Fabrication of TiAl-based Nanocrystalline Powders Using Polyphase Microstructure Formation Process

Abstract

Phase transformation processes and thermal stability of nanocrystalline microstructures in TiAlbased alloy powders have been studied. Ti₅₀Al₄₅Mo₅(at.%) alloy powders prepared by a plasma rotating electrode process were ball-milled for 96 h. The as-milled powders are composed of nanocrystalline grains of α(hcp-A3) or β(bcc-A2) phase smaller than 50 nm in diameter. In very early stages of annealing at 873-1373 K, the milled powders transform into an equilibrium three-phase state of α₂(D0₁₉)+ β₂(B2)+γ(L1₀), followed by the formation of the γ phase from the α (or α₂) phase. The annealed powders have an equiaxed grain structure and exhibit a small increase in grain size after the heat treatment. This result indicates that the coexistence of fine α₂, β₂ and γ grains with the different crystal structures and compositions significantly retards the grain coarsening in the annealing processes.