Abstract
Zn is usually added to dental casting gold alloys as deoxidizer and to improve the castability. We observed in the previous study that the addition of Zn to AuCu lowers both liquidus and solidus temperatures of the alloy and this increases the age-hardening rate in low temperature like 37°C [1]. But the effect of Zn addition to dental casting gold alloys on age-hardening behaviors have not been clarifed yet. The purpose of this study is to examine the effect of Zn addition on the full stage of age-hardening behaviors and to clear the relation between ordering behaviors and age-hardening of the AuCu-Zn pseudobinary alloys by hardness test, X-ray diffraction (XRD) and transmission electron microscope (TEM) observation.
With Zn content, the age-hardening rate was increased and overaging was delayed.
From XRD study, it was revealed that with Zn content the AuCu II phase become stable and antiphase domain size of the AuCu II superstructure markedly decreased. This means that the addition of Zn to AuCu effectively increases the density of antiphase boundaries per unit volume of the AuCu II superstructure. This is suggested to be the main cause for the retardation of the overaging in the alloys containing 5at.% Zn or more. It was revealed by TEM observation that the fast age-hardening of Zn added alloys is due to the coherent strain by lattice distortion which is made during the very early stage of ordering.
