Non-proportionally multiaxial cyclic deformation of AZ31 magnesium alloy: Experimental observations

The non-proportionally multiaxial cyclic deformation of extruded AZ31 magnesium alloy with a hexagonal close-packed (HCP) crystal structure is investigated at room temperature by performing the strain- and stress-controlled axial-torsional cyclic tests with different loading paths and using thin-walled tubular specimens. The cyclic hardening/softening and multiaxial ratchetting of the AZ31 alloy are discussed. The AZ31 alloy shows significant cyclic hardening in both axial and torsional stress-strain responses. An additional softening occurs in the axial response, while an additional hardening presents in the torsional one under the multiaxial (i.e., combined axial-torsional) cyclic loading conditions. The multiaxial ratchetting mainly occurs in the direction of non-zero mean stress, and the ratchetting strain greatly depends on the shapes of multiaxial loading paths. The traditional equivalent stress-strain responses cannot reasonably characterize the multiaxial cyclic ones of the AZ31 alloy, since different mechanisms of plastic deformation are involved in the axial and torsional deformations. The axial and torsional stress-strain responses should be discussed separately.