Abstract: The chemical boundaries inside the ultrafine spinodal decomposition structure in metastable β-Ti al-loys 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 βTi-4.5Al-4.5Mo-7V-1.5Cr-1.5Zr (wt.%) alloy. Utiliz-ing the spinodal decomposition structure, the aged Ti-Al-Mo-V-Cr-Zr alloys showed multi-architectured αprecipitates spanning from micron-scale (primary αp) to nano-scale (secondary αs) that were uni-formly distributed in the β-domains. Being compared with the forged sample, the multi-scale heteroge-neous 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 ap-proach based on chemical boundary engineering to construct multi-architectured αprecipitates provided an effective strategy for achieving an outstanding combination of ultra-high strength and ductility in metastable β-Ti alloys.

Key words: Ti alloys, Multi-architectured αprecipitates;, Spinodal decomposition, Mechanical properties, Strengthening mechanisms