The effect of pressure on metastable phase formation in the undercooled Bi-Sn system

A metastable phase was produced by the solidification of highly undercooled Bi-48.6 at% Sn alloy droplet samples. During heating the metastable phase was observed to melt at 116° C at ambient pressure. The onset of the metastable endotherm was found to increase with increasing pressure, while the liquidus and eutectic temperature for the structure stable at ambient pressure decreased with increasing pressure. Based on the pressure dependence of the melting trend, the metastable phase will be stable at the expense of the stable ambient pressure structure under high hydrostatic pressure conditions (above ∼ 1 GPa). Both microstructural observations and X-ray examinations at ambient pressure revealed that the metastable phase was present in droplet samples and that the X-ray diffraction pattern was close to that of the high-pressure stable phase previously reported as a rhombic cell. High-pressure thermal analysis has also allowed for identification of the effect of pressure in promoting favourable formation kinetics and the kinetic transition from the equilibrium phases to the metastable phase at high undercooling.