Experimental investigation and simulation on stress rupture behavior of a Ni-based DS superalloy affected by initial elastic-plastic multi-axial stress state

In this study, the stress rupture behavior of a Ni-based directionally solidified (DS) superalloy affected by initial elastic-plastic multiaxial stress state at 850 °C has been investigated. Smooth and three types of U-shape notched plate specimens have been adopted. The notch effect on stress rupture lives is exhibited as notch softening, and there is a positive correlation between the degree of notch softening and theoretical stress concentration factor. Scanning electron microscopy (SEM) observation is conducted to reveal failure mechanism. For both smooth and notched specimens, thickness dependence on crack initiation due to the changed crack tip stress state is exhibited. The initiation of microcracks is mainly caused by interfacial decohesion at the carbide/matrix interface, non-uniform deformation field at eutectic/matrix interface and stress concentration around cavities. The plastic fiber zone of smooth plate is located in the intermediate region, which is occurred throughout a wide region near notch root in notched specimens. Finally, a finite element analysis (FEA) is performed based on the transverses elastic viscoplastic constitutive model with the Kachanov-Rabotnov damage rule. The simulation results show that the effect of notch softening is due to the fact that the redistributed tensile stress is higher than the value of net section stress in a very wide region.
Source: Materials Science and Engineering: A - Category: Materials Science Source Type: research