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Materials Science and Engineering A 492 (2008) 221229

Contents lists available at ScienceDirect

Materials Science and Engineering A

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m s e a

Synthesis and properties of bulk metallic glasses

in the ternary NiNbZr alloy system

Z.W. Zhu a,b, H.F. Zhang a,, B.Z. Ding a, Z.Q. Hu a

a Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences,

72 Wenhua Road, Shenyang 110016, Chinab Graduate School of the Chinese Academy of Sciences, Beijing 100039, China

a r t i c l e i n f o

Article history:

Received 6 November 2007

Received in revised form 11 March 2008

Accepted 8 April 2008

Keywords:

Bulk metallic glass

Ni-based alloy

Thermal property

Mechanical property

Corrosion resistance

a b s t r a c t

Bulk metallic glasses (BMGs) with high thermal stability, good mechanical properties and high corrosion

resistance were synthesized in the NiNbZr system. A large bulk glass-forming region with 60 < Ni < 64,

28< Nb< 38 and 0 < Zr< 9 (in at.%) was found. The critical size for the glass formation is 3 mm. These

investigated Ni-based BMGs process high glass transition temperature of about 880900 K and high on-

set crystallizationtemperature of 915932K as wellas highcompressive fracture strengthof approximate

3.03.2 GPa along with some compressive plasticity of about 2%. Electrochemical measurements indicate

they also exhibit high corrosion resistance, i.e., large passive region above 1.5 V (vs. saturated calomel

reference electrode, SCE). The influence of the Zr content on the glass-forming ability (GFA) and corro-

sion behaviors was carefully studied, indicating that some Zr addition improves the GFA and corrosion

resistance. 2008 Elsevier B.V. All rights reserved.

1. Introduction

Bulk metallic glasses (BMGs, typically referred to a minimum

casting dimension larger than 1 mm) have been greatly concerned

in the past fewdecades because they areof particular scientificand

engineering interests [13]. Some progress in both glass-forming

ability (GFA) and mechanical properties has been made, for exam-

ple, the amorphous samples with the critical size over 10 mm were

successfully prepared in Mg [4], Zr [5], Fe [6], Ti [7], Cu [8], Pd [9],

etc., based alloys, and some Cu [10,11], Zr [12,13], Ti [7,14] based

BMGs samples display very large compressive plastic strain in com-

pression tests. It is exciting even though some problems are still

puzzled. Meanwhile, due to various desirable properties, including

high yield strength, hardness and elastic strain limit in addition to

reasonably high fracture toughness, fatigue resistance and corro-sion resistance, etc., BMGs have been tried to be made into some

itemssuch as sporting goods, surgical instruments,and strong, thin

cases forelectronic devices such as mobile phone andU-disc [3]. To

satisfy the requirements of commercial applications, it is urgent to

improve the known BMGs plasticity or to develop new BMGs with

higher GFA and better mechanical properties, especially based on

common metals, such as Al, Cu, Fe, Ni, etc.

Corresponding author. Fax: +86 2423971783.

E-mail address: [email protected] (H.F. Zhang).

In the case of Ni-based alloys, bulk metallic glasses wereprepared in the complex alloy systems, such as NiNbCrMoPB

[15], NiTiZr(Si,Sn) [16], NiNbTiZrCoCu [17],

NiNbTiZrSiSn [18,19], NiNbSn [20,21], NiCuTiZrAl

[22], NiTaSn [23], etc. However, with compared to that of Cu-,

Zr-, Ti- and Fe-, etc., based BMGs [49], the GFA of Ni-based amor-

phous alloys is a challenging subject. Because so far the maximum

dimension of Ni-based BMGs samples is only 5 mm [22,24], the

development of new Ni-based glass former with higher GFA is

imperative. In the period of the conventional amorphous alloys,

NiNb and NiZr systems were famous for their GFA. Very recently,

BMG samples up to 2-mm thick were fabricated in binary NiNb

alloy system [25,26]. Some reported works also indicate that

Ni-based Nb-bearing BMGs possess better mechanical properties

than other Ni-based ones [16,27]. According to Miracles efficientcluster packing model [28], NiNbZr system with good atomic

