Crack propagation paths and fracture of Ni-based single crystal

The fracture behaviour of compact tension (CT) specimens of nickel-based single crystal superalloys DD3 has been studied at 760°C, 850°C and 950°C and three different crack crystallographic orientations of (001)[100], (011)[100] and (111)[0-11]. The macro crack growth paths, the fracture toughness and the micro fracture morphologies have been examined by optical microscope (OM) and scan electron microscope (SEM). A crystallographic plastic finite element method (FEM) has also been used to analyze the resolved shear stress distribution ahead of the cracks. The experimental and FEM results show that the crystallographic orientations have much influence on the crack growth path, the fracture toughness of specimen, and the stress characterization ahead of the cracks. At low temperature, e.g. 760°C or 850°C, the macroscopic crack growth path appears on the zigzag wave, but all the fracture surfaces are flat along the crack direction at high temperature, e.g., 950°C. FEM analysis shows that the deformation and fracture ahead of the crack take place in the specific slip planes for different crack orientations. For the three specimens tested at 760°C, the deflected angles of crack initiation direction with respect to crack plane are 45°, 53.7° and 90° in the order of crack orientations of (001)[100], (011)[100] and (111)[0-11], respectively. The growth path of crack is agreement with the theoretical analyses by FEM. Furthermore, the temperature is another factor affecting the fracture of specimens. It is clear that the fracture mode of the CT specimens transfers from brittle to ductile with the temperature increasing.