chemistry optimization, and controllable3D geometry.74 In this method, multiple guns co-sput-tering setup is used to make BMGs of required composi-tion and geometry. The variations in nanopatterning canbe brought via variation of certain parameters such asthe angle of individual gun, angle between the normal ofsubstrate, and incoming flux of sputtered species, as wellas the rotation angle of the substrate, called the azi-muthal angle shown in Figure 6. By varying azimuthalangle, uniformity of the films can be controlled as well ascomplex 3D structures can be synthesized. Compositionof the resulting glassy structure is controlled by both

Figure 6. Multiple targets COAD (left) and the overview of hybrid nanostructures (right). (a) Multiple-guns deposition system whereeach gun is controlled independently. (b) Morphology of a template with »100 nm gold nanoparticles. The scale bar is 500 nm. (c) Theshadowed growth is illustrated. (d) Hybrid nanostructures of two different MGs, the inset is the XRD spectra of the deposited nanostruc-tures. Scale bar, 2 mm (e–h) Shows hybrid nanostructures with different lateral dimensions. Scale bar, 500 nm. (© Nature PublishingGroup (NPG). Reprinted with permission from Liu et al.74 Permission to reuse must be obtained from the rightsholder.)


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deposition rate of individual guns and rotation speed oftarget holder. Using this multipurpose technique, hybridnanoarchitectures of two different BMGs, namelyZr-Cu-Al and Ni-Nb-Sn—both exhibiting low thermo-plastic formability—is fabricated by the variation ofvapor flux chemistry. Nanopatterns can be achieved, viathis method, on BMG systems possessing poor combina-tion of thermal stability and GFA.

During nanoimprinting, a truly demanding task is thesynthesis of desired nanostructures for specific applica-tions. In this regard, a novel method, the sacrificialimprint lithography, has been demonstrated to achievenanotextured and multiscale patterned surfaces on metal-lic glasses.73 In this strategy designed by Singer et al., thecomplexity of the fabrication of desired nanostructureson complex geometries, is effectively invalidated, by theintroduction of sacrificial template imprinting using ZnOnanostructures that can be produced via economicalroutes. Good control can be achieved over tuning themorphologies of the nanostructures by changing the

growth conditions on variety of substrates. The nano-structures fabricated via this facile method, are transferredto the BMG samples via thermoplastic forming followedby rinsing to achieve multi-patterned BMG, as shown inFigure 7. Apart from using sacrificial templates, metallicglasses have also been patterned using polymer templates,an economical method as opposed to disposable silicontemplates.124

Another issue during imprinting that appears whenthe sample is exposed to higher temperatures is the initi-ation of crystallization. This limitation can either benegated by room temperature imprinting or patterningmethods capable of short heating times and fast forming.In the former case, Kim has, very remarkably, carriedout the imprinting of Hf-Cu-Ni-Ti-Al amorphous sys-tem—one that generally shows zero macroscopic plastic-ity—at room temperature, opening a new researchdimension of room temperature BMG imprinting.79

Micro-viscous deformability, as shown in this work, hasbroad implications for room temperature ductility and

Figure 7. Formation of a 3D nano/microstructured pattern by sacrificial templating. (Top) A macro/microstructured mold undergoescyclical steps of (i) Showing hydrothermal growth of ZnO nanostructure, (ii) multiscale imprinting via thermoplastic forming of BMG, (iii)mechanical detachment from the mold, and (iv) rinsing of the mold to etch away the template. (Bottom) Scanning Electron Microscopyimages with high magnification insets of (left) a fabricated multiscale mold after growth of the sacrificial template, ZnO and (right) thefinal rinsed multiscale patterned BMG exhibiting textured stent geometry. Scale bars, 200 mm in main images and 200 nm in insets.(© Nature Publishing Group (NPG). Reprinted with permission from Singer et al.73 Permission to reuse must be obtained from therightsholder.)


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other engineering applications. In the latter paradigm, anovel fast and energy-efficient strategy is designed byKaltenboeck125 in which a Zr-Ti-Cu-Be metallic glassstrip is ohmically heated by the simultaneous applicationof electric current pulse perpendicular to a magneticfield. The specimen is shaped by the resulting Laplaceforce exerted on the strip beneath which a ceramic diewith the desired pattern is placed. This method canshape BMGs in extremely short millisecond durations inthe absence of any conventional heating sources andmechanical loading. Another approach to bypass crystal-lization during high temperature forming operation isthe electric discharge heating126 in which BMGs are uni-formly heated to viscosity states enabling thermoplasticshaping for time duration short enough to avoid theinception of crystallization. Likewise, a rapid-capacitivedischarge technique with homogeneous heating ratesaround 105 K/s can bypass the crystallization of theamorphous system127 for the same reasons of short timeduration. High precision metallic glasses, even in case ofmarginal glass formers, can be fabricated, using theaforementioned method, in an optimal viscosity range.

Since the size of nanopatterns is restricted by grainsize in crystalline materials,128 the nano-patterned mate-rials, therefore, usually result in rough surfaces. BMGs,however, produces extremely smooth features (seeFigure 8). 76,129 For instance, nanoarchitectured Pt-Cu-Ni-P BMG samples130 have been produced by forcing aBMG sample over a preheated nanoporous alumina dis-playing smooth nanoscale features. Another example isthe Zr-Al-Cu-Ni system that displays good nanoscalepatterning ability as observed after Focused Ion Beam(FIB) etching done in comparison to a crystalline Pt thinfilm. Very smooth etching is observed in the case ofglassy system owing to its isotropic nature. Using FIB,patterns of line width as small as 12 nm have beenobtained on BMG surface.76 Some BMGs with reactivenature, usually oxidize in the process of nanomolding,and develop an oxide layer that, consequently, affects thenanomolding. The introduction of a wetting layer131

may work very well during processing of such glasses toavoid these complications. Isotropic and homogeneoushigh strength features ranging from around 30 nm tocentimeters can be achieved on BMG surfaces withoutthe presence of any porosity or residual stresses. Suchfascinating features in the nano/micro range can ensureperformance enhancements in current MEMS, NEMSdevices, and other nanostructures.132

5. Bulk metallic glass composites

Structural applications require high performance engi-neering materials in which a combination of certain

properties like strength, fracture toughness and ductilityare indispensable. BMGs are lucrative engineering mate-rials but the Achille’s heel is their inconsiderable plastic-ity and defencelessness to catastrophic failure thatobstructs their role as structural materials. BMGs, moreoften than not, display brittle nature and to dodge thisshortcoming, different impressive findings have beenpublished. Because of the conflicting nature of the simul-taneous presence of high strength and high ductility inany crystalline or amorphous system, combinations ofuseful mechanical properties are, indeed, harsh to design.

Figure 8. Atomic force microscopy of BMG surfaces and the cor-responding height profiles along the indicated lines: (a) Pt-BMGmolten droplet quenched in water (Z-contrast: 156 nm). Theheight profile reveals a maximum peak-to-valley roughness ofabout 25 nm for the as-cast Pt-BMG. (b) The mechanically pol-ished Pt- BMG surface (Z-contrast: 145 nm) displays a higherpeak-to-valley roughness of 60 nm. (c) In contrast, Pt-BMG ther-moplastically formed on silicon is as smooth as the silicon (Z-con-trast: 2.5 nm), with a maximum peak-to-valley roughness of3.5 A

!. (d) An atomically smooth surface is obtained after thermo-

plastic forming of Pt-BMG on cleaved mica (Z-contrast: 2 nm).The peak-to-valley roughness in the 2 mm horizontal scan is lessthan 2 A

!. (© AIP Publishing LLC. Reprinted with permission from

Kumar et al.129 Permission to reuse must be obtained from therightsholder.)


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The composite effect, nonetheless, is one alternative toachieve such combinations, which are otherwise quiteimprobable. The BMG-based composites are broadlydivided into three classes as follows.

1. In-situ composites with in-situ precipitated nano-crystalline phases either via controlled annealing ordeformation-induced devitrification (DID), etc.

2. Ex-situ composites with BMGs either as reinforce-ments or as matrix.

3. Multi-layered composites with alternating crystal-line-amorphous nanolayers.

In this section, different classes of, and current trendsin, BMG composites are chronicled. In Table 2, the dif-ferent classes of BMG composites, their mechanicalproperties, and the peculiar phenomena responsible forthe attained properties are summarized.

