Microstructural evolution and mechanical behavior of metastable β-type Ti–25Nb–2Mo–4Sn alloy with high strength and low modulusMicrostructural evolution and mechanical behavior of metastable β-type Ti–25Nb–2Mo–4Sn alloy with high strength and low modulusr

This paper presents a systematic study of newly developed metastable β-type Ti–25Nb–2Mo–4Sn (wt%) alloy with high strength and low elastic modulus, with focus on the microstructural evolution and mechanical behavior associated with aging. The pre-treatment (solution treatment or cold rolling) prior to aging exerts substantial influence on the subsequent aging response including microstructural evolution and mechanical behavior. Even under the same aging treatment, the aging products could be (β+ω), or alternatively (β+α), depending on the pre-treatments. This interesting aging response was discussed on the basis of the mechanism for ω formation. High-density dislocation tangles and grain boundaries induced by severe cold rolling play a key role in hindering the transition from β to isothermal ω, favoring the precipitation of α phase on aging. By aging cold-rolled specimen for short time, superior mechanical properties, i.e. high ultimate strength of ∼1113 MPa and low elastic modulus of ∼65 GPa, achieved in Ti–25Nb–2Mo–4Sn alloy. The characterization of microstructural evolution and compositional change indicated that the precipitation of fine α does not cause the enrichment of β-stabilizers in β matrix upon a short-time aging, guaranteeing low elastic modulus of the short-time aged specimen. Meanwhile, fine α precipitates as well as dislocations play a crucial part in strengthening, giving rise to its high yield strength and high ultimate tensile strength.