Effect of composition, annealing temperature, and high pressure torsion on structure and hardness of Ti-V and Ti-V-Al alloys

The severe plastic deformation strongly changes the microstructure and properties of titanium-based alloys. The structure and microhardness of four binary and ternary titanium-based alloys (Ti-4wt.% V, Ti-4wt.% V-6wt.% Al, Ti-4wt.% V-3wt.% Al, and Ti-5wt.% V-6wt.% Al) have been studied after preliminary annealing and following high pressure torsion (HPT). After HPT, the Ti-4wt.% V alloy contains much less (omega Ti) phase than Ti-4wt.% Fe and Ti-4wt.% Co alloys. The addition of aluminum to the binary Ti-V alloys completely suppresses the formation of the high-pressure (omega Ti)-phase. HPT leads to the partial decomposition of the annealed (alpha Ti) solid solution and "purification" of alpha-phase similar to that in the Ti-Fe alloys. After HPT of the studied ternary alloys, the (beta Ti)-phase completely disappears and nanoparticles of Ti2Fe form instead. This fact explains why the addition of aluminum leads to the increase of microhardness of alloys after annealing between 600 degrees C and 950 degrees C and after HPT-treatment. The increase of the temperature of the preliminary annealing also increases the hardness of all alloys after HPT-treatment.