Ni-Mo co-alloying for strength-ductility synergy in equiaxed laser additive manufactured Ti-6Al-4V alloys

In laser additive manufacturing (LAM) of titanium alloys, conventional strategies for promoting equiaxed grains can effectively weaken crystallographic texture but may simultaneously deteriorate intragranular microstructures and grain-boundary stability, leading to a strength-ductility trade-off. Therefore, achieving equiaxed grains while preserving favorable intragranular microstructures remains an important challenge for improving the mechanical performance of additively manufactured titanium alloys. Based on previous findings regarding the role of Mo in regulating intragranular α phase evolution, this work investigates a Mo-Ni co-alloying approach to coordinate grain morphology and intragranular microstructure. A series of Ti-6Al-4V-1Mo-xNi alloys (x = 0, 2, 3, 4 wt%) were fabricated using direct energy deposition (DED). The results show that the addition of 2-3 wt% Ni significantly enhances constitutional undercooling and the grain growth restriction factor (Q), promoting the columnar-to-equiaxed transition (CET) while preserving the refined α lath structure associated with Mo addition. As a result, the DED Ti-6Al-4V-1Mo-3Ni alloy exhibits a tensile strength of 1234.6 MPa and an elongation of 12.1%. In contrast, excessive Ni addition (4 wt%) leads to Ni/Al segregation at grain boundaries, resulting in a mismatch in strengthening between grain interiors and grain boundaries and causing premature boundary-dominated fracture with a significantly reduced elongation (0.7%). Microstructure-based finite element simulations further reveal a composition-dependent deformation mode transition from intragranular-dominated multiscale deformation (2-3 wt% Ni) to boundary-controlled brittle failure (4 wt% Ni). These findings provide useful insights into alloy design strategies for achieving balanced strength-ductility combinations in equiaxed LAM titanium alloys.