Achieving superior strength-ductility balance in a novel heterostructured strong metastable β-Ti alloy

Abstract

In metastable β-Ti alloys, the primary deformation mechanisms, including ordinary dislocation plasticity (ODP), deformation twinning (DT), and phase transformation, are primarily controlled by the stability of the β phase. Based on the Bo-Md diagram, here, we prepared a novel metastable β-Ti alloy with a nominal composition of Ti-1.8Al-7.8Mo-3.7Cr-6.7Zr, and further manipulated its microstructural heterogeneities (in terms of the grain size and the dislocation density) via thermomechanical processing to achieve a controllable activation sequence of deformation mechanisms. It was unveiled that in the materials with heterogeneous laminated structures (HLS), ODP was the first deformation mechanism to be activated, whereas, in the gradient equiaxed grained (GEG) alloys, as well as those with heterogeneous bimodal structures (HBS), DT was found to precede ODP. Compared with the GEG alloys, which exhibited an inferior strength-ductility combination (yield strength ∼639 MPa, total elongation ∼18.6%, uniform elongation ∼5%), both alloys with heterostructures, the HLS and HBS samples, manifested simultaneous enhancement in yield strength and tensile ductility. The former, with a high pre-existing dislocation density, showed an outstanding total elongation of ∼34% (uniform elongation ∼18%). The latter, with a large back stress, displayed, on the other hand, ultra-high yield strength of ∼910 MPa. This heterostructure-induced property enhancement was rationalized by a newly proposed strengthening model, which takes into account the grain size distribution and considers both the isotropic and the kinematic hardening. The present results imply the combination of dislocation engineering and heterostructure design being an effective strategy to achieve excellent strength–ductility balance in metastable β-Ti alloys.