Materials Science and Engineering A304–306 (2001) 151–158

The science and technology of mechanical alloying

C. Suryanarayanaa,∗, E. Ivanovb, V.V. Boldyrevc

a The George S. Ansell Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401-1887, USAb Tosoh SMD, Inc., Grove City, OH 43213-1895, USA

c Institute of Solid State Chemistry and Mechanochemistry, Russian Academy of Sciences, Novosibirsk, Russia

Abstract

Mechanical alloying (MA) is a powder metallurgy processing technique involving cold welding, fracturing, and rewelding of powderparticles in a high-energy ball mill, and has now become an established commercial technique to produce oxide dispersion strengthened(ODS) nickel- and iron-based materials. MA is also capable of synthesizing a variety of metastable phases, and in this respect, the capabilitiesof MA are similar to those of another important non-equilibrium processing technique, viz., rapid solidification processing (RSP). However,the “science” of MA is being investigated only during the past 10 years or so. The technique of mechanochemistry, on the other hand, hashad a long history and the materials produced in this way have found a number of technological applications, e.g., in areas such as hydrogenstorage materials, heaters, gas absorbers, fertilizers, catalysts, cosmetics, and waste management. The present paper discusses the basicmechanisms of formation of metastable phases (specifically supersaturated solid solutions and amorphous phases) by the technique of MAand these aspects are compared with those of RSP. Additionally, the variety of technological applications of mechanically alloyed productsare highlighted. © 2001 Elsevier Science B.V. All rights reserved.

Keywords:Mechanical alloying; Supersaturated solid solutions; Amorphous phases; Rapid solidification processing; Applications of mechanically alloyedproducts

1. Introduction

Non-equilibrium processing of materials has attracted theattention of a number of scientists and engineers due to thepossibility of producing better and improved materials thanis possible by conventional methods [1]. Rapid solidificationprocessing (RSP) and mechanical alloying (MA) are twosuch processing methods with somewhat similar capabilities.RSP has started as an academic curiosity in 1960 and ma-tured into an industrially accepted technology with its use forthe production of amorphous ferromagnetic sheets for trans-former core applications by AlliedSignal. On the other hand,MA started as an industrial necessity in 1966 to produceoxide dispersion strengthened (ODS) nickel- and iron-basedsuperalloys for applications in the aerospace industry and itis only recently that the science of this “apparently” simpleprocessing technology has begun to be investigated. Whilethe scientific basis underlying RSP was investigated in de-tail from the beginning, applications for the products of RSPwere slow to come about mostly because of the limitationson the size and shape of the RSP materials. Thus, inventionof the melt spinning technique to produce long and con-

∗ Corresponding author. Tel.:+1-303-273-3178; fax:+1-303-273-3795.E-mail address:schallap@mines.edu (C. Suryanarayana).

tinuous ribbons and of the planar flow casting method toproduce wide ribbons accelerated the applications of RSPalloys. Even today use in transformer core laminations re-mains the major (and most voluminous) application of theRSP alloys. The developments in the science of RSP andthe applications of RSP products can be found in severalmonographs and proceedings of the RQ conferences (see,e.g., [2–6]). In contrast, the technique of MA was used forindustrial applications from the beginning and the basic un-derstanding and mechanism of the process is beginning tobe understood only now. There have been several reviewsand conference proceedings on this technique too [7,8], butthe present status of MA has been most recently reviewed bySuryanarayana [9]. The MA literature available up to 1994has been collected together in an annotated bibliography[10]. Fig. 1(a) presents the growth of publications in the fieldof MA during the period 1970–1994 and Fig. 1(b) comparesthe growth trends of publications between MA and RSP. Itmay be noted that even though the trends for MA and RSPare similar, that for MA is offset by about 15 years fromthat of RSP. In the present paper, we will first discuss thecurrent scientific understanding of the metastable effects —specifically solid solution formation and amorphization phe-nomena — achieved by MA and then describe the presentand potential applications of MA. A critical comparison will

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Fig. 1. (a) Growth of publications in the area of MA during 1970–1994. (b) Comparison of the growth trends of publications in MA and RSP. Eventhough the trends appear similar for both the techniques, the growth for MA is offset by about 15 years from that of RSP.

be made on the effects achieved by MA and RSP, wheneverpossible.