size distribution (the Goldschmidt atomic radius of Zr is 0.160 nm,

which is larger than 0.146 nm and 0.128 nm for Nb and Ni, respec-

tively) could have high GFA [29,30]. Additionally, some believe that

some valve metals, such as Nb and Zr, etc., enrich in the surface

film to prevent the materials from corrosion [3134]. As a result,

the ternary NiNbZr system might be a good candidate to develop

new Ni-based BMGs with better combination among GFA, good

mechanical properties and high corrosion resistance. Nevertheless,

systematic investigation of BMGs in ternary NiNbZr system is

hardly reported [29,34,35].

0921-5093/$ see front matter 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.msea.2008.04.021


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222 Z.W. Zhu et al. / Materials Science and Engineering A 492 (2008) 221229

In this paper, we reported that bulk glasses can be formed in

quite wide composition range of ternary NiNbZr system. The 3-

mm diameterglassy samples were successfully prepared by copper

mold injection casting method. They displayed good mechanical

properties, high thermal stabilityand high anti-corrosion property.

Theeffect ofthe Zrconcentrationon thephase transformationupon

casting and corrosion behavior is also carefully discussed.

2. Experimental

Master alloy ingots were prepared by arc melting a mixture of

ultrasonicallycleansedNi, Nband Zrwith a purityof above99.9%on

a water-cooled copper hearth under Ti-gettered high purity argon

atmosphere. The chemical homogeneity was obtained by repeated

melting at least four times. The ingots were then remelted under

high vacuum in a quartz tube by using induction heating coil and

injected through a nozzle with 0.51mm in diameter into the cop-

per mould with a cavity of 24 mm diameter.

The as-cast samples were characterized with X-ray diffraction

(XRD, Philips PW1050, Cu K), transmission electron microscopy

(TEM, JEOL 2010, 200 kV) and differential scanning calorimetry

(DSC; Netzsch DSC 404C). The specimens used for XRD measure-ment were cut from the middle part of the as-cast rods. Thin slices

from the2.5-mm diameteras-cast rods were usedfor preparing the

TEM samples, which were ground and mechanically dimpled with

a GATAN precision dimple grinder as well as polished using argon

ion milling as the final thinning process using a GATAN precision

ion polishing system(PIPS). DSC measurements were performed in

a flowing argon atmosphere at a heating rate of 0.33 K/s.

Mechanical properties were measured with the samples of 2-

mm diameter and 4-mm length on a servo-hydraulic materials

testing system (MTS 810). To perform compression tests under a

constant strain rate of 2104 s1, a MTS strain gauge was used.

Fracture surface was examined by scanning electron microscopy

(SEM, Hitachi S3400N).

Corrosion behavior of Ni-based BMGs in 1 M HCl aqueous solu-tion open to air was studied by electrochemical measurements on

an Advanced Electrochemical System (Princeton Applied Research

PARSTAT 2273) at 300K. Prior to the corrosion tests, the specimens

were mechanically polished in cyclohexane with silicon carbide

paper up to No. 2000, degreased in acetone, washed in the dis-

tilled water, and dried in air. Electrochemical measurements were

conducted in a three-electrode cell a platinum counter electrode

anda saturatedcalomelreference electrode(SCE). Potentiodynamic

polarization curves were measured at a potential sweep rate of

0.333 mV/s after immersing the samples for several minutes, when

the open-circuit potential became almost steady.

The surface of the samples exposed to air after mechanical

polishing and conducted by potentiodynamic polarization mea-

surements up to 1 V (vs. SCE) in 1 M HCl solution was examined

by X-ray photoelectron spectroscopy (XPS) using a photoelectron

spectrometer with Al K radiation (h = 1486.6 eV). From thesespectra, the composition of the passive film and the underlying

alloy surface was quantitatively determined.

