Dependence of the distribution of deformation twins on strain amplitudes in an extruded magnesium alloy after cyclic deformation

Cyclic deformation characteristics and the distribution of deformation twins in a recently developed magnesium alloy AM30 after strain-controlled low cycle fatigue tests were studied in the loading direction along the extrusion direction. The alloy was basically cyclically stable at a lower strain amplitude of 0.2% with low stress amplitude and small plastic strain amplitude, while it exhibited strong cyclic hardening at a higher strain amplitude of 0.6%. In addition to the initially high stress amplitude and large plastic strain amplitude, the stress amplitude increased and plastic strain amplitude decreased significantly as cyclic deformation proceeded at the higher strain amplitude. Twinning in compression and detwinning in tension occurred at both strain amplitudes, and the distribution of the residual twins observed near the fracture surface was a consequence of competition between twinning and detwinning, depending on the strain amplitude, cyclic hardening-softening characteristics, and the plastic zone size ahead of the crack tip. In the stable cyclic deformation at the lower strain amplitude, deformation twins appeared mostly in the area prior to the final rapid fracture, while at the higher strain amplitude with cyclic hardening the extensive and well-developed twins were mainly observed in the crack initiation area.