In-situ synchrotron high energy X-ray diffraction study on the internal strain evolution of D019-α2 phase during high-temperature compression and subsequent annealing in a TiAl alloy

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.