Micro-mechanisms of Cyclic Plasticity at Stress Concentrations in a Ni-Based Single-Crystal Superalloy

Abstract

Ni-based single-crystal superalloys are high-temperature materials used for turbine blades in jet engines. Fatigue damage can pose a major threat to the integrity of such components in operation. Traditionally, the fatigue behaviour of superalloys has been studied by investigating cyclic plasticity in the bulk of the material. When the cyclic loads are nominally elastic, however, such investigation may not contribute to the understanding of the alloy’s fatigue behaviour, since plastic micro-strains are confined to regions near stress raisers such as microstructural defects and are therefore randomly distributed. In turn, the plastic micro-strains near the ‘critical’ stress raiser, i.e. the one that acts as nucleation site for the dominant crack, govern fatigue life by inducing early crack initiation, and are therefore the key to capture the material’s fatigue behaviour. Hence, this work is concerned with the experimental characterisation of cyclic plasticity at the initiation site in a Ni-based single-crystal superalloy at 800 °C tested with nominally elastic cyclic loads. Such investigation was carried out by focused ion beam (FIB) lift-outs and subsequent transmission electron microscopy (TEM) studies. It is shown that deformation is significantly more pronounced near the ‘critical’ stress concentration; in addition, deformation is rather homogeneous across large regions surrounding the stress raiser, with remarkably different deformation modes compared to those observed in the bulk of the specimens and from those expected in superalloys tested in similar conditions. The investigation of local cyclic plasticity at stress concentrations promises therefore to provide new insight into fatigue crack initiation in Ni-based superalloys.