Precipitation of α2 phase, β decomposition-induced HCP-FCC phase transformation, and formation of abnormal nanotwins in an additively manufactured multicomponent α + β titanium alloy

Abstract

An α + β titanium alloy with 9 components was processed by laser directed energy deposition (LDED). Such a complex alloying strategy led to the precipitation of coherent α₂ (Ti₃Al) particles, phase transformation from hexagonal close packed (HCP) to face centered cubic (FCC), and formation of abnormal nanotwins in α lamellae. Specifically, the FCC nano laths and abnormal nanotwins were formed due to the shearing of the prismatic plane of the α phase during β decomposition. To the best knowledge of authors, such phenomena were found in α + β titanium alloys for the first time. All of these fine microstructures in α lamellae are coherent, so the restriction of dislocation movement by coherent interface and twin boundary is critical strengthening mechanism for the development of α + β titanium alloys.