Synergic strength-ductility enhancement in bimodal titanium alloy via a heterogeneous structure induced by the 〈c + a〉 slip

In this work, a heterogeneous structure (HS) with an alternating distribution of coarse and fine α lamella is fabricated in bimodal Ti6242 alloy via insufficient diffusion of alloying elements induced by fast heating treatment. Instead of a distinct interface between the primary α phase (αp) and β transformation microstructure (βt) in the equiaxed microstructure (EM), all αp/βt interfaces are eliminated in the HS, and the large αp phases are replaced by coarse α lamella. Compared to the EM alloy, the heterostructured alloy exhibits a superior strength-ductility combination. The enhanced strength is predominantly attributed to the increased interfaces of α/β plates and hetero-deformation induced (HDI) strengthening caused by back stress. Meanwhile, good ductility is ascribed to its uniform distribution of coarse and fine α lamella, which effectively inhibits strain localization and generates an extra HDI hardening. This can be evidenced by the accumulated geometrically necessary dislocations (GNDs) induced by strain partitioning of the heterostructure. Significantly, the HDI causes extra 〈c+a〉 dislocations piling up in the coarse α lamella, which generates an extra strain hardening to further improve the ductility. Such hetero-interface coordinated deformation mechanism sheds light on a new perspective for tailoring bimodal titanium alloys with excellent mechanical properties.