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

用透射电子显微镜和高分辨电子显微镜研究了AuCu14at中AuCu有序化和两相分解引起的结构构型的变化。%Ag伪二元合金。在643K时，通过形核和生长过程形成了两种不同类型的析出物，即纺锤形和链状。这些沉淀物被称为复合沉淀物，因为它们具有由富银fcc相组成的立方块和沿其纵轴排列的LPAPB AuCuII有序薄片组成的夹心结构，并在上下工作台平面上覆盖着AuCuII片层。它们的侧面没有被覆盖，也没有打开以连接基质。在略低于旋节温度的老化过程中，面心立方相的方块也被AuCuII有序的薄片夹在中间。这种三明治结构周围环绕着AuCuII有序相。提出了这些构型的模型。该合金的早期低温时效为调制结构。含铂、钯的商品齿科金合金Au-15wt的时效硬化机理。%Cu-6wt.%Ag-5wt.%铂-3wt.%PD-3wt.%锌，也进行了研究。在临界有序化温度T_C=825K以下的等温时效相变顺序为：无序固溶体α_0面心立方-亚稳态AuCuI‘有序相(FCT)+亚稳态α_2’无序相(FCC)-平衡AuCuI有序+平衡α_2无序相。硬化是由于AuCuI‘晶片与基体界面处引入共格应变所致。这些有序的血小板具有相互垂直的c轴，以补偿其四边形带来的应变。界面粘结性的丧失会导致合金的软化，即过时效。

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.