Co-existence of γ′N phase and γN phase on nitrided austenitic Fe-Cr-Ni alloys- I. experiment

The formation of a metastable and nitrogen-supersaturated f.c.c. interstitial solid solution layer on Fe-Cr-Ni austenitic stainless steel at a moderate temperature around 650-720 K is not entirely understood. In the present work, three groups of austenitic Fe-Cr-Ni alloys, containing systematic variations of chromium, nickel and iron contents were nitrided by plasma-based low-energy ion implantation at 653 K for 4 h and investigated with light-optical microscopy (LOM), electron probe microanalysis (EPMA), (grazing incidence) X-ray diffraction (XRD) and transmission electron microscopy (TEM). Commercial AISI 304L was included for comparison. For the austenitic alloys with a Cr content below 12 wt.%, a duplex layer is observed in the nitrided case where the interface between the top layer, consisting of gamma-Fe4N like ordered gamma'(N), and the disordered gamma(N) (nitrogen enriched austenite) zone underneath is associated with a decrease in N content. For the alloys with a Cr-content over 12 wt.%, a featureless continuous zone is observed with LOM and a gradual decrease in nitrogen content is measured with EPMA. Nevertheless, a similar duplex structure of outer gamma'-Fe4N like ordered gamma'(N) and inner gamma(N) is confirmed by XRD and TEM for all nitrided alloys, irrespective of the Cr content. The results are discussed in terms of the short-range order (SRO) promoted by the Cr-N interaction and long-range order (LRO) caused by the Fe-N interaction. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.