Fatigue damage development in pure polycrystalline magnesium under cyclic tension–compression loading

Cyclic deformation and fatigue damage development of pure polycrystalline Mg were investigated by conducting fully reversed strain-controlled tension–compression experiments along the extrusion direction at two strain amplitudes of 1% and 0.12%, respectively, in ambient air. At a strain amplitude of 1%, the tensile peak stress exhibited significant cyclic softening while the compressive peak stress showed moderate cyclic hardening. At a strain amplitude of 0.12%, marginal cyclic hardening was observed symmetrically at the tensile and compressive peak stresses. To study the fatigue damage development, a series of experiments using companion specimens were interrupted at the specified loading cycles. The morphology of fatigue damage on the specimen surface was examined by high resolution scanning electron microscope (SEM). With a strain amplitude of 1%, grain boundary cracking and twin tip damage were detected right after the second loading cycle. During the initial ∼80% fatigue life, microcracks were incessantly initiated with limited propagation that was observed to inhabit on both the grain boundary and the twin boundary. Final fracture surface was characterized by densely distributed cleavage facets along twin lamella boundaries together with tearing/shearing ridges. At a strain amplitude of 0.12%, initiation of microcrack by grain boundary cracking occurred at the loading cycle between ∼50% and ∼70% fatigue life. Both intergranular and transgranular propagation modes were observed. The early-stage transgranular propagation was dominated by slip-induced cleavage cracking on both basal and non-basal slip planes. Crystal sugar-like structure and cleavage facets were found on the initiation site on the final fracture surface. Early-stage initiation and transgranular propagation modes were discussed on their dependences of cyclic loading magnitudes.