Experiment Study On The Effect Of Ni Content On The Diffusion Behavior Of Nitrogen In FeCrNi Austenitic Alloys

Plasma-based low-energy ion implantation was applied to treat the FeCrNi austenitic alloys under the condition of 380? and 450? for 4 h.The effects of Ni content on the diffusion behavior of nitrogen in theg_N phase and the stability of theg_N phase were studied.Metallographic microscope,SEM,EPMA,XRD and microhardness tester were used to investigate metallographic structure,thickness of the modified layer,nitrogen concentration-depth profiles,surface nitrogen concentration,phase structure,microhardness of the modified layers on the austenitic alloys treated by plasma-based low-energy ion implantation.This work is an experiment basement for the effect of Ni content on the diffusion behavior of nitrogen in the austenitic alloys.On the nitrided surface of FeCrNi austenitic alloys,a N supersaturated metastable FCC phase?g_N?,also named expanded austenitic forms.A modified layer with dense structure and no second phase precipitated is obtained in the austenitic alloys after treated for 4 h at 380?.The thickness decreases regularly as Ni content increased,which is mainly owing to the increasing of stacking fault energy.Nickel element can stabilize the austenite structure and increase the resistance to dislocation sliding of the austenitic alloys,which can affect the strain rate and stress level of the modified layer.Moreover there are repulsion interactions between Ni and N,which maybe prevent further dissolution of nitrogen atom in the alloys.XRD results show that there are diffraction peaks of the HCP phase in lower Ni-content austenitic alloys,and the peaks intensity decreases with higher Ni content.Nickel element can increase the stacking fault energy which prevent the process of plastic induced HCP transformation during nitriding.EPMA results show that the nitrogen concentration-depth profiles have a nitrogen concentration plateau-type decreasing followed by a leading edge.There is no influence of Ni content to the surface nitrogen concentration,because of the existence of the Cr-N strong interactions.The surface nitrogen concentration may mainly relate with Cr content.A seriously precipitated layer is obtained in the low-nickel austenitic alloys after treated for 4 h at 450?.The XRD results show that the diffraction intensity of CrN andg'-Fe₄N precipitates decrease as Ni content increased.Through the EDS analysis of the second phase,it is found that the second phase is mainly a Cr-rich region.Nickel element may stabilize theg_N phase,so CrN precipitates gradually disappears with Ni content.At the same time,the thickness of the modified layer also has a tendency of decreasing with the increasing of Ni content.