2. The technique of mechanical alloying

MA is a dry powder processing technique and has beenused to synthesize both equilibrium and metastable phasesof commercially useful and scientifically interesting mate-rials. The technique was developed by Benjamin [11,12]around 1966 to develop an alloy combining oxide disper-sion strengthening withγ ′ precipitation hardening in a

nickel-based superalloy intended for gas turbine applica-tions. Since the oxides cannot be dispersed in the liquidstate, a solid-state processing technique was necessary.Thus, MA owes its origin to an industrial necessity. MAis a simple and versatile technique and at the same timean economically viable process with important technicaladvantages. One of the greatest advantages of MA is in thesynthesis of novel alloys, e.g., alloying of normally immis-cible elements, which is not possible by any other tech-nique including RSP. This is because MA is a completelysolid-state processing technique and therefore limitationsimposed by phase diagrams do not apply here.

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The process of MA consists of loading the powder mixand the grinding medium (generally hardened steel or tung-sten carbide balls) in a stainless steel container sealed undera protective argon atmosphere (to avoid/minimize oxidationand nitridation during milling) and milling for the desiredlength of time. About 1–2 wt.% of a process control agent(PCA) (usually stearic acid) is normally added to preventexcessive cold welding amongst the powder particles, espe-cially when powders of ductile metals are milled. The typesof mills generally used are SPEX mills (wherein about 10 gof the powder can be processed at a time), attritors (wherea large quantity of about a few pounds of powder can beprocessed at a time), or Fritsch Pulverisette mills (wherepowder in more than one container can be processed si-multaneously). The times required for processing are shortin the SPEX mills whereas they are longer in the attritorsor Fritsch mills. A description of the MA processing anddetails of the different mills can be found in [9,13].

3. Attributes of mechanical alloying

The formation of an amorphous phase by mechanicalgrinding of a Y–Co intermetallic compound in 1981 [14],and in the Ni–Nb system by ball milling of blended ele-mental powders in 1983 [15] brought about the recognitionthat MA is a potential non-equilibrium processing technique.Since the mid-1980s, a number of investigations have beencarried out to synthesize a variety of alloy phases includingequilibrium and supersaturated solid solutions, crystallineand quasicrystalline intermediate phases, and amorphous orglassy alloys. Nanocrystalline materials (with a grain size offew nanometers, usually<100 nm) are also produced by MAof powder mixtures. Additionally, it has been recognizedthat this technique can be used to induce chemical (displace-ment) reactions in powder mixtures at room temperature orat much lower temperatures than normally required to syn-thesize pure metals [16]. For example, pure copper metalpowder was produced by ball milling a mixture of CuO andCa powders at room temperature. These attributes of MA arevery similar to those of RSP and so frequently comparisonsare made of the relative efficiencies of these two techniques.

While the suitability of a particular technique to developa material depends on the eventual applications for whichthe product is intended, one fundamental way of compar-ing these two non-equilibrium processing techniques is toevaluate the departure from equilibrium achieved. Calcula-tions show that the maximum departure from equilibrium is24 kJ/mol in RSP and it is 30 kJ/mol in MA suggesting thatit is possible to achieve greater departure from equilibrium(or “more” metastable effects) during MA.

4. Solid solubility limits

Extended solid solubilities have been achieved by bothMA and RSP techniques in several alloy systems. An

Table 1Comparison of solid solubility limits (at.%) achieved by MA and RSP insome alloy systems

Solvent Solute Equilibrium value By MA By RSP

At RT Maximum

Ag Cu 0.0 14.0 100 100

Al Mn 0.4 0.62 18.5 9.0Nb 0.0 0.065 25 2.4Ni 0.0 0.11 10 7.7Ru 0.0 0.008 14 4.5Ti 0.0 0.75 36 2.0