5.1. In-situ composites

In this class of BMG composites, nanocrystalline domainsare purposely introduced inside amorphous matrix usingvarious techniques. In this regard, a very imposing work isdone by Hofmann: achieving high toughness and tensileductility in BMG-based composites with inhomogeneousmicrostructure containing crystalline dendrites in glassymatrix. The dendrites, produced as a result of a two-stepprocess, stabilize the overall system and arrest the cata-strophic failure, thus resulting in improved global plasticity.The system displays fascinating mechanical properties withroom-temperature tensile ductility higher than 10%, yield-strength around 1.2–-1.5 GPa and extremely high values ofK1C and G1C nearly-equal-to or exceeding those of thetoughest known steel and titanium alloys.9 Such incredibleproperties substantiate the claim of BMG-based composites

as modern structural engineering materials. The engineer-ing stress-strain curves of Zr36.6Ti31.4Nb7Cu5.9Be19.1,Zr38.3Ti32.9Nb7.3Cu6.2Be15.3, and Zr39.6Ti33.9Nb7.6Cu6.4Be12.5(DH1, DH2, and DH3) are shown in Figure 9.

Identically, the in-situ precipitation of nanocrystalshas been successful in an endeavor to enhance the com-pressive plasticity of Cu-Zr-based bulk metallicglasses.133 In this study, various systems having fullyamorphous, partially amorphous regions and largeembedded crystals are comparatively investigated formechanical properties. Excellent plasticity is exhibited bythe partially amorphous system containing small nano-crystals, both under compression and bending.133 Suchbehavior is attributable to the presence of nanocrystalsthat play a prominent role in the nucleation and multi-plication of shear bands. In another study, Khanolkaret al. probed the effect of partial devitrification on shockwave response of Fe-based metallic glass matrix compo-sites. Yet again, the addition of nanocrystals results insignificant increase in yield strength and shear strengthretention, by obstructing the propagation of failurefronts.134 The same strategy of in-situ precipitation hasworked equally well in another Fe-based BMG compos-ite. A significant improvement in strength, over 3.0 GPa,and plastic strain exceeding 30%—by the introduction ofin-situ ductile a–Fe dendrites—is witnessed in compari-son to fully amorphous brittle system.8 Such tremendousimprovements are attributed to the resulting dendritesthat hamper the propagation of main shear band as wellas activate multiple shear band generation, thus effec-tively circumventing catastrophic failure. During in-situfabrication, the reinforcing phase is carefully chosen soas not to degrade other useful properties. For instance, inmany cases the elastic strain limit of BMG composites is

Table 2. The different classes of BMG composites, their mechanical properties, and the peculiar phenomena responsible for theenhanced mechanical properties.

S No. Classification System/s Mechanical Properties Peculiar Phenomena Reference(s)

1 In-situ formation of dendrites Zr-Ti-Nb-Cu-Be Tensile ductility: > 10%, Yieldstrength: 1.2–1.5 GPa

Stabilization of glass againstcatastrophic failure by isolateddendrites in a BMG matrix

9

2 In-situ precipitation ofnanocrystals

Cu-Zr-based Fracture strength: 2.3 GPa,Plastic strain: 9%

Multiplication of shear bands 133

3 In-situ precipitation ofcrystalline phases

Fe-Cr-Mn-Mo-W-B-C-Si

Hugoniot elastic limit: 8.58 GPa forSAM2£ 5–600 and 11.76 GPa forSAM2£ 5–630

Barriers (nanocrystallites) to thepropagation of failure fronts (shearbands and cracks)

134

4 In-situ formation ofcrystalline phases

Fe-based Fracture strength: 3 GPa, Plasticstrain: > 30%

Hampering main shear band andactivation of multiple shear bands

8

5 Metallic glass reinforcement-crystalline matrix

Zr-based MG fibers-Al 7075

Yield strength: 366 MPa (composite)and 168 MPa (alloy)

Crack blunting by reinforcing phase 19

6 Dual phase crystalline-amorphous composite

TiNi— Cu-Zr Max. Fracture strength: 2600 MPa,Plastic strain: 25%

Transformation induced plasticity, TRIPeffect

47

7 B2-reinforced amorphousmatrix

Cu-Zr-based Max. Fracture strength 1.5 GPa,Plastic strain: 19.3%

Reversible B2–B19’ Transformation 45

8 Nanolayered crystalline/amorphous

Cu/Cu-Zr Fracture strength:> 2.0 GPa,Plastic strain: 30%

Deformation-induced devitrification(DID)

88

9 Nanolayered crystalline/amorphous

Cu/Cu-Zr Ultimate tensile strength: 2.1 GPa,Tensile deformability: 4%

Size-dependent deformation-modetransition

89


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not preserved, though with successful achievement ofstrength and ductility. For such cases, NiTi, for instance,exhibits two excellent properties of strain hardening andreversible B2–B19 transformation, both of which canequip any system with superior strength and superelas-ticity. The existence of such phase in amorphous matrixhas yielded fascinating results.45 Wu et al. investigated aB2- in-situ reinforced bulk metallic glass composite (B2-BMGC) showing large elastic strain limit and excellentplastic deformation capability under tensile loading. Theas cast system shows a plastic strain of 19.3% before frac-ture while being tested in tension—a property that isattributed to the strain-hardening characteristic of B2phase and the activation of multiple shear bands. Nano-crystals have also popped up in various systems inresponse to applied deformation, a phenomenon calledin-situ deformation-induced devitrification (DID). Thispeculiar phenomenon, too, leads to incredible plastic-ity135 on the same grounds, as discussed earlier.

5.2. Ex-situ composites

BMGs have been used both as reinforcing materials in acrystalline matrix136–139 and as matrix material140,141 andsuch systems are emerging as new class of compositematerials. As an example, Wang et al. reinforced Al7075

alloy with Zr-based metallic glass fibres via spark plasmasintering.19 A thin interdiffusion layer is formed at theinterfaces justifying the excellent bonding characteristicsbetween the two systems. The reinforcing phase inhibitsthe plastic deformation and retards crack propagation bycrack blunting, subsequently improving the yield strengthby a substantial amount, from » 168 MPa of the alloy to» 366 MPa of the composite. In other studies, TRIPeffect—the transformation of austenite to martensite withthe application of stress—is incorporated into amorphoussystems via composite formation. The TRIP effect leads toredistribution of applied stresses and ameliorate the plasticproperties of amorphous materials. Dual-phase (Ti-Ni—Cu-Zr) crystalline-glassy composites exhibit goodmechanical properties such as yield strength, total plasticstrain, and transformation induced plasticity (TRIP). Inthe compression testing of this system, the reversiblestress-induced transformation plays a vital role in increas-ing the plasticity of the dual-phase systems.47 Transforma-tion induced plasticity is also found in other BMGcomposites via in-situ neutron diffraction.142

5.3. Nanolayered crystalline/amorphous composites

Nanolayered composites, too, possess good combinationof mechanical properties. A very intriguing finding,

Figure 9. Improved tensile ductility of DH1, DH2, and DH3 at room temperature: Backscattered SEM image of the microstructure of DH1(a) and DH3 (b) where the dark contrast is due to the glass matrix and the light contrast is from the dendrites. (c) Engineering stress-strain curves of Vitreloy 1 and DH1, DH2, and DH3 at room-temperature tension tests. (d) Optical micrograph showing the necking inDH3. (e) Optical micrographs of undeformed tensile specimen with deformed DH2 and DH3 specimens. (f) SEM micrograph showingthe tensile surface of deformed DH3, the inset shows the higher magnification. SEM micrographs show the necking in DH2 (g) and DH3(h). (i) Representative brittle fracture for monolithic BMG samples. (© NPG. Reprinted with permission from Hofmann et al.9 Permissionto reuse must be obtained from the rightsholder.)


