Some aspects of the annealing behavior of indium
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IN a recent paper Balluffi el al. I refer to the work of Bredt and Koehlere on the annealing behavior of copper deformed at subzero temperature. 99.999 pct pure Cu was deformed at -195~ and significant changes were reported to have been observed during holding at -195~ for long periods. The authors also make the important observation that such aging at -195~ could substantially alter the subsequent annealing at higher annealing temperatures. Essentially similar results were obtained on specimens of indium of >99.9 pct purity and the present paper describes briefly the important observations recorded. Specimens of indium were deformed in compression by 30 pct at liquid nitrogen temperature. The deformed specimens were then given two types of treatments. In treatment a, the specimen designated a, was rapidly brought up to room temperature, +20~ In treatment b, the specimen designated b, was kept at -193~ in a I liter dewar until the liquid nitrogen had evaporated over a period of 24 hr. At the end of this stage, the specimen temperature rose slowly to +20~ Specimen a, was metallographicallyprepared first. Specimen b was prepared after all the liquid nitrogen had evaporated and the specimen temperature reached 20~ Details of sample preparation and etching procedure have already been described in the literature,s After this stage, the specimens were kept in a dessicator at +20~ for 3 weeks, during which period they were examined metallographicallyat regular intervals. The above set of experiments, i.e., treatments a and b and aging of specimens a and b for 3 weeks at +20~ were repeated three times and the results reported below were found to be reproducible. The microstructure of specimens at the end of treatments a and b are shown in Figs. l(a) and (b). The structures are free of deformation traces and are fully recrystallized,being made up of a range of grain sizes. The presence of many annealing twins can also be seen clearly in the figures. Etch pits, faintly outlining some form of prior grain structure, e.g., Fig. l(b), were observed in both specimens and their significance will be discussed in the following section. The important differences between Figs. l(a) and (b) a r e as follows: Specimen a is made up p r e d o m i n a n t l y of v e r y c o a r s e g r a i n s with only a few s m a l l g r a i n s . In s p e c i m e n b, there a r e m a n y m o r e s m a l l g r a i n s than in s p e c i m e n a and the a v e r a g e g r a i n size of the c o a r s e r g r a i n s a p p e a r s to be s m a l l e r than in s p e c i m e n a. Both s p e c i m e n s were e x a m i n e d at r e g u l a r i n t e r v a l s d u r i n g the holding p e r i o d . The m i c r o s t r u c t u r e of specimen a remained essentially stable, apart from c e r t a i n m i n o r changes to be d e s c r i b e d l a t e r . However, s i g n i f i c a n t c h a n g e s in the m i c r o s t r u c t u r e of s p e c i m e n b were o b s e r v e d d u r i n g holding. D i s t i n c t C. DASARATHYis Senior Research Officer, Research Centre, Brit
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