On the microstructural-textural characterization and deformation analysis of a nano/ultrafine grained Fe-20Cr-8Mn-0.3N duplex alloy with superior mechanical properties

The microstructure of a Fe-20Cr-8Mn-0.3N duplex stainless steel was tailored by combination of cold deformation and subsequent martensite reversion annealing. It was shown that by reversion treatment at proper temperature and time nano/ultrafine grained (NG/UFG) with superior mechanical properties (1GPa yield strength and 40% elongation to fracture) can be achieved. Supplementary dilatometry analysis and thermodynamic studies proved that the shear reversion mechanism of ά-martensite to austenite was responsible for the formation of NG/UFG microstructure. Detailed transmission Kikuchi diffraction (TKD) characterization revealed that austenite grain size distribution was broad and narrowed by increasing the reversion annealing time. The observed inhomogeneity in grain size distribution was ascribed to the large variation in morphology and deformation level of martensite. Additionally, it was proved that certain stable original orientations of austenite grains i.e. 110//ND are highly resistant to martensitic transformation, causing stability of originally large austenite grains. The austenite texture after reversion annealing in the present duplex alloy was found to be similar to that of single phase austenitic steels. It was shown that Brass and Goss textures, which were formed during cold rolling, persist after reversion annealing irrespective of austenite metastability degree. Transmission electron microscopy (TEM) revealed that superior mechanical strength observed in the NG/UFG material, stems from both grain refinement and chromium nitride precipitates which form during revision annealing. The extraordinary plasticity of the UFG microstructure was recognized to be due to the complex deformation mechanisms of NG/UFG austenite, specifically nano size twin/twin-like bands, as well as deformation accommodation by ferrite phase.