High resolution transmission electron microscopy of age-hardenable Au-Cu-Zn alloys for dental applications

Microstructures of age-hardenable AuCu-Zn pseudobinary alloys for dental applications were studied by means of high resolution transmission electron microscopic (HRTEM) observation and X-ray diffraction study. HRTEM study revealed that the appearance frequency of antiphase boundaries (APBs) per unit volume of the AuCu II superstructure effectively increased by Zn addition to AuCu, which may be the reason for that high hardness was maintained for a long time in AuCu-Zn alloys. The disordered APBs zone in the AuCu II superstructure had wavy characteristics and fluctuated within regular range. With increasing Zn content in AuCu-Zn alloys, the fluctuation range of APBs' width became narrower, thus random APBs' spacing and irregular APBs' shape of AuCu II superstructure changed to comparatively regular APBs' spacing and shape. Due to the APBs' wavy characteristics, spacing between successive APBs, M, was not constant but scattered, and the magnitude of the scattering of M value decreased with increasing Zn content. By Zn addition to AuCu, phase transformation from a disordered alpha phase to AuCu II phase was greatly accelerated, which made it possible for the AuCu-Zn alloy to have excellent age-hardenability at relatively low temperature like intraoral temperature.