Fatigue of ZK60 magnesium alloy under uniaxial loading

Abstract

Uniaxial fatigue experiments were conducted on ZK60 magnesium (Mg) alloy in ambient air. They include fully reversed strain-controlled experiments with strain amplitudes ranging from 0.24% to 6.0%, strain-controlled experiments with a large compressive minimum strain of −9.4%, and stress-controlled experiments. For fully reversed strain-controlled experiments, fatigue process consists of crack initiation, small crack growth, and final failure in tension when the strain amplitude is lower than 3.0%. Final failure occurs under compression when the strain amplitude is larger than or equal to 3.5%, where the fracture surface shows characteristics identical to these observed from monotonic compression. A kink point corresponding to a strain amplitude of 0.35% is observed in the strain-life curve from the fully reversed strain-controlled fatigue experiments. This strain amplitude demarcates the influence of twinning–detwinning deformation on fatigue of the material. Asymmetric stress–strain hysteresis loops with a tensile mean stress are observed when the strain amplitude is larger than 0.35%. When the material is pre-compressed to −9.4%, the initial texture is changed from a strong basal texture to a strong (0001) or c-texture. The subsequent strain-controlled fatigue loading results in compressive mean stresses. The fatigue lives are similar to those of the basal textured state with identical strain amplitude for strain amplitudes larger than 0.4%. The Smith, Watson, and Topper fatigue parameter does not correlate the fatigue experiments with large compressive mean strains.