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deformation-induced devitrification (DID), is revealedby Zhang et al. while investigating the plastic deforma-tion behavior of crystalline Cu-amorphous Cu-Zr nano-layered heterogeneous system. Again, an otherwisemutually inconsistent combination of strength anddeformability, is attained in this system with very highstrength value of 2.0 GPa and exceptionally high deform-ability of »30% (shown in Figure 10). This superiordeformability is achieved by the concoction of two plas-tic-enhancing phenomena, i.e., the deformation induceddevitrification in the glassy layers aided by externallysupplied dislocations from the adjacent crystallinelayers.88 Since glassy layers act as sinks for dislocations,the absorbed dislocations provide strain-softening insidecrystalline layers by preventing dislocation debris as wellas stimulate atomic arrangements in the glassy layersthat subsequently activate the DID process. Essentially,the heterogeneous structure helps in hampering the gen-eration of shear bands. Homogeneous plasticity alongwith high strength values are successfully achieved innanolaminates of Cu-Zr to Cu system.89

All such studies conducted on the different BMGcomposites and the promising mechanical properties

obtained give an indication that BMGs are emerging assuitable candidate materials for structural applications.Moreover, the issues of brittleness and susceptibility tocatastrophic failures are diminishing with such novelstrategies coming forward.

6. Biocorrosion

In spite of the fact that there are numerous crystallinematerials exhibiting decent in-vitro and in-vivo biocor-rosion and biocompatibility—for instance, the widelyemployed biomedical 316L stainless steel, cobalt-chro-mium alloys, Ti-based, Ta-based, and Zr-based sys-tems—they, however, possess certain shortcomings suchas leaching of bio-toxic ions owing to the presence ofbio-toxic elements Ni, Al, etc. as well as high elastic mod-ulus that may lead to stress shielding.143 In contrast, bulkmetallic glasses have garnered immense research enthu-siasm as potential biomaterials endowed with excellentcorrosion resistance properties, mechanical properties,wear resistance, and low Young’s modulus. The highercorrosion resistance is ascribed to the absence of crystal-line defects and microstructural inhomogeneities. The

Figure 10. SEM images of nanolayered Cu/Cu-Zr micropillars: (a) before and (b) after the uniaxial compression tests, showing barrelingof the micropillar and extrusion of individual Cu layers. (c) The cross-sectional image of the deformed pillar, displaying the layered struc-ture. (d) The true stress-strain plot for Cu/Cu-Zr micropillars with three different layer thicknesses showing strain hardening-to-softeningtransition. Inset is the true stress–strain curves of amorphous pillars associated with shear banding induced-strain burst. (© NPG.Reprinted with permission from Zhang et al.88 Permission to reuse must be obtained from the rightsholder.)


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engrossing aspect of BMGs that render them as highlycorrosion-resistant systems is the flexibility in the selec-tion of corrosion resistant species from a wider composi-tional space which provide an efficient way to improvecorrosion resistance—for instance, by choosing elementsthat form protective oxide layers.

In metallic materials, crystalline and amorphous alike,alloying elements can go into the surrounding environ-ment as a result of corrosion—a prominent mechanismalong with wear—and can produce toxic effects, adversebiological reactions, site accumulation, and eventualimplant failure. Thus, corrosion process, apart frombeing detrimental to mechanical properties, dictate bio-compatibility as well.144 As such, the biocorrosion inves-tigation is very crucial to ensure safe performance of thebioimplants in-vivo. Excellent bio-corrosion resistance isvitally important for biomedical implant materials to tol-erate the aggressive human body environment and sup-press the release of corrosion-induced metallic ions.

Extensive research is carried out on the corrosion-resistance properties of different BMG systems, chiefly,in three different dimensions: (1) comparative study ofamorphous systems and their crystalline counterparts;

(2) comparison of two or more BMGs with differentcompositions but belonging from the same system withsame principal species; and (3) corrosion variation inBMGs caused by the changing corrosive media or minoraddition of elements. This section reviews the bio-corro-sion investigation of three important classes of BMGs—namely Zr-, Ti-, and Fe-based—in different biologicalmedia such as phosphate buffered saline (PBS), artificialsaliva, Hank’s solution, and Ringer’s solution in compar-ison to crystalline biomaterials.

Zr-based BMGs are well known for their high GFA,excellent corrosion resistance and biocompatibility. Forexample, one noteworthy study is the electrochemicalpolarization measurements of Ni-free Zr-Cu-Fe-Al-Agin PBS (see Figure 11a) in comparison to pure Zirconiummetal and Ti-6Al-4V. The BMG shows a spontaneouspassivation with extremely low passive current density,in fact orders of magnitude lower than pure Zr and Ti-6Al-4V alloy.53 Furthermore, the BMG also exhibits awide passive region of around 0.92 V, although lowerthan that of Ti-6Al-4V.

Zr-Pd system in Hank’s solution has also displayedpromising results. Anodic polarization studies of this

Figure 11. Potentiodynamic polarization curves of various BMG systems in comparison to know biomaterials: (a) Zr-Cu-Fe-Al-Ag in SBFsolution at 37!C, in comparison with well-known systems, Ti6Al4V alloy and pure Zr metal. The BMG shows spontaneous passivationand possesses the lowest passive current density along with a wide passivation potential around 0.92 V. (© Elsevier. Reprinted with per-mission from Liu et al.53 Permission to reuse must be obtained from the rightsholder.) (b) Comparative study of pure Ti and Ti–Zr–Sithin film metallic glasses in SBF at 37!C. (© Elsevier. Reprinted with permission from Ke et al.143 Permission to reuse must be obtainedfrom the rightsholder.) (c) Potentiodynamic polarization curves of the Fe-based BMGs, in comparison to 316L SS and TC4 open to air at37!C, in Hank’s solution. (© Elsevier. Reprinted with permission from Li et al.145 Permission to reuse must be obtained from therightsholder.)


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system show a self-passivation behavior and formation ofZrO2 protective oxide film.23 The system exhibits veryhigh resistance to pitting corrosion throughout the widepotential region of around 1000 mV. The average currentdensity value shown by this system is comparativelylower than many Ti-based, Zr-based, Fe-based glassyalloys, and few crystalline systems. Another comparativestudy is done by Lu et al. using Ni-free Zr–Cu–Al–Nb–Pd system in PBS along with Ti-6Al-4V and 316L stain-less as reference materials.55 The BMGs show spontane-ous passivation behavior and lower passive currentdensities than both reference materials. Furthermore, theBMG shows a higher pitting resistance as compared to316L stainless steel. Such excellent properties resultmainly from the ZrO2-rich surface films which is moreprotective than the one formed on stainless steel. Com-parison of amorphous versus crystalline materials havealso corroborated the former as relatively more corrosionresistant. For instance, Tabeshian et al. have comparedthe corrosion behavior of bulk crystalline and amor-phous Zr-Cu-Ni-Al systems in PBS via polarization andelectrochemical impedance measurements. Pitting corro-sion is detected on both systems, however, the passiv-ation region in case of crystalline system is substantiallysmaller.146

Of all the metallic glasses developed, Ti-based BMGs,in comparison to others, have attracted special attentiondue to their low density, high specific strength, excellentcorrosion resistance, and biocompatibility. Fabricatingmetallic glasses without the presence of toxic elementssuch as Cu, Be, Al, and Ni—which are necessary forgood GFA—is a challenging task. In this regard, a Ti-Zr-Si, without undesirable bio-toxic elements, has beenreported that possesses excellent biocorrosion propertiesin SBF, even bypassing pure Ti, justified by the lowercorrosion current and passive current densities. On topof that, the system exhibits lower modulus and highhardness making it more appropriate for bioimplants. Inthis study, the appearance of nanocrystalline regimes isshowed to aggravate corrosion resistance. Both potentio-dynamic (see Figure 11b) and AC impedance spectros-copy justify the higher corrosion resistance of the saidfilms in comparison to pure Ti.143 In the same chain isthe Ti60Nb15Zr10Si15 system, free of bio-toxic elements,reported by Calin et al. exhibiting extremely low corro-sion current and passive current density, during electro-chemical testing in Ringer’s solution—lower than cp-Tiand much lower than cast polycrystalline species.144

Although copper is notorious for biomedical applica-tions, still its inclusion in BMGs is, sort of, inescapable.Wang et al. did the potentiodynamic polarization of Ti-Cu-Hf-Si in Hank’s solution, displaying better corrosionresistance than pure Ti and Ti–6Al–4V alloy.147

Furthermore, Ti–Zr–Cu–Co–Sn–Si–Ag148 and Ni-freeTi–Zr–Cu–Fe–Sn–Si149 both possess a higher corrosionpotential (Ecorr) and lower corrosion current density, incontrast to Ti–6Al–4V, in PBS.

