Transient cyclic stress-strain response and cumulative damage in Cu-16at.% A1 single crystals fatigued under variable straining

Abstract

In order to investigate the transient cyclic stress-strain response and cumulative damage behavior of Cu-16at.% A1 alloy, single crystals with a common single slip orientation were cyclically deformed under variable straining. Studies were made of hardening behavior, strain bursts, hysteresis loops, friction and back stresses, dislocation structures, damage accumulation and life behavior. During ascending step tests, hardening naturally occurred and the strain was found to be accommodated not by increasing the plastic strain of current active slip bands but by increasing the volume fraction of active slip bands. During descending step tests, however, the plastic strain per individual slip band decreases with decrease of strain amplitude. The transient response after a reduction of strain amplitude in Cu-16at.% Al single crystals is not softening as commonly occurs but hardening. Hardening in this case seems to be caused both by hardening of individual slip bands and by the decrease of the volume fraction of active slip bands, which is also an indication of hardening. With regard to damage behavior, in low-high step tests, the number of cracks was found to increase remarkably during high amplitude cycling whereas the number of cracks did not change much after the step to low amplitude cycling during a high-low test. Miner summations of fatigue life for H-L step tests were found to be smaller than unity because the stresses associated with the structure produced at high amplitude were high and the plastic strain localized in active slip bands increased with the decrease of the volume fraction of active slip bands, caused by continuous cycling at the reduced strain amplitude. In L-H step tests, the Miner summations for fatigue life were found to be greater than unity because the hardening and the damage accumulation associated with low amplitude cycling were relatively insignificant as compared to that at high amplitude and fresh damage developed during the high amplitude sequence. The Miner summation behavior in these alloy single crystals was found to be more typical of that in commercial metals, and different from behavior in copper.