Precious Metal Dental Casting Alloy Aged Structure And Performance

In present paper, a type IV noble metal dental casting alloy Au -12.3 wt %Ag -12.2 wt % Cu -4.0wt% Pt-0.5wt% Zn-0.14wt % Ir, which has high strength and is age-hardenable, was systematically studied. After isothermal ageing at 37, 100, 200, 350 and 500℃, the microstructure and hardness of the alloy was investigated by means of optical microscopy, micro hardness testing, X-ray diffraction (XRD) and scaning electron microscopy (SEM). The transformation process was analyzed and the age-hardening mechanism was discussed. The results will provide theoretical basis for the practical application of this alloy.After solution treatment, the microstructure of rolling specimens consisted of α 0 single phase of face-centered cubic crystal. The matrix α0 phase underwent different phase decomposition depending on ageing temperatures. At low temperature of 37℃ and 100 ℃, the matrix α 0 phase underwent a spinodal decomposition and transformed into the α 0' modulation structure. The spinodal decomposition might take place by the help of Cu atoms migration with the aid of quenched-in excess vacancies. Early stage of ageing at 350 ℃ and 500 ℃, the matrix α0 phase transformed α 0' modulation structure. Increasing the ageing time, the α 0' at the grain boundaries decomposed to the equilibrium Ag-rich α 2 phase which is a disordered face centered cubic structure and the equilibrium AuCu I ordered phase which is a face centered tetragonal structure. The Ag-rich α 2 phase and AuCu I ordered phase existed as thick platelet-like microstructure. While the α 0' modulation structure in the interior of the grains kept largely unchanged even when the ageing time exceeded 8640 minutes at 350℃. For the specimens ageing at 500℃, the α 0' modulation structure in the interior of grains could completely transformed into equilibrium Ag-rich α 2 disordered phase and AuCu I ordered phase by independent nucleation. That resulted microstruture was coarse cellular at the grain boundaries and fine cellular inside the grains.At all ageing temperature, the hardness of the alloy increased with the formation of the α 0' modulation structure. The highest hardness value was Hv270~Hv290 at 37-350℃, while the value for 500℃ was lower at Hv240~Hv250 The hardness of the alloy decreased as the occurrence of the Ag-rich α 2 disordered phase and AuCu I ordered phase, suggesting the formation of the equilibrium decomposition phases attributed to the overageing.There was coarse dendritic structure and obvious dendritic segregation in the as casting alloy. The as casting alloy represented a typical age-hardening nature in suitable temperatures but its age-hardening effect was not as obvious as that of rolling alloy. The hardness values of as casting alloy were discrete. At 500℃, lamellar structure was obviously observed at the grain boundaries. The precipitations at the boundaries were irregularly distributed because of dendritic segregation. It is necessary for the casting alloy to take homogeneous heat treatment before the practical application in order to reach a homogeneous hardness.