Solidification of undercooled Sn-Sb peritectic alloys: Part I. Microstructural evolution
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I.
INTRODUCTION
TIIE
establishment of rapid solidification processing (RSP) as an innovative approach to materials processing has been based to a large extent on the development of favorable microstructural features, t~,2j As a broad generalization of the microstructures associated with RSP, there appear to be two categories of characteristics. In one case, the influence of RSP treatment can be viewed in terms of a decrease in local solidification time and the concomitant reduction in microstructural scale. Another category of microstructural development is the evolution of novel structures comprised of phase mixtures that are not associated with the equilibrium features of the phase diagram. During the development of RSP, emphasis has been placed on the attainment of high cooling rates, but it is now recognized that rapid solidification conditions can be achieved with slow cooling rates provided the liquid is substantially undercooled prior to nucleation. Therefore, RSP techniques either involve delayed nucleation of the solid followed by unconstrained growth or constrained growth in which solid phase formation is limited by the rate of heat extraction. Although the application of rapid cooling rates has yielded a variety of nonequilibrium solidification products, this approach offers limited possibilities for direct temperature measurement during processing so that information regarding the undercooling and cooling rate must be calculated or inferred by microstructural analysis.
The droplet emulsion technique, which involves delayed nucleation of the solid, can be used to analyze formation of metastable phases during RSP in a controlled manner, t3'4'5~ This technique was applied in the present investigation to study microstructural evolution in undercooled, Sn-rich Sn-Sb alloys. As shown by the Sn-Sb phase diagram (Figure 1), t6j the body-centered tetragonal (bct) Sn-rich solid solution (/3, Strukturbericht A5) forms via a peritectic reaction between liquid and e (B 1) at 250 ~ The 3' phase exhibits a NaCl-type structure but with a slight rhombohedral distortion (a = 89.38 deg at stoichiometry).171The rapid solidification of these alloys is of interest, since this system can serve as a model for the analysis of microstructural evolution in other alloys which involve a peritectic reaction. The previous application of RSP techniques to Sn-Sb alloys has been reported to result in reduced segregation and solubility extension, ts,9,~~ In fact, complete supersaturation of/3-tin has been achieved through splat quenching for alloys containing up to 22 at. pct Sb. [91 Information obtained from the present analysis has been coupled with the earlier rapid quenching work to provide a more complete understanding of rapid solidification behavior. II.
EXPERIMENTAL PROCEDURE
High-purity Sn and Sb (99.999 pet) were used to prepare the alloys investigated. The alloys were encapsulated in PYREX* ampoules under vacuum, melted, and *PYREX is a trademark of Coming Glass Works, Coming, NY.
W.P. ALLEN, formerly Graduate Student,
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