Sequentially-activated multiple deformation mechanisms enable a hierarchically duplex titanium alloy with high strength-ductility synergy

Abstract

The HCP α-precipitates in duplex titanium (Ti) alloys are quite important to their mechanical properties in terms of accommodating plastic deformation for large ductility on the one hand and hindering dislocation motion for high strength by α-precipitate/β-matrix interfaces on the other hand. However, the intrinsic limited slip systems of low symmetric α-precipitates lead to progressive deformation localization even cracking due to high stress concentrations, becoming the origin of the strength-ductility conflict in Ti alloys. Here, a tri-modal Ti-4.5Al-4.5Mo-7V-1.5Cr-1.5Zr (wt.%) alloy as a model is decorated by hierarchically multi-scaled and multi-polymorphic α precipitates to introduce multiple plasticity mechanisms for high strength-ductility synergy. The sequentially-activated plasticity mechanisms (SAPMs) dominantly involve prismatic 〈a〉 and pyramidal 〈c + a〉 dislocation slip and cross-slip in globular micron-α_g, the P-type HCP-to-FCC transformation and stacking faults/nanotwins in rod-like submicron-α_r, and prismatic 〈a〉 dislocation slip transfer in secondary nano-α_s. The SAPMs in the α precipitates are strongly size-dependent and explained in terms of the critical resolved shear stress (CRSS). The contribution of multi-polymorphic α-precipitates to the yield strength is quantified with their distinguished strengthening capabilities of α-precipitates in multi-scales. The present hierarchical Ti alloy with the tri-modal microstructure achieves a good combination of yield/ultimate tensile strength of 1550/1614 MPa and ductility of ∼8.7 %.