Hardness and Composition of Solution-Treated Ag-Pd-Cu-Au Alloy Microstructures

An Ag-Pd-Cu-Au casting alloy was subjected to solution treatment at various temperatures. The dynamic hardness of each resultant microstructure and the Vickers hardness of the whole alloy were measured to investigate their relationship with changes in metal and crystal structure of the alloy and microstructural elemental concentration. The samples following solution treatment at 650, 700, 750, and 800 degrees C consisted of a fine-layer structure (Structure A), a rough-layer structure (Structure B), and an island-like structure (Structure C). Structures A and B, after treatment at 850 degrees C, were indistinguishable. The composition of Structure A was very similar to that of the alloy. Structures B and C were Ag-rich/Pd-poor/Cu-poor and Ag-poor/Pd-rich/Cu-rich, respectively. The composition of Structures B and C approached that of Structure A with increasing treatment temperature. The dynamic hardness, which was highest in Structure A, lower in Structure C, and lowest in Structure B, increased with increasing treatment temperature. These findings suggest that solid solution strengthening, due to elemental diffusion between structures during heating, contributes to hardening at elevated solution treatment temperatures. [doi: 10.2320/matertrans.MRA200819]