CYCLIC RESPONSE AND DISLOCATION-STRUCTURES OF AISI 316L STAINLESS-STEEL .2. POLYCRYSTALS FATIGUED AT INTERMEDIATE STRAIN AMPLITUDE

Polycrystalline AISI 316L stainless steel has been cycled at intermediate strain amplitudes. Cyclic response was analyzed in terms of the cyclic stress-strain curve (CSSC), softening behavior, the friction stress and the back stress. Dislocation structures were determined by transmission electron microscopy (TEM). It was found that cyclic softening behavior occurred and this has a significant influence on the dislocation structures by stimulating the activity of cross slip. The cross slip system becomes the second most important slip system, and causes the development of dislocation wall structure. Compared with the cyclic response of 316L single crystals, which are planar in slip character, polycrystals show a tendency towards wavy slip, despite the low stacking fault energy and high friction stress. It is concluded that grain boundaries, together with dislocation ''starvation'', have a major influence on the fatigue behavior of polycrystals by increasing the tendency to multi-slip, decreasing the capacity of strain transfer from grain to grain and from band to band, and changing the limit of stress or strain sustainable by a given dislocation structure in the sequence that occurs.