Coarsening behavior of Sb-InSb eutectic alloy of two starting particle sizes

227

METALLOGRAPHY 18:227-234 (1985)

Coarsening Behavior of Sb-lnSb Eutectic Alloy of Two Starting Particle Sizes

G. GRAISS AND G. SAAD

Physics Department, Faculty of Education, Ain-Shams University, Cairo, Egypt

An investigation has been made of the thermal stability of Sb triangular rods in the S b InSb eutectic subjected to different annealing temperatures. Specimens from material with two different particle sizes were held at the temperatures of 480, 485, and 490°C --_ I°C for times of up to 400 hr. The results indicated that the Sb particle diameters coarsened more rapidly in specimens of smaller initial particle diameter (0.9 p,m) than those of larger particle diameter (3.4 ~xm). The activation energy of the coarsening process in the material of the smaller starting particles was found to be two thirds of that needed for the alloy of the larger starting particle diameter. Alignment of Sb particles was found in the later stages of annealing.

Introduction Much attention has recently been devoted to investigating the suitability of unidirectionally solidified eutectic alloys for use as high temperature materials. One of the most important requirements in this context is that the composite be capable of retaining its aligned character during service, and much work has been aimed at determining the high temperature stabilities of aligned composites [1-5]. It is important to ascertain to what extent aging of the eutectic alloys under high temperature may affect the alignment of composites during their coarsening, leading to their high temperature stability. Accordingly, the present work describes an experimental determination of the effect of aging time and aging temperature on the coarsening behavior of the eutectic Sb-InSb. This system was selected for study because of its isothermal stability [6-8] and also because of its suitability for experiment. However, few studies on the coarsening behavior of this alloy have been made. Hence, the present study has been undertaken to establish the effect of starting particle size as well as annealing temperature and annealing time on its coarsening behavior. © Elsevier Science Publishing Co., Inc., 1985 52 Vanderbilt Ave., New York, NY 10017

0026-0800/85/$03.30

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G. Graiss and G. Saad

Experimental PREPARATION OF THE EUTECTIC SPECIMENS The Sb-InSb eutectic, made from antimony and indium of 99.999% purity, was melted under a vacuum of 10 - 6 torr in a quartz tube at 600°C. The melt was kept at this temperature for 4 hr before being streamed to a narrow quartz tube of diameter 3 mm and length 250 mm. The melt was then solidified unidirectionally in a vertical resistance furnace. Two growth rates, of 15 and 30 mm/hour, were employed to obtain samples containing different antimony particle sizes (designated A and B, respectively). The test pieces were sectioned transverse to the growth direction using a jeweller's saw. ISOTHERMAL EXPERIMENTS In order to investigate the effect of annealing temperature on coarsening behavior, three test pieces, of length 5 mm, were cut from both samples A and B and were annealed in pairs at 480°C (A1 and B1), 485°C (A2 and B2) and 490°C (A3 and B3). These annealing temperatures correspond to 96, 97, and 98% of the eutectic temperature. A tubular furnace was provided with a thermo-regulator which helped to control the annealing temperature to _ I°C. Isothermal annealing was carried out for different lengths of time up to 400 hr. Specimens were then air cooled, and prepared for metallographic examination. METALLOGRAPHY The test pieces were prepared for optical metallography by grinding down to the finest grade of silicon carbide paper and polishing to 1-p,m diamond compound. A solution of ferric chloride in alcohol was used for etching. The specimens were then examined in transverse cross-section using an optical microscope and the antimony particles were observed. Since antimony particles have a quasiregular shape [7], statistical diameters were measured for individual particles using a conventionally accepted method [9].

Results and Discussion E F F E C T OF STARTING PARTICLE DIAMETER As a result of the two different growth rates employed in preparing the eutectic alloy, two different starting particle sizes of average diameters

Coarsening of Sb-lnSB Eutectic

229

of 3.4 ~m (A) and 0.9 ~m (B) were obtained. By annealing the two samples A and B at the same temperature and measuring the particle diameter of each at various times, growth curves were obtained as shown in Fig. 1. Transverse microstructures associated with the various annealing times at 480°C are shown in Fig. 2, and those for two long-term annealing periods at 485°C are shown in Fig. 3.

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i

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I

i

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A2

7 A1

0

I.

i

I,,

I

1

80

160

240

320

400

Annealing

time

(hours)

FIG. 1. The mean particle diameter D (mm) as a function of annealing time at 96% (A1 and B 0 , 97% (A2 and B2), and 98% (A3 and B3) of the eutectic temperature for directionally solidified Sb-InSb eutectics. (A and B refer to different starting particle diameters.)

230

G. Graiss and G. Saad B]

A1

~4

30 hours

i00 hours

....... ~ 160 hours

.....:

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~;~ ..... ,,

FIG. 2. Transverse microstructures of Sb-InSb eutectic annealed at 96% of the eutectic temperature (480"C) showing the change of coarsening rate during the first week of annealing. Specimens (A) have starting particle diameter larger than specimens (B).

