Grain refinement and improved properties of an as-cast Ti-47Al-2Cr-2Nb alloy by pre-deformation heat treatment

The mechanical properties of as-cast TiAl alloys have been greatly limited by their original coarse microstructure. In this study, a low-temperature pre-deformation heat treatment was proposed to optimize an as-cast Ti-47Al-2Nb-2Cr alloy. The microstructure was significantly refined through the pre-deformation below eutectoid temperature, followed by a heat treatment in the α+γ phase area, producing two types of microstructures: duplex (DP) and nearly lamellar (NL). The average size of lamellar colonies was reduced to 5.3% and 20% of the as-cast microstructure, respectively. Investigation of the microstructural evolution process reveals that the decomposition of coarse lamellar colonies and the formation of newly fine grains are primarily dominated by γ recrystallization, γ to α phase transformation, and α recrystallization. The remnant colonies after deformation can be completely eliminated by heat treatment, improving the microstructural homogeneity and mechanical properties of the alloy. Moreover, the DP structure alloy exhibits an ultimate tensile strength (UTS) of 537 MPa and an elongation (EI) of 6.8% at 800 °C, which were increased by 44.7% and 518.2%, respectively, compared to the as-cast alloy. Analysis of crack propagation and deformation mechanisms reveals that refining the grain size and lamellar structure contributes to enhancing the deformation ability of TiAl alloys.