3. Results and discussion

3.1. Glass-forming ability

The NiNbZr alloy system shows good GFA. Fig. 1 illustrates

XRD patterns of the as-cast NiNbZr alloys rods with a diameter

of2 mm. In Fig. 1a, when a increase from 1 to9, the 2-mmdiameter

as-cast samples of Ni61.5Nb38.5aZra alloys display only a series of

diffuse maxima around 2= 42

, while for a = 11 the 2-mm diame-

Fig. 1. XRD patterns of the as-cast rods with a diameter of 2mm for (a)

Ni61.5 Nb38.5aZra (a =1, 3, . . ., 11at.%) [29] and (b) Ni100b(Nb0.85Zr0.15)b (b =35, 36,

. . ., 40 at.%).

teras-cast rodsexhibitapparent crystalline Bragg peaks.It indicates

that when Zr content is below 9 at.%, the glassy samples with 2 mm

in diameter can be synthesized in the alloys Ni61.5Nb38.5aZra [29].

Likewise, it is also found out that the glass can be also formed in a

quite large composition range as the Ni concentration is changed.

It is shown in Fig. 1b. When Ni content increases from 60 at.% to

65 at.%, the patterns of the 6164at.% Ni-bearing samples consist

of only a broad peak without any observable crystalline diffrac-

tion peaks, indicating that the 2-mm diameter glassy sample can

be made. Outside of this range, crystallization occurs to the sam-

ples with 2 mm in diameter. Identification of the crystalline phases

will be discussed in detail in Section 3.3. Through tens of alloys

experiments, it is discovered that there exists a wide BMG formingregion of 60< Ni< 64, 28 < Nb< 38 and 0 < Zr< 9, in at.%, in ternary

NiNbZralloy system, as shown in Fig.2. In thelarger green ellipse

region, thesampleswith at least 2 mm in diameter canbe manufac-

tured. It is necessary to point out that the 3-mm diameter as-cast

rods arecapableto be producedin thesmaller purpleellipse region.

XRD patterns of the 3-mm diameter samples are shown in Fig. 3,

displaying the typical characteristics of those of the amorphous

phase.

3.2. Thermal property

Characterization of the thermal property of the investigated

NiNbZr glassy alloys, especially determination of the onset of

glass transition temperature (Tg) and the onset of crystallization


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Z.W. Zhu et al. / Materials Science and Engineering A 492 (2008) 221229 223

Fig. 2. Sketch of the glass formation and variation of Tg and Tx in the NiNbZr

system. The solid green ellipse corresponds to the region where BMGs with at least

2 mm in diameter can be formed, the solid purple ellipse corresponding to 3-mm

diameter BMGs.

Fig. 3. XRD patternsof theas-cast rods with a diameter of 3mm in thethree alloys.

temperature (Tx) as well as liquidus temperature (Tl) was con-

ducted by DSC measurements. Fig. 4 shows the high temperature

DSC profiles recorded at a heating rate of 0.33 K/s. And Table 1

lists the concerning thermal data of the typical alloys. The sam-

ples used for DSC measurements were cut from the middle section

of the 2-mm diameter as-cast rods. When DSC traces and the ther-

mal properties are considered, the alloys with the Zr content of

19 at.% or Ni content of 6164 at.% show the distinct glass transi-

tion, while for60 at.% Niand 65at.%Ni alloys, no endothermicevent

can be observed on the traces, implying no glass transition occurs,

as shown in Fig. 4a and b. Tg, defined as the onset of the endother-

mic event, is relatively high and above 880 K. Compared with the

known Ni-based BMGs [1522], it can be inferred that these BMGs

have high thermal stability.

