The influence of microstructure on CTOD fracture toughness of DZ125 base metal and brazed joint

The microstructure of the base metal zone and the weld zone of the DZ125 directionally solidified nickel-based superalloy brazed joint is studied by means of scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and optical microscope. The CTOD fracture toughness of the base metal zone and the weld zone at 950 °C was tested respectively. The results show that the microstructure of the DZ125 alloy base metal grows with dendrites, the arrangement of the dendrites is close, the divergence angle is small, the dendrites are distributed with a uniform and fine γ' phase, script-shaped MC carbides and petal-shaped γ/γ' eutectic phases are distributed in the inter-dendrite zone. The uniform γ' phase is distributed in the weld zone. At 950 °C, the CTOD fracture toughness of base metal and brazed joint is 0.080 mm and 0.048 mm, respectively. The fracture toughness of the base metal is higher than that of the brazed joint. The fracture morphology shows that the base metal has good toughness at 950 °C, while the weld zone has poor toughness at this temperature. It can be inferred that the long needle-shaped M₆C carbides will weaken the structure performance.