PLANAR SLIP AND DEFORMATION-INDUCED STRAIN INSTABILITIES IN CYCLICALLY DEFORMED AL-LI SINGLE-CRYSTALS

The cyclic deformation behavior of Al-8.5at.%Li single crystals has been investigated at room temperature (RT) in plastic strain control (Repsilon= - 1) at 0.3 Hz. Additionally, tests have been carried out under stress control (Rsigma= - 1) at 1 and 100 Hz. In specimens stored at RT for about 100 h as well as in underaged specimens pronounced strain instabilities, i.e. strain bursts, have been found at frequencies below 1 Hz under stress and strain controlled conditions, where the measured plastic strain amplitude gamma(b) was about 10 times higher than that desired. For overaged specimens, however, no strain bursts could be observed. It is shown that macroscopic strain bursts result from highly mobile dislocations which propagate on isolated slip planes. In all cases the intensity of strain burst is related to the degree of slip planarity. For wavy slip, in particular, where local cross-slip is possible, no macroscopic strain bursts have been found.