Overcoming the strength-ductility trade-off in additive manufacturing of titanium alloy by in situ fabrication of heterogeneous lamellar microstructure

The strength-ductility synergy in heterogeneous materials offers significant advantages, though their scalable and controlled fabrication remains challenging. This study introduces an in situ fabrication strategy for heterogeneous lamellar titanium (HLT) alloy via laser powder bed fusion of a powder mixture consisting of Ti6Al4V (TC4) and 3 wt% Fe. By periodically varying the scanning velocity between layers, a heterogeneous lamellar microstructure is achieved due to the unique Fe distribution originating from the various volumetric energy densities (VEDs). Consequently, the HLT achieves high yield strength (1 036 MPa) and ultimate tensile strength (1 419 MPa) without compromising uniform elongation (UE), surpassing most TC4 alloys. The high strength may be attributed to precipitation strengthening originating from the nano-sized α and ω precipitates, while the high UE and work hardening arise from the strain-induced martensite (SIM) and strong hetero-deformation induced (HDI) stress. The denser dual-phase interfaces and smaller grains in the low VED layers contribute to the higher sensitivity to the SIM. A strain gradient between soft and hard layers evolves during loading, and it further enhances the HDI strengthening and SIM behavior. Through this work, the in situ fabrication method and the deformation mechanism of lamellar heterostructure could offer valuable reference for the optimization and application of heterogeneous materials.