On the evaluation of stability of rare earth oxides as face coats for investment casting of titanium

Abstract

Attempts have been made to evaluate the thermal stability of rare earth oxide face coats against liquid titanium. Determination of microhardness profiles and concentration profiles of oxygen and metallic constituents of oxide in investment cast titanium rods has allowed gradation of relative stability of rare earth oxides. The relative stability of evaluated oxides in the order of increasing stability follows the sequence CeO₂ → ZrO₂ → Gd₂O₃ → didymium oxide → Sm₂O₃ → Nd₂O₃ → Y₂O₃. The grading does not follow the free energy data of the formation of these oxides. A better correlation with the experimental observations is obtained when the solubility of the metallic species in titanium is also taken into consideration.