Development of low-cost Ti alloys with a balanced strength and ductility with generation of ultra-fine microstructures

This study aims to understand the interplay between strength and ductility in metastable β-Ti alloys based on eutectoid and neutral elements. A low-cost ternary Ti-7Cr-4Sn alloy was prepared by furnace cooling from the single β region at the end of the primary processing. Although isothermal ω, a nano-precipitation generally considered to embrittle the material, is present in the obtained ultra-fine microstructure, the material still exhibits a balanced strength and ductility, with a yield stress of 1067 MPa and elongation of about 10%. The obtained tensile properties surpass traditional primary processed Ti-6Al-4V, and are comparable to a range of expensive commercial high-strength aerospace Ti alloys. Multiple microstructural features, including grain boundary α (α_GB), short rod shape primary α (α_P), isothermal ω (ω_iso) and ω assisted secondary α (α_s) are characterised within the room temperature microstructure. Microstructural analysis reveals that strong Cr segregation in the β phase and slight partitioning of Sn between the α and β phase strengthens the β phase while also preserving ductility in the alloy. This results in a microstructure dominated by the ductile α phase and sub-micron α grain boundaries. This study also discusses the evolution of these microstructural features during different stages of cooling from β matrix, substantiating a promising alloy design strategy for affordable high-performance new Ti alloys.