Interfacial characterization and properties of Ti6Al4V/NiTi laser additive manufactured functional gradient materials

Ti6Al4V/NiTi bionic function graded materials (BFGM) with dense and defect-free microstucture were prepared by laser additive manufacturing technology, and their interfacial microstructure, precipitation phase characteristics and mechanical properties were investigated. The results show that the Ti6Al4V/NiTi BFGM exhibits a non-uniform microstructure consisting of various grain morphologies and irregular and abnormal eutectic tissues, which are mainly titanium-rich and nickel-rich solid solutions and (Ti, Ni) compounds. As the content of NiTi alloy increases, the number and morphology of precipitated phases in different deposition layers change significantly. The microstructure of BFGM undergoes a series of transformations: α + β biphasic microstructure → columnar crystals + irregular eutectic structure → columnar crystals → equiaxial crystals → equiaxial crystals + columnar crystals. Phase aggregation, separation and segregation during solidification have significantly affected the mechanical properties of BFGM. The maximum microhardness of BFGM is 730.9 HV, which is attributed to the presence of brittle Ti₂Ni phase. The tensile strength is 202 MPa and the elongation is 6.5%, which is significantly higher than that of the directly connected Ti6Al4V/NiTi heterogeneous material. The tensile fracture is characterized by brittle fracture with multiple secondary cracks extending along the crystal.