In-situ synchrotron X-ray tomography investigation of damage mechanism of an extruded magnesium alloy in uniaxial low-cycle fatigue with ratchetting

This paper elucidates the relationship between the primary deformation processes and associated damage mechanisms operating in an extruded AZ31 Mg alloy under uniaxial low-cycle fatigue with an occurrence of ratchetting. Different mean stresses and stress amplitudes were prescribed to study twinning/detwinning-dominated (TDD), twinning/detwinning and slip-dominated (TDSD), and slip dominated (SD) cyclic deformations, respectively. In each case, the damage was examined by synchrotron radiation micro-computed tomography and scanning electron microscopy. The plastic deformation mechanism for each load case was then characterized by electron backscatter diffraction and transmission electron microscopy. Profuse twins were detected in the TDD and TDSD samples, while more complex structures and higher dislocation densities were found in the TDSD and SD samples where dislocation slip was plentiful. The TDSD sample had the most damage initiation sites owing to the transgranular damage caused by twins and the intergranular damage induced by the combined effect of < c + a > dislocations and twins, followed by the TDD sample because of the low dislocation density, while the SD sample had the fewest due to the lack of twins. Shear linkage between the isolated cracks was observed only in the TDSD sample. This provides rich evidence for the modelling of damage evolution of extruded Mg alloys. (c) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.