On the grounds of disordered nature, Fe-based BMGs,too, manifest better corrosion resistance properties ascompared to conventional crystalline materials, forinstance, 316L SS biomedical steel, in the form of lowervalues of Icorr and higher values of pitting potential.150 Inaddition, they possess excellent mechanical properties,biocompatibility, relatively higher GFA, and low cost.Thus, they are also emerging as potential materials forbiomedical implant applications. In one study, Li et al.reported the biocorrosion of a series of Fe80 ¡ x ¡ yCrx-Mo

yP13C7 BMGs in Hanks solution and artificial saliva

with Fe55Cr20Mo5P13C7 system demonstrating outstand-ing corrosion resistance. The Icorr of the Fe-based BMGsis lower than that of 316L SS while the pitting potentialEpit and the width of passivation region are markedlyhigher than 316L SS, suggesting relatively more stablepassive films on BMGs145 (see Figure 11c). Another sys-tem Fe55-xCr18Mo7B16C4Nbx has been comparativelyinvestigated in Ringer’s solution. with 316L stainless steelas reference. The said BMG system exhibits a lower cor-rosion current, lower passive current density, wider pas-sive region, and higher polarization resistance. Amongthe tested compositions, the Fe51Cr18Mo7B16C4Nb4 pos-sesses higher polarization resistance value as comparedto both 316L stainless steel and Ti–6Al–4V testifying totheir excellent corrosion resistance properties.151 In thesame series with principal Fe, Cr, Mo species is Fe-Co-Cr-Mo-C-B-Y, that displays higher values of polarizationresistance and pitting corrosion potential as compared to316L SS in Hank’s solution and artificial saliva solutionmaking it a safe alternative for biomedical applications.56

Table 3 summarizes the biocorrosion studies of differentBMGs in comparison to conventional biomaterials.

In short, Zr-, Ti-, and Fe-based systems, in most cases,exceed the corrosion resistance of conventional biomate-rials such as commercially pure titanium, zirconium,316L SS and Ti-6Al-4V alloy in biological media. Addi-tionally, the reported studies offer a compelling evidencethat such amorphous systems—after more rigorous in-vivo and in-vitro cytocompatibility investigations—can,in the near future, replace conventional biomaterials.

7. Magnetic properties

The role of magnetic materials in rapid technologicalrevolution is, undoubtedly, very decisive. There havealways been stringent needs of materials with concoctionof excellent strength and remarkable magneticproperties152–154 for future functional and structural


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applications. Metallic glasses, with distinct mechanical,physical, and chemical properties, are attractive materialsin fields like MEMs, NEMs devices where fusion of elec-trical, magnetic, and optical components, are required.For example, amorphous high Curie temperature mag-netic semiconductor is demonstrated in one study withcombined functionalities: Negative temperature depen-dence of resistivity, ferromagnetism, and high transpar-ency, all in one system fabricated by the introduction ofoxygen into ferromagnetic metallic glass Co-Fe-Ta-B.This seminal work offers a fertile ground for novel appli-cations and device concepts. Other than combined func-tionalities, another distinct feature of amorphousmagnetic systems is that they allow direct one-stepmanufacturing of final shape products. Moreover, prop-erties can be easily tailored by exploiting the flexibilityoffered by BMGs in terms of composition, size, andshape. Commercial applications in the area of magneticrecording, hard magnets, high density rewritable devices,magnetic refrigerants, and magnetostrictive devices arehighly anticipated. Figure 12 summarizes various mag-netic properties discovered in BMGs, with their potentialapplications. In the following paragraphs, a brief over-view of magnetic properties displayed by BMGs andtheir potential applications are reviewed.

7.1. Soft and hard magnetic properties

BMGs, usually, exhibit soft magnetic properties becauseof the absence of structural inhomogeneities such as

grain boundaries and crystal defects that hinder thedomain wall motion during magnetization. Fe-, Ni-, andCo-based metallic glasses have been extensively studiedhaving excellent soft magnetic properties in the form oflow coercive force and high magnetic permeability.90,155

Soft magnetic properties have been exploited for applica-tions in different fields such as cores, transformers, canti-levers, and sensors. As opposed to BMGs’ characteristicnature of soft magnetic properties, the appearance ofhard magnetic properties—primarily because of the pres-ence of nanocrystalline phases, precipitates or otherdefects—is, also, very appealing and therefore, haveattracted wide-scale research. BMGs with hard magneticproperties have potential applications in recording disks,actuators, and permanent magnetic motors, etc.156 Nd-Fe-Al,156 Pr-Fe-Al,157 and Sm-Fe-Al158 are some of theamorphous systems with decent combination of bothappreciable GFAs and hard magnetic properties.

7.2. Magnetostriction

BMGs possess magnetostriction behavior—change indimensions of a material in response to external magneticfield—which expands their scope in the field of sensors66

and actuators. In one study, for instance, Fe64Co17Si7B12has been used as a magnetostrictive material along with apiezoelectric element to fabricate a multiferroic magneto-electric composite for energy harvesting.159 Even a weakelectromagnetic signal can deform the metallic glass rib-bon because of high magnetostriction value » 25 £ 10¡6.

Table 3. Biocorrosion study of various BMG systems in comparison to conventional crystalline biomaterials.

Classificationof BMGs BMG System/s

Corrosiveenvironment

Conventional biomaterialsfor comparative study

Interesting electrochemical aspects of BMGs ascompared to the widely employed biomaterials Reference

Zr-based Zr-Cu-Fe-Al-Ag PBS Zr & Ti-6Al-4V Lower passive current density by many orders ofmagnitude

53

Zr-Pd Hank’s solution … Lower current density (Icorr)23

Zr-Cu-Al-Nb-Pd PBS 316L Stainless steel &Ti-6Al-4V

Spontaneous passivation and lower passive currentdensities

55

Zr-Cu-Ni-Al PBS Zr-Cu-Ni-Al bulk crystalline Higher passivation region 146

Ti-based Ti-Zr-Si SBF Pure Ti lower corrosion current (Icorr) and passive currentdensities

143

Ti-Nb-Zr-Si Ringer’s solution cp-Ti & cast polycrystalline extremely low corrosion current (Icorr) and passivecurrent density

144

Ti-Cu-Hf-Si Hank’s solution Pure Ti & Ti–6Al–4V Higher corrosion potential (Ecorr) 147

Ti–Zr–Cu–Co–Sn–Si–Ag PBS. Ti–6Al–4V Higher corrosion potential (Ecorr) and lower corrosioncurrent density (Icorr)

148

Ti–Zr–Cu–Fe–Sn–Si PBS. Ti–6Al–4V Higher corrosion potential (Ecorr) and lower corrosioncurrent density (Icorr)

149

Fe-based Fe-Co-Cr-Mo-C-B-YFe-Cr-Co-Mo-Mn-C-B-Y Fe-Cr-Mo-C-B-Er

Hank’s solution andartificial saliva

316L Stainless steel Lower values of corrosion current density (Icorr) andhigher pitting potential (Epit)

150

Fe-Cr-Mo-P-C Hank’s solution andartificial saliva

316L Stainless steel Lower corrosion current density (Icorr), higher pittingpotential (Epit) and width of passivation region

145

Fe-Cr-Mo-B-C-Nb Ringer’s solution 316L Stainless steel andTi-6Al-4V

lower corrosion current (Icorr), lower passive currentdensity, wider passive region, and higherpolarization resistance

151

Fe-Co-Cr-Mo-C-B-Y Hank’s solution andartificial saliva

316L Stainless steel Higher values of polarization resistance and pittingpotential (Epit)

56


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Ni-Mn-Ga has also been used as a magnetostrictive mate-rial in another composite.160 The same effect of magneto-striction coupled with soft magnetic properties of BMGs,has been exploited in the fabrication of a high-perfor-mance biosensor.161 Magnetostriction can be tuned usingvarious approaches, for instance, Fe-Co-B-Si-Nb is dem-onstrated to improve with the addition of carbon showinga linear correlation between applied magnetic field andresulting displacement162 which is highly important forsensing applications.

