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Oak Ridge, TN, United States

Pierce D.T.,Colorado School of Mines | Jimenez J.A.,CSIC - National Center for Metallurgical Research | Bentley J.,Microscopy and Microanalytical science | Raabe D.,Max Planck Institute Fur Eisenforschung | Wittig J.E.,Vanderbilt University
Acta Materialia

Understanding the relationship between the stacking-fault energy (SFE), deformation mechanisms, and strain-hardening behavior is important for alloying and design of high-Mn austenitic transformation- and twinning-induced plasticity (TRIP/TWIP) steels. The present study investigates the influence of SFE on the microstructural and strain-hardening evolution of three TRIP/TWIP alloys (Fe-22/25/28Mn-3Al-3Si wt.%). The SFE is increased by systemically increasing the Mn content from 22 to 28 wt.%. The Fe-22Mn-3Al-3Si alloy, with a SFE of 15 mJ m-2, deforms by planar dislocation glide and strain-induced εhcp-/αbcc-martensite formation which occurs from the onset of plastic deformation, resulting in improved work-hardening at low strains but lower total elongation. With an increased SFE of 21 mJ m-2 in the Fe-25Mn-3Al-3Si alloy, both mechanical twinning and εhcp-martensite formation are activated during deformation, and result in the largest elongation of the three alloys. A SFE of 39 mJ m-2 enables significant dislocation cross slip and suppresses εhcp-martensite formation, causing reduced work-hardening during the early stages of deformation in the Fe-28Mn-3Al-3Si alloy while mechanical twinning begins to enhance the strain-hardening after approximately 10% strain. The increase in SFE from 15 to 39 mJ m-2 results in significant changes in the deformation mechanisms and, at low strains, decreased work-hardening, but has a relatively small influence on strength and ductility. © 2015 Acta Materialia Inc. Source

Pierce D.T.,Vanderbilt University | Bentley J.,Microscopy and Microanalytical science | Jimenez J.A.,CSIC - National Center for Metallurgical Research | Wittig J.E.,Vanderbilt University
Scripta Materialia

Since the stacking fault energy significantly influences the deformation mechanisms of Fe-Mn-Al-Si twinning-induced plasticity steels, two methods for its experimental determination by transmission electron microscopy of dislocations, namely the size of extended nodes and the separation of Shockley partials, were evaluated for an Fe-24.7Mn-2.66Al-2.92Si (wt.%) alloy. Measurement of partial dislocation separation provided the most reliable results, yielding a stacking fault energy of ∼16 mJ m -2, which is comparable to recent experimental and theoretical values for similar alloys. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Source

Pierce D.T.,Vanderbilt University | Pierce D.T.,Colorado School of Mines | Jimenez J.A.,CSIC - National Center for Metallurgical Research | Bentley J.,Microscopy and Microanalytical science | And 3 more authors.
Acta Materialia

The stacking fault and interfacial energies of three transformation- and twinning-induced plasticity steels (TRIP/TWIP) (Fe-22/25/28Mn-3Al-3Si wt.%) were determined by experimental and theoretical methods. Analysis of Shockley partial dislocation configurations in the three alloys using weak-beam dark-field transmission electron microscopy yielded stacking fault energy (SFE) values of 15 ± 3, 21 ± 3 and 39 ± 5 mJ m-2 for alloys with 22, 25 and 28 wt.% Mn, respectively. The experimental SFE includes a coherency strain energy of ∼1-4 mJ m-2, determined by X-ray diffraction, which arises from the contraction in volume of the stacking fault upon the face-centered cubic (fcc) to hexagonal close-packed (hcp) phase transformation. The ideal SFE, computed as the difference between the experimental SFE and the coherency strain energy, is equal to14 ± 3, 19 ± 3 and 35 ± 5 mJ m-2, respectively. These SFE values were used in conjunction with a thermodynamic model developed in the present work to calculate the free energy difference of the fcc and hcp phases and to determine a probable range for the fcc/hcp interfacial energy in the three Fe-Mn-(Al-Si) steels investigated. In addition, the interfacial energies of three Fe-18Mn-0.6C-0/1.5(Al/Si) TWIP and five Fe-16/18/20/22/25Mn binary alloys were also determined from experimental data in the literature. The interfacial energy ranged from 8 to 12 mJ m-2 in the TRIP/TWIP steels and from 15 to 33 mJ m-2 in the binary Fe-Mn alloys. The interfacial energy exhibits a strong dependence on the difference in Gibbs energy of the individual fcc and hcp phases. Accordingly, an empirical description of this parameter is proposed to improve the accuracy of thermodynamic SFE calculations.© 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Source

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