Time-lapse photography of the β-Sn/α-Sn allotropic transformation

Abstract

When tin transforms from the β to α phase it undergoes a dramatic process. The crystal structure changes from tetragonal to diamond cubic; the material properties transform from a ductile metal to a brittle semiconductor; however the most notable change is the decrease in density, which goes from 7.31 g/cm³ to 5.77 g/cm³ [1]. It can be calculated that this decrease in density is equivalent to an increase in volume of about 26% [2]. Due to this volume increase and the brittleness, the transformed material progressively cracks and eventually falls apart. This could potentially be a threat for tin-rich alloys used in electronics in low temperature applications. Due to the optimal transformation temperature of approximately 240 K and the long time required for the transformation, a direct observation of the phenomenon has not been possible. This study proposes a new method for observing the β/α transformation in situ using a time-lapse photographic technique. This study concentrates on pure tin, but the applicability of the method opens new possibilities for studying the phenomenon for other tin alloys, such as the two commonly encountered eutectics of SnCu and SnAgCu.