7.3. Magnetocaloric effect

Magnetocaloric effect, a reversible temperature changingphenomenon, that occurs when a material is imposed to achanging magnetic field, usually accompanied by a changein magnetic entropy, is also displayed by variousBMGs.163–165 This effect can be employed for magneticrefrigeration—an energy efficient and environmentallyfriendly approach. Although attractive candidate materialssuch as Gd5Si2Ge2 compound or other rare earth based

systems—showing first-order or second-order magnetictransition—exist but they have major shortcomings suchas low corrosion resistance, hysteresis losses, and poormechanical properties. Fe-based amorphous systems, onthe other hand, possess captivating combination ofmechanical properties, soft magnetic properties—withmagnetic hysteresis approaching zero—, tunable curietemperatures and excellent corrosion resistance.166 In onestudy for instance, Li et al. demonstrated Fe-Mn-P-B-Cmetallic glass exhibiting both tunable magnetic entropychange and Curie temperature by varying Mn content.The said system reaches a highest refrigerant value of147.09 J Kg¡1 and can be exploited for low-cost and near-room-temperature magnetic refrigerant.166 Likewise, Liet al. studied (Fe0.76 – xTmxB0.24)96Nb4 BMG by dopingthe system with Thulium. The doping enhances the mag-netic caloric response as well as makes the Curie tempera-ture tunable.167 This system, too, has the potential to beused as room-temperature magnetic refrigerants. Addi-tionally, Re-based BMGs are also opted out as excellentcandidate materials for magnetic refrigerants.41

Figure 12. Schematic showing the novel magnetic properties exhibited by different BMGs and their potential applications.


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7.4. Spin dynamics and heavy fermion behavior

Spin dynamics (SD) and heavy fermion (HF) behavioursare also exhibited by BMGs. Spin glass (SG)—orienta-tions’ freezing of magnetic moments—is observed inPr60Al10Ni10Cu16Fe4, a model system displaying multipleSG transitions. BMGs display distinctive characteristicsfrom conventional SG materials. A noncollinear-ferro-magnetic spin-glass state is observed in Tb-Co-Al ternarymetallic glass system showing a large magnetic entropychange of 9.75 J K¡1 Kg¡1.168 Gd-Ni-Al also shows spin-glass behavior, large magnetocaloric effect, and magneticentropy values.169 Another property in BMGs, the heavyfermion behavior—in which electrons exhibit extremelyhigh effective masses—have attracted large attentionbecause of some appealing properties that such systemspossess. Such properties arise as a result of competitionbetween two prominent effects, the Kondo resonanceand magnetic ordering.170 Ce-La-Al-Cu-Co BMG systempossesses substantial heavy fermion behavior.41 Further-more, the addition of minor amount of Gd can transformCu-Zr-Al BMG into a heavy-fermion system.171

7.5. Novel magnetomechanical interactions

Magnetic properties of BMGs can be exploited to changethe microstructure. Magnetomechanical interaction in

the form of induced magnetic anisotropy resulting fromhigh-load indentation is witnessed in Fe-C-B-Si-P, asshown in Figure 13. This shows that apart from bringingmicrostructural variations in metallic glasses via changesin flow defect, microstructures can also be controlled andmodified using magnetic effect as a stress-induced mag-netic anisotropy can give rise to elastic heterogeneity.172

7.6. Piezomagnetism

Piezomagnetism—change in magnetoconductivity aris-ing from the application of stresses at the surface ofamorphous systems—is another fascinating phenome-non found in BMGs 173,174 that can be used for highlysensitive, non-contact, strong anti-interference, and fastresponse piezomagnetic sensors.64 Amorphous alloy rib-bons with excellent magnetic properties are suitable can-didate materials for such force-sensitive sensors.

8. Recent applications of BMGs

BMGs possess excellent combination of properties suchas high values of yield strength, elasticity, corrosion resis-tance; low elastic moduli; polymer-like thermoplasticformability, superplasticity; and excellent magnetic prop-erties which render this class of material very suitable for

Figure 13. Schematic of magnetomechanical interactions in Fe-C-B-Si-P BMG: The phase images of (a) as spun Fe-C-B-Si-P ribbon sys-tem, (b) region I, (c) region II, and (d) region III of deformed system taken using Magnetic Force Microscopy. The circles S, A, B, C, and Dmark the position of nanoindentations. (© Elsevier. Reprinted with permission from Liu et al.172 Permission to reuse must be obtainedfrom the rightsholder.)


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many technological applications. Figure 14 depicts theexcellent features of BMGs that can be exploited forapplications in different fields.

8.1. Applications in catalysis

Electrocatalysis play a fundamental role in energy storageand conversion devices such as batteries, fuel cells, andelectrolysis cells. The widespread commercialization ofelectrochemical devices to fulfill the rising energydemands has been, largely, impeded by the poor effi-ciency and durability of catalysts. Noble-metal-basedmetallic glasses, on the other hand, display phenomenalelectrocatalytic activity and durability, making thempromising candidate materials for next-generationenergy storage and conversion devices.61,175,176 Their dis-ordered atomic configuration result in a complex elec-tronic structure that contribute to enhanced catalyticproperties. For this reason, the research interest in inves-tigating BMGs, as high-performance catalysts, is swiftlymounting. Designing tailored BMG systems for electro-catalysis can impart significant improvements in a greatmany fields such as batteries, micro-reactors, sensors,and fuel cells. In the following paragraphs, some capti-vating BMG-based catalysts with applications in fuelcells, water purification, and hydrogen evolution reaction(HER) are explored.

In electrocatalysis, multicomponent systems—asopposed to single-component catalysts—are more appro-priate for enhanced activity, efficiency, and durability.Such systems have been developed via synthesis of core-shell nanoparticles, atomically dispersed alloys, and sur-face modification strategies, but there are many challengesand processing complexities associated with them. In con-trast, however, nanostructured bulk metallic glasses—mul-ticomponent systems with no limits on solubility of

alloying elements as well as capable of nanomolding—canbe tuned to ameliorate catalytic function and durabilityvia compositional and morphological control, as discussedby Doubek et al.78 In this study, Pt-based BMGs havebeen synthesized with desired composition and uniquemorphological structures for enhanced electrocatalysisusing subtractive modification and additive manufactur-ing. In the former, a component, especially a less nobleone, is preferentially leached out from the surface in orderto enrich it with more noble elements; while in the latter,processes such as galvanic displacement and underpoten-tial deposition are implemented to deposit a newly desiredspecies on the active surface of the specimen (seeFigure 15). The BMG system is further studied usinghydrogen evolution, methanol oxidation, and oxygenreduction all of which testify to the highly active and dura-ble nature of BMG-based catalyst.78 The BMG in compari-son to Pt/C catalysts, acts as a “self-improving” systemattributed to the process of dealloying that exposes higherand higher amounts of active sites in response to corro-sion. It is noteworthy that the BMG-based catalyst dis-plays impressive stability at higher temperatures, incontrast to Pt/C catalyst, that demonstrate complete deac-tivation near 80!C.

BMG-based catalysts, owing to their huge potential,have been studied as catalyst materials in different fields.In the area of fuel cells, for instance, the conventional cat-alysts are required to exhibit two critical features—a highsurface area and a multicomponent system. As a result ofthe broad applicability of high surface area nanostruc-tures for energy conversion, previous efforts were focusedon using 1D carbon nanotubes for enhancing the utiliza-tion and dispersion of catalysts until the presence of car-bon was found to deteriorate catalytic performance.Multicomponent nanowire catalysts are required for fuelcell catalysis to ensure higher activity and utilization and

Figure 14. Some remarkable features exhibited by different BMG systems and their potential applications in various fields.


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therefore, various nanoporous, nanotubular, and nano-wire structures have been reported.177–179 But they sharethe common shortcoming of complex synthesis routesowing to the difficult fabrication of metallic nanostruc-tures. BMGs negate such shortcomings by offering betterformability and compositional flexibility.70 In this regard,Pt-Cu-Ni-P system is fabricated using an economical andscalable top-down nanoimprinting to form nanowiresand subsequently studied for electrochemical perfor-mance. Selective dissolution via dealloying makes thenanowires enriched with platinum thus making thosesites highly conductive which, in turn, aids in augment-ing the overall performance of the system. Excellentdurability and electrochemical activity are exhibited bythe catalysts towards CO, methanol and ethanol oxida-tion. In this BMG system, the synergistic effect of Ni andCu, that promotes the formation of hydroxyl species,leads to enhanced activity of Pt-BMG.70 After 1,000cycles the Pt-BMG is capable of maintaining 96% of theperformance which exceeds more than twice for Pt/Ccatalysts.

