A study of the mechanisms of cyclic deformation in f.c.c. metals using strain rate change tests

Plastic strain rate change tests were performed during low cycle fatigue (LCF) of 7075-T6 aluminum and Type 304 stainless steel using plastic strain as the control variable. The evolution of dislocation interactions was observed by evaluating the activation area and true stress as a function of cumulative plastic strain. Activation areas for 7075-T6 aluminum at each of three plastic strain amplitudes, 0.2, 0.4, and 0.6%, have initial values of approximately 250–450 b2 which decrease to 70–115 b2, respectively, during cyclic loading to saturation. Activation areas for 304 stainless steel at both amplitudes tested, 0.4 and 0.6%, exhibit initial values of 90 b2 which increase slightly to 130 b2 at large cumulative strains. Both materials show a deviation from the Cottrell–Stokes law during cyclic hardening and softening. Tests performed at saturation reveal a mild dependence of activation area on plastic strain amplitude for aluminum but no such relationship for stainless steel. These results reflect a contrast between wavy slip for pure copper and 7075 aluminum versus planar slip for 304 stainless steel tested at room temperature. In addition, the Cottrell–Stokes law holds in both alloys at saturation. Dislocation motion in 7075 aluminum and 304 stainless steel is controlled by obstacles that are characteristically more thermal than forest dislocations; obstacles in 7075-T6 aluminum are identified as solutes from re-desolved particles; for 304 stainless steel, the obstacles are also in the form of solutes.