Interfacial Oxidations of Pure Titanium and Titanium Alloys with Investments

Abstract

External oxides of a commercially pure titanium (cpTi), Ti6A14V alloy, and an experimental β-type titanium alloy (Ti 53.4wt%, Nb 29wt%, Ta 13wt%, and Zr 4.6wt%) were characterized after heating to 600, 900, 1150, and 1400°C in contact with three types of investments (alumina cement, magnesia cement, and phosphate-bonded) in air. XRD studies demonstrated that MgO, Li₂TiO₃ and/or Li₂Ti₃O₇ were formed through reactions with the metal and the constituents in the magnesia cement-investment after heating to 900, 1150, and 1400°C. Except for these conditions, TiO₂ (rutile) was only formed on cpTi. For titanium alloys, the other components apart from Ti also formed simple and complex oxides such as Al₂O₃ and Al₂TiO₅ on Ti6Al4V, and Zr_0.25Ti_0.75Nb₂O₇ on the β-type titanium alloy. However, no oxides containing V or Ta were formed. These results suggest that the constituents of titanium alloys reacted with the investment oxides and atmospheric oxygen to form external oxides due to the free energy of oxide formation and the concentration of each element on the metal surface.