Effect of Oxygen Content on Microstructure and Mechanical Properties of Cast and HIP’ed γ TiAl Alloys

To clarify the inherent effect of oxygen content on mechanical properties of cast and HIP’ed γ TiAl alloys, two series of alloys containing an extremely low level (0.008 mass% to 0.013 mass%) and a medium level (0.051 mass% to 0.083 mass%) of oxygen were prepared. Aluminum contents of each series of alloys were varied from 44.4 to 50.0 at%. Beside these series of alloys, Ti-48 at%Al alloys containing different amounts of oxygen from 0.009 mass% to 0.230 mass% were prepared. In tensile test at room temperature, no marked ductility improvement was observed even though reducing the oxygen content below 0.01 mass%. Microstructures of the studied alloys consisted of equiaxed γ and lamellar form grains and the volume fractions of lamellar form grains and α2 phase increased with increasing oxygen content. The 0.2% proof stresses of the alloys were analyzed by the rule of mixtures by regarding the alloys as a kind of composite material consisting of equiaxed γ and lamellar form grains. The ductility depended on the volume fraction of lamellar structure and exhibited superior ductility at around 20 vol%. Tensile properties of the alloys were strongly controlled by the microstructural factors. The oxygen content affects it through changing microstructures. In the Ti–Al phase diagram, the α(α2)+γ two phase region was expanded toward the aluminum rich side along with the increase of oxygen content, which resulted in increasing the Al content and decreasing the hardness of the γ phase.