Optimizing strength-ductility of laser powder bed fusion-fabricated Ti–6Al–4V via twinning and phase transformation dominated interface engineering

Abstract

Titanium alloys produced by laser powder bed fusion (LPBF) are known to possess fine microstructures and multi-scale interfaces. Here, we introduce a high density of α'/β, α/β phase interfaces, {10 1‾1} twin boundaries, and basal stacking faults (BSFs) in a Ti–6Al–4V alloy through LPBF and annealing. The LPBF-fabricated alloy consists of fine acicular α′ martensite with numerous {10 1‾1} twins and BSFs, which results in ultrahigh strength (>1300 MPa), but very low ductility (<5%) due to the massive interface strengthening. Subsequent annealing treatments decrease the strength while increasing the ductility, as the α′ martensite partially or fully decomposes into a lamellar (α+β) structure. The 955 °C-annealed alloy possesses a good strength-ductility combination (yield strength of 1000 MPa, tensile strength of 1078 MPa, and total elongation of 20%). During the stages of uniform plastic strain (up to ∼12%), deformation occurs by dislocation slips, leading to significant dislocation accumulations around the α/β interfaces. At later stages (∼20% strain), when necking occurs, a high density of fcc-γ bands is observed in the hcp-α phase, indicating the onset of deformation-induced martensitic transformation (DIMT) from hcp-α to fcc-γ. The occurrence of DIMT may be attributed to the decreased stacking fault energy and the cohesive energy difference between the hcp and fcc phases, due to the segregation of Al elements in the hcp-α phase. Low-loss electron energy loss spectroscopy reveals that the deformation-induced fcc phase is not Ti-hydride, but a new allotrope of Ti.