Macroscopic response and microscopic substructure have been studied for the cold-worked and stress-released Zr-4 alloy under
biaxial
out-of-phase loading with phase angles of 30°, 60°, 90° and equivalent strain amplitudes of 0.4%, 0.6%, 0.8%. The results show that the delay angle between stress deviation and strain increment vectors firstly exhibits a large variation range, and then drops to saturation as the plastic deformation processes. The variation range of delay angle depends on the curvature of loading path, and the variations have the minimum and maximum values at 90° and 30° phase angles, respectively. The average value of equivalent stress increases, however, its variation range decreases and reaches stability as the phase angle and equivalent strain amplitude increase. Zr-4 alloy displays an initial hardening followed by cyclic softening under out-of-phase loading. The Mises equivalent stress response curve of the alloy under out-of-phase loading lies above that under both uniaxial and in-phase loading. The alloy exhibits cyclic
additional hardening under out-of-phase loading. TEM shows that the typical dislocation configuration changes from individual lines to tangles and embryonic cells as the phase angle and the equivalent strain amplitude increase. The isotropic hardening mechanism plays an important role in producing cyclic additional hardening.