Mechanism of unique hardening of dental Ag–Pd–Au–Cu alloys in relation with constitutional phases

The objective of this research was to investigate the effect of constitutional phases on the unique hardening behavior of as-solutionized dental Ag–Pd–Au–Cu alloy fabricated by cold rolling. The commercial dental Ag–Pd–Au–Cu alloy fabricated by cold rolling consists of Cu-rich α1, Ag-rich α2, and β phases. On the other hand, the Ag–Pd–Au–Cu alloy fabricated by the liquid rapid solidification (LRS) method consists of single α phase. They were subjected to various heat treatments, respectively. The microstructures were observed by scanning electron microscope, transmission electron microscope and X-ray diffraction. The hardness was evaluated by a Vickers micro-hardness tester. In the Ag–Pd–Au–Cu alloy fabricated by cold rolling, the fine L10-type-ordered β′ phase is precipitated and the coarse β phase is remained after solution treatment at 1123 K. The hardness increases drastically. On the other hand, in the Ag–Pd–Au–Cu alloy fabricated by LRS method, the single α phase was decomposed into the α1 phase and the α2 phase after solution treatment at 1023 K and its hardness change was small. However, after aging treatment at 673 K, the fine β phase is precipitated in the α phase and the hardness increases greatly even in the Ag–Pd–Au–Cu alloy fabricated by LRS method. It is considered that the precipitation of the fine L10-type-ordered β′ phase may contribute strongly to the unique hardening in the as-solutionized dental Ag–Pd–Au–Cu alloy fabricated by cold rolling.

► The unique hardening behavior on Ag–Pd–Au–Cu alloys by solution treatment was investigated.
► The Cu-rich α1 phase and Ag-rich α2 phase decomposed by solution treatment contributed weakly to the change of hardness.
► The fine β phases precipitated by aging treatment caused great increase in hardness.
► The fine L10-type ordered β′ phase precipitated by solution treatment may contribute to the unique hardening behavior.