Dependence of strength and ductility on secondary α phase in a novel metastable-β titanium alloy

As a novel metastable β titanium alloy , Ti-6Mo-5V-3Al-2Fe, it is essential to establish the correlation among its microstructure, strength and ductility. In this paper, transmission electron microscope (TEM) and scanning electron microscope (SEM) were used to observe the secondary α phase (α s ) of the aged alloy. The strength and ductility of the alloy were characterized by a room temperature tensile test . The dependence of strength and ductility on the α s of the alloy was investigated. Results indicate that the minimum width and spacing of α s within β grains can be obtained after being aged at 470°C for 4h. As the increase of aging temperature and time, the average width and average spacing of α s increase. A relational equation among precipitation strengthening and spacing length of α s is established to analyze the strengthening mechanism . It is demonstrated that the average spacing length of α s plays a predominant role in the strengthening of the alloy. The aged alloy exhibits poor ductility due to the formation of continuous grain boundaries α phase (GB α). After being aged at 620°C for 4h, Widmanst a ¨ t t e n α phase nucleates along β grain boundaries and grows toward β matrix (WGB α). The combination of α s spacing widening and WGB α formation obviously improves the ductility of the alloy.