Characterization and Prediction of Flow Behavior in High-Manganese Twinning Induced Plasticity Steels: Part II. Jerky Fl
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THE jerky (serrated) flow behavior of high-manganese twinning induced plasticity (TWIP) steels has been studied recently by several investigators.[1–7] However, the concept of the reorientation of single or clusters of solute substitutional and interstitial atoms in the cores of dislocations as the cause for dislocation pinning and dynamic strain aging (DSA) was introduced by Dastur and Leslie[8] and Cuddy and Leslie,[9] and it was approved by Chen et al.[7] Kandarpa and Spretnak[10] studied austenitic manganese steel and found that the distortion of the octahedral sites by manganese atoms activates stress-induced ordering of the single carbon atoms. Ha¨hner et al.[11–16] highlighted that along with DSA, cooperative dislocation interaction is the second necessary condition for the onset of pronounced serration, which is known as the Portevin Le-Chatelier (PLC) phenomenon. It has been reported that the attraction A. SAEED-AKBARI, Post-Doctoral Researcher and Research Team Leader, is with the Department of Ferrous Metallurgy, RWTH Aachen University, 52072 Aachen, Germany. Contact e-mail: Alireza. [email protected] A.K. MISHRA, Project Engineer, formerly with the Department of Metallurgical & Materials Engineering, Indian Institute of Technology, Kharagpur 721302, India, is now with SIEMENS Corporate Research and Technologies, Bangalore 560100, India. J. MAYER, University Professor and Department Head, is with the Central Facility for Electron Microscopy, RWTH Aachen University. W. BLECK, University Professor and Department Head, is with the Department of Ferrous Metallurgy, RWTH Aachen University. Manuscript submitted August 30, 2011. Article published online February 11, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
between carbon-manganese pairs causes local ordering[17] and that the presence of carbon as the solute atom at the octahedral sites results in pronounced planar glide, leading to superior work hardening.[7,18] However, at room or sufficiently lower temperatures, the mobility of carbon atoms is restrained because the required activation energy for diffusion in high-manganese TWIP steels is high. Therefore, to explain the room-temperature aging behavior of TWIP steels, Kim et al.[6] concluded that the carbon-vacancy pairs—because of a comparatively lower activation energy required for the movement of vacancies—rotate under the influence of the strain field surrounding the dislocations. This rotation produces sufficient asymmetric lattice distortion for Snoek-type locking and the final strain aging phenomenon. However, the exact mechanisms leading to a jerky flow in TWIP steels are still the subject of ongoing research. The latest proposed theory by Lee et al.[1] on the occurrence of the DSA phenomenon suggests the single diffusive hop of a carbon atom within the point-defect complex, interacting with the stacking fault, as the governing mechanism. DSA was detected when the carbon atom reorientation time was shorter than the residence time of the stacking fault at the location of the point-defect
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