Study On Microstructure And Phase Stability Of Supersaturated Nitrided Layer On Austenitic Stainless Steel

The supersaturated nitrided(10-35 at.%)layer on austenitic stainless steel produced by plasma-based low-energy ion implantation has a remarkable compound performance of wear and corrosion resistance compared with austenitic stainless steel.This nitrided layer is a metastable phase with inhomogeneous structure and has broad application prospects in the nuclear industry.However,the understanding of the crystal structure of the nitrided layer is still controversial,and the detailed structural model of the nitrided layer at atomic level is lacking.Furthermore,the controversy and lack restrict the understanding of the phase stability and the structural stability of the nitrided layer.Since the structure is closely related to the performance,the research about the microstructure of the nitrided layer is essential for the application in the nuclear industry.This work focuses on the microstructure of supersaturated nitrided layer on austenitic stainless steel,and quantitatively characterizes the organizational structure,defect structure,crystal structure and short-range ordered structures of the supersaturated nitrided layer.Based on these results,a crystal structure model and the formation regularities of short-range ordered structure were proposed.Moreover,an atomic structural model of supersaturated nitrided layer was established.The phase stability and the structural stability of the nitrided layer were explored at atomic level.The main conclusions are drawn as follows:1.Besides isolated stacking fault and twin,clustered stacking fault composed of high dense stacking faults can also be found in the supersaturated nitrided layer on austenitic stainless steel.The clustered stacking fault consists of a series of stacking faults on every second {111} atomic planes.In addition a grain refinement process occurs during the nitriding process except for the grain rotation.2.A crystal structure model of the supersaturated nitrided layer including clustered stacking faults was proposed based on the characterization results of the defect structure and crystal structure.The supersaturated layer is a face-centered cubic structure including isolated stacking faults,twins and clustered stacking faults.This layer bears a plane stress due to the constraint of matrix and prefers(111)orientation.An isolated stacking fault,twin and clustered stacking fault can lead to a shift,broading and asymmetric of the peak in X-ray diffraction pattern.At the same time,they can also lead to a diffuse scattering in selected area electron diffraction pattern.However,it is impossible to reproduce the X-ray diffraction pattern and selected area electron diffraction pattern of the supersaturated nitrided layer by using any of the above defect alone.Using the crystal structure model of the supersaturated nitrided layer including clustered stacking faults to fit the X-ray diffraction pattern,it has been found that the nitrided layer produced by plasma-based low-energy ion implantation includes 10-20%stacking fault,in which 50-70%is clustered stacking fault.Furthermore,this crystal structure model is applicative in similar nitriding method and similar matrix.3.The characterization results of the short-range ordered structure indicate that the short-range ordered structure is more obvious in the region without stacking fault although these structures can be found in both regions with or without stacking fault.The short-range ordered structure can be consisted of an octahedral cluster denoted Fe6-nCrnN according to the first-principles calculations.The Fe6-nCrnN clusters with more Cr atoms have a greater energetical preference.Moreover,the Cr atoms distributed in pairs at the opposite octahedron vertexes are the energetically favored configurations.The number of Cr atoms and their detailed location in the Fe6-nCrnN cluster are dominated by the charge transfer and the consequent Coulombic interactions in the clusters.The Fe6-nCrnN octahedrons may combine into clusters with larger scale when the N concentration increases.In the combination with high energetical preference,the ESP is as high as possible.Cr atoms prefer the shared sites of octahedral clusters,and N atoms prefer to locate on the planes with different magnetic moments.The atomic structure of the region without stacking fault in the supersaturated nitrided layer can be described as the combination of face-centered cubic Fe and all energetical favored short-range ordered clusters which dispersively embed in the face-centered cubic Fe.4.The phase stability of the nitrogen supersaturated metastable phase is related to the short-range ordered structure introduced by high concentration of Cr and N atoms,temperature and the release of residual stress.The residual stress acts on the phase stability in synergy with temperature.The influence of high concentration of Cr and N is more obvious than the influence of the residual stress and temperature.The stabilization of the metastable phase is mainly a chemically-driven mechanism by Cr and N atoms forming short-range ordered clusters.The structural stability of supersaturated nitrided layer can be improved by the short-range ordered structure formed by nitrogen and vacancy.