Catalysts have also been explored in the area of waterpurification, for instance, in the degradation of AZO dye.In this area, noble metals (Pt, Pd) and transition metal(Fe, Ni) systems have been reported to effectivelydegrade organic water pollutants and AZO dye, however,they face certain limitations. The former system is

expensive, scarce, and prone to catalytic poisoning whilethe latter is bio-toxic, less durable as well as highly reac-tive. Such shortcomings, therefore, limit their potentialapplications in this field. In contrast, BMGs possessexcellent durability, chemical inertness, catalytic activity,and compositional control. The higher catalytic activityof BMGs is usually ascribed to the lower activationenergy for electron transfer, uniform dispersion of activecomponents, and presence of low coordination atoms.69

Currently, Fe-180,181 and Mg-based67,68 BMGs areexploited for the effective degradation of organic chemi-cals. As an illustration, an Al-based BMG has been inves-tigated by Das et al. for degradation of AZO dye and theresults have been compared to zerovalent iron par-ticles—a state-of-the-art catalyst. Al-BMG, as opposed toits crystalline counterpart and zero valent iron particles,showed excellent durability, faster and complete degra-dation of AZO, absence of by-products and morphologi-cal stability69 (see Figure 16). The crystallized BMG isalso tested and the results showed no catalytic activity,probably due to the formation of less active intermetal-lics. The reported BMG system is well-tailored and eachcomponent is added keeping in view its potentialbenefits.

Highly active and durable electrocatalysts are alsorequired in electrochemical water splitting, an economi-cal route for Hydrogen synthesis. Apart from Pt-based

Figure 15. Evolution of the surface of Pt-BMG after dealloying: (a) Schematic showing the generation of nanoporous surface upon deal-loying. (b) The change in Pt content due to dealloying (DCPt) as a function of initial Pt content, characterized via EDX. Inset: (left) co-sputtering system, (right) Pt–Ni–Cu alloy compositional spread on Si wafer. (c) Evolution of the electrochemical surface area (ECSA) ofPt-BMGs during cycling in 0.5 M H2SO4. (© John Wiley and Sons. Reprinted with permission from Doubek et al.78 Permission to reusemust be obtained from the rightsholder.)


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catalysts, various other catalysts such as metal oxides,carbides, sulfides, and their combinations with graphiticnanocarbons have been developed for such purposes, butthey possess relatively low activity and durability, in gen-eral as well as in comparison to Pt-based.71 Therefore,highly active and durable catalysts are also needed forefficient electrochemical hydrogen generation to tacklethe current energy crisis as Hydrogen is a suitable alter-native. In this regard, a multicomponent Pd-Ni-Cu-Pmetallic glass catalyst exhibits a unique self-stabilizingperformance over a long-cycling life attributable to theself-optimized active sites. The self-optimization in thesaid system appears as a result of dealloying on the mul-ticomponent amorphous surface layers.71 The BMGshows enhanced performance and less activity degrada-tion as compared to Pt/C catalysts for long term usage.The incredible performance of the BMG is also attrib-uted to the presence of several types of active sites on thedisordered surface. Figures 17a, b, the overpotential ofBMG is 76 mV driving a current density of 10 mA cm¡2

after 10,000 cyclic voltammogram (CV) cycles whileunder the same condition, the potential of the commer-cial Pt/C catalyst approaches 108 mV which justifies thesuperior HER performance of MG to that of Pt/C cata-lyst, especially in terms of the long-term activity. It is

important to notice that when the onset potential of Pt/C reaches 35 mV after 10,000 CV cycles, the quantity ofthe MG diminishes to 14 mV which is indicative of thefact that Pt/C loses activity during the long time CV test,as opposed to the self-improved behavior from MG. Theexchange current density (j0) of the amorphous catalystextrapolated from the Tafel plot (0.217 mA cm¡2), isalso much larger than that of the Pt/C catalyst ("0.146).

Apart from metallic glasses in bulk form, thin filmamorphous materials have also been studied for catalysisin the field of hydrogen generation. A thin amorphouslayer of MoS on dealloyed nanoporous gold, a core-shellcomposite, has resulted in a six-fold improvement in cat-alytic activity over other MoS-based materials for hydro-gen evolution reaction. Large surface area and highlyconductive nature of the nanoporous electrode alongwith a good charge transfer between gold-amorphousfilm are considered as the basis of such exceptional cata-lytic performance.72

8.2. Biomedical applications

Research on BMGs in terms of their biomedical applica-tions is heading at a tremendous speed as safer andstronger biomaterial are demanded. Studies conducted

Figure 16. Degradation of AZO dye by different systems: (a) Color change of AZO dye (left picture) from deep blue to transparent (rightpicture) after reaction with Al-BMG. The middle picture shows the AZO dye after reaction with zero valent iron powder, and the brown-ish solution is a clear indication of the formation of reaction by-products, testifying that Fe is unable to degrade the dye completely. (b)UV–Vis absorption spectrum shows that the solution became almost completely transparent after reacting with Al-BMG system. (c) nor-malized intensity vs. time plot demonstrating that amorphous Al-BMG particles leads to faster degradation of the AZO dye in compari-son to their crystalline counterpart as well as zero valent iron powder. (d) Raman spectrum of pure AZO dye before and after reactionwith Al-BMG particles certifying the complete dissociation of AZO dye. (e) Infrared spectrum before and after reaction demonstratingthe complete breaking of ─C─H─ and ─C─N─ bonds of AZO dye. (© John Wiley and Sons. Reprinted with permission from Daset al.69 Permission to reuse must be obtained from the rightsholder.)


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so far on the cytocompatibility and biocorrosion of manyZr-, Ti-, Mg-, and Fe-based BMGs show promisingresults.10,54,182 BMGs, their biocompatibility analysis,corrosion analysis, and potential biomedical applicationsare extensively reviewed in several articles42,44,183 thatreinforces the claim of BMGs as prospective candidatematerials for biomedical applications. The biocorrosionand biocompatibility, more often than not, exceeds thatof conventional crystalline biomaterials such as 316L SS,Ti, and commercial Ti-6Al-4V. This section, however, isdevoted to distinct BMG systems that embodies thestriking combination of desired biomedical characteris-tics and that have the capabilities to substitute conven-tional biomaterials.

There has been a substantial increase in cardiovascu-lar diseases and statistical data for the next decade showseven more depressing results. Among the treatmentsavailable, cardiovascular stent implantation is one of themost compelling strategy.184 In the area of stent implan-tation, the widely known stent materials are 316L SS andNiTi both of which contain nickel—an infamous mate-rial that may generate local immune responses. More-over, the lower strength values of these materialsnecessitate thicker strut structures for reliable stents. Tocounter such shortcomings, a Ni-free Zr-based BMGs isa promising candidate for future stent applications withexcellent characteristics.185 The system exhibits goodmechanical properties that allows the fabrication of

Figure 17. Comparison of Pd-Ni-Cu-P MG vs Pt/C catalyst: (a, b) Polarization curves of the MG catalyst (a), and the 10 wt. % Pt/C (b) in0.5 M H2SO4 after 0, 1,000, and 10,000 CV cycles. The insets show the overpotential at 10 mA cm¡2 after 10,000 cycles. The arrows indi-cate the evolution trends of the curves. (c, d) Comparison of the onset potential at 0.6 mA cm¡2 and the overpotential driving 10 mAcm¡2 with increasing cyclic number. (e) Tafel plots obtained from the polarization curves. (f) Variations of the percentages of initial cur-rent at 300 mV vs. time. (© John Wiley and Sons. Reprinted with permission from Hu et al.71 Permission to reuse must be obtained fromthe rightsholder.)


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stents having comparatively thinner struts. The thinnerdesign, might, subsequently lower the chances of reste-nosis. Finite element analysis, conducted in this study,also certifies the mechanical advantage of amorphousstents. Furthermore, this system possesses good biocor-rosion resistance and cytocompatibility that further aug-ments its potential.

