Effect of small additions of Ir on properties of a binary Au–Ti alloy

Objectives: Previous investigations in the binary system Au–Ti have shown that a Au based alloy with 1.7 wt% Ti reveals properties, which are appropriate to dental applications. Aim of this investigation was to refine the coarse grained dendritic microstructure of the alloy. Derived from experience with traditional high gold alloys and based on theoretical considerations, Ir was chosen as potential grain refiner and its influence on grain size, hardness, and thermal distortion was investigated. Method: Seven different Au–Ti alloys with Ir contents of 0–0.5 wt% were prepared in an arc furnace and test pieces cast by the dental casting technique. Microstructures of the alloys were documented by light microscopy. Grain size was assessed by a comparison procedure, hardness was measured by the indentation test, and thermal distortion by a three point bending test in a modified dilatometer. Results: It revealed that Ir has a grain refining effect on the alloy with smallest grain sizes at a concentration of at least 0.3 wt%. It seems that the alloy with 0.5 wt% Ir shows slight grain growth during firing. Little homogenization was observed after firing. The influence of Ir on hardness was negligible. Thermal distortion is minimized with a concentration of 0.1 wt% Ir. In order to profit by the high thermal distortion resistance, the alloy Au98.2Ti1.7Ir0.1 (wt) was chosen as definite composition, although 0.1 wt% Ir does not satisfactorily refine the alloy. Significance: The alloy Au98.2Ti1.7Ir0.1 (wt) revealed properties appropriate for the crown and bridge technique.