Fracture behaviour of the compact tension specimens of nickel-based single crystal superalloys at high temperatures

The fracture behaviour of compact tension (CT) specimens made of nickel-based single crystal superalloys DD3 has been studied by experiments at 760, 850 and 950 °C. Three different crack crystallographic orientations, (0 0 1)[1 0 0], (0 1 1)[1 0 0] and (1 1 1)[0 −1 1], of the specimens have been considered. The macro crack growth and micro characteristics of the fracture surfaces have been examined by optical microscope (OM) and scan electron microscope (SEM). The emphasis has been put on the crack growth path and fracture toughness as well as the micro characteristics of the fracture surface. The crystallographic plastic finite element method (FEM) has also been used to analyze the resolved shear stress distribution, normal stress and slip systems activated of the specimens, especially ahead of the cracks, in order to have a deep understanding with crack. The experimental and FEM results show that the crystallographic orientations of the CT specimens have a great influence on the crack growth path and fracture toughness of the specimens as well as the stress characteristics ahead of the cracks. The experiments show that at low temperature, e.g. 760 and 850 °C, the macroscopic crack growth path appears as zigzag wave, while at high temperature, e.g. 950 °C, all the fracture surfaces are flat along the crack direction except the crack initiation. FEM analysis shows that deformation and fracture ahead of the crack take place in specific slip planes in different crack orientations. The resolved shear stress of the slip system is found to be the main factor of specimens fracture. Also, the normal stress of slip planes has some influence on the crack growth. To 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 (0 0 1)[1 0 0], (0 1 1)[1 0 0] and (1 1 1)[0 −1 1]. The growth path of the crack observed in experiment is agreement with the theoretical analyses by FEM. Furthermore, temperature is the other 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.