Ni Ta 3.0 17.2 30 16.6

Ti Si 0.0 3.5 37.5 6.0

exhaustive list of the solubility extension values can befound in [9]. Even though the limits of supersaturation havenot been established in all the systems by both RSP andMA, based on the available results, a selective comparisonis presented in Table 1. It may be seen that a greater extentof supersaturation is obtained by MA than by RSP. A gen-eral observation is that the maximum solubility extensionachieved by MA is higher than that by RSP when the roomtemperature solubility is very small or zero [9]. Further, it isalso possible to achieve supersaturation in some alloy sys-tems by MA that is not possible by RSP; this is especiallytrue in liquid immiscible systems [9]. Fig. 2(a) comparesthe maximum equilibrium and extended solid solubilities ofdifferent elements in Cu achieved by MA. It may be seenthat there is a significant increase in the solid solubilityachieved by MA. The increase in solid solubility may berationalized by the Hume-Rothery rules, where the relativeatomic sizes, crystal structures, and electronegativities ofthe solvent and solute play an important decisive role. Acorrelation between the solid solubility and atomic radius ispresented in Fig. 2(b). It may be noted that increased solidsolubility is achieved only when both the atomic size factor(<15%) and similar crystal structure conditions are satisfied.The solid solubility is very limited when these conditionsare not satisfied or when the crystal structure is different.Similar results have been obtained in other cases also.

The scientific basis for the formation of supersaturatedsolid solutions through RSP is reasonably well established.For example, the limit of supersaturation obtainable by RSPhas been explained using the concept ofT0, the temperatureat which the solid and liquid phases have the same freeenergy. It is possible to obtain a supersaturated solid solutionif the alloy melt during RSP is cooled to a temperaturebelowT0. Since the liquid phase is not involved in the MAprocessing, this concept may not be applicable to explainthe formation of supersaturated solid solutions by MA. But,it will be useful to relate the formation of solid solutions byMA to some parameter, say energy input. Since the solidsolubility limits increase with increasing solidification ratein RSP alloys, it will be instructive to check whether the

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Fig. 2. (a) Comparison of the equilibrium and extended solid solubilities of different solute elements in Cu by MA. (b) Correlation between atomicradius and equilibrium, and extended solid solubilities in Cu by MA.

solid solubility limits increase with increasing energy inputduring MA.

Several theories have been proposed to explain the for-mation of supersaturated solid solutions by MA. It wasreported that the solid solubility could be increased up tothe composition when an amorphous phase starts forming,i.e., the solid solubility limit is decided by the metastableequilibrium between the supersaturated solid solution andthe amorphous phase [17]. Another theory is that the solidsolubility limit is determined by the balance between inter-mixing due to shearing forces during milling and the de-composition of the solid solution due to thermally activatedjumps. If the ratio of these two, referred to as theγ factor, isvery small thermally activated diffusion will drive the sys-tem to equilibrium and no supersaturation is obtained. Onthe other hand, for very large values ofγ , a fully randomsolid solution is obtained [18]. It has now been acceptedthat the most important reason for the increase in solid solu-bility is the formation of nanostructures during milling [19].

The large volume fraction of atoms in the grain boundariesin these ultrafine-grained materials is expected to enhancediffusion and consequently increase the solid solubility.Even though the above theories explain the solid solubilityincreases observed in some instances, all the theories arenot able to explain the observations in all the alloy systems.

5. Amorphization by mechanical alloying/milling

Amorphization is one of the most frequently reportedphenomena in mechanically alloyed powder mixtures.Amorphous phases have been obtained starting from blendedelemental powders or mixtures of intermetallics (MA), orstoichiometric compounds (pre-alloyed intermetallics oreven pure elements) (referred to as mechanical milling,MM). The number of amorphous phases synthesized byMA is too numerous to list here, but a complete list maybe found in [9]. In fact, it has been reported that any alloy

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can be made amorphous under the appropriate milling con-ditions. But, it should be noted that powder contaminationduring milling could be an important contributing factor inthe formation of an amorphous phase by MA, at least insome cases.

Amorphous phases can form from blended elementalpowders either directly or via the formation of an inter-metallic phase. For example,

mA + nB → (AmBn)amorphous,

or mA + nB → (AmBn)crystalline→ (AmBn)amorphous

In some instances it has been reported that a solid solutionforms in the initial stages of mixing of blended elementalpowders, which on continued milling, becomes amorphous.In some other instances it has been reported that thesequence of phase formation is

blended elemental powder→ intermetallic

→ amorphous phase

Whether an intermetallic or a solid solution forms prior toamorphization depends on the relative free energies of thetwo competing phases. On the other hand, amorphizationin ordered alloys seems to follow the sequence:

ordered phase→ disordered phase(loss of long-range order)

→ fine-grained(nanocrystalline) phase

→ amorphous phase

The times required for amorphization are dependent on theprocess variables, but they are usually shorter during MMthan in MA, because alloying need not occur during MM.Thus, there appear to be many pathways for the formationof an amorphous phase by MA/MM.

