High-Resolution Electron Microscopy Study of Phase Transformation in Dental Precious Metals Alloys

用透射电镜和高分辨电镜研究了AuCu-14 at中AuCu有序化和两相分解引起的结构变化。%Ag伪二元合金。在643 K下，发现了两种不同类型的沉淀物，即纺锤形和链状沉淀物，它们是通过形核和生长过程形成的。这些沉淀物被命名为复合沉淀物，因为它们具有由富银fcc相的立方体块体和LPAPB AuCu II有序薄片组成的三明治结构，并且在顶部和底部台面上覆盖有AuCu II薄片。它们的侧面没有被覆盖并且打开以与基质连接。通过在低于亚稳温度下老化，面心立方相的立方块也被AuCu II有序的薄片晶夹在中间。这种三明治结构被AuCu II有序相包围。提出了这些构型的模型。研究了含铂、钯、Au-15 wt. %Cu-6wt. %Ag-5wt. %Pt-3wt. %Pd-3wt. %的Zn。在低于有序化临界温度（T_C= 825 K）的等温时效过程中的相变顺序为：无序固溶体α_0 fcc →亚稳AuCu I'有序相（fct）+亚稳α_2'无序相（fcc）→平衡AuCu I有序相+平衡α_2无序相。硬化是由于在AuCu I'片晶和基体之间的界面处引入了共格应变。这些有序的片晶具有相互垂直的c轴，以补偿由它们的四方性引入的应变。界面粘结性的丧失导致合金的软化，即界面粘结强度的降低.例如，过度老化

Changes in structural configuration arose from AuCu ordering and two-phase decomposition were studied by means of TEM and HREM observations in a AuCu-14at. %Ag pseudobinary alloy. Two different types of precipitates, spindle-type and chaintype, were found at 643K formed by nucleation and growth process. These precipitates were named as composite precipitate because they had a sandwich structure consisted of a cuboidal block of silver-rich fcc phase and an LPAPB AuCu II ordered thin platelets across their longitudinal axis, and were covered with the AuCu II Platelets on the top and bottom table planes. Their lateral faces were not covered and opened to connect with the matrix. By aging just below spinodal temperature, the cuboidal blocks of fcc phase were also sandwiched by the AuCu II ordered thin platelets. This sandwich structure was surrounded by the AuCu II ordered phase all around. Models of these configurations were proposed. A modulated structure was found by aging at low temperature range for early stage in the present alloy.The age-hardening mechanism of a commercial dental gold alloy containing platinum and palladium, Au-15wt. %Cu-6wt. %Ag-5wt. %Pt-3wt. %Pd-3wt. %Zn, was also studied. The sequence of phase transformations during isothermal aging below the critical temperature for ordering, T_C=825K, was described as follows : disordered solid solution alpha_0 fcc --- metastable AuCu I' ordered phase (fct) + metastable alpha_2' disordered phase (fcc) --- equilibrium AuCu I ordered + equilibrium alpha_2 disordered phase. The hardening was due to the introduction of coherency strain at the interface between the AuCu I' platelet and the matrix. These ordered platelets had mutually perpendicular c-axes to compensate for the strain introduced by their tetragonality. A loss of coherency at the interface brought about softening of the alloy, i. e., over-aging.