Effects of pre-existing dislocations on regulating deformed substructures in a partially-recrystallized metastable β-titanium alloy with good strength-ductility combinations

A heterogeneous partially-recrystallized state was prepared in a metastable Ti-V-Cr-Al alloy through controlled post-annealing, following severe cold deformation. The samples, designated as 760-10, 760-60, and 850-10, were annealed at 760 degrees C and 850 degrees C for 10 and 60 min, respectively. In the 760-10 sample, the presence of recovered grains with a considerable dislocation density resulted in the highest yield strength (YS) of similar to 700 MPa, due to cooperative grain boundary and Taylor hardening, but at the expense of reduced tensile elongation at room temperature (RT). The combined effects of pre-existing dislocations and grain refinement significantly increased the critical resolved shear stress (CRSS) for twinning or stress-induced martensitic transformation (SIMT), leading to the occurrence of microbands and limited strain hardening capability. In contrast, the 760-60 and 850-10 samples, with lower dislocation densities, exhibited pronounced twinning- and transformation-induced plasticity (TWIP and TRIP) effects. The combined TWIP, TRIP, and the strain hardening from geometrically necessary dislocations (GND) contributed to a higher uniform elongation (similar to 30 %) compared to the 760-10 sample. At cryogenic temperatures, the primary deformation modes of the 760-10 sample transited from microbands to {332}< 113 > twins. The abundant TWIP effect, occurring at a high-stress level, facilitated notable cryogenic ductility (similar to 30 %) and strain hardening while maintaining a high YS (similar to 1100 MPa). Conversely, the 760-60 sample experienced brittle fracture in the elastic regime due to deformation heterogeneity arising from multiple texture components, and the 850-10 sample fractured prematurely after yielding, attributed to local dislocation piling-ups during extensive twinning across weak interfaces at cryogenic temperatures.