Microstructure and mechanical properties evolution of Ti–5Al–5Mo–5V–1Cr–1Fe alloy under rapid heat treatment

Abstract

Starting microstructure with nonuniform micro-scale grains is designed for the study of microstructure and mechanical properties after rapid heat treatment (RHT) and subsequent aging. RHT was used to retard β grain growth in single-phase β region, while RHT and aging parameters affected size of secondary α and, hence, tensile strength and ductile characteristics. Heating rate for aging is important parameter affecting ω phase precipitation, which, in turn, affects the size of secondary α. Fast heating for aging avoided formation of ω phase, while ω phase formed at slow heating and duplex aging conditions results in refined α precipitations. After single-step aging with fast heating rate, proved by the calculation of strength based on the strengthening mechanism, the RHT temperature for the highest strength is at the bottom of single-phase β region. However, the nonuniformity of starting grain size results in the nonuniform β grain size after RHT, which affects the strengthening and elongation negatively. Proper selection of RHT parameters and fast heating for subsequent aging leads to refined secondary α and high balance of tensile properties (1415 MPa, 5.2%). Formation of ω phase at slow heating and at duplex aging resulted in marked drop in ductility with some variation of strength level (1344 MPa, 2.2%, and 1433 MPa, 0.7% correspondingly). Homogenising of the starting grain size by decreasing the β solution treatment temperature leads to achievement of the best tensile properties (1480 ± 12.5 MPa, 8.2% ± 0.99%).

Keywords

• Near-β titanium
• Rapid heat treatment
• ω phase
• Secondary α precipitation
• Strengthening mechanism