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Thermomechanical response of a TWIP steel during monotonic and non-monotonic uniaxial loading
Majidi, O.,De Cooman, B.C.,Barlat, F.,Lee, M.G.,Korkolis, Y.P. Elsevier Sequoia 2016 Materials science & engineering. properties, micro Vol.674 No.-
<P>The tensile properties of a Fe-18%Mn-0.6%C-1.5%Al Twinning-Induced Plasticity (TWIP) steel were investigated at different strain rates in three loading modes, i.e. uniaxial monotonic loading, stress relaxation and loading-unloading-reloading. Infrared thermography was used to investigate the effect of the dynamic strain aging, the strain rate and the temperature on the flow stress. In addition to the standard, i.e., non-isothermal tensile tests, isothermal uniaxial tensile tests were performed at 25 degrees C, 45 degrees C and 65 degrees C. While the non-monotonic loading modes resulted in an increase of the total elongation at a low strain rate of 10(-3) s(-1), no increase was observed for strain rates higher than 6 x 10(-3) s(-1). The temperature gradients observed during non-isothermal tests were reduced when non-monotonic loading conditions were used. Temperature changes were found to influence the hardening behavior, and consequently the ductility, of the TWIP steel. Deformation twinning also had a significant influence on the results as its kinetics in TWIP steel are determined by the temperature dependence of the stacking fault energy. (C) 2016 Elsevier B.V. All rights reserved.</P>
Latypov, M.I.,Shin, S.,De Cooman, B.C.,Kim, H.S. Elsevier Science 2016 Acta materialia Vol.108 No.-
<P>In the present contribution, a phenomenological constitutive model of medium manganese steels, in which both twinning-induced (TWIP) and transformation-induced (TRIP) plasticity enhancing mechanisms are activated, is implemented in the finite element framework. The implementation is utilized for the analysis of the full-field strain partitioning in dual-phase microstructure maps obtained from electron backscattering diffraction. The results of the finite element analysis suggest that the strain localization in the studied steel has an alternating character. Specifically, in the low strain region, most of the externally imposed deformation is accommodated by the initially softer austenite. The higher strain hardening rate of austenite due to deformation twinning (TWIP effect) and the mechanically-induced transformation to martensite (TRIP effect) results in a shift of the strain localization to ferrite. This alternating strain localization is a key feature that distinguishes the medium manganese TWIP+TRIP steel. It is shown that this alternating strain localization contributes to the superior mechanical behavior of medium manganese TWIP+TRIP steel reported in the literature. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.</P>
Effect of micro-alloying elements on the stretch-flangeability of dual phase steel
Lee, J.,Lee, S.J.,De Cooman, B.C. Elsevier Sequoia 2012 Materials science & engineering. properties, micro Vol.536 No.-
The present paper investigates the effect of micro-alloying elements on the stretch-flangeability of a conventional dual phase steel by means of microstructural analysis and hole expansion tests. The addition of micro-alloying elements suppressed the formation of new ferrite from the intercritical austenite and induced a finer grain size which increased the strength of the steel. The increased amount of intercritical ferrite formed at higher annealing temperature resulted in an improved stretch-flangeability by simultaneously increasing the elongation and decreasing the strength. The addition of 200ppm Nb was beneficial for the hole expansion performance whereas a detrimental effect on hole expandability was found after the addition of 400ppm Nb to the dual phase steel.