Enhanced strength-ductility synergy and mechanisms of heterostructured Ti6Al4V-Cu alloys produced by laser powder bed fusion

Limited slip systems of α structure always play a major role in hindering strength-ductility synergy improvement of titanium (Ti) alloys. To overcome the strength-ductility trade-off of Ti alloys, heterostructure is introduced into Ti6Al4V-xCu (x = 0, 1, 3, 5 wt.%) alloys produced by laser powder bed fusion (LPBF), and the formation mechanism and deformation behavior of heterostructures were investigated. The results showed that the monolithic α' in the LPBF-produced Ti6Al4V-xCu (wt%) alloys is decomposed into dual α and α" after heat treatment at 600–900 °C through α' → α + Ti2Cu → matastable phase → α + α". The multistage transformation of α' to α and α" is driven by thermal activation (temperature > 800 °C), Cu addition and rapid cooling of water quenching. The heterostructure with α and α" in the LPBF-produced Ti6Al4V-5Cu alloy after heat treatment at 800 °C results in high tensile strength (∼1.3 GPa) and large elongation (∼15%). The enhanced strength-ductility synergy is attributed to the decomposition of brittle α' and Ti2Cu, as well the soft-hard heterostructure of α" and α. Moreover, the deformation twins (DTs) in α" and the heterogeneous interfaces of α and α" can also improve the strength and ductility of the LPBF-produced Ti6Al4V-xCu alloys. These findings elucidate the influence of heterostructure (α and α") on strength and ductility, which is helpful for designing Ti alloys with excellent mechanical properties.