Amorphization of single composition powders by mechanical milling

Scripta Materialia, Vol. 34, No. I, pp, 21-27, 1996 Elsevier Science Ltd Copyright 8 1995 Acta Metallurgica Inc. Printed in the USA. All rights reserved 1359-646Z96 $12.00 + .@I

Pergamon 0956-716X(95)00466-1

AMORPHIZATION OF SINGLE COMPOSITION POWDERS BY MECHANICAL MILLING C. C. Koch Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 276957907 (Received September 1, 1994) (Revised April 15, 1995) Introduction

Of the various methods which have been used for over 30 years to prepare amorphous materials, such as vapor phase deposition and rapid solidification, mechanical grinding or milling has also been noted as another but much less studied technique. It has been observed to produce amorphous structures in, eg., crystalline forms of SiO, (1). The more recent interest in the use of ball milling as an amorphization method for single composition materials was stimulated by the work of Ermakov et al (2,3) who observed amorphization of the intermetallic compounds YCo,, Y&o,, YCo, and Y$o~~ and partial amorphization in GdCo, and Gd,Co,. Subsequently Schwarz and Koch (4) demonstrated amorphization of the intermetallic compounds NiTi, and Ni,,SNb,, which were the same compositions as amorphous alloys they prepared by “mechanical alloying” of the elemental powders. The ball milling of powders with different compositions, in which material transfer occurs, is termed “mechanical alloying” (MA) while ball milling of single composition powders such as compounds or elements has been termed “mechanical milling” (MM) or “mechanical grinding” (MG) in the literature. The author prefers the term MM since “grinding” is conventionally used to denote the metal cutting methods which involve material removal by chip formation due to shear stresses. The intermetallic compounds which were amorphized by MM were described by Weeber and Bakker (5) in their review published in 1988. Since that time numerous additional intermetallic compounds, inorganic non-metallic compounds, and even several elements have been reportedly amorphized by MM. This paper will describe selected aspects of the phenomenology of amorphization by MM in single composition materials and then discuss some possible mechanisms for such amorphization. Phenomenolow

of AmorDhization

bv MM

Amorphization by MM consists of energizing the crystalline solid by the severe cyclic deformation that ball-milling provides. It is significant that ball-milling has been found to induce extensive plastic deformation even into extremely brittle materials such as Al 5 intermetallic compounds like Nb,Sn (6). This is presumably due to the partly hydrostatic stress states existing during milling. Irradiation by energetic particles can produce amorphization m many crystalline materials by the introduction of defects. This solid state amorphization phenomenon has been extensively studied and amorphization by MM may exhibit many similarities to it. In fact several models for solid-state amorphization (to be discussed in the 21

