Induction mechanisms of high-density nano twins during solidification process: reducing stacking fault energy of γ phase by Re and forming highly mismatched B2(Re)/α2 interface

Abstract

It is extremely difficult to introduce high-density nano twins during the solidification process of TiAl alloy. In this study, high-density nanotwins are inducted in the as-cast Ti48Al2Cr alloyed by adding Re element. Phase transformation, morphology characteristics of nano twins, compressive and tensile properties, and the related mechanisms have been studied. Results show that B2 phase enriched with Re tends to precipitate along the α2/γ interface within lamellar colony. The stacking fault energy (SFE) of γ phase decreases from 43 mJ/m2 to 16 mJ/m2 as Re content increases from 0 at.% to 0.6 at.%, decreasing the critical shear stress for twin formation. Compared to the mismatch value of α2/γ interface (0.004), which of B2/α2 and B2/γ interfaces increase to 0.247 and 0.149, respectively. Driven by high interfacial stress, high-density dislocations are generated at the B2/α2 interface, providing the dislocation slip channel for the formation of stacking faults (SFs) and nanotwins at the B2/γ interface. Therefore, the mechanism of inducting high-density nanotwins is to reduce the stacking fault energy of γ phase by Re and form highly mismatched B2/α2 interface. Compressive strength and the strain increase from 1723 MPa to 2398 MPa and 29% to 39% as Re content increases from 0 at.% to 0.6 at.%, respectively. Tensile strength increases from 356 MPa to 452 MPa without sacrificing plasticity. The improvement in strength and plasticity are attributed to the nano-twinning strengthening and interfacial thermal mismatch strengthening. Forming nanotwins during solidification process serve as the nucleation sites for newly formed twins during deformation process, increasing the deformation tolerance of TiAl alloy.