Biaxial low cycle fatigue properties and dislocation substructures of zircaloy-4 under in-phase and out-of-phase loading

Biaxial fatigue tests were performed for Zircaloy-4 (Zr-4) under in-phase (IP) and out-of-phase (OP) loading. Effects of the principal strain ratio (ε₃/ε₁), phase angle (Φ) and the equivalent strain range (Δε_eq) on the biaxial cyclic hardening, fatigue life and deformation substructure were examined. The cyclic stress-strain curves of Zr-4 fatigued under IP and OP cyclic loading were shifted to a higher stress level than those in monotonic deformation. Cyclic hardening was more accelerated and fatigue life was shorter under OP loading than under IP loading. As the Φ and Δε_eq increased, the fatigue life decreased. Parallel dislocation bands were frequently observed under IP loading but dislocation tangles and embryonic dislocation cells were formed with increasing Δε_eq. Under OP loading the dislocation configuration changed from parallel dislocation walls to well-developed dislocation tangles as the Φ and Δε_eq increased. The mechanisms of additional cyclic hardening and fatigue life drop under OP loading are discussed.