Factors influencing heterogeneous nucleation in an NiSiB glass

Journal of the Less-Common Metals, 145 (1988)

FACTORS INFLUENCING Ni-Si-B GLASS* M. II. ZUERCHER

167

167 - 174

HETEROGENEOUS

NUCLEATION

IN AN

and D. G. MORRIS

University of Neucha’fel, fnstitute of Structural Metallurgy, 2000 ~euch~tel

~S~ifzerland~

(Received May 31,1988)

Summary A crystallization study of isothermally annealed Ni-7at.%Si-17at.%B ribbon using qu~ti~tive me~o~aphy is presented. Particular attention was devoted to the investigation of the possible factors influencing nucleation and growth rates. In spite of the care taken during sample preparation, crystal nucleation at low temperature was dominated by impurity effects, independent of the quenching rate used to prepare the ribbon. At higher temperatures, nucleation and growth were continuous and constant, irrespective of the quenching rate. The high sensitivity to contaminants (earlier observed by rapid crys~llization on the ribbon surfaces) was demonstratid by deliberate internal oxidation. Once more, the main effect of these impurities was observed at low temperature, adding to the preexisting impurity effect, resulting in a significant increase in nucleation rate. In order to quantify the influence of impurities on cryst~l~ation, fine dispersoid particles were voluntarily added to the alloy before spinning. These particles present the advantage of being directly observable, and nucleation behaviour can be related to known properties such as stability, interfacial energy, structure or shape. From the high melting temperature carbides, nitrides and oxides chosen, several illustrations of the crystallization behaviour are shown and discussed. The aim is to develop an understanding of the influence of such dispersoid or impurity particles on overall crystallization kinetics, This information may be useful in enhancing or inhibiting heterogeneously nucleated crystallization and may lead to wellcontrolled transformations and structures.

1. Introduction Whilst cryst~Iization mechanisms, usually divided into nucleation and growth, are now well established, the controlling factors are not completely understood. The reasons why a particular nucleation mechanism occurs and *Paper presented at the Symposium on the Preparation and Properties of Metastable Alloys at the E-MRS Spring Meeting, Strasbourg, May 31 -June 2,1988. 0 Elsevier Sequoia/Printed

in The Netherlands

168

the parameters determining the kinetics of reaction may remain obscure. A given crystallization process is not always clear and a complete understanding usually requires extensive study of the structure, the impurity concentration and efficiency, the quenching rate necessary to avoid the formation of quenched-in embryos etc. Several analytical and experimental techniques can be used but the accuracy is obviously limited when the aim is to explore the atomic environment of nanometre-sized quenched-in structures. Another approach can be used when the preparation conditions are well known; it is possible to vary one experimental parameter deliberately and to evaluate the resulting effects. This possibility has been used in some experiments by varying the quenching speed of a given alloy and relating it to the crystallization kinetics [ 1, 21. Other studies have reported the influence of quenching atmosphere and of preparation conditions on the resulting crystallization [3, 41. However, in many investigations where heterogeneous nucleation has been supposed or established on the basis of kinetic measurements, usually at the surface [5, 61, no clear explanation has been given on nucleation sites and embryo location. Experiments where macroscopic impurities or inoculants are voluntarily added to induce heterogeneous nucleation are more rare and these particles are usually introduced to improve mechanical properties [ 7, 81. This study follows an earlier report [9] on an Ni-Si-B alloy of the same composition in which two different nucleation processes were detected depending on the annealing temperatures and at the same time showing a strong sensitivity to contamination (especially visible at the ribbon surfaces). A new step in the understanding of the crystallization mechanism of this alloy was made in this study using different experimental conditions such as quenching rates and impurity content. Heterogeneous nucleation was induced by the addition of micrometre-sized particles, the efficiencies of which are directly observable and can be interpreted in terms of stability, interfacial energy and perhaps structure or density of defects in the particle surface. 2. Experimental

procedure

The alloy Ni-7at.%Si-17at.%B (considered here has exactly the same composition and was produced following the same experimental procedure as described in detail previously [9, lo]. The master alloy was cast by melt spinning at three different wheel speeds (17 m s-l, 26 m s-l and 36 m s-l), corresponding to increasing quenching rates. An alloy containing oxygen impurities was also prepared by induction melting in a quartz tube in 0.5 atm of air instead of vacuum. The samples containing deliberately added particles were obtained by mixing a piece of the master ingot with the corresponding powder in a sealed quartz tube under vacuum, followed by energetically shaking at a temperature about 50 “C above the melting temperature until solidification. The conditions of subsequent melt spinning were strictly similar to those of particle-free alloys.

