Low-cycle biaxial fatigue of a polycrystalline magnesium-lithium alloy

We analyze the life duration of a magnesium-lithium alloy subjected to biaxial low-cycle fatigue under out-of-phase combined tension-compression and torsion. It was discovered that the life duration depends on the angle of the phase shift phi between the two perpendicular cyclic loads. The maximum life duration is attained in the case where the phase shift is absent, i.e., phi = 0. It was also shown that deformation always remains plastic if phi exceeds a critical angle phi(c), i.e., no unloading of the specimen may occur. Fatigue properties are studied using the von Mises equivalent stresses and strains. We also present data on the evolution of the density of deformation twinning. It is assumed that the decrease in life duration is explained by permanent changes in the direction of loading in the presence of a phase shift, which inhibits the process of stabilization of plastic deformation in the constituent grains of polycrystals.