It is easilyfound out that for a series of alloys with the composi-

tion of Ni61.5Nb38.5aZra, Tg decreases abruptly by about 9 K when

the concentration of Zr increases from 3 at.% to 5 at.% (Fig. 4a and

Table 1). When Zr content is below 3 at.% or above 5 at.%, Tg nearly

remains equal, around 893K or 882 K, respectively.The similar phe-

nomena on glass transition does not occur to Ni100b(Nb0.85Zr0.15)balloys. Fig. 4b and Table 1 show that Tg falls continuously as Ni con-

tent decreases. Additionally, it is observed from Fig. 4a and b that

there exist significant differences in crystallization behaviors. The

alloys transform from the beginning three-stage crystallization to

double-stage one as the Zr content or Ni content rises. Whether the

Zr content increasesor Ni content falls, Tx alwaysdeclines. The vari-

ation ofTg and Tx is roughly drawn in Fig. 3. Further, it is found that

the variation ofTg is related to the crystallization behavior for the

Ni61.5Nb38.5aZra alloys, since the abrupt decrease in Tg occurs at

the transition from the three-stage crystallization to double-stage

one. As shown in Figs. 2 and 3, the alloy with the 5 at.% Zr exhib-

ited the best GFA of the Ni61.5Nb38.5aZra alloys. As a result, the

GFA is inferred to have a close relationship with the crystallization

process, which will be discussed in detail in Section 3.3.

It is thought that the atomic arrangement configuration is

attributed to Tg or Tx dependence of Zr content. As known, the dif-

ferent interaction exists among Ni, Nb and Zr atoms, indicated by

different mixing enthalpy values among them, i.e., 49 kJ/mol for

NiZr, 30 kJ/mol for NiNb, and 4 kJ/mol for NbZr [36]. Due to

the different interaction, it leads to atomic reconfiguration to intro-

duce Zr atoms into the NiNb alloy. Extended X-ray absorption fine

structure experiments [35] reveal that the bonds like NiNi and

NiZr around Ni atoms and NbNi and NbNb around Nb atoms

are chemically preferred to be formed as Zr is added into NiNballoys. Difference in atomic configuration contributes to the dif-

ferent behaviors, including different Tg, Tx, etc., during reheating

process. It also affects the subsequent crystallization as mentioned

above.

Fig. 4c and d exemplify the melting behaviors of the NiNbZr

alloys. The liquidus temperature, Tl, and the melting temperature,

Tm, decrease with increasing the Zr amount or reducing the Ni

amount. The extent of the decline of Tl is faster than that of Tm.

Similarly, Trg [37] deduced from the thermal parameters, are pro-

posed to be correlated well with the GFA. They are also given in

Table 1. Trg exhibits high values, but does not either possess a good

correlation with the GFA in the NiNbZr system. The supercooled

liquid regionT= Tx Tg, which reflects thethermal stability of the

Table 1

Thermal properties of the as-cast samples with a diameter of 2 mm, except that the diameter of samples in Ni61.5 Nb38.5 alloy is 1.5mm, deduced from the high temperature

DSC measurement at a heating rate of 0.33 K/s

Alloy (at.%) Tg (K) Tx (K) Tp (K) Tm (K) Tl (K) Tx (=Tx Tg , K) Trg (=Tg/Tl)

Ni61.5 Nb38.5aZra a = 0 894 932 1455 1519 38 0.589

a = 1 893 926 935 1441 1510 33 0.591

a = 3 892 921 928 1426 1456 29 0.613

a = 5 883 918 926 1414 1444 35 0.612

a = 7 882 913 923 1390 1439 31 0.613

a = 9 882 913 918 1384 1420 31 0.620

Ni100b(Nb0.85Zr0.15)b b = 36 899 935 1408 1448 36 0.621

b = 37 890 933 1412 1450 43 0.616

b = 38 886 924 1416 1487 38 0.599

b = 39 884 915 1420 1513 31 0.584


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Fig. 4. DSC scans corresponding to glass transitions and crystallizations, melting behaviors of the as-cast rods with a diameter of 2 mm, (a) and (c) for Ni61.5 Nb38.5aZra (a = 1,

3, . . ., 9 at.%) [29], (b) and (d) for Ni100b(Nb0.85Zr0.15)b (b = 35, 36, . . ., 40 at.%).

supercooled liquid towards crystallization, varies slightly with the

composition (Table1) and ranges from30 K to40 K.It is not directly

related to the GFA (shown in Fig. 3) in the current work although

Twas suggested to characterize the GFA [2].

