An oxygen-rich face-centred cubic (FCC) Ti phase was engineered in the microstructure of a Ti-6Al-4V
alloy via additive manufacturing using laser powder bed fusion. Designated 'C', this oxygen-rich FCC
phase has a lattice parameter of 0.406 nm and exhibits an orientation relationship with the parent a0
phase as follows: (00 01)a0//{111}C, and h1 2

10ia0 //h1 1

0iC. We propose that the formation of the C
phase is facilitated by the combined effect of thermal gradients, deformation induced by the
martensitic transformation, and local O enrichment. This enables an in-situ phase transformation from
the hexagonal close-packed a0 phase to the C phase at elevated temperatures. Our density functional
theory calculations indicate that oxygen occupancy in the octahedral interstices of the FCC structure is
energetically preferred to corresponding sites in the a0 phase. The in-situ mechanical testing results
indicate that the presence of the FCC phase significantly increases the local yield strength from 1.2 GPa
for samples with only the a0 phase to 1.9 GPa for samples comprising approximately equal volume
fractions of the a0 and FCC phases. No loss of ductility was reported, demonstrating great potential for
strengthening and work hardening. We discuss the formation mechanism of the FCC phase and a
pathway for future microstructural design of titanium alloys by additive manufacturing