Influence of orientation and temperature on the fatigue crack growth of a nickel-based directionally solidified superalloy

Fatigue crack growth (FCG) behaviors of a widely used nickel-based directionally solidified (DS) superalloy have been investigated. Standard compact tension (CT) specimens in longitudinal, transverse and diagonal directions are cast and tested at 25 °C, 600 °C and 850 °C to reveal the orientation and temperature dependence. The post-test fractography is observed through scanning electron microscope (SEM) and optical microscope (OM) to understand the underlying mechanism responsible for the fracture modes. Results indicate that cracks in all three orientations exhibit a similar propagating behavior, while the temperature shows a significant effect on the crack propagation regardless of the influence of orientation. It has been found that a higher temperature leads to a faster propagation rate in the initial stage due to the cyclic softening response of materials. However, the FCG rates of specimens at lower temperature speed up more rapidly and exceed those at higher temperature in the following stage. This is attributed to the crack closure effect induced by the oxidation at a much higher temperature. Therefore, a new model based on thermal activation is proposed to get a better ability for the FCG rate prediction of the DS superalloy under different temperatures.