Non-uniform deformation response of Ti6Al4V/TiC functionally graded material under different composition gradient designs by direct energy deposition

Metal/ceramic functionally graded materials (FGMs) have garnered considerable attention due to their capability to improve heat resistance while maintaining excellent mechanical strength. However, the non-uniform spatial gradient distribution of ceramic particles makes the deformation and fracture behavior of FGMs very complicated during static loading. Optimizing the composition gradient designs to enhance mechanical properties and fracture toughness while maintaining heat resistance has been a longstanding puzzle. Herein, three ceramic composition gradient transition methods including convex function mode, linear mode and concave function mode were designed, and Ti6Al4V/TiC FGMs were fabricated by laser direct energy deposition (LDED), with TiC mass fraction gradually changing from 0 % to 30 %. To reveal the non-uniform deformation response and fracture mechanism under different composition gradient modes, comprehensive analyses of microstructure characterization, in-situ DIC tensile measurement and fracture morphology of as-deposition Ti6Al4V/TiC FGMs were conducted. As a result, the strain along the central axis exhibits a wavy non-uniform gradient distribution during the longitudinal tensile process, while low-gradient layers with good plastic deformation capability alleviate the stress concentration of high-gradient layers through localization bands. Compared with the convex function mode, the tensile strength of concave function transition mode along with the Z-direction exhibits a distinct increase of 201 MPa, while the elongation improves by 1.11 %, which reflects the synergistic improvement in strength and ductility. This is due to the increased occurrence of plastic deformation in more areas, resulting in a stronger gradient synergistic strengthening effect. Meanwhile, the stress intensity factor of the crack tip under different gradient transition modes was obtained through integral transformation and dual integral equation solving, further verifying the above conclusion theoretically. The fractures tend to occur in high-gradient ceramic regions, primarily due to the fracture of unmelted particles leading to premature failure. This work emphasizes the importance of nonlinear composition design strategies in the development of metal/ceramic functionally graded materials.