Comparison of in-phase and 90° out-of-phase multiaxial low-cycle fatigue behavior of a Cu-Ni alloy

Axial-torsion multiaxial low-cycle fatigue (LCF) experiments were conducted on a Cu-Ni alloy under strain-controlled in-phase (IP) and 90° out-of-phase (OP) loadings at room temperature to compare the cyclic hardening and softening behaviors of the alloy under different loading conditions. The cyclic stress responses (CSRs) exhibited multiple stages of hardening and softening. The OP loading resulted in higher cyclic hardening and softening, and a notable reduction in fatigue life, as compared to IP loading. Microstructural characterization using electron backscattered diffraction (EBSD) revealed that OP loading produced more uniformly distributed and higher kernel average misorientation (KAM) values, indicating a uniformly distributed higher dislocation density compared to IP loading, for the same applied axial and shear strain amplitudes. Schmid factor calculations revealed that a greater number of slip systems are activated in each grain under OP loading than under IP loading, leading to a greater and more uniform KAM. Internal stresses determined from stress-strain hysteresis loops reveal that the CSR under IP loading is characterized by friction stress hardening and back stress softening. In contrast, the CSR under OP loading involves both friction stress and back stress hardening during primary hardening and majorly friction stress softening for cyclic softening. The resulting dislocation substructure of IP loading exhibited clear cell formation with lower cell density and low-angle grain boundaries (LAGBs), in contrast to OP, which exhibits a higher density of cells of smaller size filled with dislocations and a higher fraction of LAGBs, rationalizing higher hardening and softening in the OP loading.