Twinning–detwinning behavior during the strain-controlled low-cycle fatigue testing of a wrought magnesium alloy, ZK60A

The twinning and detwinning behavior in a strongly textured magnesium alloy was investigated using in situ neutron diffraction during the cyclic deformation along the prior extrusion direction at the fully reversed total constant strain amplitude of 1.2% at room temperature. The initial preferred orientation places the c-axis in most grains perpendicular to the loading axis, and this favors extensive { 1 0 1 ¯ 2 } 〈 1 0 1 ¯ 1 〉 twinning under compressive loading. In contrast, the grains are not favorably oriented to undergo such twinning during monotonic tensile loading along the prior extrusion axis. This is the reason for the well-known tension–compression strength asymmetry of wrought magnesium alloys. The strength in compression is controlled by the stress required to activate twinning, while the strength in tension is controlled by the harder non-basal slip mechanisms. The unique orientation relationship between the parent grains and the twin grains favors detwinning during the subsequent loading reversal. In situ neutron-diffraction results indicate that such twinning and detwinning alternates with the cyclic loading, i.e. most of the twins formed during compression are removed when the load is reversed. However, a small volume fraction of residual twins gradually increases with increasing cycles, which may be an important factor in dictating the low-cycle fatigue behavior of the magnesium alloy.