Structure and phase transformation of nanometer-sized particles of alloys examined by TEM and electron diffraction

We prepared nanometer(nm)-sized particles of several alloys undergoing order-disorder or martensitic transformations in their bulk states by vacuum evaporation, and examined their atomic structures and phase transformations therein by transmission electron microscopy and electron diffraction. As a result, remarkable size effects were observed on both the two kinds of phase transformations, ie., large depressions of their transformation temperatures. A computer calculation of the ground state energy of nm-sized particles of the Cu3Au alloy by the modified embedded atom method and the Monte Carlo simulation of the order-disorder transformation indicated that the depression of the critical temperature T-c is essentially due to surface effects. Nm-sized austenite particles of Fe-Ni alloys around 25 at.% Ni were observed to be stabilized to a great extent, as previously reported. It was found, however, that the well-stabilized austenite particles transformed into the martensite ones when kept at 77 K for a prolonged period. This result revealed that contrary to previous reports even the nm-sized austenite particles undergo the martensitic transformation at subzero temperatures, although isothermally. The causes giving rise to the drastic stabilization brought about by the reduction of size are discussed.