2015 年 79 巻 5 号 p. 265-272
Dental casting Au-Ag-Pd alloys exhibit a singular phenomenon that they get hardened after softening solution treatment when the temperature is higher than 800℃. The purpose of this study is to clarify the hardening mechanism in terms of the concentration distribution of the constituent elements. Two types of commercial dental alloys with different Ag/Cu ratios were subjected. Samples were solution-treated using a vertical furnace and dropped into ice water to quench the specimens. During the solution treatment, the specimens were kept in a vacuumed silica tube or in Ar-gas flow. The cooling rate was slightly different between both situations and the cooling rate in the former was slightly slower than in the latter one. The samples stayed soft after solution treatment below 750℃. At 800℃, only the vacuumed samples got hardened. Elemental maps using SEM-EDX were not able to find distinguished difference in the microstructure between the softened and hardened specimens, even the difference based on the composition of the commercial dental alloys was detected. Higher-Ag-containing alloy revealed two-phase structure in which one is α phase of fcc matrix and the other is rod-like β phase of B2 type ordered CuPd. Higher-Cu-containing alloy did three-phase structure in which the fcc matrix was separated into a fine eutectic structure composed of Ag-rich α1 and Cu-rich α2 phases. Coherent precipitation of β′ phase with L10 type ordered structure was detected in the α or α1 fcc matrix from the hardened specimens. STEM-EDX results revealed rapid evolution of concentration fluctuation between Ag and Cu that occurred during quenching. The rapid evolution of the concentration fluctuation was considered to spontaneously proceed from spinodal decomposition between Ag and Cu in the α or α1 fcc matrix. The spinodal decomposition caused the concentration modulation, and the Cu-concentrated part immediately formed the coherent β′ precipitation by coupling with Pd.