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following section) have been recently proposed which claim to be applicable to both irradiation and MM (7-9). Brimhall et al (10) suggested that the best criterion for predicting amorphization in intermetallic compounds by high energy ion irradiation was the compositional range of the compound. With a few exceptions, compounds with narrow compositional ranges, i.e. “line compounds” exhibited amorphization and those with wide compositional ranges did not. Luzzi and Meshi (11) subsequently presented conditions for amorphization by electron irradiation. Their empirical criteria were: l] the order/disorder temperature, T,, must be > T,, the melting temperature; 21 the fraction of A atoms F, (F, < FB) must be t l/3; 31 A and B should be separated in the periodic table by more than two groups; 41 the compound should have a complex crystal structure. The criterion of narrow compositional range of Brimhall et al (10) does not appear to hold in general for amorphization induced by electron irradiation (11). This also appears to be the case for amorphization by MM. For example, Nb,Ge and Nb,Sn have relatively wide compositional ranges while NiZr, and Ni,,Zr, are line compounds, but all four compounds can be amorphized by MM (12). While the criteria presented by Luzzi and Meshii for electron irradiation induced amorphization appear to generally apply to amorphization by MM, the criterion of F, t l/3 has several exceptions since a number of compounds with AB, stoichiometry exhibit amorphization by MM. While the other three criteria appear to hold for compounds which amorphize by MM, there are many examples of compounds which meet these criteria but don’t amorphize. For example, the three compounds in the Nb-Ge system, Nb,Ge, Nb,Ge,, and NbGe, all fulfill the three criteria but only Nb, Ge can be amorphized by MM (13). These criteria may therefore be necessary but not sufficient for amorphization by MM, and as will be discussed in the next section, amorphization is presumably controlled by the relative energetics of the defected crystalline and amorphous phases. Several variables of the milling process can be critical in controlling amorphization. These include mill energy, milling temperature, and impurity contamination from the milling atmosphere or balls. There are several examples of systems in which amorphization occurs at lower milling intensities but only crystalline phases are observed at higher intensities. For example, this behavior was observed for ZrNi alloys (14) and Nb,Sn (15). It was assumed that higher mill intensities produce heat and higher temperatures in the powder during milling. Yamada and Koch (16) carried out MM of TiNi intermetallic powder in two different mills. The more energetic Spex shaker mill provided a higher degree of lattice strain and rapidly refined the grain size to the nanometer regime. Amorphization was observed for the Spex mill when the nanocrystalline grains reached diameters of about 5 nm but not in the less energetic vibratory mill where the grain size saturated to a constant value of - 15 nm after long milling times. The lattice strain vs. grain size for MM in the two mills is shown in Figure 1. It was suggested that the more energetic Spex mill provided more lattice strain and this is related to the development of the finer grain size and amorphization. Pathak et al (17) have recently summarized the results of varying the temperature of the mill on the kinetics of amorphization in intermetallics. In most experiments reported, a lower milling temperature was found to accelerate the amorphization process. It was believed this is due to a nanocrystalline grain boundary structure as the defect driving the crystalline-to-amorphous phase transformation, and the experimental evidence for more rapid development of the nanocrystalline structure at lower milling temperatures. If the development of a nanocrystalline grain structure by MM involves a competition between defect creation by the cyclic plastic deformation and defect recovery by thermal activation, then lower temperatures should favor fine nanocrystalline grain formation. The milling time for attaining an amorphous structure at various milling temperatures is illustrated for TiNi, CoZr, and NiZr, intermetallics in Figure 2. While most of the limited number of studies involving amorphization of intermetallics as a function of milling temperature follow the above behavior, there is at least one counter-example (18) where amorphization kinetics were faster at higher milling temperature. More studies are needed to clarity these discrepancies. One serious problem with the milling of fine powders is the potential for significant contamination from the milling media or atmosphere. Contamination of intermetallic compound powder can influence

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23

30x10-4

.E

z

“‘\, Spex Mill

v)

z ._ t: 1

10 /

0

20

40

60

80

Grain Size (nm) Figure 1. Lattice strain versus grain size for Spex milled and vibratory milled TiNi.

the crystalline-to-amorphous phase transformation or the stability of the amorphous phase on continued milling. If, as ia often the case, the milling vial and balls are steel, significant iron contamination may result. This is a particular problem for hard, strong intermetallic compound powders which can produce severe wear of lthe steel milling media and iron contamination. Therefore it is imperative to carry out chemical analyses on the milled powder to determine the extent of iron contamination and the level to which one is dea.ling with ternary rather than binary phase equilibria. If iron contamination is a problem other milling media, eg. tungsten carbide, may be used. The other major source of contamination is from the milling atmosphere - i.e. oxygen or nitrogen. These impurities can be avoided/minimized by milling in high purity inert atmospheres such as argon. However, practically, and with reactive metal components such as Ti or Zr some contamination may be observed. The influence of contamination on amorphization of intermetallic compounds presumably depends on the phase equilibria of the given multicomponent system. While additional alloying elements added to a binary system usually favor amorphization, both enhancement anldprevention of amorphization have been reported for contaminant additions during MM. Oxygen has been shown to stabilize the amorphous phase for NiZr, (19) in terms of raising the crystallization temperature of the amorphous phase produced by MM. Conversely, oxygen contamination was found to stabilize the crystalline Nb, Ge, compound against amorphization by MM (13). The MM of the NiZr intermetallic in nitrogen resulted in formation of an amorphous nitride of composition NiZrN, 15 (20). The milling time for amorphization was about half (24h) that (48h) for amorphization in a pure argon atmosphere. The continued milling of amorphous TiAl has been reported to result in an fee phase (21). Analysis of the powder after milling and lattice parameter measurements strongly suggest the fee phase is TiN iduced by nitrogen contamination during milling. Therefore, interstitial atom contamination in solid solution can either favor or discourage amorphization and in extreme cases can result in an impurity phase such as TiN (2 1). While most studies of amorphization of single composition materials by MM have focused on intermetallic compounds or, in some cases, non-metallic inorganic compounds (22), there are also a few

