High-Resolution Electron Microscopy Study on Characterization of Interfaces induced by Phase Transformation in Dental Gold Alloys

(1)通过电阻率、硬度测试、X射线衍射、电子衍射、透射电镜和扫描电镜观察，研究了AuCu-14 at合金的等温淬火硬化特性及其相变。%Ag伪二元合金。有序化的临界温度为650 K，失稳点为640 K。623 K时效硬化是由亚稳相AuCu Ⅱ '的形核和长大过程引起的，但时效温度低于亚稳相的失稳温度，而低温区的时效硬化分为两个阶段，前者是由亚稳相AuCu Ⅰ'和AuCu Ⅱ '的失稳分解引起的，后者是由亚稳相AuCu Ⅰ'和AuCu Ⅱ '的有序化引起的。过时效是由AuCu I和/或AuCu II有序相和富银α_2平衡相的晶界析出引起的。(2)AgCu-40at. %Au合金的研究。在高于有序化临界温度Tc = 620 K的648 K以下等温时效时，发现了两个硬化阶段。硬化的第一阶段发生的调幅分解产生的调制结构的形成。进一步的硬化是通过有序化产生亚稳态AuCu I'和/或AuCu II'有序片晶从调制结构的富铜部分生长而引起的。即使在648 K以下（高于本合金的有序化温度T_c），也存在明显的第二阶段硬化的过渡有序化。通过有序过渡相的消失也发现了剧烈的软化。在773 K时效时，虽然观察到调制组织，但没有硬化。

(1) Age-hardening characteristics and the associated phase transformations were studied by means of electrical resistivity, hardness tests, X-ray and electron diffractions, TEM and SEM observations in a AuCu-14at. %Ag pseudobinary alloy. Critical temperature of the ordering was determined to be at 650K, besides spinodal locus was estimated at 640K. Age-hardening at 623K was attributed to the nucleation and growth process of AuCu II' metastable ordered phase though the aging temperature was just below spinodal temperature, while the 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 aging temperature. The overaging was induced by grain boundary precipitation of AuCu I and/or AuCu II ordered and silver-rich alpha_2 equilibrium phases.(2) Age-hardening mechanism in a AgCu-40at. %Au alloy was also studied same experimental procedure the above. Two stages of hardening were found by isothermal aging below 648K which was higher than the critical temperature of ordering, T_c = 620K. The first stage of hardening took place by formation of a modulated structure resulting from spinodal decomposition. Further hardening was brought about by ordering yielding metastable AuCu I' and/or AuCu II' ordered platelets grown from copper-rich portion of the modulated structure. Transitional ordering which gave rise to a marked hardening of second stage was found even though the temperature below 648K which was higher than T_c for ordering in the present alloy. Drastic softening was also found by disappearance of the transitional ordered phases. Although the modulated structure was observed by aging ta 773K, there was no hardening.