Secondary cyclic hardening and dislocation structures in type 316 stainless steel at 600°C

Abstract

Polycrystalline type 316 stainless steel has been fatigued in air and in vacuum at 600°C at two constant plastic strain amplitudes. Although in aire the curve of cyclic stress versus number of cycles is prematurely interrupted by failure, in vacuum it is sufficiently extended to allow secondary cyclic hardening to occur. Transmission electron microscopy observations show dislocation configurations and evolutions of configurations (during the fatigue life) which are similar for both strain amplitudes. During the saturation plateau, several configurations coexist: regular walls, a labyrinth structure and persistent slip bands (PSBs) with a ladder structure. At failure (in vacuum) a cellular structure widely prevails while a labyrinth structure and PSBs with a ladder structure no longer exist. We propose different mechanisms to explain the evolution of the dislocation configurations: the formation of a labyrinth structure, the change of walls into cells and the change of PSBs into cells. We also explain the secondary cyclic hardening by the evolution of microstructure.