An investigation on Ni₄Ti₃ phase precipitation and its effects in laser directed energy deposition of TiC–NiTi composites

The failure of the NiTi alloys due to their low mechanical strength is the major problem inhibiting their further applications. Adding ceramic reinforcement materials could not only enhance the strength but also tribological characteristics of NiTi alloys. Among various adopted ceramic reinforcement materials including Al₂O₃, SiC, TiN, TiB₂, TiO₂, and TiC, TiC is preferred since it little affects the unique shape memory properties of NiTi. For the first time, this paper reports the fabrication of TiC–NiTi composites using laser DED process. Microstructural characterizations are presented to understand the Ni₄Ti₃ phase precipitation mechanisms with the addition of TiC particles. Nanoindentation tests are conducted to evaluate the mechanical properties. The addition of TiC particles could alter the phase formation since Ti atoms tend to migrate towards the TiC particles. The nucleation and growth of Ni₄Ti₃ precipitates are then effectively restricted. The typical thicknesses of the Ni₄Ti₃ precipitates are reduced by 20% (about 30 nm) at lower levels of laser power or eliminated at a higher level of laser power. The addition of TiC also leads to a reduction of about 10% in the volume fraction of the Ni₄Ti₃ precipitates and an increase of 6%–17% in the volume fraction of the NiTi₂ phases. The refinement of the Ni₄Ti₃ precipitates improves the pseudoelasticity by about 10% (0.05 in recovery energy ratio). Besides, the nanohardness and Young's modulus are also strengthened. These experimental results indicate that TiC has the capability to refine the Ni₄Ti₃ precipitates, enhance the strength of NiTi, and improve the pseudoelasticity.