Application of a Dislocation Density-Based Constitutive Model to Al-Alloyed TWIP Steel
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HIGH manganese twinning-induced plasticity (TWIP) steel is a new type of structural steel, characterized by both high strength and superior formability.[1–3] The TWIP effect offers an extraordinary possibility to adjust the mechanical properties of steel by modifying the strain hardening. Therefore, TWIP steel research efforts have focused on the relationship between microstructural evolution and strain-hardening behavior during deformation. The stacking fault energy (SFE) plays a key role in determining the deformation mechanisms and the strain-hardening behavior in high Mn austenitic steels. A single-phase, stable austenitic microstructure showing the TWIP effect is obtained when the material has a SFE in the appropriate range, JINKYUNG KIM, von Humbodt Research Scholar, is with the Department of Microstructure Physics and Alloy Design, Max Planck Institute for Iron Research (MPIE), Du¨sseldorf, Germany. YURI ESTRIN, Professor, is with the Centre for Advanced Hybrid Materials, Department of Materials Engineering, Monash University, Clayton, VIC, Australia. BRUNO CHARLES DE COOMAN, Professor, is with the Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, San 31, Hyosa Dong, Nam Gu, Pohang 790-784, South Korea. Contact e-mail: [email protected] Manuscript submitted December 21, 2012. Article published online May 7, 2013 4168—VOLUME 44A, SEPTEMBER 2013
although the critical stacking fault region to achieve twinning-induced plasticity is still not established.[2,4,5] Achieving a single-phase austenitic microstructure showing the TWIP effect is important for the development of TWIP steels. The addition of Al is necessary in order to improve the mechanical performance of TWIP steels, as Al can be used (a) to control the SFE[6], (b) to reduce the tendency for localized deformation,[7] and notably (c) to improve the resistance to delayed fracture.[8] The role of Al in the mechanical properties of TWIP steel, especially in the case of Fe18Mn0.6C steel, has been investigated in detail. Thus, its effect on the SFE,[9–11] microstructure and tensile properties,[10,11] and texture development[12] is well documented for this steel. However, no constitutive model correctly describing the strain-hardening behavior and the effect of Al on the mechanical properties of Fe18Mn0.6C TWIP steel is currently available. Kim et al.[13] used the Kubin–Estrin model[14,15] to compute the strain hardening from the evolution of the coupled densities of the mobile dislocations, qm, and immobile forest dislocations, qf. In this model, the two dislocation densities are coupled via terms which simultaneously appear as annihilation terms in the evolution equation for qm and production terms in the evolution equation for qf. Both densities tend to saturate at large strains. The contributions of all relevant deformation METALLURGICAL AND MATERIALS TRANSACTIONS A
Table I.
Chemical Compositions of the 18 pct Mn TWIP Steels Used in the Present Work
Composition (Mass Percent) Fe18Mn0.
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