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 are severely limited by their coarse as-cast microstructure. This study proposes a low-temperature pre-deformation heat treatment to refine the microstructure of an as-cast Ti-47Al-2Nb-2Cr alloy. The process involves pre-deformation below the eutectoid temperature followed by heat treatment in the α+γ phase field, yielding two distinct microstructures: duplex (DP) and nearly lamellar (NL). The average lamellar colony size was reduced to 5.3% and 20% of the as-cast size for DP and NL structures, respectively. Microstructural evolution analysis reveals that the decomposition of coarse lamellar colonies and the formation of fine grains are governed by γ recrystallization, γ→α phase transformation, and α recrystallization. Remnant colonies after deformation are completely eliminated by subsequent heat treatment, enhancing microstructural homogeneity and mechanical properties. The DP alloy exhibits an ultimate tensile strength (UTS) of 537 MPa and an elongation (EI) of 6.8% at 800°C, representing increases of 44.7% and 518.2% over the as-cast alloy, respectively. Analysis of crack propagation and deformation mechanisms indicates that grain refinement and lamellar structure refinement contribute to improved deformation ability. This work provides a viable pathway for optimizing as-cast TiAl alloys for high-temperature applications.