Biodegradable Mg-, Ca-, Zn-, and Sr-based BMGsare emerging as a new class of materials that can beused for wide range of applications where temporaryimplants are required.44 Contrary to the recent enthu-siasm in crystalline Mg alloys for bioresorbable appli-cation, they, however, suffer from faster degradationand lower strength and fatigue values. Mg-basedBMGs, on the other hand, display—in comparison tothe crystalline counterparts—higher strength values,low elastic moduli, enhanced corrosion resistance,and flexibility in the addition of alloying elements.The clinical trials conducted on Mg-based biodegrad-able orthopedic implants provide promising resultsindicative of their importance as potential biomateri-als.58 For instance, a novel biodegradable system Ca-Mg-Zn is a potential material for skeletal applica-tions59 owing to the promising properties displayedin in-vitro tests and cytotoxicity analysis. AnotherBMG system with intriguing combination of biode-gradability, high compression strength, biocompatibil-ity, osteoconductivity, and Young’s moduluscomparable to that of human bones, can be a promis-ing biomaterial for orthopedic applications.182 To befully considered for clinical applications, there aresome shortcomings faced by Mg-based biodegradablesystems such as size limitation and lack of plasticity.In several attempts, nevertheless, they are counteredby different strategies such as the addition of minorelements and composite effect respectively but stillthere is a lack of rigorous in-vivo characterization.

Some load bearing biomedical applications necessitategood fracture toughness and fatigue strength. In thisregard, Ni-free Zr-Cu-Fe-Al-Ag biocompatible systemwith impressive values of fatigue strength, fracture tough-ness and low Young’s modulus can be an excellent candi-date.53 Problems like stress shielding and implant failure,that occur in metallic crystalline system because of rela-tively higher Young’s Modulus and lower strength values,respectively, are well known in case of osteosynthesisdevices (implants used for bone fracture managementthat demand removal in future). Zr65Al7.5Ni10Cu17.5BMG have been studied in-vivo for osteosynthesis devi-ces such as bone screws, plates, and intramedullary nails.The Zr-based BMG, in comparison to Ti-6Al-4V, showsinactive nature, less bonding characteristic, easierremoval as well as faster bone healing.60 With more

detailed studies in-vivo, this system might, therefore,provide promising substitute materials for osteosynthesisdevices.

Glassy structure is also exploited for cancer theranos-tics—treatment of tumours via pre-designed tumour spe-cific agents that are responsive to the localized tumourenvironment. In the conventional approach, ferrous ionscause the disproportionation of H2O2 in a specifictumour environment to form reactive oxygen speciesthat subsequently kill the cancerous cells. Ferrocene andits derivatives have been used in several studies for thisanticancer activity but the molecular nature of FeII car-riers are prone to early bio-oxidation. Moreover, thesespecies may also result in side effects in cancerousregions due to overproduction of H2O2. The reactivenature of metallic glass iron nanoparticles, made via hub-ble-bubble method, is exploited to ensure rapid ioniza-tion of iron particles in acidic tumors57 whichsubsequently induce localized Fenton reaction. Amorph-ization of these particles results in improvements inproperties as compared to the crystalline counterpart.Additionally, the disordered structure can be used tovary the biodegradability and tune the magnetic proper-ties for magnetic targeting systems. The intratumoralinjection of amorphous iron nanoparticles completelyinhibit the growth of tumor cells exhibiting a superioranticancer effect, as shown in Figure 18.

In general, the promising findings, so far gathered invarious investigations, testify to the fact that BMGs areemerging as new-generation high-performance biomate-rials for applications like cardiovascular stents, boneimplants and skeletal applications etc.

8.3. BMG-based micro and macro devices

In this section, some of the recently reported BMG-basedmicrodevices and macro-components are discussed. Inmany devices, the crucial components are replaced withBMGs resulting in improved properties. The perfor-mance of such devices, in general, bypass those made ofconventional crystalline materials.

8.3.1. Fuel cellThe main challenge in the designing of fuel cells is theavailability of low-cost, effective materials along withtheir appropriate manufacturing routes. Si is a commonchoice for most of the recently developed fuel cells but itpossesses low shock resistance and poor electrical con-ductivity.186 Additionally, the Si-based Micro Fuel Cells(MFCs) demand expensive and complex processing tech-niques. The thermoplastic formability of BMG, which isa low-cost and economical fabrication method andwhich enables the fabrication of hierarchical structures


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with length scales ranging from centimeters down tonanometers, is exploited to make a BMG micro fuel cell.In this cell, the crucial components such as the catalystlayer, diffusion layer, and flow fields are made of bulkmetallic glasses, using thermoplastic forming (TPF) (seeFigure 19). The flow fields are made of Zr-based BMGembossed via TPF while the catalyst layer is made of Pt-based BMG that exhibits remarkable catalytic perfor-mance. The MFC made via this route offers the advan-tage of processing route as well as allows fabrication ofcell components from a single material. Remarkably, theZr-based current collectors show advantages over theconventionally used stainless steel, silicon, and graphite.TPF-based embossing on BMGs—that are durable andelectrochemically active—thus provides a versatile, fast,and economical route for the production of novel microfuel cells.61

8.3.2. MicrocantileverThe microcantilever sensors have applications in numer-ous fields such as nanotribology, catalysis, magnetic forcemicroscopy, MEMS, etc. Moreover, with the rise of scan-ning probe microscopes, high-performance microcanti-levers are demanded for the precise measurements of

force and displacement. Conventional systems made ofsilicon, nitrides, and other crystalline materials possesslow quality factors as well as suffer from certain process-ing limitations. Likewise, cantilevers made of metalsexhibit low quality factors because of the presence of dis-locations, slip planes, and grain boundaries. Bulk metal-lic glasses, on the contrary, are free from such defectsand possess the superior advantage of polymer-like pro-cessability. One system, Pt-Cu-Ni-P BMG, has beenemployed for the fabrication of a microcantilever owingto its outstanding formability, low processing tempera-ture, and inherent functionalization. The BMG microre-sonators offer tunable performance and wide range ofsensing.77 The said system is tested at different annealingtemperatures to investigate the effect of structural relaxa-tion. Annealing results in an increase in quality factorand resonant frequency with the former approaching apeak value of 8100 in vacuum. Quality factors approach-ing 8100 in vacuum and 2000 in ambient conditions areachieved, which are the highest reported values for anysystem having this geometry. The most convincing attri-bute of this system is the performance tunability whichis achieved by varying annealing temperature andprocessing conditions to optimize the cantilever for

Figure 18. Schematic showing the anticancer effect of amorphous iron nanoparticles (AFeNP): (a) Coronal T1-weighted Magnetic Reso-nance images of 4T1 tumor-bearing mice taken before and one hour after intravenous (i.v.) injection of AFeNPs with (right, C) or with-out (left, ¡) magnetic targeting. (b) Change in the H2O2 concentration in the tumor after intratumoral AFeNP injection ((i.t.)) andintravenous AFeNP injection without ((i.v.)) and with magnetic targeting ((i.v.) C magnet; n D 3, mean § s.d., #P < 0.05,and ###P < 0.001). (c) Change in the relative tumor volume after different treatments (n D 5, mean § s.d., #P < 0.05, ##P < 0.01, and###P < 0.001), growth inhibition of tumour cells is evident (d) H&E staining images, LDI-MS Fe mapping, and merged images of tumortissues after i.v. AFeNP injection with or without magnetic targeting. The circles show the isolated pockets. (© John Wiley and Sons.Reprinted with permission from Zhang et al.57 Permission to reuse must be obtained from the rightsholder.)


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different circumstances. In a different work, twistedmicrocantilevers have been fabricated via surface micro-machining of thin-film metallic glasses.187

8.3.3. MicroscannerThe silicon-based microscanners used in MEMS possesscertain limitations since silicon has a stiff, brittle nature,and contain defects when microprocessed. Such undesir-able characteristics create problems during torsionalstresses in microscanners which subsequently limits theresulting scanning performance. Again, BMGs come tothe rescue by offering desired properties such as lowYoung’s modulus, high fracture toughness and strength.A microscanner is developed in which the moving tor-sion bars are made of metallic glass (see Figure 19). Alarge rotating angle with diminished power consump-tion, better sensing and actuation performances, as com-pared to both single and polycrystalline silicon, have

been noted. So far, the lowest values for power consump-tion, and highest for rotating angle, are reported usingthis MG-based microscanner.189 Optical coherencetomography (OCT) reveals one order of magnitudehigher lateral resolution as compared to OCT imagestaken via microscanners containing silicon torsionalbars. The incredible features and simultaneous presenceof ultra-small power consumption, driving voltage aswell as ultra-high resolution and scanning angle makethis system truly remarkable for a wide range ofapplications.