The mechanism of amorphization by MA is not clearlyunderstood, but it is now believed that a solid-state reactionsimilar to that observed in thin films occurs in mechanicallyalloyed powders. During MM, however, destabilization ofthe crystalline phase is thought to occur by the accumula-tion of structural defects such as vacancies, dislocations,grain boundaries, and anti-phase boundaries. The continu-ous decrease in grain size (and consequent increase in grainboundary area) and a lattice expansion would also contributeto the increase in free energy of the system. Additionally,the theories invoked for the formation of amorphous phasesby RSP methods [2–5] (e.g., a minimum solute concen-tration required for the formation of an amorphous phase,significant size difference between the solvent and soluteatoms, etc.) have also been used to explain the amorphiza-tion phenomenon by MA. But, in the case of formationof amorphous phases starting from ordered compounds,it was noted that two conditions should be satisfied, viz.,that Tc is higher thanTm (whereTc is the critical orderingtemperature andTm the melting point) and that the ratio of1Emismatch/1Eordering (where1E is the energy) should be

high. Powder contamination is a matter of concern duringmilling and at least some of the reports on the formationof amorphous phases can be traced to the presence of sub-stantial amounts of impurities. Avoiding the contaminationresulted in the formation of only a crystalline phase [20].

The amorphous phases synthesized by MA and RSPtechniques have been found to display similar radial dis-tribution functions. The crystallization temperatures of theamorphous phases obtained by MA and RSP are againfound to be similar, even though the activation energy forcrystallization of RSP alloys is much higher than for MAalloys. But, it has been noted that the composition ranges ofthe amorphous phases are much wider in alloys producedby MA than in those obtained by RSP. Further, while RSPproduces amorphous phases usually around deep eutecticcompositions, the composition range in MA amorphousalloys is generally around the equiatomic composition [9].

There has been lot of activity in recent times on thesynthesis/processing of bulk amorphous alloys [21]. Thesealloys, characterized by a wide supercooled liquid region(large temperature difference between the crystallizationand glass transition temperature), can be easily synthe-sized at extremely low solidification rates. Such alloys havealso been synthesized by MA in multicomponent Zr andMg-based alloys [22]. Applications for such bulk amor-phous alloys include the 3 mm thick faceplate inserts forhigh-end golf club heads [23]. It is usually observed that itis easier to obtain a metastable phase by MA than by RSP.Additionally, synthesis of nanocrystalline materials is mucheasier by MA than by RSP.

6. Technological applications of mechanical alloying

6.1. Present applications

The most important application of mechanically alloyedproducts is in the form of ODS alloys. These alloys havecomplex compositions and it is difficult to process themthrough conventional ingot metallurgy (IM) methods.

Mechanically alloyed materials are strong both atroom and elevated temperatures. The elevated tempera-ture strength is derived from more than one mechanism.First, the uniform dispersion (with a spacing of∼100 nm)of very fine (5–50 nm) oxide particles (commonly usedare Y2O3 and ThO2) which are stable at high tempera-tures, inhibit dislocation motion in the metal matrix, andincrease the resistance of the alloy to creep deformation.Another function of the dispersoid particles is to inhibitthe recovery and recrystallization processes, which allowa very stable grain size to be obtained. These large grainsresist grain rotation during high-temperature deformation.The very homogeneous distribution of alloying elementsduring MA gives both the solid solution strengthened andprecipitation-hardened alloys more stability at elevatedtemperatures and overall improvement in properties. INCO

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Table 2Typical applications of mechanically alloyed ODS alloys

