Microstructural development in copper-interlayer transient liquid phase bonds between martensitic NiAl and NiTi

Abstract

Transient liquid phase (TLP) bonding of Ni–24 at% Al–16 at%Cr and Ni–30 at% Al with a tetragonal L10 type martensitic microstructure to nominally stoichiometric NiTi with a monoclinic (distorted B19 type) martensitic matrix is investigated in this paper. The TLP bonds described in this article employed 50 μm thick copper interlayers. Bonds were prepared using holding times of between 20min and 2 h at a bonding temperature of 1150°C. A holding time of 1 h at 1150°C was sufficient to remove the liquid-phase from the bond-line, however a layer of Ni2AlTi (L21 type Heusler phase) was left at the bond line. This layer remained present with further holding up to 2 h at 1150°C. With 50 μm thick copper interlayers, 2 h holding at 1150°C resulted in significant titanium diffusion, from the NiTi substrate, to the joint. Depletion of titanium from the NiTi substrate, in turn, reduced the solidus temperature of the NiTi substrate to the point where localized melting occurred on the surface of the NiTi substrate. Extensive microstructural changes were observed in the NiTi substrate after TLP bonding and these are discussed in detail. In contrast, the NiAl substrate was largely unchanged after TLP bonding.