8.3.4. Current sensorHere the magnetic properties of BMGs are exploited.Fe-B-Nd-Nb system that exhibits good fracture tough-ness, vast super-cooled liquid region, and excellent mag-netic properties, is used for the fabrication of a currentsensor (cantilever). The cantilevers made via high-yield

Figure 19. Schematic of different BMG-based devices: (A & B) Micro fuel cell fabricated using Zr-BMG flow field/current collector platesand the porous Pt-BMG nanowire catalytic layer architecture. (© John Wiley and Sons. Reprinted with permission from Sekol et al.61 Per-mission to reuse must be obtained from the rightsholder.) (C) SEM images of the Fe-B-Nd-Nb cantilevers arrays, and (D) magnified viewof the cantilevers. (© Elsevier. Reprinted with permission from Liu et al.188 Permission to reuse must be obtained from the rightsholder.)(E) Zr-Cu-Al-Ni metallic glass based microscanner, with MG as the tension bar material for enhanced mechanical performance. PZT actu-ation beams are used to generate mechanical torque to the mirror plate. (© John Wiley and Sons. Reprinted with permission from Linet al.189 Permission to reuse must be obtained from the rightsholder.)


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micromachining, is capable of good magnetic detectionmaking it feasible for non-contact current sensors.188

8.3.5. Cardiovascular stentBMGs possess simultaneous existence of higher yieldstrength, high elastic strain limit, and better corrosionresistance as compared to conventional crystalline mate-rials. Such properties allow the fabrication of flexiblestents with high load-bearing ability and capable of sus-taining high flexural strains. Ti/Zr based metallic glassstents have been fabricated190 with combination of prop-erties such as corrosion resistance, biocompatibility, andparamagnetic properties (for MRI compatibility). Fur-thermore, finite element analysis (FEA) is conducted byseveral researchers to evaluate the suitability of BMG-based cardiovascular stents as feasible materials for thetreatment of heart blockage. For example, Kumar et al.tested the feasibility of a Zr-based prototype self-expand-ing stent using finite element modelling and have com-pared the results with widely used nitinol stents as wellas experimental failure strength of human arteries. Com-parable arterial stresses and penetration, to that of NiTi,are observed with induced stresses well below the failurelimit of arteries. The results justify the safe deploymentof the BMG-based stent in human descending aorta.191

In another study of Zr-based stent, carried out usingfinite element analysis and molecular dynamic simula-tion, different stent designs for percutaneous applica-tions are examined for feasibility purpose. This study,additionally, provides insight to the design of futurestents with superior mechanical properties.192

8.3.6. Wear-resistant gearMetallic glasses, either in bulk form or as coatings, canproduce highly wear resistant surfaces. Hoffman opti-mized the wear resistance of BMG gears using differenttesting methods such as pin-on-disk and gear-on-geartesting. A gear with a superior wear resistance and excel-lent surface finish, as compared to EDMed gears, are fab-ricated in centimeter scale after carefully selecting fromthe tested BMG compositions.50 The gear-on-gear testingreveals that Cu-Zr based BMG shows a 60% improve-ment in wear resistance properties in comparison to Vis-comax C300, which is a very special high-performancesteel used by NASA for Mars rover Curiosity.

9. Current issues and outlook

Notwithstanding the excellent properties exhibited byBMGs, they also possess certain critical limitations thathave put a damper on their widespread commercializa-tion. Although researchers have been, up to some extent,successful in overcoming the various limitations, still

there is a long way to go. Here, the key issues faced byBMGs, some proposed strategies and future researchtrends are briefly described. Figure 20 illustrates the cur-rent challenges faced by BMGs, outlook, and proposedsolutions.

The first limitation of BMGs is their complex fabrica-tion routes that need tremendously high cooling rates,high material purity, expensive principal species, andsophisticated equipment. An economical and fast routeis indispensable for BMGs to triumph over the existinghurdles. In almost all the fabrication methods, a veryclose control over the experimental variables isdemanded to ensure completely amorphous systems. Orelse, crystalline phases are precipitated that, dependingon the system, might improve or degrade the final prop-erties. Combinatorial fabrication methods, being highlyversatile, have promising future benefits for the develop-ment of BMGs and can greatly expedite the BMG com-mercialization. More and more systems, in future, needto be analyzed via this extremely fast, simultaneous fabri-cation-characterization route to dig up vital composi-tions with admirable physical, functional, andmechanical properties. The second issue is the size limi-tation. This issue is, mainly, a result of the first limitationsince larger sizes result in low heat dissipation and makethe formation of amorphous system increasingly diffi-cult. For BMGs to substitute structural materials, thislimitation is very decisive and need to be eliminatedsoon. So far, rods and other samples with different geom-etries have been successfully synthesized using modestcooling rates, but that’s inadequate. One solution toachieve large sample sizes is via welding techniques or,as discussed earlier, the liquid-solid joining of BMGs.This strategy, to some extent, might lessen the severity ofthe second issue but many other convincing techniquesneed to be designed to achieve BMGs with larger sizesfor industrial applications. The third issue is the onset ofcrystallization during thermoplastic forming—anunmatched characteristic, that has rendered the BMGsystem very special for a multitude of applications. Nev-ertheless, if this feature is to be exploited, the onset ofcrystallization needs to be avoided or substantiallydelayed. The fourth issue is their brittle nature that hasbeen, to some extent, successfully overcome by the fabri-cation of composite BMG materials. The variousapproaches adopted in this regard so far have been veryeffective in fabricating materials possessing paradoxicalcombination of properties—features that were once con-sidered mutually exclusive. But still, fracture mechanicalstudies for such composite materials need to be done tocertify their importance as high strength materials.Mathematical modeling and simulation using advancedsoftware like ANSYS, may very well, optimize the


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amorphous systems with desired combination of charac-teristics. Novel computational techniques are needed tobetter incorporate the parameters of glassy systems andprovide useful results. Other than this, modeling of theco-sputtering parameters is much needed to preciselyoptimize the chemistry of the metallic glass coatings/membranes and enhance the high-throughput nature ofthis versatile combinatorial technique.

BMGs have the capability to be effectively used asporous implant materials and drug delivery systems. ABMG-based stent or other bioimplants can be madeporous via subtractive modification or dealloying thatleads to self-improving surface behavior as well as theresulting nanoporosity can be exploited for controlled-drug-release drug delivery applications. Thus, a drug-eluting BMG-based stent can be a future new-generation

stent for cardiovascular interventions. The redeemingquality of BMGs that makes them excellent candidatesfor strong and porous biomaterials is the freedom ofmulticomponent elemental system where any elementwith specific features such as good preferential leaching,corrosion resistance or antibacterial properties can beadded. As opposed to this approach, the porous materi-als made via other methods such as powder metallurgy,etc., are either brittle and weak or the porosity induced isinappropriate to ensure controlled drug-release. PorousBMGs can also be studied for bone implantation asstrong and biocompatible materials with low Young’smodulus. The porosity can be tailored accordingly tomatch the young’s modulus of human bone. This, alongwith the capability of drug delivery, reinforces the claimof BMGs as new generation orthopedic implants.

Figure 20. Schematic of various issues faced by BMGs and outlook. Extreme right column suggests further research exploration in spe-cific areas of BMGs.


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Thin-film BMGs have the potential to emerge as mod-ern high-performance coatings. Since in thin films, thesize limitations, processing complexities, and brittlenessissues—as faced in bulk formation—can be circum-vented. Multiple elements in the BMG coatings can beadded for multitude of purposes and multifunctionalcoatings. For instance, for biological applications, Ag canbe added for antibacterial properties, Pd or Pt for corro-sion resistance, Zr, Ti for corrosion resistance and bio-compatibility, and yet another species such as Cu—withother noble species—for preferential leaching. Thus, mul-tifunctional coatings for biomedical, catalysis, wear resis-tance and other applications can be designed this way.

Last, but not the least, BMGs can improve the perfor-mance of many of the commercial microdevices as theycan potentially replace the crucial elements such as cata-lysts, cantilevers, sensors, magnets, etc. But advancedcharacterization techniques are required to study theirfeasibility as substitute materials.

ORCID

Muhammad Mudasser Khan http://orcid.org/0000-0003-3669-1547Ali Nemati http://orcid.org/0000-0002-4842-2988Zia Ur Rahman http://orcid.org/0000-0003-0629-5287Umair Hussain Shah http://orcid.org/0000-0002-7865-3269Hassnain Asgar http://orcid.org/0000-0002-8427-5188Waseem Haider http://orcid.org/0000-0003-4235-3560

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