Kinetics and Microstructural Aspects of the Allotropic Transition in Tin.

The allotropic phase transition in tin has attracted rather heterogeneous interests for the practical consequences it has in wide and different ranges of applications, spanning from the degradation of organ pipes to the failure of microelectronic soldering alloys. One of the main aspects that any experimental work on this issue had to face is the extremely low kinetics of the transition from the high temperature body centered tetragonal polymorph, beta-phase, to the low temperature diamond cubic, alpha-phase. In an attempt to speed up the transition and get further insight into its kinetic aspects, we used an approach based on high-energy ball-milling. Indeed, the heavy plastic deformation and the associated high density of microstructural defects are expected to be effective in reducing most of the kinetic constraints to the transition. Moreover, in view of the claimed heterogeneous character of the nucleation of the transition, we investigated this aspect by preparing ball-milled powder mixtures in which 1 wt.% of InSb was added to tin, together with 2 wt.% graphite as process control agent, in order to achieve a fine dispersion of this nucleant (InSb) substance. Cooling and heating experiments, conducted on the ball-milled powder mixtures: Sn-2C and Sn-1InSb-2C, demonstrated that, under the adopted experimental conditions, heterogeneous nucleation in the presence of InSb was the only mechanism capable to induce the beta -> alpha phase transition. The transition could be followed by differential scanning calorimetry, although the limited scanning times were not sufficient for a full accomplishment of the transition. The results obtained so far indicate that a further speed up of the transition kinetics may come from a more prolonged milling, capable to drive the average crystallite size below the value achieved in the present investigation