Spinodal decomposition-mediated multi-architectured α precipitates making a metastable β-Ti alloy ultra-strong and ductile

Abstract

The chemical boundaries inside the ultrafine spinodal decomposition structure in metastable β-Ti alloys can act as a new feature to architect heterogeneous microstructures. In this work, we combined two semi-empirical methods, i.e., the d-electron theory and the e/a electron concentration, to achieve the spinodal decomposition structure in a metastable β Ti-4.5Al-4.5Mo-7V-1.5Cr-1.5Zr (wt.%) alloy. Utilizing the spinodal decomposition structure, the aged Ti-Al-Mo-V-Cr-Zr alloys showed multi-architectured α precipitates spanning from micron-scale (primary αp) to nano-scale (secondary αs) that were uniformly distributed in the β-domains. Being compared with the forged sample, the multi-scale heterogeneous microstructure enables the aged β-Ti alloy to have ultra-high strength (yield strength ∼1366 MPa and ultimate tensile strength ∼1424 MPa) and an appreciable ductility (∼9.3 %). Strengthening models were proposed for the present alloys to estimate the contribution of various microstructural features to the measured yield strength. While the solid solution strengthening, β-spinodal strengthening, and back stress strengthening made comparable contributions to the strength of the forged alloy, the back stress strengthening was the predominant strengthening effect in the aged alloy. This alloy design approach based on chemical boundary engineering to construct multi-architectured α precipitates provided an effective strategy for achieving an outstanding combination of ultra-high strength and ductility in metastable β-Ti alloys.