Enhancing high-temperature strength and ductility in laser powder bed fusion Ti–6.5Al–2Zr–1Mo–1V alloy via heat treatment optimization

Abstract

Heat treatment (HT) is an effective way to attain the desired microstructure and mechanical properties in laser powder bed fusion (LPBF) Ti–6.5Al–2Zr–1Mo–1V (TA15). However, previous studies only obtained the high tensile ductility of LPBF TA15 but were accompanied by a large loss of strength, which was unable to achieve balanced mechanical properties. Here, we evaluated the tensile properties of LPBF TA15 at their usage temperatures (500 °C), and a superior combination of high-temperature strength and ductility in LPBF TA15 was successfully achieved via a single-step heat treatment. After the heat treatment close to the β transus temperature (950 °C as used in this study), the tensile strength of the sample was ∼30% higher than that of as-cast TA15, accompanied by the obvious enhancement of ∼67% in elongation. The high strength originated mainly from the secondary α-phase and discontinuous grain boundary α-phase (GB-α), whereas the large ductility correlates to the complete decomposition of brittle α’ martensite. Discontinuous GB-α could inhibit crack initiation at the grain boundary and the fine secondary α-phase impeded the dislocation motion, which could enhance the tensile strength. Meanwhile, the α lath directions after heat treatments were far away from the 45° of the load direction, which also avoided the localized deformation within α laths and thus improved the fracture resistance of LPBF TA15.