Martensite decomposition and its effect on mechanical performance in laser powder bed fusion Ti–6.5Al–2Zr–1Mo–1V alloy

Heat treatment (HT) is a useful post-AM processing method for laser powder bed fusion (LPBF) near-α titanium alloys that could decompose the martensite and attain the desired mechanical properties. However, the completeness of martensite decomposition and its effects on the mechanical properties of LPBF Ti alloys have rarely been reported. In this study, post-AM heat treatment with a relatively low temperature (750 °C) and short dwell time was carried out in LPBF Ti-6.5Al-2Zr-1Mo-1V to study the martensite evolution. Results show that α’ martensite tends to decompose at the twin boundary and martensite boundary during the post-AM HT process, as evidenced by the observed fine β ligament at these sites. Such β ligaments could act as barriers to obstruct dislocation across the martensite boundary and impede the dislocation motion within the martensite lath, leading to a significant improvement in yield strength (increased by over 100 MPa) as compared to the as-built state. These findings could contribute to a fundamental understanding of microstructure evolution during the post-AM heat treatment and present a promising method that could facilitate further mechanical performance manipulation in AM Ti alloys.