Phase transformations in the NbAu system by ball milling and the study of the metastable phases

Abstract

A detailed study on the NbAu binary alloy system upon ball milling is reported. An analysis of the energy levels of the NbAu alloy reveals that the solid solution has a lower enthalpy than the amorphous state for all compositions. The ball milling experiments were performed on five compositions of the NbAu alloys. All five alloys transformed to solid solutions after 40h of grinding. The structures of these solid solutions are different depending on the composition of the starting material. In further experiments, the properties of the metastable phases obtained by ball milling were studied. The lattice parameter of the solid solutions turns out to obey Vegard's law. The exothermic heat of the phase restoration appears to be approximately proportional to the enthalpy as predicted by Miedema's semi-empirical model. The magnitude of the peak temperatures and the activation energies follows the same trend as the melting temperaures. Examination of particle size and shape for two composition gives more insight in the milling process. Finally, the results for prolonged milling are presented.