169

The ribbons were isothermally annealed at three different temperatures: 400 “C and 450 “C under argon protection and 470 “C in a salt bath. Crystallization analysis was performed by optical and scanning electron microscopy on metallographically well-prepared samples.

3. Results and discussion 3.1. Effect of different quenching rates and of oxygen impurities Figure 1 shows the general morphology of the crystals. Randomly distributed twinned crystals with an Ni,B structure are observed in the bulk of all ribbons. The growth rates of these nearly spherical crystals remain constant throughout the crystallization process at all annealing temperatures and are independent of the quenching rate used, increasing with increasing temperature.

Fig. 1. General

morphology

of the randomly

distributed

crystals

with an Ni3B structure.

At 450 “C and 470 “C, the quenching rate has no evident influence on the nucleation rates and the global kinetics remain unchanged. The continuous and uniform nucleation rate corresponds to a thermally activated, uniform nucleation process. At 400 “C, the saturation observed earlier for one speed disappears for the others (Fig. 2) but with no significant difference in the nucleation kinetics. The absence of a logical variation with the quenching rate leads to the conclusion that nucleation occurs on preexisting, extrinsic impurities or contaminants, the concentration of which is susceptible to variation. As illustrated in the following figures, nucleation and growth rates are markedly increased by the introduction of oxygen impurities. Figures 3(a) and 3(b) show, respectively, the growth rates and the crystal volume densities of the original ribbon and of the ribbon containing oxygen at 450 “C. Although the growth rate remains constant with time, the effect of oxidation increases with decreasing temperature, especially for the nucleation rate. These kinetic effects are certainly related to the composition and structural modifications caused by the presence of oxygen atoms. It would

170

I 0

5

I

I

I

I

20

40

60

80

TIME

(H)

Fig. 2. Crystal number density observed after annealing at 400 “C for the three quench rates studied. I- = 45OY

~~:~~~i~”

I

0 0 (a)

~~-~~i~n

20

8 40 TIME (Min)

t 0

\ 60 @I

20

1 40

TIME (Mb)

Fig. 3. (a) Crystal size after annealing the original ribbon and the ribbon containing oxygen at 450 “C. (b) Crystal volume density after annealing the original ribbon and the ribbon containing oxygen at 450 “C.

seem that silicon atoms are bound, at least partially, to oxygen atoms to form silica molecules. There are two consequences of this: (i) the local composition of the alloy is modified and (ii) the formation of silica particles could play a role as nucleation sites, The first assertion can explain the approximately threefold increase in the crystal growth rate, since the crystal phase formed is always Ni,B as revealed by electron diffraction. Secondly, molecules such as SiO, are potentially favourable sites for nucleation and could be added to the preexisting or the~~ly activated sites. This leads to a corresponding rise in the nucleation rate as observed. This interpretation is supported by the fact that the effect of oxidation is greater at lower temperature where there is limited thermal activation and where the favourable nucleation sites have been shown to be limited [9].

171 3.2. Heterogeneous nucleation on dispersed particles The addition of welldefined particles to the alloy makes it possible to examine their effect on crystallization directly. In order to establish the more relevant parameters determining the particle efficiency, the particles were chosen so as to cover a wide range of properties such as stability, nature of chemical bonds, crystal structure, interfacial energy, etc. Moreover, to maintain an unchanged alloy composition, particles with high melting temperat~e and s~bility were chosen to avoid possible dissolution. According to classical nucleation theory [ 11,121 the homogeneous and heterogeneous nucleation rates can be written as I horn

I hetero -

T

exp(- AyFero]

where L) is the atomic diffusion coefficient across the interface, iV, is the average volume concentration of atoms, JV, is the average surface concentration of active atoms for nucleation, a, is the average atom diameter and AG* is the activation energy required to form a critical nucleus. A geometrical calculation shows that AGhetero differs from AGhom only by a function of 8, the so-called wetting angle. This angle is determined by the equilib~um of the interfacial energies at the gl~s-c~s~l-p~icle interface and f( 0) can be expressed as f(e) = 32

-. 3 cos e + c0s3e)

