Interface microstructural evolution, microhardness variation mechanism and 750 °C tensile behavior of the HIPed/FGed PM superalloy plastic bonded joints

In present research, a novel 3rd-generation powder metallurgy (PM) superalloys with different initial microstructures (hot isostatic pressed (HIPed) and fine grained (FGed)) were selected for plastic bonding (PB). Non-uniform deformation of the HIPed/FGed PB joint can be obtained due to the significant difference in flow behavior of the studied PM superalloys. Microstructure observation reveals that interface recrystallization dominates the successful healing of bonding interface. The lower dynamic recrystallization (DRX) degree accompanied with higher dislocation density can be detected in HIPed side, manifests that incomplete grain refinement would reduce the efficiency of interface healing and induce the generation of strain gradient at bonding interface. In addition, the microhardness variation of HIPed/FGed PB joint is fully dependent on the interface microstructural evolution. The average microhardness at the bonding interface about 501.0 HV is greatly associated with the neutralization effect of strain hardening from HIPed side and fine grain strengthening from FGed side. Special attention was focused on the fracture behavior and underlying mechanism of the HIPed/FGed PB joint in 750 °C tensile test. Owing to the grain size difference and huge strain gradient, the dislocations pile-up caused by localized stress concentration at bonding interface should be responsible for the brittle fracture of the HIPed/FGed PB joint. The whole process from damage initiation to final fracture includes the microvoids nucleation at grain boundaries, microvoids grow and coalesce into the microcracks, the extending microcracks aggregate to form main crack and then the brittle fracture occurs.