Cyclic deformation behavior and microscopic mechanism of cold-worked Zr-4 alloy under biaxial loading

Biaxial fatigue behaviors of the Zr-4 alloy under proportional and non-proportional loading with the phase angles of 30°, 60° and 90° were studied. The cyclic Mises stress response curves as a function of the number of cycles consist of the first cyclic hardening and the following saturation stages at lower cyclic strain amplitudes under proportional and non-proportional loading. The initial cyclic hardening is followed by a continuous cyclic softening as cyclic strain amplitude increases. The striking cyclic softening is displayed at high cyclic strain amplitudes. The non-proportional cyclic additional hardening is displayed by the fact that the cyclic Mises stress curves under non-proportional loading lie above that under proportional loading at the same cyclic strain amplitudes. Zr-4 alloy exhibits the highest degree of additional hardening, when the phase angle is 30° and the equivalent strain equals to 0.769%. The biaxial fatigue life decreases as the cyclic strain amplitude increases under proportional and non-proportional loading. The fatigue life under non-proportional loading is less than that under proportional loading at the same equivalent strain amplitudes. Deformation substructure examination by TEM reveals that the dislocation structure tends to form dislocation cells as well as tangle configuration as the loading mode changes from proportional to non-proportional loading. The isotropic hardening mechanism plays an important role in inducing an additional hardening in the cold-worked Zr-4 alloy.