A MONTE-CARLO STUDY OF THE STRUCTURE AND COMPOSITION OF (001) SEMICOHERENT INTERPHASE BOUNDARIES IN CU-AG-AU ALLOYS

Monte Carlo simulation, in conjunction with embedded atom method, has been used to model the composition and structure of semicoherent (001) interphase boundaries separating coexisting Cu-rich and Ag-rich phases in a ternary Cu-Ag-Au alloy. This approach has also been used to re-examine previous results obtained on two-phase binary Cu-Ag alloys. Results of computations performed at 600 and 900 K on both binary and ternary alloys show good agreement between predicted bulk phase composition and the experimental phase diagrams. A statistical structural and compositional analysis of the atomic configurations at interfaces has shown that the (001) interface is unstable and tends to break down into {111} facets. This faceting manifests itself as the formation of silver-rich pyramids which penetrate deeply into the copper-rich phase. Possible sources for the instability of the (001) interface are discussed. It is also found that while this faceting is well defined at 600 K, it is substantially smeared out at 900 K. Segregation of gold occurs along the facet surfaces, and as a result appears to be weak in composition profiles calculated along the [001] direction. This conclusion about the nature of the composition profiles in confirmed by comparisons between the results obtained in the early stages of the simulation, when the (001) interface is metastable, and from a regular solution model of the composition profile across a coherent (001) interface.