Transmission electron microscopy and electron diffraction studies on martensitic transformations in nanometre-sized particles of Au-Cd alloys of near-equiatomic compositions

摘要

Martensitic transformations in nanometre-sized particles of Au-Cd alloys of near-equiatomic compositions were studied by means of transmission electron microscopy and electron diffraction. The alloy particles with an average size of several nanometres were prepared by high-vacuum electron-beam deposition. In the as-deposited state the particles were crystalline and possessed a B2-type structure at room temperature, similar to that of the beta(2) phase in the corresponding bulk. The beta(2)-phase particles with an average composition of Au-44.5 at.% Cd were transformed at 76K into an orthorhombic structure similar to that of the gamma(2)' phase in the bulk, while those with an average composition of Au-50.2 at.% Cd were transformed at 60 K into a trigonal structure similar to that of the zeta(2)' phase in bulk. The temperature parameter in the Debye-Waller factor measured at room temperature for the beta(2)-phase particles was roughly four times that for Au in the bulk. It was thus found that the M-S temperatures for the beta(2)-->gamma(2)' and beta(2)-->zeta(2)' transformations in the nanometre-sized particles were lowered far below room temperature despite the increasing surface area and the enhanced lattice softening. Their reverse transformations, however, took place on heating to room temperature. It was thus seen that the lowering of the transformation temperatures was due to the decrease in the thermodynamic equilibrium temperatures T-0 between the beta(2) and gamma(2)' and between the beta(2) and zeta(2)' phases. The decrease in T-0 indicated that the free energies of the beta(2), gamma(2)' and zeta(2)' phases of the alloys in the small-particle state are definitely different from those in the bulk state. [References: 46]