Effect of initial twins on the stress-controlled fatigue behavior of rolled magnesium alloy

The effect of initial twins on the fatigue behavior of rolled AZ31 alloy was investigated by introducing {10-12} twins into the alloy through pre-deformation and then conducting fully reversed stress-controlled fatigue tests along the rolling direction (RD) and the direction normal to the rolling plane (ND). It was found that the fatigue life of the alloy is higher under RD loading than under ND loading, but when initial twins exist, the fatigue resistance along the RD decreases, whereas the ND fatigue resistance increases, eventually reversing the fatigue resistance behavior under the ND and RD loading conditions. The tensile mean strain generated under ND loading is also changed to compressive strain by initial twins, whereas the reverse is true under RD loading. The fatigue strength of samples with and without twins is inversely proportional to the logarithm of their plastic strain energy density, which corresponds to the inner area of the hysteresis loop at half-life, irrespective of the loading direction.