On the origin and stability of the metastable phase in rapidly solidified Sn-Bi alloy particles embedded in Al matrix

The paper reports the synthesis of nanoscaled tin-bismuth alloy particles embedded in an aluminum matrix by rapid solidification. The observed microstructure and composition analyses suggest the co-existence of tetragonal (beta)-Sn and rhombohedral Bi phase within the nanoparticles. Additionally, another phase, primarily a tin-based solid solution, gets stabilized and co-exists with the equilibrium bismuth phase in some of the smaller particles. In order to understand the kinetic stability of this phase we have carried out detailed thermal analyses using differential scanning calorimetry, high temperature X-ray diffraction and transmission electron microscopy. The results suggest that at small length scales this phase can be more stable than the eutectic phase. We have tried to explain the formation and stability of this new phase in terms of favorable nucleation kinetics, and the pressure developed in the melt within the cavity during solidification.