Low cycle fatigue behaviour in vacuum of a 316L-type austenitic stainless steel between 20 and 600 degrees C .2. dislocation structure evolution and correlation with cyclic behaviour

In this second part, we have reported the evolutions of the dislocation structures of an austenitic stainless steel cycled in vacuum for different temperatures between 20 and 600 degrees C. For each temperature. constant plastic strain amplitudes Delta epsilon(p)/2 ranging from 6 x 10(-4) to 5 x 10(-3) have been considered, The dislocation evolutions have been correlated to the cyclic behaviour described in Part I. The slip character appears to be more wavy at 20 and 600 degrees C than at intermediate temperatures, with dislocation structures composed, in the main, of cells and walls-and-channels. Between 200 and 500 degrees C, the number of cycles to failure is higher than at 20 and 600 degrees C and, at the same time, the dislocation arrangements are more planar from the first cycles, up to failure, but cells, walls-and-channels and tangles are less numerous. Instead, another structure appears, which we have called the corduroy structure, composed of alignments of very small defects such as loops, debris and cavities. This structure develops progressively with cycling and is all the more extended as cycling is performed at low strain amplitudes. The optimum temperature for its formation is 400 degrees C. It is shown that the extension of the curduroy structure can be directly correlated to the amount of secondary hardening observed in vacuum between 200 and 500 degrees C. In contrast, the degree of formation of corduroy structure is not directly correlated with fatigue-life evolution. Fatigue Lifetime results from the competition between a beneficial effect (planar slip) and a detrimental one (the cyclic stress level). The planar behaviour has been associated with dislocation interactions with C and N solute atoms, which also determine dynamic strain ageing at intermediate temperatures. (C) 1997 Elsevier Science S.A.