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n

0 .

o-0.0

0.1

0.2

Milling Temperature/Melting

Figure 2. Milling

time for amorphization

0

NiTi

l

CoZr

m NiZr2

0.3

0.4

Temperature

versus milling temperature

normalized

by the intermetallic

compound melting

temperature.

reports on amorphization of elements. Metallic elements in general do not exhibit amorphization. Exceptions are certain transition metals (Ni, Co, Cr, Fe, Mn) which can exhibit amorphization when vapor deposited on liquid helium temperature (4.2K) substrates (23). However, very high purity Ni could not be amorphized (24) even at 4.2K, suggesting some impurity concentration is needed for amorphization in pure metals. In any event, the amorphous metal films formed at 4.2K generally crystallized at temperatures well below ambient. However, Pavlov et al (25,26) have given evidence for the partial amorphization of Ni and Pt after extreme degrees of plastic deformation ( 1 99.95%) by multiple-pass rolling or drawing. The microstructure was found to break up into “fragments” - nanocrystallites - and eventually some volumes exhibited an image and selected area diffraction pattern consistent with an amorphous structure. To the author’s knowledge these observations have not been reproduced. No evidence for amorphization by MM of pure metallic elements has been presented. Instead, MM has been found to produce a nanocrystalline microstructure (27-29). In contrast to metals the covalent semiconducting elements, Si and Ge, are more easily amorphized by a variety of methods including ion implantation (30) vapor condensation (3 l), or indentation (32). Gaffet and Harmelin (33) have reported the partial amorphization of Si by MM. They based this conclusion mainly on fitting functions to the broadened x-ray diffraction lines in milled Si. In order to explain their data about 10% of the structure was assumed to be amorphous. They also presented TEM and DSC data which suggested partial amorphization. However, in contrast to the above conclusions Bokhonov et al (34) have suggested the amorphous phase observed after MM of Si is the suboxide SiO. They propose that oxygen contamination during milling results in formation of SiO, which in turn reacts with Si and amorphizes to SiO which is structurally similar to amorphous Si. However, the crystallization temperature, T,, for amorphous SiO was found to be about 840°C while Gaffet and Harmelin (33) reported at T, = 690°C in good agreement with the T, of amorphous Si prepared by ion implantation (30) of 692°C. Subsequently Shen et al (35) have also obtained evidence for the partial amorphization of Si by ball-milling using XRD, TEM, DSC, and Raman spectroscopy. In particular a Raman peak attributable to amorphous-Si appears to be strong evidence in favor of its existence after MM.

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MILLING

25

8.0 % "0 -

60 .

a

4.0

N-0

2.0

0.0

0.2

0.4

1

0.6

0.8

1.0

-s*

Figure 3. Dependence of the Debye temperature I&,on long-range-order (1 - S2) calculated for NiZr (after reference 9).

