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Behjati, P.,Kermanpur, A.,Najafizadeh, A.,Samaei Baghbadorani, H.,Karjalainen, L.P.,Jung, J.G.,Lee, Y.K. Scientific and Technical Press ; Elsevier Science 2014 MATERIALS AND DESIGN Vol.63 No.-
A conceptual approach was used to design a new Ni-free austenitic stainless steel with a unique combination of ultrahigh strength and ductility. The concept was based on the alloying of the 0.05C-18Cr-12Mn (wt.%) steel by 0.39%N and heavy warm rolling (84% reduction) at 1173K (900<SUP>o</SUP>C) to achieve the yield strength of minimum 1GPa and high tensile strength and elongation due to a proper stability of the austenite as a result of the optimized stacking fault energy (SFE). The yield strength of 1010MPa, tensile strength of 1150MPa and high fracture strain of 70% were measured for the steel designed. Dislocation and solid solution hardening mechanisms are introduced as the main contributors for the ultrahigh yield strength of the steel. The strain hardening is gradual and the hardening rate reaches a high level of ~2400MPa at a high true strain of 40% due to slow α'-martensitic transformation and mechanical twinning. Consequently, the ductility of the designed steel is excellent.
Behjati, P.,Kermanpur, A.,Najafizadeh, A.,Baghbadorani, H.S.,Jung, J.G.,Lee, Y.K. Elsevier Sequoia 2014 Materials science & engineering. properties, micro Vol.614 No.-
In this work, the influence of precooling on deformation-induced α'-martensitic transformation and mechanical properties of Fe-16Mn-4Cr-0.1C (wt%) austenitic steel is investigated. It is shown that precooling in liquid nitrogen enhances the volume fraction of ε-martensite. During tensile testing, ε-martensite acts as a nucleation site for α'-martensite and promotes the strength and ductility of the steel. The precooled steel exhibits an excellent combination of mechanical properties (tensile strength of 1240MPa, uniform elongation of 35% and total elongation of 42%). Further, it is shown that tensile strength and elongation of the steel strongly depend on the deformation temperature. This is explained by the influence of temperature on stacking fault energy (SFE) and accordingly governing deformation mechanisms of the steel.
Behjati, P.,Kermanpur, A.,Najafizadeh, A.,Samaei Baghbadorani, H.,Jung, J.G.,Lee, Y.K. Elsevier Sequoia 2014 Materials science & engineering. properties, micro Vol.610 No.-
In this work, a specific thermomechanical treatment was used to enhance mechanical properties of a high-manganese austenitic steel. Simple uniaxial tension tests revealed that the treated steel exhibits an outstanding strength (yield strength: 970MPa and tensile strength: 1384MPa) and ductility (fracture strain: 37%) synergy. Microstructural studies showed that several strengthening mechanisms contribute to mechanical properties of the treated steel, including nano grains, nano twinned austenite grains, nano carbides and TRIP. Stacking fault energy and strain-induced martensitic transformation were found to have an important role in appropriate adjustment of the strengthening mechanisms to achieve the optimum properties.