Effect of strain amplitude on low-cycle fatigue behavior of an alpha Cu-Zn alloy

Low-cycle fatigue (LCF) tests were performed at ambient temperature on annealed single-phase Cu–30 wt% Zn
(70/30 brass) at strain amplitudes of 0.4 %, 0.7 %, 1.0 %, and 1.2 % to investigate the effect of strain amplitude
on the cyclic stress response. The cyclic stress response exhibits primary hardening, softening, and secondary
hardening. The primary hardening and softening increases, while the secondary hardening decreases with
increasing strain amplitude. Internal stresses determined from stress-strain hysteresis loops show that back-stress
hardening and softening dominate the primary hardening and softening, whereas friction stress is almost constant, leading to Masing behavior. Electron backscatter diffraction (EBSD) analysis revealed that Kernel Average
Misorientation (KAM) and low-angle grain boundary (LAGB) fraction increase with increasing strain amplitude,
indicating an increase in geometrically necessary dislocation (GND) density and recovery of dislocation substructure with increasing strain amplitude. The recovered substructure determined by softening of back stress
reveals the formation of planar slip bands, and the slip band intersection increases with increasing strain
amplitude, indicating that Cu-Zn 70/30 is a purely planar slip alloy. During secondary hardening, friction stress
increases, and unrecovered corduroy substructure forms. The back stress components were determined based on
a modified Chaboche model for different strain amplitudes to calculate the intergranular and intragranular back
stresses evolution with increasing cycles and increasing strain amplitudes, and correlated with microstructural
evolution. There is competition between the evolution of intergranular and intragranular back stress due to
heterogeneous dislocation distributions during cyclic softening