3.3. Phase transformation dependence of the Zr content upon

solidification

Upon solidification, the glass is formed by competing against

the nucleation and growth of the crystals in the undercooled melt.

The glass can be fabricated under the condition that the nucleation

and growth of the primary competing crystals are completely sup-

pressed while the melt is cooled through the temperature interval

from Tl (below which crystallization is thermodynamically possi-

ble) to Tg (below which the melt is frozen into the solid). Therefore,

the GFA is always thought to be associated with the competing

crystals [38,39]. In order to make sure the relationship among the

glass formation, the Zr content and the competing crystals in the

current study, extensive XRD and TEM investigations were per-

formed.

Fig. 5 compares XRD patterns of as-cast rods with 2.5 mm in

diameter in the Ni61.5Nb38.5aZra alloys. It indicates phase transfor-mation dependence of Zr concentration upon solidification at the

similar condition. For the alloys with the 1 at.% and 3 at.% Zr, the

position of the crystalline diffraction peaks remains almost identi-

cal, but their difference only exists in the intensity. It suggests that

the structures of the precipitating crystals in 1 at.% and 3 at.% Zr

samples are the same. By carefully matched with the data in the

Power diffraction files, the crystals were indexed as the hexagonal

NiNb and orthorhombic Ni3Nb phases, which were also confirmed

in Fig. 6a and b. But with increasing the Zr content from 1 at.% to

3 at.%, the size of the crystals decreases dramatically, from400nm

to 20 nm. The grain refinement, which causes the broadening

of the peaks in XRD patterns of the 3at.% Zr alloy as shown in

Fig. 5, reveals that the Zr addition can retard the precipitation of

the crystals and be conducive to the glass formation. When the


5/9


Fig. 5. XRD patterns of the as-cast Ni61.5 Nb38.5aZra (a = 1, 3, . . ., 9 at.%) rods with a

diameter of 2.5mm.

Zr content rises to 5 at.%, the competing crystals were completelysuppressed and the amorphous phase were formed as indicated by

the unique broad diffuse halo in XRD patterns of the 5 at.% Zr alloy

in Fig. 5 and the homogeneous contrast of TEM image in Fig. 6c.

When the Zr content reaches 7 at.%, a new weak peak (marked by

the red dash line) appears at 2= 38.9 in the XRD patterns, denot-ing that the new primary crystalline phase is produced so as to

decrease the GFA. Fig. 6d shows that some orthorhombic Ni10(Nb,

Zr)7 crystals were formed in the glassy matrix and the size is about

50 nm. When the Zr content is further added to 9 at.%, the peak at

2=38.9 in the XRD patterns in Fig. 5 largely enhanced, indicat-ing that the sample crystallized a lot. It is consistent with Fig. 6e.

The size of the grain of the primarily precipitated orthorhombic

Ni10(Nb, Zr)7 phase reaches 500 nm. Meanwhile, some unidenti-

fied phases could be formed, as marked bythe capital A in Fig. 6e.

When combining the present results with our previous work that

the competing crystals against the glass formation are NiNb and

Ni3Nbphasesin the binary Ni61.5Nb38.5 alloy, it is known that intro-

ducing minor Zr below 3 at.% in the alloys of Ni61.5Nb38.5 cannot

change the type of the competing crystals but retards nucleation

and growth process, which would improve the GFA, and when the

Zr content increases up to 7 at.%, the new primary competing crys-

talline phase is produced so as to deteriorate the GFA. As a result,

the alloy ofthe 5 at.%Zr exhibit the bestGFA ofthe Ni61.5Nb38.5aZraalloys, as shown in Figs. 1 and 3. The trend of the phase transfor-

mation with the Zr content also agrees well with thecrystallization

shown in Fig. 4.

From a view of solidification, the crystals form through nucle-

ation and growth of the nuclei. It would suppress the formation of

the crystals to increases thermodynamically Gibbs free energy bar-rier G* for the nucleation. G* for a critical spherical nucleus isexpressed by [40,41]:

G 163

3(Gv +E)2.

