Influence of cyclic stress amplitude on mechanisms of deformation of a high nitrogen austenitic stainless steel

The tension-tension fatigue behavior of Fe-18Cr-18Mn-0.63N high nitrogen austenitic stainless steel was investigated focusing on the effect of the imposed stress amplitude on the deformation and damage characteristics. It was found that the S-N curve of the fatigue life vs. the stress amplitude meets a specific bilinear Basquin relationship. With the increase in stress amplitude, the distinctive damage features on the specimen surface change from slip cracking into intergranular cracking. The internal microstructure is mainly composed of planar single-slip dislocation arrangements at low stress amplitudes. In contrast, at high stress amplitudes, the microstructures primarily comprise planar multiple-slip dislocation structures, deformation twins, twin-like bands together with a few wavy slip dislocation arrangements. Such a variation in deformation micromechanisms with the applied stress amplitude is believed to be the major reason for the occurrence of a specific bilinear Basquin relationship in the S-N curve.