Uncovering the effect of aging temperature on micro deformation incompatibility and strength-ductility evolution of a novel high-strength titanium alloy with bimodal microstructure

The plastic deformation mismatch between primary α_p and transformed β matrix (β_t, β matrix precipitated with secondary α_s) significantly influences the mechanical performance of titanium alloy with bimodal microstructure. This work investigated the influence of aging temperature on α_p, β_t properties, micro deformation incompatibility and strength-ductility evolution of bimodal-structured titanium alloy. A novel high-strength titanium alloy Ti-5Al-7.5V-0.5Mo-0.5Zr-0.5Si (TC5751S) was 880 ℃ solution treated and 500 ℃/550 ℃/600 ℃ aged to obtain various bimodal microstructures. With the aging temperature increases from 500 ℃ to 600℃, the average yield strength (YS) of TC5751S decreases from 1218 MPa to 1131 MPa, while the average ductility remains changeless (overall ε_f: 10 ± 0.1 %). The drop of strength was ascribed to the softening of β_t. α_s coarsening induced by aging temperature increase weakened the impedance of dislocation motion inside β_t, resulting in the softening of β_t and decrease of tensile strength. The ductility invariance was ascribed to the dynamic equilibrium between β_t softening and α_p/β_t interface weakening. As the aging temperature increased, void nucleation within β_t was mitigated owing to β_t softening. However, the softening of β_t simultaneously exacerbated the plastic deformation mismatch between hard α_p and soft β_t during tension, thereby promoted void nucleation at α_p/β_t interface. The detrimental effect of α_p/β_t interface damage on ductility was counterbalanced by the positive effect of the reduced damage tendency within β_t, thus maintaining the ductility of TC5751S alloy unchanged. Our findings enhance the understanding of deformation incompatibility and strengthening-ductilizing mechanism of titanium alloy with bimodal microstructure.