Here, is the interfacial energy and Gv is Gibbs free energy dif-ference between the crystal and liquid; E is the strain energyinduced by atomic mismatch. Gv is usually negative when the

melt is undercooled. In contrast, Eis positive and increases withthe increase of the supercooling. Apparently, when Zr is introduced

into NiNb alloy, Zr atomswouldlocate the positions which should

belong to Nb or Ni atoms. Due to the large size difference between

Zr andNi or Nb atoms, it increases distinctly theatomic-level strain

energy E, thereby, elevating G*. As a result, it would postponethe process of the nucleation to some extent. Minor Zr below 5 at%

effectively retardsthe nucleationand growth of the NiNband Ni3Nb

phases and makes the grain decrease to 20nm (Fig. 6a and b);

whenthe Zr content is 5 at.%, the crystalsare completely suppressed

(Figs. 5 and 6c); but excessive Zr addition above 7 at.% prompts the

separation of a new phase (Figs. 5 and 6d) so as to deteriorate the

GFA [8,40].

3.4. Mechanical property

In order to evaluate the mechanical performance of the studied

NiNbZr BMGs, the quasi-static compression tests were carried

out. Five samples with 2 mm in diameter and 4 mm in length were

measured at a strain rate of 2104 s1 for each alloy. Fig. 7 shows

the stress curves as a function of strain of the three alloys with

the highest GFA (Figs. 2 and 3). The data of mechanical properties

of the alloys are tabulated in Table 2. It is seen that the measured

the samples all displayed an elastic deformation up to the yield

strain of 1.92.3% at the yield stress of about 2.7 GPa, followed by a

plastic elongation by about 2% prior to the ultimate fracture.All the

alloys display considerable ultimate fracture strengths as high asapproximate 3.2 GPa. For a comparison, the mechanical properties

of binary NiNb BMGs are also listed in Table 2 [25]. It is easy to

be found out that Zr addition slightly reduces the strengths of the

alloys from 3.4 GPa for Ni61.5Nb38.5 to 3.03.2 GPa for the NiNbZr

alloys. It is attributed to slight reduction of Tg [42]. As illustrated

in Tables 1 and 2, the maximum strength, m, is proportional toTg. The higher Tg, the higher m. It is reasonable that the NiNbZrglassy alloys with high Tg possess high ultimate fracture strength.

Besides, to our knowledge, the NiNbZr BMG alloys are one series

of those exhibiting the highest strengths in metalmetal BMGs.

SEM observations indicate thatthe NiNbZrBMGs mainly frac-

tured in a shear mode and well-developed vein patterns were

formed on the fractured surfaces. Some multiple shear bands are

also seen on the lateral surface of the fractured specimens. How-

ever, fractographically, 9 at.% or more Zr makes the NiNbZr BMGs

transit from the ductile to the brittle [29].

3.5. Corrosion resistance

Corrosion property of the NiNbZr bulk metallic glasses in 1 M

HCl aqueous solution wasinvestigated. No weight loss wasdetected

for NiNbZr BMGs after immersion in aqueous solution open to

air for 1 week, indicating that the corrosion rate is very low.

For a further understanding of the corrosion behaviors of

NiNbZr BMGs and studying the influence of the Zr content on

the corrosion property, electrochemical measurements were per-

formed. Fig. 8 shows the representative curves of the cathodic and

anodic potentiodynamic polarization of the BMG Ni61.5

Nb38.5

aZra

alloys in 1 M HClaqueous solution at 300K. In evaluating thecorro-

sion property of the materials, the most important parameters are

passive region and passive current density. A wide passive region

with low passive current density corresponds to the better corro-

Table 2

Mechanical parameters of the glassy (a) Ni61.5Nb38.5, (b) Ni61.5 Nb33.5 Zr5 , (c)

