CYCLIC RESPONSE AND DISLOCATION-STRUCTURES OF AISI 316L STAINLESS-STEEL .1. SINGLE-CRYSTALS FATIGUED AT INTERMEDIATE STRAIN AMPLITUDE

Cyclic response of AISI 316L stainless steel has been studied at ambient temperature by using monocrystalline specimens. The cyclic stress-strain (CSS) curve, obtained by conducting step ascending tests, was observed to contain a plateau in the plastic strain range from epsilon(pl) = 5 x 10(-5) to epsilon(PL) = 2 x 10(-3), with the saturated plateau stress about 59 MPa. Cyclic softening was observed in both step ascending tests and constant plastic strain tests, and is attributed to the following three factors: dislocation starvation; interaction between dislocations and interstitial solute atoms; and transformation of dislocation structures. Dislocation structures in the steel, revealed by TEM, were found to consist mainly of planar edge dislocations arranged into tangles on the primary slip plane. It was found that both the distance between slip bands and the width of the dislocation tangles decrease with applied number of cycles. On prolonged cycling, the dislocation structure was found to transform from planar arrays into a structure similar to ''loop patches'' observed in pure copper. Despite the low stacking fault energy of the steel, cross slip was frequently observed and was considered to play an important role in the transformation of the dislocation structures.