Cyclic deformation and fatigue of extruded AZ31B magnesium alloy under different strain ratios

Abstract

Cyclic deformation and fatigue behavior of extruded AZ31B magnesium (Mg) alloy were investigated under strain-controlled loading along the extrusion direction at different strain amplitudes with three strain ratios (Rε=0, −1, −∞). With a strain ratio of Rε=−∞, partial twinning-partial detwinning occurs at all strain amplitudes, leading to accumulation of deformation twins as the loading cycle is increased. With strain ratios being Rε=0 and −1, partial twinning–complete detwinning is the cyclic deformation mechanism when the strain amplitude is higher than 0.35%. At a strain amplitude lower than 0.35%, dominant cyclic deformation mechanism is dislocation slips irrespective of the strain ratio. Compared with the cases of Rε=0 and Rε=-1, stronger cyclic hardening is exhibited at all the investigated strain amplitudes for Rε=−∞. Fatigue fracture surfaces show regions with lamellar-like and dimple-like features. Lamellar-like feature existing in the crack initiation and stable propagation region are mainly due to twinning–detwinning during cyclic deformation. The dimple-like feature formed in the unstable crack propagation and final rupture region mainly arises from severe intra-granular plastic deformation by dislocation slips. At the same strain amplitude, the lamellar-like traces become more aggravated with a lower stain ratio. The strain–fatigue life curves exhibit a distinguishable kink at strain ratios Rε=−∞, −1 and 0. The fatigue life with Rε=−∞ shows the highest and that with Rε=0is the lowest at the same strain amplitude. The Smith–Watson–Topper (SWT) fatigue criteria can correlate well the fatigue experiments of AZ31B Mg alloy under different strain ratios.