Effects of microstructural inhomogeneity on deformation behavior and fracture mechanism of cast K447A nickel-based superalloy at room and elevated temperatures

The microstructural inhomogeneity of the K447A nickel-based superalloy significantly affects its tensile properties and fracture behavior. The tensile properties and fracture mechanism of a K447A casting with variable-wall-thickness, which was prepared via vacuum investment casting, were systematically investigated at room temperature and an elevated temperature under different applied strain rates. This was done to reveal the relationship between microstructural inhomogeneity and mechanical properties. The results presented that the small columnar grains appeared in the thinner part of the casting and large equiaxed grains existed in the thicker part. Coarse equiaxed grains resulted in higher variability in tensile properties at different temperatures than small columnar grains. Tensile strength and ductility were negatively correlated with grain size, unfavorable grain orientation, and the content of large-angle grain boundary. As the test temperature increased to 760 °C, the tensile strength changed minimally, suggesting that the anti-phase boundary (APB) reinforcement mechanism in the L1 2 ordered phase was effective. The elongation rate increased significantly as the test temperature and strain rate increased. Moreover, the microstructural inhomogeneity also induced significant variations in fracture behavior and fractographic characteristics, primarily governed by the size and orientation of grains. Local stress concentration resulting from slip lines shearing through carbides was found to trigger brittle fracture of carbides and initiate internal cracks propagation along slip lines in the alloy.