Transient Liquid Phase Bonding of NiTi Using Cu- and Nb-base Interlayers

Transient liquid phase (TLP) bonding was examined as an approach for joining NiTi to achieve a high joint efficiency while minimizing chemical variance within the joint region. Two bonding interlayer chemistries (Cu-base and Nb-base) were identified by screening thermodynamic criteria for TLP in ternary alloys using the CALPHAD method. These two systems were then experimentally evaluated with respect to their impact on solidification kinetics, microstructure in the joint region, and performance during quasistatic and cyclic tensile loading. For both interlayer chemistries, the composition profile and microstructure in the joint region confirmed an isothermal solidification mechanism. In addition, the joints were found to be fully dense and contain at most 1.2% intermetallic phases. Tensile testing showed excellent load transfer across the joints with approximately 4% recoverable strain and martensite onset stresses reaching 94% and 89% of the unbonded, annealed NiTi values for Cu-base and Nb-base interlayers, respectively. Lastly, a stable superelastic response was observed under cyclic loading for both bond chemistries, with spatial variation in the strain evolution linked to enhanced stiffness and hardness in the joint region arising from the substitutional Cu and Nb solutes, as confirmed via nanoindentation. This study demonstrates that TLP bonding of NiTi can produce high-strength and nearly intermetallic-free joints without sacrificing functional performance, such as the superelastic response.