Abstract: The residual stress in the D0₁₉-α₂ phase is known to be significantly higher than that in the L1₀-γ phase in TiAl alloys after deformation due to the poor plasticity and strong mechanical anisotropy of the α₂ phase. However, the internal stress accumulation and relaxation in the α₂ phase during high-temperature deformation and annealing are scarcely investigated. In this study, for the first time, the internal strain evolution and load partitioning between the α₂ and γ phases at high temperatures are characterized by in-situ synchrotron high energy X-ray diffraction (HEXRD) technique. The plastic deformation is at least initiated at a stress of roughly 200 MPa in the γ phase and 775 MPa in the α₂ phase. The intergranular strains in the α₂ phase are generated by the onset of dislocation glide in the γ phase, and accentuated with the accumulated dislocations and the ensuing twinning activity. After unloading, great intergranular strains are preserved in the α₂ phase constrained by the heavily plastically deformed γ phase. During subsequent heating from 400 to 1000 °C, the internal strains in the α₂ phase are almost fully relaxed by substantial dislocation annihilation and rearrangement in the γ phase. During annealing at 800 °C, the internal strain relaxation is rapid in the initial 10 min, whereas considerably retarded subsequently. The extent of relaxation after holding at 800 °C for 1 h is equivalent to that of heating in an effective temperature range of 680-880 °C for 10 min. The in-situ lattice strain measurements with various thermal relaxation schemes provide guidance for the stress relief annealing of TiAl components.

Key words: TiAl alloys, Synchrotron radiation, Intergranular strain, Stress relaxation