期刊
ACTA MATERIALIA
卷 59, 期 19, 页码 7406-7415出版社
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.actamat.2011.08.012
关键词
Multiphase steel; NbC; (Nb, Ti)N; Precipitation; Transmission electron microscopy
资金
- Materials Innovation Institute [MC5.06280a]
Multiphase steels utilising composite strengthening may be further strengthened via grain refinement or precipitation by the addition of microalloying elements. In this study a Nb microalloyed steel comprising martensite, bainite and retained austenite has been studied. By means of transmission electron microscopy (TEM) we have investigated the size distribution and the structural properties of (Nb, Ti)N and NbC precipitates, their occurrence in the various steel phases, and their relationship with the Fe matrix. (Nb, Ti)N precipitates were found in ferrite, martensite, and bainite, while NbC precipitates were found only in ferrite. All NbC precipitates were found to be small (5-20 nm in size) and to have a face centred cubic (fcc) crystal structure with lattice parameter a = 4.36 +/- 0.05 angstrom. In contrast, the (Nb, Ti)N precipitates were found to have a broader size range (5-150 nm) and to have a fcc crystal structure with lattice parameter a = 8.09 +/- 0.05 angstrom. While the NbC precipitates were found to be randomly oriented, the (Nb, Ti)N precipitates have a welldefined Nishiyama Wasserman orientation relationship with the ferrite matrix. An analysis of the lattice mismatch suggests that the latter precipitates have a high potential for effective strengthening. Density functional theory calculations were performed for various stoichiometries of NbC(x) and Nb(x)Ti(y)N(z) phases and the comparison with experimental data indicates that both the carbides and nitrides are deficient in C and N content. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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