The Research Of Refining Grain And Grain Boundary Engineering Of γ-TiAl Based Alloy Assisted By MIF

y-TiAl based alloy not only has excellent high-temperature strength, high temperature creep and oxidation resistance, but also has low density and high strength-to-weight ratio, having extensive application prospect in the aerospace and automobile industry. However, y-TiAl based alloy is in short of the plasticity at room temperature, leading to the poor deformation processing ability which becomes a major obstacle to application. The multidirectional isothermal forging (MIF) combined with annealing heat treatment is used to adjust the grain size and grain boundary character distribution (GBCD) in y-TiAl based alloy to improve its room temperature plasticity. The effect of MIF process and annealing teatment parameters on the grain size and grain boundary character distribution of y-TiAl based alloy were studied, major conclusions are as followed:(1) Static recovery and recrystallization were occured in TiAl based alloy after appropriate multidirectional isothermal forging and annealing heat treatment, so grain size was refined and the proportion of annealing twin was increased at the same time, forming uniform fine equiaxed grain structure. The proportion of special grain boundary was increased by 10% compared to the base material, which is helpful for improving mechanical properties of TiAl alloy.(2) The microstructure of TiAl based alloy was influenced by the deformation process. Lower deformation temperature or faster deformation rate were beneficial to increase the proportion of Σ3 boundary in TiAl based alloy. However, lower deformation temperature, faster deformation rate and lager deformation caused the higer deformation resistance, and the surface of the alloy was easier to crack, so the optimal deformation process for TiAl based alloy was deformation temperature of 800℃, strain rate of 0.01 s-1 and true strain of 0.67.(3) The microstructure of TiAl based alloy was influenced by annealing process. The level of recovery recrystallization and grain growth was directly proportional to annealing temperature. And the proportion of special grain boundary was increased firstly and then decreased as the annealing time extending. TiAl based alloy could form uniform equiaxed structure and more annealing twins in a short time at 1300℃ annealing temperature, that refining grain size and optimizing grain boundary character distribution simultaneously. But TiAl based alloy was difficult to recrystallize when the annealing temperature was down to 900 ℃. The Σ1 subgrain boundaries were formed in early stage of annealing, and then the proportion of special boundaries was increased due to the twinning along with the grain growth. However, the special grain boundaries were swallowed by large random angle grain boundaries over the annealing time, reducing the proportion of special grain boundaries.(4) The compression strength and plasticity of TiAl based alloy after multidirectional isothermal forging and annealing heat treatment were improved obviously relative to the base material. So the combination of grain refining and grain boundary character distribution optimization could enhance the failure resistcane of TiAl based alloy.