Ni62Nb32.3Zr5.7 and (d) Ni63Nb31.45 Zr5.55 at.% samples under an unaxial compressive

loading at a strain rate of 2104 s1

Alloy y (MPa) y (%) m (MPa) f (%) E(GPa)

a 3000 1.7 3450 3.7 170

b 2730 2.1 3000 4.1 130

c 2750 2.2 3080 3.7 128

d 2700 2.1 3170 3.5 127


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Fig. 6. Bright-field TEM images of the as-cast 2.5mm rods: (a) Ni61.5 Nb37.5Zr1 , (b) Ni61.5 Nb35.5 Zr3, (c) Ni61.5 Nb33.5 Zr5, (d) Ni61.5 Nb31.5Zr7 and (e) Ni61.5 Nb29.5Zr9 , indicating the

dependence of the microstructures on the Zr content for the NiNbZr alloys.

sion resistance. In Fig. 8, the similar polarization behaviors were

observed among the Ni61.5Nb38.5aZra BMG alloys except some dif-

ferences in the magnitude of the passive region andpassive current

density. They were spontaneously passivated with extremely wide

passive region and relatively low passive current density. For a = 1,

the Ni61.5Nb38.5aZra glassy alloy has the passive region of approx-

imately 1.7 V (vs. SCE), which is from 0.18 V (vs. SCE) to 1.5 V (vs.

SCE), and passive current density of about 1 A m2

. It will undergo

locally rapid dissolution when the potential exceeds 1.63 V (vs.

SCE). With further increasing the Zr content, the passive region

abruptlyreduces to about 1.5V (vs. SCE) and keeps stable while the

passive current density declines by one or two magnitude order,

which is about 0.1 A m2 for a =3, 5 alloys, 0.05A m2 for a = 7 alloy,

and 0.1 A m2 for a = 9 alloy. Accordingly, the addition of Zr is only

slightly reduce the passive region but obviously decreases the pas-

sive current density, implying that the addition of the appropriate


7/9


Fig. 7. Nominal compressive stress-strain curves of the as-cast samples with 2 mm

in diameter and 4mm in length at a strain rate of 2104 s1, a, b and c for

Ni63Nb31.45 Zr5.55, Ni62Nb32.3Zr5.7 and Ni61.5Nb33.5Zr5, respectively.

amount of Zr is conducive to the corrosion resistance of the inves-

tigated alloys. In the potential range higher than 1.5V (vs. SCE),the current density of the Ni61.5Nb38.5aZra with a = 39 increases

rapidlywith the potential, whichmay be attributed to the evolution

of O2 and/or Cl2. The results indicate that the NiNbZr BMG alloys

have high corrosion resistance in the aggressive acid solution.

High corrosion resistance has been regarded as one of the

superior merits of metallic glasses since the discovery of amor-

phous FeCrPC alloy with the extremely high anti-corrosion

property [13,43]. Herein, to clarifying the origin of the high anti-

corrosionpropertyof thecurrent NiNbZr BMGalloys, thesurface

Fig.8. Potentiodynamicpolarization curvesof theBMG Ni61.5 Nb38.5aZra (a = 1,3, . . .,

9)in at.% alloys measuredat a potentialsweep rateof 0.333mV/s in1 M HCl aqueous

solution open to air at 300 K.

films formed in air and in 1 M HCl solution were characterized byXPS.

XPS spectra of the NiNbZr BMG alloys consisted of the peaks

of alloy elements in addition to those of oxygen and carbon. The

weak Cl 2p peak was also observed on the XPS spectra of the spec-

imens potentiodynamically polarized till 1 V (vs. SCE) in 1 M HCl

solution. The C 1s peaks resulted from the unavoidable contami-

nant carbon on the top surface of the specimens. The O 1s spectra,

shown in Fig. 9d, is comprised of the peaks arisen from the oxygen

in metalOmetal bond, metalOH and/or bound water. The peaks

Fig. 9. XPS spectrum of the Ni61.5 Nb33.5Zr5 BMG alloys after potentiodynamically polarized till 1 V (vs. SCE) in 1 M HCl aqueous solution open to air at 300 K: (a) Ni 2p, (b) Nb

3d, (c) Zr 3d and (d) O 1s.


