Tensile deformation and fracture behavior at high temperature of the TLP joint for Hastelloy X superalloy with laminated interlayer

Abstract

Transient liquid phase diffusion bonding of Hastelloy X superalloy was performed using laminated foils of BNi-5 / BNi-5 and BNi-5 / Ni / BNi-5, respectively. A comprehensive analysis, combining experimental observations and crystal plasticity finite element analysis, was conducted to examine the microstructure, deformation, and fracture behavior of the joints. Both joints, bonded at 1170 °C for 4 h comprised an isothermal solidification zone (ISZ) and a diffusion affected zone (DAZ). The ISZ of the joint employing BNi-5 / BNi-5 exhibited columnar characteristics, with fine grains distributed on its flanks. During tensile testing, stress concentration arose at the ISZ-DAZ interface due to grain structure inhomogeneity. Furthermore, large size silicides were precipitated in the DAZ and at the the ISZ-DAZ interface. Under the stress concentration, cracks initiated at silicides and propagated along the interface, leading to a brittle fracture. The joint bonded with BNi-5 / BNi-5 possessed a tensile strength of 310 MPa and an elongation of 12.03 % at 750 °C, which amounted to 81.3 % and 32.5 % of the respective properties of the base material. The addition of Ni significantly increased the microstructure uniformity and inhibited the precipitation of silicides. The plasticity of the joint was improved and the fracture mode changed to intergranular ductile fracture. The tensile strength and elongation at 750 °C of the joint bonded with BNi-5 / Ni / BNi-5 were 305.4 MPa and 17.87 %, corresponding to 80.0 % and 48.2 % of the respective properties of the base material.