Characterisation of strain localisation processes during fatigue crack initiation and early crack propagation by SEM-DIC in an advanced disc alloy

Abstract

Fatigue failure processes in metallic materials are closely related to the evolution of strain localisation under cyclic loading. Characterisation of this strain localisation is important in understanding the mechanisms of fatigue crack initiation and propagation, and provides critical validation data to develop appropriate crystal plasticity models for prediction of these processes. In this study, strain localisation during fatigue crack initiation and early crack propagation in an advanced Ni-based superalloy for turbine disc application has been characterised at the grain level with a sub-micron resolution by digital image correlation on SEM images using secondary γ' themselves as the speckle pattern. The obtained full-field strains have been analysed in global coordinates associated with the applied loading direction and in terms of the local coordinates associated with individual slip bands. Deformation arising from in-plane and out-of-plane dislocation slip can be identified by a combination of shear strain ɛ xy and transverse strain ɛ yy in the local slip band coordinates in combination with EBSD analysis. Cracks preferentially initiate from slip/strain bands adjacent and parallel to twin boundaries and then propagate along the slip/strain bands, leading to the onset of significant transverse strain ɛ yy in the local band coordinates as a consequence of crack opening. Crack propagation is closely related to strain accumulation at the crack tip which is determined by the grain orientation and grain size. Transverse strain ɛ yy in local slip band coordinates together with the inclination angle between dislocation slip direction on an activated {111} plane and the slip trace of this {111} plane at the specimen surface is proposed to be a cracking indicator/fracture criterion.

Role of oxygen in enhanced fatigue cracking in a PM Ni-based superalloy: Stress assisted grain boundary oxidation or dynamic embrittlment?

“…The maximum applied strain was of ~ 0.9% based on an FE elastic-plastic simulation. The residual strain distribution in this preloaded CG LSHR alloy at room temperature and for 30000 cycles was measured by using the SEM-DIC technique, which is detailed in our recent publication [52]. After pre-loading at room temperature, this specimen was re-polished and then oxidised at 725 o C for 24 hours, followed by SEM-EDS analyses.…”

“…9 (b). Details of the strain measurement procedure using SEM-DIC can be found in our recent publication [52]. As shown in Fig.…”

Measurements of plastic localization by heaviside-digital image correlation

“…While there is still much progress to be made for better electron beam control to minimize beam drift and defects, these phenomena have been well characterized and methods for their correction exist [6,30,31]. These developments give insight to strain localization induced by physical slip bands during plastic deformation of metallic materials [6,27,30,32]. Moreover, investigated regions are large enough to be representative of the microstructure of the material.…”

Effect of pre‐corrosion on damage evolution and crack propagation in aluminum alloy 7050‐T7651

“…Among the different types of experimental methods, digital image correlation (DIC) is an effective method for obtaining the displacement and strain fields on the surface of materials, with the advantages of providing full‐field, real‐time, non‐contact measurements as well as flexibility . This optical technique has been extensively applied to various materials, such as metals, composites, and rock . DIC was utilized to study the growth of multiple intergranular cracks as a colony on an Alloy 600 in a tetrathionate solution .…”