Cavity self-healing mechanism at the interface of high-Cr ferritic steel/austenitic steel dissimilar diffusion-bonded joint during cyclic phase transformation treatment

Abstract

In this work, cyclic phase transformation treatment (CPTT) was developed to achieve self-healing of interfacial voids in high-Cr ferritic steel/austenitic steel dissimilar diffusion-bonded joints. The evolution of voids was analyzed based on microstructural characteristics, and mechanical properties of joints were assessed through lap-shear tensile tests. The results indicate that, in contrast to isothermal heat treatment (IHT), CPTT significantly enhances efficiency of cavity healing, leading to substantial improvements in both interface bonded ratio and shear performance of joints. By considering equivalent interfacial internal stress, a kinetic model for cavity healing was proposed, incorporating coupled the interface and surface diffusion, and the power-law creep mechanism. Simulation results demonstrate that diffusion predominates during cavity healing with negligible contribution of plastic flow. The actual cavity healing can be divided into two stages: in initial stage the large penny-shaped cavities become shorter in length with negligible change of height, while in the final stage, nearly circular voids shrinkage with a significant decrease of void size due to the enhanced effect of local surface diffusion. Moreover, it suggests that tensile internal stresses can impede healing or even promote residual void growth. Conversely, normal compressive internal stresses within cavity healing zone induced by the cyclic α↔γ phase transformation during CPTT intensify chemical gradients around void neck. This promotes accelerated atomic diffusion adjacent to void neck region, thereby resulting in a notable reduction in the duration required for complete cavity healing.