Stability of refractory oxides for mold material of Ti-6Al-4V alloy precision casting

It has been proved that yttrium oxide has superior properties for the mold material in precision casting of Ti-6Al-4V alloy through basic studies on the reaction between one of plasma sprayed layers of oxides and liquid Ti-6Al-4V alloy during solidification. The reactivity estimated in terms of the maximum hardness and amount of oxygen absorbed in surface hardened layer formed at the surface of simple castings has good correlation with the high temperature stability of oxides evaluated by the standard free energy changes of formation of oxides, solution of oxygen and metallic constituents of oxides into liquid Ti-6Al-4V alloy. The hardness profile of the surface hardened layer has been quantitatively explained by the mechanism in which oxygen evolved from oxide instantaenouly after contacting with liquid Ti-6Al-4V alloy is absorbed and diffuses through liquid and solid layers of Ti-6Al-4V alloy castings.