Phase Stability and Deformation Modes in Functionally Graded Metastable Austenitic Stainless Steel; A Novel Approach to Evaluate the Role of Nitrogen

An austenitic stainless steel AISI 304 plate was functionally graded by interstitial alloying with nitrogen by high-temperature solution nitriding, resulting in a symmetrical nitrogen concentration profile over the plate thickness. The responses to plastic deformation and austenite stability were investigated by applying cold rolling up to 70 pct overall thickness reduction of the plate. Electron probe microanalysis, X-ray diffraction, electron microscopy, and hardness indentation were applied for characterization of the evolutions of nitrogen concentration profile, phase distribution, deformation microstructure, and hardness developing upon plastic deformation. The results demonstrate that the critical nitrogen content necessary to prevent deformation-induced martensite formation increases in the low-to-medium strain range, while it dramatically increases at high strain levels. With increasing nitrogen content, the dominant deformation mode evolves from deformation-induced martensite formation to a mixture of martensite and twin formation, and, eventually twinning and dislocation glide. The plastic strain regimes for the various deformation modes depend strongly on the nitrogen content. The results are discussed in relation to the effect of nitrogen content on the stacking fault energy of austenite in Fe-Cr-Ni alloys.