Elucidation of Characteristic Strengthening Mechanisms in Dental Gold Alloys by High Resolution Electron Microscopy

通过电阻率测量、硬度测试、X射线和电子衍射、透射电子显微镜和扫描电子显微镜观察，研究了Au-14at.%Ag准二元合金的时效硬化特征及其相变。有序化的临界温度为650K，调幅温度为640K。尽管时效温度略低于调幅温度，但623K的时效硬化归因于AuCuII亚稳有序相的形核和生长过程。在较低温度范围内的硬化分为两个阶段，前者是由于调幅分解，后者是由于AuCuI‘和/或AuCuII’亚稳定相随时效温度的变化而有序产生的。本文还研究了一种含铂和钯(Au-32at.%Cu-7.6at.%Ag-6.2at.%Zn-3.8at.%Pd-3.5at.%Pt)的商品牙科金合金的时效硬化机理。在临界有序化温度(T_c=825K)以下等温时效过程中的相变顺序为：无序固溶体α_0(FCC)--亚稳AuCuI‘有序相(FCT)+亚稳态α_2’无序相(FCC)--平衡AuCuI有序相(FCT)+平衡α_2无序相(FCC)。高分辨电子显微镜观察表明，硬化是由于在AuCuI‘有序晶片与周围基体的界面处引入共格应变所致。用直接时效等原子AuCu合金的电子显微镜、SAED和X射线衍射仪测试证实了CVM预测的调幅有序化。通过不同温度时效过程中显微结构特征的差异和X射线衍射谱的变化，确定了共格相界T_c‘和调幅有序化温度T_s。测得T_c‘和T_s分别为648K和630K。

Age-hardening characteristics and associated phase transformations were studied by means of electrical resistivity measurements, hardness tests, X-ray and electron diffraction, transmission electron microscopy and scanning electron microscopic observations in an AuCu-14at.%Ag pseudobinary alloy. The critical temperature for ordering was determined to be 650 K and the spinodal temperature was estimated to be 640 K.Age-hardening at 623 K was attributed to nucleation and growth processes of the AuCu II' metastble ordered phase even though the ageing temperature was just below the spinodal temperature. While hardening in the lower temperature range took place at two stages, the former was due to spinodal decomposition and the latter was brought about by ordering of AuCu I' and/or AuCu II' metastable phases depending on ageing temperature.The age-hardening mechanism of a commercial dental gold alloy containing platinum and palladium (Au-32at.%Cu-7.6at.%Ag-6.2at.%Zn-3.8at.%Pd-3.5at.%Pt) was also studied. The sequence of phase transformations during isothermal ageing below the critical temperature of ordering, Tc=825 K, 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 phase (fct) + equilibrium alpha_2 desordered phase (fcc). High resolution electron microscopic observation revealed that the hardening was due to the introduction of coherency strain at the interface between the AuCu I' ordered platelet and the surrounding matrix.The spinodal ordering which was predicted by the CVM was confirmed by means of TEM, SAED and XRD examinations in the equiatomic AuCu alloy using direct ageing. The coherent phase boundary, Tc', and the spinodal ordering temperature, Ts, were identified from differences of microstructural features and changes in XRD patterns during ageing at verious temperatures. The Tc' and Ts were determined to be 648 K and 630 K.