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 (Δεeq) 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 Δεeq 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 Δεeq. Under OP loading the dislocation configuration changed from parallel dislocation walls to well-developed dislocation tangles as the Φ and Δεeq increased. The mechanisms of additional cyclic hardening and fatigue life drop under OP loading are discussed.