Formation Of Surface Nanostructure And Microstructure Engineering Of Fe-23Mn And Fe-12Mn Alloys

Due to the unique structural characteristics and many excellent mechanical and physical properties,the nanostructured metals have attracted much attention from material scientists and engineers during the past 30 years.So far,the material scientists have developed a series of processing techniques of nano-metals,invovling both the "top-down" and "bottom-up" approaches.Thus,it is allowed to investigate,in details,the novel physico-chemical phenomena in nanostructured metallic systems and reveal the complex structure-property relationship.At the present time,there are still important issues in the field of nano-metal research.They are listed as follows: firstly,the ultrafine-grained or nanograined metals usually present poor tensile ductility due to the lack of working hardening capability;secondly,there is a lack of understanding of limit of structural refinement caused by plastic deformation and the limit of material strengthening.These important scientific problems need to be solved through in-depth and systematic experimental research and theoretical analysis.In this Master thesis,two Fe-Mn binary alloys are chosen as model materials.Nanostructured Fe-Mn alloys were prepared by the surface mechanical treatments.They are used to explore the effects of higher degree of alloying on the processing and microstructure engineering of nanometals.Among them,the Fe-23 Mn alloy is a model alloy of high-Mn TRIP steels,which involves the FCC-HCP martensitic transformation.The nanostructured Fe-23 Mn alloy can provide opportunity for the study of the martensitic transformation in nanostructures and the influence of TRIP effect on the mechanical properties of nano-metals.The Fe-12 Mn alloy is a model alloy of medium-Mn steels.The microstructure at room temperature has ultrafine substructure and high dislocation density.Therefore,severe plastic deformation is conducted to explore the limits of structural refinement and strengthening.The main results of this thesis are listed as follows:1)When the heating temperature of Fe-23 Mn alloy exceeds 200?,it reaches the single-phase Austenitic region.Therefore,the Austenite pre-deformation can be performed at a low temperature.In order to improve the efficiency of structure refinement,the author applies the technique of surface mechanical rolling treatment during heating(W-SMRT).The results show that the gradient nanostructure is formed on the surface of Fe-23 Mn alloy after W-SMRT.The average grain size of the upmost surface layer is 70 nm,and the major constituent phase is the Austenite phase.In contrast,the room-temperature microstructure of coarse-grained materials is dominated by the ?-Martensite phase.This indicates that the nanostructure induced by plastic deformation significantly improves the stability of Austenite phase.After the tensile deformation of the gradient samples at room temperature,the nanocrystalline Fe-23 Mn layer is strained and the deformation-induced martensitic transformation occurs,accompanied by the increase of hardness.Therefore,strain hardening is resulted in.The lamellar ?-Martensite grows significantly during the deformation.These results demonstrate a novel method for the preparation of metastable nanocrystalline metals by combining alloy composition design and plastic deformation.It is shown that the strain hardening capability of nanocrystalline metals can be obtained by activating dynamic phase transformation.2)At room temperature,the Fe-12 Mn alloy presents a martensitic microstructure with ultrafine substructure and high dislocation density.This provides a special opportunity for the pursuit of nanostructure through plastic deformation.In this regard,the author treats the Fe-12 Mn bar with the technique of surface mechanical grinding treatment(SMGT).the gradient nanostructure is obtained.Due to the excellent plasticity of Fe-12 Mn alloy,it can undergo 8 passes of SMGT.We have performed systematic structure characterization and hardness measurement.It is found that the upmost surface layer of Fe-12 Mn alloy gradient samples presents a nanolamellar structure,and the average thickness of the lamination is 30 nm.The size of the nanolamellar of Fe-12 Mn alloy is larger than that of IF steel treated with the same parameters.However,the microhardness of nanolamellar Fe-12 Mn alloy is as high as 8.5 GPa,which is much higher than the nanolamellar IF steel(5.3 GPa)with a size of 22 nm.This indicates that the size of the laminate is not the only parameter determining the strength.These results indicate that the combination of alloy design and severe plastic deformation provides a new opportunity to explore the strength limit of nanostructured metals.