8/9


Fig. 10. Cationic contents in the surface films for the NiNbZr BMG alloy exposed

to air and those potentiodynamically polarized till 1 V (vs. SCE) in 1 M HCl aqueous

solution open to air at 300 K: (a) Ni61.5 Nb37.5 Zr1 and (b) Ni61.5 Nb31.5Zr7.

of Ni 2p, Nb 3d and Zr 3d, shown in Fig. 9ac, respectively, corre-

spond to their oxidized states in the surface film and their metallic

states in the underlying alloy surface [3133].

Fig. 10 shows cationic contents in the surface films for the

NiNb-Zr BMG alloys exposed to air and those potentiodynami-

cally polarized till 1 V (vs. SCE) in 1 M HCl aqueous solution open

to air at 300 K. The Nb and Zr were enriched in the surface films

when the specimens of the investigated alloys were exposed to

air. When polarized in the 1 M HCl solution, the contents of Nb

and Zr in the surface film further increased. The formation of Nb-

and Zr-enriching surface films would be responsible for the high

corrosion resistance of the NiNbZr BMG alloys, like Nb, Zr, Ti-enriching surface films leading to the high corrosion resistance

of the NiNbTiZrCo(Cu) glassy alloys [32,33]. In further ana-

lyzing the effect of the addition of Zr, it was found out that the

Ni content was identical, about 37 at.% (shown in Fig. 10), while

Zr would partially substitute for Nb in the surface films formed

in air with increasing the amount of Zr. In contrast, the surface

films formed in 1 M HCl solution exhibited completely different

behavior with increasing the content of Zr. The Nb content main-

tained equal while the Ni content dramatically decreased by Zr

partial substitution for Ni. Thus, with increasing the content of

Zr, the decline of the content of Ni of metallic state (Fig. 9a) in

the surface films is thought to contribute into the decease (shown

in Fig. 8) by two magnitude order in the passive current density

in the potentiodynamic polarization measurements, which obvi-

ously enhances the corrosion resistance of the NiNbZr BMG

alloys.

4. Conclusions

The systematic investigations of the GFA, thermal, mechanical

and corrosion properties of the NiNbZr BMGs lead us to draw

some following conclusions:

(1) Bulk metallic glasses were successfully prepared in wide com-

position range of NiNbZr system. There exists a wide BMG

forming region, in at.%, 60< Ni< 64, 28< Nb< 38 and 0 < Zr< 9.

Themaximum diameter of theas-castglassyrodsreaches3 mm

by using copper mould injection casting method. The Zr addi-

tion improving the GFA is attributed to suppress the nucleation

of the hexagonal NiNb and orthorhombic Ni3Nb phases.

(2) NiNbZr BMGs exhibit high thermal stability with high Tgof 880900 K and high Tx of 915932 K. The variation of Tg is

related to the crystallization behavior for the Ni61.5Nb38.5aZraalloys.

(3) These Ni-based BMGsexhibitgood mechanicalproperties along

with high compressive fracture strength, 33.2 GPa and somecompressive plastic deformation of about 2%.

(4) NiNbZr BMGs possess high corrosion resistance and were

spontaneously passivated with extremely wide passive region

above 1.5 V (vs. SCE) and relatively low passive current density

in the 1M HCl aqueous solution open to air at 300K, espe-

cially for Ni61.5Nb31.5Zr7, whose passive current density is

on the magnitude of 102 A m2. High corrosion resistance is

due to the formation of the Nb and Zr enriched surface films,

whichpreventthe alloys fromfurthercorrosion. The Zr addition

enhances the corrosion resistance by its partially taking place

of metallic Ni in the surface films.

Thus, the development of the NiNbZr BMGs with high GFA,

high thermal stability, good mechanical properties and high corro-

sion resistance would help expand the application of the BMGs as

structural materials.

Acknowledgements

The authors gratefully acknowledge S.J. Zheng and G.M. Cheng

for the assistance of TEM experiments, and the financial support

from the Ministry of Science and Technology of China (Grant Nos.

2006CB605201 and 2005DFA50860), the National Natural Science

Foundation of China (Grant No. 50731005).

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