ProDosed Mechanisms

for AmorDhization

bv MM

Most of the explanations for amorphization by MM originated with the suggestion from Swanson et al (36) for radiation-induced amorphization that amorphization can occur when Gc + G, > GA. Here Gc = free energy of the crystalline phase; G, = free energy increase due to the defects (introduced by irradiation); and G, = free energy of the amorphous phase. This idea was first applied to amorphization by MM by Koch and Kim (37) for Nb,Sn and Nb,Ge compounds first prepared by mechanical alloying of the elemental powders and subsequently amorphized on continued milling. From the viewpoint of thermodynamics, the first question which must be addressed is: what defects can supply the required free energy to permit Gc + G, > GA? The common defect introduced by mechanical deformation - the dislocation - does not appear to provide sufficient energy and in general the measured values for stored energy during severe cold working rarely exceeds about l-2 kJ/mole (38). The free energy differences between the crystalline and amorphous states are usually L 5 kJ/mole. In the case of ordered intermetallic compounds, disordering energies can be significant and anti-site disorder and/or antiphase boundaries may store sufficient energy to drive amorphization. Bakker and co-workers (eg. 39) have reported a number of studies of disordering of intermetallic compounds. They conclude that energy storage in a ball-milled intermetallic cornpound is mainly due to anti-site chemical disorder. This energy storage may or may not result in amorphization, depending on the relative free energies of the metastable disordered solid solution and the amorphous state. These conclusions are probably appropriate for many of the systems which have been studied but there are examples of other systems in which another defect may be the important energy storage entity. In MM of Nb,Sn the long-range-order (LRO) parameter, S, was found to become zero at milling times well before amorphization was observed (6). Conversely, no decrease in LRO was observed, to within experimental error, in MM of CoZr prior to amorphization (40). In these cases it was concluded that the defects responsible for amorphization were the nanocrystalline grain boundaries. In recent years there have been several theories for the crystal-glass transformation which follow the general idea of theLindemann phenomenological melting criterion. Johnson and co-workers (eg. 4 1) have suggested that there is an ultimate limit to disorder in crystals which may be defined by a critical concentration in solid solutions or a critical level of disorder in intermetallic compounds. Lam and Okamoto (9) propose that “melting” of a defective crystal occurs whenever the sum of the thermal and static atomic displacements reaches a critical value identical to that for the “melting” of the defect-free

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crystal. A consequence of their model for amorphization is that it provides a basis for ranking the tendency of compounds to become amorphous in terms of their elastic properties as measured by Debye temperatures, BD.They illustrate (Figure 3) the dependence of B,, on disorder (1 -S) calculated for NiZr along with experimental data for the undamaged crystalline compound and the glassy alloys. Krill et al (42) have determined 13~from low temperature specific heat measurements in a series of Nb,,,_,P& alloys. Elastic neutron diffraction measurements also provided values for atomic mean-square displacements (MSD). Generalizing the Lindemann criterion to alloys by including the static contribution to the total MSD allowed for a prediction of an absolute limit to supersaturation of the bee crystalline phase. Experimental measurements of &, etc. as a function of milling time would be useful in determining the applicability of the above theories to amorphization by MM as anti-site disorder and/or a nanocrystalline microstructure develops. Johnson (43) has suggested that materials which exhibit a decrease in volume on melting (eg. H,O, Si, Ge etc.) might be amorphized by high hydrostatic pressure at low temperatures. Indeed it has been demonstrated (44) that solid-state amorphization can occur by the pressure exceeding the metastable extension of the liquidus curve on the T-P phase diagram and that either the amorphous phase is formed directly on loading and persists on unloading because of insufficient thermal energy to crystallize or a high pressure crystalline phase transforms to a metastable amorphous phase on unloading. These pressure induced mechanisms have been proposed for mechanically driven amorphization of Si and Ge by indentation (32) and low-load scratching (45). It is possible that similar mechanisms may apply to the partial amorphization induced by MM in Si and Ge (33,35).

Acknowledvements The author’s research on this topic was supported by the National Science Foundation.

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