Cyclic deformation and fatigue behaviour of 7Mo-0.5N superaustenitic stainless steel-slip characteristics and development of dislocation structures

The present work concerns the development of dislocation structures and sur face slip markings during cyclic straining of a superaustenitic stainless s teel. The composition of the tested material was Fe-25Cr-22Ni-7.6Mo-3Mn-0.4 6N (wt%). Two total strain amplitudes, 2.7x10(-3) and 1.0x10(-2), were empl oyed and specimens were investigated at specific numbers of cycles correspo nding to certain stages on the cyclic hardening/softening curve. For both s train amplitudes, the developed dislocation structures are strongly planar and with increasing strain amplitude, the slip mode gradually changes from single slip to multiple slip. The short range ordering between Mo and N, as indicated by an atom probe investigation, is broken down during strain cyc ling leading to increased slip planarity. Early stages of cycling show disl ocation multiplication. With increasing number of cycles, the dislocations are gradually grouped together in planar bands with high dislocation densit y, surrounded by dislocation-poor areas. The evolution of such bands is ass ociated with decreasing effective stresses, while the internal stresses are only slightly reduced. Macroscopic slip bands, similar to PSBs, are formed upon prolonged cycling at the high amplitude. The slip markings created on the specimen surface show strong similarities with the bands of localised slip observed in the dislocation structures of the bulk. (C) 2001 Acta Mate rialia Inc. Published by Elsevier Science Ltd. All rights reserved.