In-situ observation of alloy phase formation in nanometre-sized particles in the Sn-Bi system

Alloy phase formation in nanometre-sized particles has been studied by in-situ transmission electron microscopy using particles in the Sn-Bi system. Observations have been carried out at a temperature (i.e. 350 K) above the eutectic temperature. For tin-rich compositions, with increasing concentration of bismuth, a particle (approximately 8 nm in size) of the terminal tin solid solution changed directly into a particle of the liquid phase, without going through the stage of solid-liquid coexistence. On the other hand, for bismuth-rich compositions, a particle (approximately 8 nm in size) of the terminal bismuth solid solution changed first to a particle with a crystal-liquid two-phase microstructure and eventually to a particle of the liquid phase, with increasing concentration of tin. Thermodynamic model calculations indicate that the contribution of the solid-liquid interfacial energy to the total Gibbs free energy of an alloy particle with a solid-liquid two-phase microstructure becomes large enough to change the phase equilibrium when the size of the particle is reduced to the nanometre range, and that the difference between the alloy phase formation for the tin-rich and bismuth-rich alloys observed here can be consistently explained in terms of the difference between the relative contributions of the solid-liquid interfacial energy for the two types of alloy.