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. • The study explores the effect of martensite decomposition completeness on the tensile performance of LPBF Ti-6.5Al-2Zr-1Mo-1V (TA15). It is conducted at a relatively low temperature of 750°C with a short dwell time. • Found that α’ martensite decomposes preferentially at twin and martensite boundaries, leading to the formation of fine β ligaments. • Tensile tests show that incomplete martensite decomposition increases yield strength from 917 MPa (as-built) to 1000 MPa after 30 minutes of annealing at 750°C, without reducing ductility. • The strengthening effect is linked to the presence of fine β ligaments at boundaries, which create additional obstacles for dislocation motion. • Martensite decomposition is enhanced at boundaries due to higher crystallographic defects, with twin boundaries influencing transformation due to Mo segregation.