Mechanisms of fatigue crack initiation and early propagation in cyclically strained nickel superalloy

In the present work, the mechanisms of fatigue crack initiation and early crack propagation of a γ′ strengthened nickel-based superalloy have been investigated. Low cycle fatigue tests have been performed on polycrystalline superalloy MAR-M247 under symmetrical strain controlled loading at ambient temperature in air. The nature of early fatigue damage, crack initiation and subsequent fatigue crack propagation was assessed for applied total strain range of 1%. Test was interrupted at given numbers of cycles corresponding to 1%, 2%, 5%, 10%, 20%, 40%, and 80% of fatigue life. The evolution of pronounced surface relief, also known as persistent slip markings (PSMs) was observed using atomic force microscopy (AFM) and high resolution scanning electron microscopy (SEM-FEG). The kinetics of extrusion growth was quantitatively measured. The regions of fast and slow extrusion growth were identified. Sharp surface PSMs arise in areas where lamellae of localized cyclic slip – persistent slip bands (PSBs) – intersect the original flat surface. Thin PSMs cut both γ matrix and γ′ precipitates. PSMs consist of extrusions accompanied by parallel intrusions. Fatigue cracks were initiated at individual PSMs. Focused ion beam (FIB) revealed fatigue crack nucleation from sharp intrusion and its subsequent propagation parallel to one of the active {111} 〈110〉 slip systems. Transmission electron microscopy (TEM) observation showed plastic strain localization into dislocation rich slabs running parallel to {111} slip planes.