Enhancing strength-ductility synergy in metastable β-Ti alloys through β-subgrains-mediated hierarchical α-precipitation

Abstract

Titanium alloys can achieve ultrahigh strength through precipitation hardening of secondary α-phase (α_s) from β-matrix but often compromise ductility due to the conventional strength-ductility trade-off. In this study, a new strategy based on β-subgrains-mediated hierarchical α-precipitation is devised to balance the conflict in Ti-6Al-2Mo-4Cr-2Fe (wt.%) alloy through a unique combination of hot rolling, short-term solid solution, and aging treatment, i.e., RSST+A. Tensile testing reveals that the RSST+A samples exhibit ultrahigh strength of ∼1581 MPa and decent ductility of ∼8.4%, surpassing ∼1060 MPa and ∼2.7% of the corresponding RSST counterparts without final aging treatment. This remarkable strengthening and counterintuitive ductilizing is attributed to the architecting of β-subgrains-mediated hierarchical α-precipitates as a result of our specific processing approach. The designed short-term solution introduces abundant β subgrains that are transformed from the retained intensive dislocations during hot rolling. The β subgrain boundaries subsequently promote a dramatic precipitation of α allotriomorphs (α_GB) and Widmanstätten side-plates (α_WGB), which effectively subdivides β grains into numerous tiny independent deformation units. Consequently, plastic strain is uniformly partitioned into a large number of small aged β subgrains during tension, which strongly impedes strain localization that would typically occur across multiple β subgrains in the fashion of long straight slip bands in the case of the RSST samples. Furthermore, the hierarchical α structure also postpones uncontrollable cracking even when structural damage occurs at the last stage of straining. These findings demonstrate that appropriately manipulating microstructure through elaborately designing processing routes enables unexpectedly ductilizing high-strength titanium alloys in the precipitation-hardening state.