Spatially heterogeneous microstructure in in-situ TiO-reinforced Ti6Al4V/316L functionally graded material fabricated via directed energy deposition

The strength–ductility trade-off is challenging for metal materials fabricated using conventional techniques. Directed energy deposition (DED), which can manufacture functionally graded materials (FGMs) with spatially heterogeneous microstructure, offers a potential solution to this problem. Here, a spatially heterogeneous microstructure which consisted of equiaxed, irregular, and columnar grains from top to bottom regions was realized in a DED-fabricated Ti6Al4V/316 L FGM. In building direction, the steep temperature gradient in the bottom region promoted the epitaxial growth of columnar grains. The columnar grains in the middle and top regions were restrained by the heterogeneous nucleation of in-situ formed TiO particles and the increasing constitutional supercooling, ultimately inducing the irregular and equiaxed grains. The equiaxed grains in the top region contributed to superior strength and wear resistance, while the irregular and columnar grains in the middle and bottom regions correlated with large elongation. Hence, FGM integrates the high strength, superior ductility, and excellent wear resistance. Furthermore, the strength–ductility synergistic mechanism and heterogeneous microstructure formation mechanism, including heterogeneous nucleation of the in-situ formed TiO particles and columnar-to-equiaxed transition, were unveiled.