Alloy Composition (wt.%) Typical applications

Ni Fe Cr Al Ti Mo W Ta Y2O3

MA754 Balance – 20 0.3 0.5 – – – 0.6 Vanes and bands for aircraft gas turbine enginesMA6000 Balance – 15 4.5 0.5 2.0 4.0 2.0 1.1 Gas turbine blades and vanesMA956 – Balance 20 4.5 0.5 – – – 0.5 Vacuum furnace muffles, trays, baskets, and in glass-processing industryMA957 – Balance 14 – 1.0 0.3 – – 0.25 Fuel cladding for fast neutron fast breeder reactors, heat exchanger tubes

produces annually about 350 t of commercial ODS alloysmainly based on nickel, iron, and aluminum. Typical com-positions and applications of MA–ODS alloys are listed inTable 2. The reasons for the application of ODS alloys forthese applications have been critically discussed in [13].

Apart from the ODS alloys, the technique of MA is alsobeing commercially used to produce PVD (physical vapordeposition) targets for the electronic industry by Tosoh,USA (about 5 t per year). This is because it is easier toproduce a chemically homogeneous product by MA ratherthan by IM methods. Yet another commercial application ofmechanically alloyed products is in the use of MRE (Meal,Ready-to-Eat) heaters which have been extensively usedduring the Desert Storm Operation by the US in 1996. Theseheaters contain finely ground mechanically alloyed pow-ders of Mg and Fe, which on contact with water, produceheat. This is a very good example of commercialization ofa simple process. Recently, it has also been reported [24]that Zoz in Germany and Fukuda Metal Foil and Powder inJapan have joined together in producing about 600 kg perday (about 200 t per year) of mechanically alloyed materialfor paints and solders. The most important advantage ofMA in all these applications is in the production of a highlyhomogeneous product without any segregation effects beingpresent in them.

6.2. Potential applications

Apart from the real applications described above, therehave been many suggested applications for MA materials.These include the production of FeSi2, a thermoelectricmaterial, which cannot be processed as a homogeneousbulk polycrystalline alloy by conventional IM methodsdue to the occurrence of eutectic and peritectoid reac-tions in the alloy system. MA offers an easy alternativefor this. The thermo-e.m.f. of the MA material is 0.2 Vagainst a value of 0.18 V for the powder produced from theingot [25].

Similarly, it has been suggested that mechanically al-loyed Mg-based materials can be used for hydrogen stor-age. Typical hydrogen storage materials should have a largehydrogen capacity, must have high rates of hydrogenationand dehydrogenation, reduced absorption temperatures, andshould require very little or no activation treatments. Allthese (except for the storage capacity) can be achieved by

having a small grain size of the material, easily achievedby MA. Even though pure Mg has a large hydrogen stor-age capacity (7.6 wt.% hydrogen as a hydride), it gets easilyoxidized and so usually Mg alloys are preferred. Produc-tion of Mg–Ni alloys containing a small amount of Ni (1–2at.%) with the Ni dispersed as discrete particles in the Mgmatrix is much easier by MA than by IM methods due tothe large difference in the melting points of the two metalsand the high vapor pressure of magnesium. Mechanicallyalloyed Mg–1 at.% Ni alloy has a hydrogen storage capacityof 6.1 wt.%.

6.3. Mechanochemistry

Mechanochemistry is the term applied to the process inwhich chemical reactions and phase transformations takeplace due to the application of mechanical energy. This isa very old process, with the first publication dating backto 1894 [26]. The applications of mechanochemistry in-clude exchange reactions, reduction/oxidation reactions,decomposition of compounds, and phase transformations.Of these, the exchange reactions have received lot of atten-tion in recent years, especially from Paul McCormick [16]in Australia, while the general phenomenon of mechanicalactivation of solids has been a popular research topic in theerstwhile USSR and eastern Europe [27].

The exchange reactions studied so far can be representedby an equation of the type

MO + R → M + RO

where a metal oxide (MO) is reduced by a more reactivemetal (reductant R) to the pure metal M. Metal chloridesand sulfides have also been reduced to pure metals this way.These reactions are characterized by a large negative freeenergy change and are thermodynamically feasible at roomtemperature. Solid-state reactions involve the formation ofa product phase at the interfaces of the reactants. Furthergrowth of the product phase involves diffusion of atoms ofthe reactant phase through the product phase, which con-stitutes a barrier layer preventing further diffusion. Hencehigh temperatures are required for the reaction to occur atreasonable speeds. MA can provide the means to increasethe reaction kinetics due to the generation of clean and freshsurfaces (a result of fracturing), increased defect density,

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and reduction of particle sizes. These mechanochemicalreactions have been utilized to produce pure metals, alloys,and compounds at room temperature both in the laboratoryand on a commercial scale.

