Journal
ACTA MATERIALIA
Volume 158, Issue -, Pages 79-94Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.actamat.2018.06.019
Keywords
Bimodal grain size; Twinning-induced plasticity; Stacking fault energy; Precipitates; Back stress hardening
Funding
- National Key R&D Program of China [2017YFB0703003]
- National Natura Science Foundation of China [U1564203, 51571141, 51201105]
- Interdisciplinary Program of Shanghai Jiao Tong University [YG2014MS23]
- Program of Shanghai Academic Research Leader [18XD1402200]
- Shanghai Key Laboratory of Materials Laser Processing and Modification, Shanghai Jiao Tong University
- Tescan China company
- Baosteel company
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Combined nanoprecipitation and grain refinement were introduced in a bimodal grained (BG) twinning induced plasticity (TWIP) high manganese steel to achieve high strength-ductility combinations. Hierarchical microstructural characteristics of heterogeneous grain size (0.2 mu m - 4 mu m) and precipitates (kappa-carbides and Nb-rich carbides) distribution was obtained by a controlled thermo-mechanical treatment. Compared with the as-received states, the BG-TWIP steels showed a significant improvement in yield strength (YS) with little loss in plasticity whether at room temperature (RT) or liquid nitrogen temperature (LNT). The multiple strengthening contributions to YS mainly originate from the combination of solid solution and grain refinement strengthening. When deformed at RT, some deformation twins nucleated in the coarse grains (CG) of the BG-TWIP steels while only numerous stacking faults formed in the fine grains (FG) at a true strain of 0.14. The twin density remained nearly unchanged with progressive deformation and the dislocation strengthening dominated in the later deformation stage. In the early deformation stage at LNT, the twin amount of the BG-TWIP steel was still small. However, the volume fraction of twins increased greatly in both the FGs and CGs when deformed to a true strain of 0.26. The occurrence of high density of nano-twins in the later deformation stage at LNT not only contributes to a strength increment of 220 MPa, but also largely increases the geometrically necessary dislocations (GND) density to enhance the forest hardening effect, corresponding to a significant higher back stress hardening at large strains. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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