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

Abstract

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 (αs) 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 αs of the alloy was investigated. Results indicate that the minimum width and spacing of αs 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 αs increase. A relational equation among precipitation strengthening and spacing length of αs is established to analyze the strengthening mechanism. It is demonstrated that the average spacing length of αs 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, Widmansta¨tten α phase nucleates along β grain boundaries and grows toward β matrix (WGB α). The combination of αs spacing widening and WGB α formation obviously improves the ductility of the alloy.

Keywords

• Titanium alloys
• Aging treatment
• Secondary α phase
• Strength
• Ductility