Phase Structure Of Plasma-Based Low-Energy Nitrogen Ion Implanted FeCrNi Austenitic Alloys At Low Temperature

A high nitrogen(>18 at.%)supersaturated face-centered cubic phase modified layer,called N-expanded austenite,can be formed on FeCrNi austenitic stainless steel by 230-450℃ low temperature nitriding.The layer has excellent combined wear and corrosion resistance properties,however,the conclusions of the study about the phase structure are not unified or even contradictory.In order to study the structural nature of the nitrogen-expanded austenite phase,20 kinds FeCrNi austenitic alloys in three groups with fixed Ni content,fixed Cr content and fixed Fe content were selected in the Schaeffler diagram,and modified by plasma-based low-energy ion implantation to study the composition,phase structure and microstructure of the nitrogen modified layer.AISI304 austenitic stainless steel was also treated as a comparation.The theoretical calculation of the composition of nitrogen-expanded austenite is consistent with the experimental results using Quasi-Cell-Model based on the interaction of nitrogen occupying the octahedral interstice.The structural characteristics and formation regulation of nitrogen supersaturated expanded austenite on FeCrNi alloy are revealed.The alloys were treated at 380℃ for 4 hours using plasma-based low-energy nitrogen ion implantation technique to obtain a low temperature nitriding modified layer.Composition and structure of the nitrogen modified layer were investigated by light microscopy(OM),electron probe microanalysis(EPMA),X-ray diffraction(XRD),grazing incidence X-ray diffraction(GIXRD)and high-resolution transmission electron microscopy(HRTEM)characterization.It is confirmaed that the nitrogen modified layers on all the alloys are composed of inner and outer duples structure.The outer layer is a fcc structure with large expansion and N has a Fe4N-like long-range ordered arrangement,which is determined as γ’N phase.The inner layer is a f.c.c.structure with small expansion and only contains Cr-N short-range ordered arrangement,that is the commonly reported γN phase.As the Cr content in the alloy increases,the N concentration and lattice constant of the two phases increase,but the increase rate of the γN phase is significantly greater than that of the y’N phase.In addition,the N concentration range and diffraction peak width of the two phases increasing,these two effects together cause the N concentration and lattice constant of the two phases to gradually approach and eventually merge.When the Cr content in alloys is less than 12 wt.%,the nitrogen modified layer exhibits a duplex layer structure in both the cross-sectional morphology and the N concentration-depth distribution,and the XRD results show two sets of f.c.c.diffraction peaks with different lattice expansion.When the Cr content is more than 12 wt.%,the cross-sectional morphology and N concentration-depth distribution of the nitrogen modified layer both shows single-layer structure,and XRD appears as a set of broad asymmetric diffraction peaks.Based to the isostructural evolution of XRD results,it is inferred that the broadened diffraction peaks are formed by the combination of the γ’N and γN diffraction peaks.TEM selected area electron diffraction and high-resolution observation results confirmed that the nitrogen-modified layer of high Cr content alloy has a duplex layer structure with outer N ordered γ’N phase and inner N disordered γN phase.The Simplified Model and the Quasi-Cell-Model of the γ’N and γN phases are respectively considered.Only the Cr-N interaction exists in the yN phase,and both the Fe4N-like interaction and Cr-N interaction are present in the γ’N phase.Partially N atoms trapped by the Cr atoms in the short-range ordered are simultaneously in the Fe4N-like long-range order positions,that is,the position sharing.The simplified model is based on the one-to-one Cr-N interaction hypothesis,it is clearly described that the position sharing of Cr-N short-range order position and the Fe4N-like long-range order position causing the N concentration and lattice constant ofγ’N and γN phases approaching.The Quasi-Cell-Model is based on multi-to-one strong and weak Cr-N interactions,N-N repulsion,and Fe4N-like Fe-N interactions.The range of N concentrations of γ’N and γN phases in different Cr content alloys are calculated,and obtains the relationship with the Cr content.It clearly describing the process that the growth rate of the yN phase is significantly larger than that of γN phase,and the widened N concentration range with high Cr.The calculated results of the Quasi-Cell-Model are consistent with the experimental results.The calculated average N atom coordination number is consistent with the extended X-ray absorption fine structure(EXAFS)spectrum and Mossbauer spectrum analysis,which further confirms the rationality of the Quasi-Cell-Model.The low-temperature nitriding results of the series alloys and the calculation results of the Quasi-Cell-Model reveal the formation regulation and structural nature of the γN phase and γ’N phase.It unifies the controversy of ordered or disordered nitrogen arrangement in nitrogen-expanded austenite phase,and provides experimental and theoretical basis for the subsequent further study of γN phase and γ’N phase.