Cyclic deformation behavior and microstructures of hydrided ZIRCALOY-4 under biaxial loading

Biaxial fatigue behavior of hydrided ZIRCALOY-4 (Zr-4) in the as-cold-worke d (CW) and recrystallized (RZ) conditions under in-phase (IP) and out-of-ph ase (OP) cyclic loading was investigated. The CW Zr-4 showed cyclic softeni ng followed by a saturation stage during biaxial cyclic loading. Additional cyclic softening was displayed in CW Zr-4 under OP loading with the phase lag of 30 and 60 deg. The additional softening level decreased as the phase lag increased. On the other hand, RZ Zr-4 showed cyclic hardening followed by a saturation stage, and additional cyclic hardening was obtained under OP loading. The additional hardening arose as the phase lag increased. Obse rvation of the fracture surface showed that the biaxial fatigue failure of the CW Zr-4 under OP loading was controlled by crack initiation and propaga tion through the hydrides, while the nucleation and coalescence of microvoi ds were dominant in the failure of CW Zr-4 under IP loading and RZ Zr-4 und er both IP and OP loading. The typical deformation substructure in CW Zr-4 specimens was composed of dislocation tangles together with parallel disloc ation lines under IP and OP loading. Whereas the parallel dislocation lines were formed by prismatic slip for RZ Zr-4 under IP loading and OP loading with the lower phase lag, they developed into dislocation networks, loops, and debris as the phase lag increased under OP loading. The additional cycl ic softening for CW Zr-4 was due to the relief of the anisotropic hardening mechanisms when the loading mode changed from IP to OP. The additional cyc lic hardening of RZ Zr-4 under OP loading is attributed to an increase in t he interaction between the primary dislocations and other dislocations from different slip systems.