An experimental study on cyclic deformation and fatigue of extruded ZK60 magnesium alloy

Cyclic deformation and low-cycle fatigue properties of extruded ZK60 magnesium alloy were experimentally investigated by carrying out fully reversed strain-controlled uniaxial tension–compression fatigue experiments along the extrusion direction. The material shows significant yielding asymmetry under monotonic tension and compression. Marginal cyclic hardening is manifested at all the strain amplitudes tested. The shape of the stress–strain hysteresis loop and the mean stress strongly depend on the strain amplitude. When the strain amplitude is higher than 0.52%, asymmetric shape of the stress–strain hysteresis loop and significant tensile mean stress develop. With intermediate strain amplitudes (0.45–0.52%), the stress–strain hysteresis loop evolves from an asymmetric shape at the initial loading cycles to a symmetric shape after approximately 10% fatigue life, accompanied by moderate saturated mean tensile stresses. Symmetric stress–strain hysteresis loop with zero mean stress is observed for the strain amplitudes lower than 0.45%. It is suggested that the twinning–detwinning process plays a dominant role for the deformation at high strain amplitudes while dislocation slip dominates at the low strain amplitudes. The strain–life fatigue curve shows a kinking plateau in the vicinity of 0.52% strain amplitude. Different crack initiation mechanisms are proposed in the two strain amplitude ranges divided by the plateau in the strain–life curve. The material displays a tensile cracking growth behavior and the Smith, Watson, and Topper (SWT) fatigue parameter is appropriate for the description of the fatigue life. The residual twins after fatigue were examined by light microscopy and the fracture surfaces were observed by scanning electron microscopy (SEM). Distinctive features were observed depending strongly on the strain amplitudes. The observed microscopic features are discussed with respect to possible crack initiation and propagation mechanisms.