INVESTIGATING DRIVERS FOR FATIGUE FAILURE VIA COUPLED X-RAY EXPERIMENTS AND SIMULATIONS

Structural metals, which form the backbone of many industries, are prone to fail after many repeated loading cycles, known to accumulate damage within the material. This work combines synchrotron X-ray techniques and crystal plasticity simulations to uncover the extreme microstructural gradients within aerospace materials that drive fatigue damage.

STRUCTURE OF MATERIALS

128 ESRF

Fatigue crack initiation, a predominant failure mechanism in many structural materials, is currently mitigated by large-scale testing programs and conservative design practices. Ultimately, these costly and time-consuming practices are required because the deterministic failure criteria for materials at the microscale (where failure initiates) are not sufficiently understood, and while detailed modelling approaches have been employed, more work is necessary to validate the failure criteria. This requires coupling specifically designed multi-modal experiments with physics-based computational models to investigate the underpinning physics that drives fatigue crack initiation.

Nickel-based superalloys are known to form fatigue cracks near specific crystalline configurations, such as coherent twin boundaries. Load applied to a crystal (grain) will elastically stretch bonds between atoms and place the material in a state of stress. With sufficient load, the crystalline lattice will plastically deform, which, eventually, can localise and produce stress concentrations and, subsequently, initiate a crack. Phenomena such as crack initiation have recently been studied

via X-ray diffraction techniques, including high-energy X-ray diffraction microscopy (HEDM). Techniques such as diffraction contrast tomography (DCT) and HEDM provide grain- averaged statistics; however, their intragranular resolution is limited. Recent advances in X-ray diffraction experiments have allowed for the acquisition of intragranular misorientation and elastic strain via dark field X-ray microscopy (DFXM), which probes a set of lattice planes within a grain of interest (GOI) with high spatial and angular resolution.

A specimen was subjected to fatigue loading and in-situ HEDM characterisation (CHESS, beamline F2), which illuminated the grain morphology of a millimetre-sized sample (Figure 110a). A GOI was identified and extracted via focused ion beam milling for further investigation, including DCT on beamline ID06 (Figure 110b). Afterwards, DFXM scans on ID06 provided information on the intragranular misorientation and elastic strain within the GOI. To compare with the DFXM results, a crystal plasticity model, based on a fast Fourier transform solver (CP-FFT), was instantiated with the microstructure from the HEDM reconstruction. The microstructure

Fig. 110: a) Reconstructed HEDM microstructure of the low solvus, high

refractory (LSHR) alloy. b) Reconstructed DCT microstructure of the extracted specimen with

DFXM spatially registered within the GOI.