The interfacial energies and wetting angles have been extensively examined for metal-ceramic interfaces [13 - 151 using the “se&e drop method”. This leads to very different wetting angles of 6 = 180” for non-wetting particles and 8 = 0” for perfect wetting (or spreading). Nevertheless, very little is known about the wettability of a crystalline phase on a ceramic particle in an amorphous metal environment: our situation is quite different since all phases are condensed. This probably leads to smaller variations in the wetting angle between wetting and non-wetting particles. The inverse relationship experimentally established between wetting and stability of particles (or enthalpy of formation) has not been clearly verified in our experiments. In Fig. 4 the surface nucleation rate Z, is reported us. the stability of particles (-AH). The relationship between them explains nothing more than the general behaviour. For example, the great difference in wetting between SiO, (a stable oxide) and the carbides can obviously be related to the high surface energies (high stability) of silica particles which leads to a weak bonding with the matrix. A more accurate analysis of the values reported in Fig. 4 underlines the importance of other param-

172

SiO,

IO’

1 loo

I 200

I 500 Enlhalpyof Formation(kJ/mol)

1

I loo0

Fig. 4. Crystal nucleation rate Is on the surface of particles as a function of their enthalpy of formation.

Fig. 5. Surface crystallization on a WC particle after annealing at 450 “C for 5 min, showing random distribution of crystals and strong anisotropy. Fig. 6. Surface crystallization on a TaC particle after annealing at 450 “C for 8 min, showing regular and uniform crystallization on all edges.

eters in explaining the individual behaviour of each particle. This can be proved explicitly by studying the kinetics of crystallization on three carbides, WC, TaC and Tic. These particles with energies of formation 37.7, 143.6 and 183.8 kJ mol-’ respectively show no simple variation in nucleation rate with these energies. Indeed, WC and TaC (Figs. 5 and 6) which have different crystal structures (hexagonal and cubic) as well as different stabilities present nearly the same rates of nucleation. TiC (cubic) is one order of magnitude slower.

173

The similarity in the kinetics of nucleation on WC and TaC is paralleled by the similar values of the angle % which lie in both cases in the region of about 65” It 10”. Using this angle to deduce the value of the function f(8) and comparing the bulk (homogeneous) and the heterogeneous nucleation rate (on the dispersoids), it is possible to determine the number N, of active sites for heterogeneous nucleation per unit area. This density was found to be 101* - 1013 m-* for both WC and TaC. This number could correspond to a density of defects or steps present on the particle surface, for example, the number of surface ledges or dislocations. The random distribution of the nucleated crystals on the dispersoid surfaces, with no preferential effect on the edges, suggests that randomly distributed point sites, for example, dislocations meeting the surface are responsible. Another interesting effect was detected on WC particles, namely the strong anisotropy of the c~s~llization (Fig. 5) which is markedly slower on the basal faces. This may be a consequence of the anisotropy of the surface energy (often observed for hexagonal structures) or it may be the result of a variation in the defect concentration dependent on the facet considered.

4. Conclusions The experiments and results presented have improved our understanding of the crystallization of the Ni-Si-B alloy. It is now well established that the quenching rate (when large enough to produce a fully amorphous sample) plays a negligible role in the low nucleation rate at 400 “C. The number of quenched-in impurities responsible for nucleation at this temperature does not vary with the quenching rate and is rather associated with the method and cleanliness of preparation. The sensitivity to oxygen contamination has been studied in the bulk. The inclusion of a higher impurity content Ieads to faster, but always constant nucleation and growth rates. The oxygen impurities are more significant at low temperature where they add to the preexisting impurities. In view of the high sensitivity of crystallization to oxidation, and probably to other contaminants, it seemed of interest to control the effect of extrinsic or injected impurities. Therefore dispersoid particles were deliberately introduced into the alloy. For some of these particles measurements were made of the surface density of crystals on their faces and the estimation of wetting angle 8. For WC and TaC, 8 was found to be near 65” and the corresponding number of active sites was approximately lOI* - 1013. Th’is number can be related to a density of point sites on the particle surface, for example, dislocations. Examination of the kinetics of crys~llization has shown that many factors have to be considered to explain the particular behaviour of each particle. Even if the surface energy or stability seems to explain the general tendency, factors such as structure, surface defects, roughness or anisotropy must not be underestimated.

174

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