Strengthening low-angle grain boundaries by twinning grids: Improving the ability of γ phase to store and block dislocations in TiAl alloy

The twin grid structure enhances low-angle grain boundaries (LAGBs) in the γ phase, effectively storing and hindering dislocations to improve the high-temperature properties of TiAl alloys. Hf was added to Ti-47Al-6Nb-0.1C-1.6Ta alloy and related mechanisms were studied. The results indicate that the addition of Hf promotes the formation of a Hf-enriched γ phase (with an L₁₀ structure, defined as the γ_Hf phase) in the TiAl alloy. The γ_Hf phase is surrounded by a γ phase containing twin grids (with an L₁₀ structure, defined as the γ_n phase). The γ_Hf and γ_n phases exhibit close orientations with LAGBs at their interfaces. The Hf and Al-rich regions in the α phase preferentially transform into the γ_Hf phase, with α grain boundary migration making γ_Hf either within the α parent phase or at its boundaries. The residual α phase around γ_Hf undergoes multiple twinning on {111} and {100} planes, ultimately resulting in the formation of the γ_n phase with twin grids. The 900 ℃ tensile strength and elongation of the Hf-added TiAl alloy reach 620.8 ± 23.1 MPa and 7.27 ± 0.6 %, respectively. The twin grids in the γ_n phases uniformly accommodate dislocations, while the substantial lattice distortion and strong atomic bonding in γ_Hf impede dislocation activation, resulting in the absence of dislocations within γ_Hf. Consequently, the LAGB between γ_n and γ_Hf effectively hinders dislocations originating from the twin grids, thereby enhancing the alloy's high-temperature strength.