Microstructure-sensitive investigation of magnesium alloy fatigue

This article presents results relating macroscopic fatigue behavior to microplasticity, twinning activity, and early fatigue crack formation in wrought magnesium alloy specimens of the AZ series. Experimental data were obtained by testing standard-sized samples prepared to be also suitable for direct microstructural quantification using scanning electron microscopy and electron back scatter diffraction for texture, grain-scale observations and fractography, as well as surface morphology measurements using white-light interferometry. In addition, in situ nondestructive monitoring of the fatigue behavior was performed by using the Acoustic Emission method. To describe the plastic anisotropy, tension-compression asymmetry, pseudoelasticity and their evolution as a function of fatigue loading, strain-control experiments of varying amplitude were conducted in several steps. Experimental measurements at different stages of fatigue life revealed repeatable occurrences of twinning-detwinning, which is further shown to be coupled with reversible surface roughening. It was also found that although tension twinning contributes considerably to overall plasticity, it could also give rise to crack initiation towards the end of the fatigue life. The role of the reported microplasticity effects was additionally explored using a Continuum Dislocation Dynamics Viscoplastic Self-Consistent (CDD-VPSC) model for the first two cycles of the fatigue life. The intention of this section was to incorporate the effect of twinning-detwinning into the CDD-VPSC model and subsequently to capture the experimental effects associated with changes in the fatigue hysteresis observed between first and second cycle. These simulation results were consistent with the hypothesis that detwinning is responsible for the anomalous hardening behavior during the tensile part of the cyclic loading in the first few cycles of loading. This observation was confirmed for several imposed strain amplitudes and was achieved by properly defining an appropriate boundary condition that allows surface morphology changes. Furthermore, the experimental test plan allowed the quantification of the fatigue life in terms of hysteresis loop parameters including plastic/elastic energy, residual stiffness, as well as mean and extreme stresses. Finally, an energy-based relationship for the evaluation of fatigue behavior based on the Ellyin-Kujawski formulation was found to provide life predictions that agree with obtained experimental information.