Strengthening and toughening mechanisms of γ-TiAl dominated by shear induced “catching bonds”

TiAl alloys, especially polysynthetically twinned (PST) TiAl, have great potential to be an advanced lightweight engineering materials for key aero-engines components like turbine blades due to their high service temperature. Most researches focus on improving the strength and ductility of TiAl, but the origin of strengthening and toughening mechanisms remain unexplored so far. This work aims at understanding the chemical bond related intrinsic mechanical behavior in essence and revealing the origin of excellent mechanical properties of TiAl alloy. We found that the excellent ductility of γ-TiAl origins from the metallic catching bonds (Ti-Ti and Al-Al) and the high strength is due to high strength Ti-Al covalent catching bond. The (111)/[11 2 ¯ ] system is most likely to be activated under pressure. The pure shearing along (111)/[11 2 ¯ ] leads to the interlayer slip of 1/6 [11 2 ¯ ] Burger vector with the breakage and reformation of Ti-Ti, Al-Al metallic bonds (“catching bonds”). The deformation mechanisms are attributed to the metallic catching bonds induced twin boundary (TB) formation, TB migration, superlattice intrinsic stacking fault (SISF) formation (detwinning) and SISF annihilation. These processes can continuously dissipate the strain energy, resulting in the excellent ductility of γ-TiAl. The structure becomes a hard slip system of (111)/[ 1 ¯ 1 ¯ 2] , and the ideal shear strength is 16.5 GPa, which is about three times higher than that along (111)/[11 2 ¯ ] , on account of the high strength covalent catching bonds (Ti-Al) dominate the deformation mechanism of 1/2 [ 1 ¯ 1 ¯ 2] full dislocation. The generalized stacking fault energy (GSFE) shows the true atomic slip path along 1/2 [ 1 ¯ 1 ¯ 2] perfect dislocation (provide a degree of ductility) is: 1/2 [ 1 ¯ 1 ¯ 2] → 1/6 [ 1 2 ¯ 1 ] + 1/6 [ 1 ¯ 1 ¯ 2] + 1/6 [ 2 ¯ 11 ] + 1/6 [ 1 ¯ 1 ¯ 2] , which agrees well with the previous experimental results.