7. Problems in mechanical alloying

In spite of the above-mentioned advantages and simplicityof MA, the technique suffers from some problems. These canbe discussed under three groups, viz., powder contamination,limited science content, and limited applications.

7.1. Powder contamination

The contamination of powders during MA is a majorconcern. The small size of the powder particles, availabilityof large surface area, and formation of new surfaces duringmilling all contribute to the contamination of the powder. Inaddition, the milling conditions (grinding medium, grindingvessel, time of milling, intensity of milling, etc.) and theatmosphere under which the powder is being milled alsocontribute to the contamination level. In many cases, espe-cially when reactive metals like titanium and zirconium arebeing milled, the levels of contamination are high and un-acceptable. These levels increase with milling time and thehighest levels reported are 44.8 at.% oxygen in an Al–6 at.%Ti powder, 26 at.% nitrogen in a Ti–50 at.% Al powder, and60 at.% iron in a W–5 at.% Ni alloy powder [9]. Althoughseveral methods have been suggested to decrease/minimizethe powder contamination level, the most effective onesseem to be (a) use of high-purity metals, (b) use ofhigh-purity atmosphere, (c) use of balls and container of thesame material that is being milled, (d) self-coating of theballs with the milled material, and (e) shortest milling timesallowed.

Fig. 3. Typical current and potential applications of mechanically alloyed products.

7.2. Limited science content

As mentioned earlier, the science base for MA has beenpoor. Although it is known that the technique works and sois useful, one is not very clear how and why the techniqueworks. This is because MA is a complex stochastic processand the number of variables involved is too many. Amongstothers, these include the type of mill; size, shape, and weightof the grinding medium; velocity, angle and frequency ofmedia impacts; ball-to-powder weight ratio; milling atmo-sphere; purity, size, shape, and hardness of the powder parti-cles; milling time; milling temperature; and type and amountof the PCA. In spite of the complexity, some attempts havebeen made to model the process of MA and limited suc-cess has been achieved. For example, it has been possibleto establish a relation between the experimentally observedphase formation and some of the process variables. But, ithas not been possible to predict the final chemical consti-tution (type and description of phases) for a given set ofmilling conditions [28,29]. Some attempts have also beenmade to conduct deterministic MA experiments in which,instead of milling the powders in an attritor or a shaker mill,one uses thin sheets of metals or rolling of powders undercontrolled conditions, to evaluate the progress of alloyingat each stage [30,31]. This has still not really gone far inenhancing our understanding of the MA process.

7.3. Limited applications

The industrial applications of MA have been few. Themost significant applications seem to be about 350 t of ODSmaterials, 200 t of solder alloy, and 5 t of PVD target (Cr–V)alloys per year. Even though other potential applicationshave been suggested, many of them have not been industrialrealities. The use of mechanochemical reactions in produc-ing pure metals, alloys and compounds, dental filling alloys,catalyst materials, inorganic pigments, and fertilizers has

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been known for some time now but needs to be exploitedfurther. Identification of some niche applications for the MAproducts is likely to accelerate the rate of growth in this field.

8. Concluding remarks

MA is a simple and versatile technique capable of produc-ing different types of metastable effects in a variety of alloysystems. The science base for this technique appears to belimited. The effects of MA are compared with those of RSP— another important non-equilibrium processing technique.The MA products find applications in various industries andthese are summarized in Fig. 3. Powder contamination ap-pears to be a serious problem during MA. Powder consolida-tion to produce bulk shapes results in loss of the metastableeffects and thus it appears wise to use the MA powder in theas-produced condition rather than trying to consolidate itinto bulk shapes. Consolidated products may be used if thealloys have a high tolerance to gaseous impurities. Search fornew applications of MA products becomes easier if one cancome up with novel designs of equipment capable of produc-ing large quantities of material, preferably in a continuousmode.

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