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

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 alpha'/beta, alpha/beta phase interfaces, {10 (1) over bar1} 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 alpha' martensite with numerous {10 (1) over bar1} 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 a' martensite partially or fully decomposes into a lamellar (alpha+beta) structure. The 955 degrees 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 similar to 12%), deformation occurs by dislocation slips, leading to significant dislocation accumulations around the alpha/beta interfaces. At later stages (similar to 20% strain), when necking occurs, a high density of fcc-gamma bands is observed in the hcp-alpha phase, indicating the onset of deformation-induced martensitic transformation (DIMT) from hcp-alpha to fcc-gamma. 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-alpha phase. Low-loss electron energy loss spectroscopy reveals that the deformation-induced fcc phase is not Ti-hydride, but a new allotrope of Ti.