Strengthening of metastable β-T9V alloy via α′ martensite regulation and α-phase introduction

Abstract

Low strength is a key limitation for the engineering applications of metastable β titanium alloys. This study aims to achieve high strength in the metastable β Ti-9V-5Zr-3Al-2Cr (wt. %) alloy by regulating the type and size of α′ martensite and the fraction of α-phase through hot-rolling. After rolling and quenching at 850 °C, 800 °C, and 750 °C, the alloys consisted of a large amount of α′ martensite. During tensile testing, the reorientation of α′ martensite enhanced the work-hardening and plastic deformation in the alloy. Rolling at 850 °C, α′ martensite is all dislocation type, i.e. dislocation α′ martensite (DM), but the martensite size is large, leading to a low tensile strength of 850 MPa. When the rolling temperature decreased to 800 °C, the α′ martensite grains were refined, and twinned α′ martensite (TM) appeared. The heterostructure formed by DM and TM generated back-stress strengthening, increasing tensile strength to 1030 MPa. Rolling at 750 °C, the two types of martensite grains were further refined, and α sub-grains were retained within the prior β grains, improving tensile strength to 1090 MPa. However, after rolling at 700 °C, α′ martensite disappeared, and the alloy was composed of α- and β-phases. Stress concentration at the phase boundaries significantly reduced plasticity. This study demonstrates the influence of α′ martensite deformation behavior and the introduction of trace α-phase on the mechanical properties of metastable β titanium alloys, providing a viable strategy for strengthening such alloys.