The relative thermal stability of colloidal silica bonded Al2O3 investment casting mold against molten Ti-Al alloy melts was investigated by means of optical microscope, EDS profiles and
hardness measurement. Ti-Al alloys with aluminum concentrations less than 30 atomic percent
reacted with Al2O3 giving the hardening layer, whereas the others have no reaction layer. In the case of the reaction between pure titanium and Al2O3 mold, the reaction was affected not only by the stability of Al2O3 mold itself but also by the effect of the binder, which was colloidal silica. As aluminum content of Ti-Al alloys increase, the thermal stability of titanium alloys is
improved because of chemical compatibility.