Martensite colony engineering: A novel solution to realize the high ductility in full martensitic 3D-printed Ti alloys

Generally, additively manufactured (AM) full martensitic titanium alloys (hexagonal-close-packed α’ phase) exhibits high yield strength (>1GPa) but poor elongation to failure (<8% usually). However, this only α’-phase microstructure is not intrinsically brittle and exhibits high ductility under certain process conditions. Therefore, it is crucial to unravel the mechanism of ductility fluctuation for process control. Here, we found the less martensite-colony microstructures (LMCM) could effectively distribute stress to variant interfaces and avoid strain localization. Therefore, it results in a 194% ductility enhancement in the AM-produced α’-Ti-6Al-4 V (Ti64) compared with the rich martensite-colony microstructure (MCM). At the same time, we also successfully improved the strength-ductility dilemma for Ti64 alloy (yield strength, Ys_0.2 = 1044 ± 10 MPa, elongation at break, EL = 15 ± 1.5%). We attribute the LMCM to refined prior-β grain and a weak variant selection during β_BCC to α’_HCP by increasing hatch spacing. What's more, to rule out the possible influence of the β phase and demonstrate the generality of our conclusion, we further apply it on α-titanium alloy (Ti-6.5Al-2Zr-1Mo-1 V, TA15 alloy) and obtain the same results.