Blended elemental P/M synthesis and property evaluation of an orthorhombic Ti-22Al-27Nb alloy

The orthorhombic (O) phase based on the compound Ti2AlNb (Ti-25Al-25Nb (mol%)) has a higher strength-to-density ratio and better room temperature ductility than conventional titanium aluminides such as TiAl and Ti3Al. In the present study, orthorhombic Ti-22Al-27Nb alloy was produced by the blended elemental (BE) powder metallurgy (P/M) method, and the microstructures and mechanical properties such as tensile and high cycle fatigue were evaluated. Homogeneous BE P/M Ti-22Al-27Nb was successfully produced by using extra low chlorine Ti powder and AI-Nb master alloy powder with a particle size smaller than 45 mu m. The microstructure in the as-HIP'ed condition consisted of O, alpha(2) and beta phases. The O and beta phases were distributed in the interior of the grains whereas the alpha(2) phase was mainly formed along the grain boundaries. The post-HIP heat treatment at 1173 K for 36 ks made the distribution of these three phases more homogeneous. Although the tensile specimen was broken before the peak tensile strength was attained in the temperature range up to 923 K, both 0.2% proof stress and fracture strength were higher than those for the Ti-6Al-2Sn-4Zr-2Mo alloy. The Young's modulus and high cycle fatigue strength at 10(7) cycles in the as-HIP'ed condition were 129 GPa and 480 MPa, respectively. High cycle fatigue strength was increased to 540 MPa by the post-HIP heat treatment.