The crack initiation and strengthening mechanism in FGH96 solid-state diffusion bonding joint by quasi-in-situ SEM-DIC and EBSD

The crack initiation and strengthening mechanism of the FGH96 solid-state diffusion bonding joint at room temperature and 50 °C were investigated based on the interaction between microscopic strain concentration and microstructure features. Digital image correlation using SEM images (SEM-DIC) and electron backscattering diffraction (EBSD) technologies were applied. The results showed that the carbides, twin boundaries (TBs) combined with the grain orientation, and the high-angle grain boundaries (HAGBs) played a critical role in the crack initiations. At room temperature, the cracks initiated at the carbide/matrix interface due to the strain mismatch. Besides, the newly formed low-angle grain boundaries (LAGBs) or HAGBs in the grain interior, which were attributed to the direct or indirect inhibition (by forming the stress-affected regions) effect on the slip movements from TBs combined with grain orientation, were developed into cracks with deformation increasing. Moreover, some grain junctions with twin grains participation were also the crack origins due to the strain compatibility and adhesive failure. The cracks at different positions propagated and coalesced to lead to the intragranular fracture. At 650 °C, the effects of carbides, TBs, and grain orientation on the strain accumulation were weakened and contributed little to the crack initiation. The microscopic strain mainly accumulated along HAGBs, leading to the cracks originating along the HAGBs and the intergranular fracture. Due to the different deformation mechanisms, the relative tensile strength and elongation of the joint reached more than 90% at room temperature, while 79.36% and merely 24.4%, respectively, at 650 °C.