It is apparent from Fig. I that, irrespective of annealing temperature, the rate of coarsening is generally much more rapid when the initial particle size is smaller. F o r example, in the case of the B2 curve (initial particle diameter 0.9 p,m), the size increased by a factor of 10 in 320 hr, whereas particles with a larger starting diameter subjected to the same annealing conditions (A2) only doubled in size. In the initial part of curve B2 (up to 160 hr) the rate of coarsening is found to be 0.045 ~m/hr, while, for curve A2, the rate is 0.012 p~m/hr. This difference in coarsening rate can be attributed to the larger surface energy associated with the smaller particles [10]. Comparison of the two sets of curves also indicates that

Coarsening of Sb-InSB Eutectic A1

231 B1

240 hours

240 hours

600 hours

320 hours

FIG. 3. Transverse microstructures of Sb-InSb eutectic annealed at 97% of the eutectic temperature (485°C) showing the change of coarsening rate during the second week of annealing. Specimens (A) have starting particle diameter larger than specimens (B). particles in sample B achieve an equilibrium size in less than 320 hr, whereas those in sample A fail to reach that size within the same time period. This also is associated with the smaller surface energy available in sample A. EFFECT OF ANNEALING TEMPERATURE F o r specimens of the same starting particle diameter (sample A or B), the rate o f coarsening (0) is, as expected, higher at higher annealing temperatures, as is clearly seen in Figure I. The energy of activation for the coarsening process (Q) can be determined based on the assumption that coarsening is controlled by an Arrhenius type equation: 0 = A e x p ( - Q/RT) where A is a constant, R is the gas constant, and T is the absolute temperature.

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G. Graiss and G. Saad

In the present case, coarsening rates were obtained for the first 160 hr at each annealing temperature for both samples A and B, and the logarithm of 0 was plotted against the reciprocal of the annealing temperature (Figure 4). The activation energy for the coarsening process was calculated from the slopes of the straight lines shown in Fig. 4. The value of Q was found to be 5.9 eV for sample A and 3.55 eV for sample B. The finding that the activation energy for sample A is 1.66 times higher than that for sample B may be due to the fact that the sample with the smaller particle diameter has higher total internal surface energy, and thus requires less energy to activate the coarsening process. In the later stages of annealing (times in excess of 320 hr), alignment of the antimony particles became more evident (Fig. 5), leading to higher thermal stability of the alloy. The mechanism by which this alignment takes place is readily explainable. Since the geometry of this alloy is known to be symmetrical triangular rods of Sb, it may be assumed that,

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-2.2

I

1.31

I

I

1.315

|

1.320

1000/T

1.325

l

1.330

(°K-l)

FIG. 4. Determinationof the activationenergy, Q, for the coarsening of Sb-InSb eutectic with two different starting particle diameters, A and B.

Coarsening of Sb-InSB Eutectic

233

Fro. 5. Transverse microstructure of Sb-InSb eutectic alloy showing the alignment of Sb particles after two weeks annealing at 98% of eutectic temperature (490°C). d u r i n g c r y s t a l g r o w t h , t h e s e rods m a y r o t a t e in a m a n n e r s u c h that the o r i e n t a t i o n r e l a t i o n s h i p k n o w n for this alloy, n a m e l y , {111} I,Sb [ I {100}Sb, p r e d o m i n a t e s [12]. T h e s e o r i e n t e d rods n o w j o i n to f o r m a p l a t e - l i k e m o r p h o l o g y , as has b e e n f o u n d b y C r o k e r et al. [7] in t h e i r w o r k o n the s a m e alloy.

The authors thank Professor M. A. Kenawy, the head of the Physics Department, University College for Women, Ain Shams University, for his kind encouragement and fruitful discussion during the course of this work.

REFERENCES 1. H. B. Smartt, L. K. Tuand, and T. H. Courteny, Elevated temperature stability of the A1-Ala Ni eutectic composite, Met. Trans. 2:2717 (1971). 2. S. Str~ssler and W. R. Schneider, Stabilityoflamellareutectics, Phys. CondensedMatter 17:153 (1974). 3. H. E. Cline, Shape instabilities of eutectic composites at elevated temperatures, Acta Met. 19:481 (1971). 4. A. R. T. de Silva and G. A. Chadwick, Thermal stability of He fibrous Fe-Fe2B eutectic alloy, Metal Sci. J. 6:157 (1972). 5. J. L. Walter and H. E. Cline, Stability of the directionally solidified eutectics Ni AICr and Ni A1-Mo, Met. Trans. 4:33 (1973). 6. W. K. Liebman and E. A. Miller, Preparation, phase-boundary energies and thermodynamic properties of InSb-Sb eutectic alloy with ordered microstructures, J. Appl. Phys. 34:2653 (1963). 7. M. N. Croker, M. McParlan, D. Baragar, and R. W. Smith, Anomalous eutectic growth, I. The determination of the eutectic structures of Bi-T1Bi2, Bi-Sn, Sb-Pb and Sb-InSb using an accelerated growth technique, J. Cryst. Growth 29:85 (1975). 8. M. N. Croker, D. Baragar, and R. W. Smith, Anomalous eutectic growth, II. The relationship between faceted/non-faceted eutectic structures, J. Cryst. Growth 30:198 (1975).

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9. Richard D. Cadle, Particle Size, Theory and Industrial Applications, Reinhold Publishing Corporation, New York (1965), p. 4. 10. R. E. Smallman, Modern Physical Metallurgy, Butterworths, London (1970), pp. 123, 431. 11. M. R. Soliman, T. H. Youssef, and H. A. Ahmed, Effect of pre-annealing and pre-cold working on isothermal annealing of AL + 0.25 wt% Fe, Fizika 2:239 (1970). 12. L. M. Hogan, R. W. Kraft, and F. D. Lemkey, Advances in Materials and Research, Vol. 5 (1971), p. 83.

Received July 1984; accepted December 1984.