Anelastic behavior of Mg-Al and Mg-Zn solid solutions

The anelastic strain measured using tensile loading-unloading loops in pure Mg and in binary solid solutions of Mg-Al and Mg-Zn, with contents of 0.5, 2 and 9 at% Al and 0.8 and 2.3 at% Zn, was correlated with available data for the area fractions and number densities of twins, for applied strains of up to 3%. For pure Mg, no anelastic strain was observed up to about 4% area fraction of twins and at an applied plastic strain of similar to 1.7 x 10(-4), after which it increased rapidly, levelling off at a plastic strain of similar to 1%. The alloys followed the same pattern, but from much smaller minimum area fractions of twinning, < 1% for Mg-Zn, and similar to 2% for Mg-Al, at applied plastic strains of similar to 2.5 x 10(-4) and similar to 3.1 x 10(-4), respectively. The anelastic strain saturated at a maximum value of similar to 0.002 for all alloys, save for the 9%Al for which it reached a much larger level (similar to 0.004). The correlation with the number density of twins followed similar patterns. For a given alloy, the magnitude of the anelastic effect can be related to the applied stress in excess of that required to initiate microplasticity. The results are discussed in terms of the solid solution hardening and softening effects upon basal and prism slip in the dilute alloys, and of short range order upon slip and twinning in concentrated Mg-Zn.