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Kappa-carbide

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κ-Carbides are a special class of carbide structures. They are most known for appearing in steels containing manganese and aluminium where they have the molecular formula (Fe,Mn)
3AlC.

Properties

Structure

Typical κ-carbide structure with Fe (red), Al (metallic) and C (black)

κ-Carbides crystallise in the perovskite structure type with the space group Pm3m (Nr. 221). This structure was, inter alia, elucidated with XRD-measurements on steel alloys containing κ-carbide precipitates but also on single crystals of manganese-κ-carbides with a molecular formula of Mn3.1Al0.9C and a lattice parameter of a=3.87Å. In steel alloys where diverse arrangements of the atoms are possible, a considerable effect of the short range ordering, e.g. of iron and manganese on the microscopic properties of the alloy, has been observed. This is especially important for the role as hydrogen-traps in steels.

Composition

A first glance at the composition of a steel alloy is achieved by analysing its surface with EDX-technique.

Depending on the content of the alloying elements of the steel, different types of κ-carbides can form. They occur in both ferritic (α-Fe) and austenitic (γ-Fe) steels. Typical alloying elements are iron, manganese, aluminium, carbon, and silicon.

Iron carbon phase diagram

Magnetism

SQUID measurements on polycrystalline Mn3.1Al0.9C revealed a soft ferromagnetic behaviour of this κ-carbide with a Curie temperature of 295±13 K, a remanent magnetic moment of 3.22 μB and a coercive field of 1.9 mT. DFT-simulations confirmed these findings and indicated that other κ-carbides behave similarly.

Occurrence

Fe3AlC κ-carbide with interstitial sites that can be occupied by H atoms

κ-carbides are typically found as precipitates in high-performance steels. A common example is the TRIPLEX steel with the generic composition FexMnyAlzC containing 18-28 % manganese, 9-12 % aluminium and 0.7-1.2 % carbon (in mass %). It is a high-strength, low-density steel consisting of austenitic γ–Fe(Mn,Al,C) solid solution, nano size κ-carbides (Fe,Mn)
3AlC
1-x and α–Fe(Al,Mn) ferrite. Other similar steels are known for their high ductility. κ-carbides are usually formed from areas enriched in carbon through spinodal decomposition and are key determinants of the properties of these steels. The low density is e.g. obtained after a hot rolling post-process. Upon cooling, different domains of austenite and ferrite are formed and κ-carbides form at the boundaries of these domains. Continuing the cooling process leads to a phase transition of austenite to ferrite and the κ-carbides are released as a result of an eutectoid transformation in form of a precipitate.

The κ-carbides can have an additional strengthening effect on steels because they can function as a hydrogen trap to counteract hydrogen embrittlement. Ab-initio DFT-simulations have shown that hydrogen can occupy the same site as carbon in the κ-carbide precipitates or an initially empty interstitial lattice site. Hereby, it was found that an increased Mn content enhances the H-trapping by attractive short-range interactions. The aforementioned short-range ordering of Fe and Mn in the κ-carbide has a significant influence on the strength of this effect. This behaviour can be used as an additional method to cope with hydrogen embrittlement which is normally prevented by simply minimising the contact of metal and hydrogen.

See also

References

  1. ^ Sozańska-Jędrasik, Liwia; Mazurkiewicz, Janusz; Matus, Krzysztof; Borek, Wojciech (6 February 2020). "Structure of Fe-Mn-Al-C Steels after Gleeble Simulations and Hot-Rolling". Materials. 13 (3): 739. Bibcode:2020Mate...13..739S. doi:10.3390/ma13030739. PMC 7041428. PMID 32041206.
  2. ^ Seol, Jae Bok (28 December 2018). "A Brief Review of κ-Carbide in Fe-Mn-Al-C Model Alloys". Applied Microscopy. 48 (4): 117–121. doi:10.9729/am.2018.48.4.117.
  3. ^ Dierkes, Hannes; van Leusen, Jan; Bogdanovski, Dimitri; Dronskowski, Richard (17 January 2017). "Synthesis, Crystal Structure, Magnetic Properties, and Stability of the Manganese-Rich "Mn3AlC" κ Phase". Inorganic Chemistry. 56 (3): 1045–1048. doi:10.1021/acs.inorgchem.6b02816. PMID 28094520.
  4. ^ Song, Wenwen; Bogdanovski, Dimitri; Yildiz, Ahmet; Houston, Judith; Dronskowski, Richard; Bleck, Wolfgang (10 January 2018). "On the Mn–C Short-Range Ordering in a High-Strength High-Ductility Steel: Small Angle Neutron Scattering and Ab Initio Investigation". Metals. 8 (1): 44. doi:10.3390/met8010044.
  5. ^ Timmerscheidt, Tobias; Dey, Poulumi; Bogdanovski, Dimitri; von Appen, Jörg; Hickel, Tilmann; Neugebauer, Jörg; Dronskowski, Richard (11 July 2017). "The Role of κ-Carbides as Hydrogen Traps in High-Mn Steels". Metals. 7 (7): 264. doi:10.3390/met7070264.
  6. Bartlett, Laura N.; Van Aken, David C.; Medvedeva, Julia; Isheim, Dieter; Medvedeva, Nadezhda I.; Song, Kai (20 February 2014). "An Atom Probe Study of Kappa Carbide Precipitation and the Effect of Silicon Addition". Metallurgical and Materials Transactions A. 45 (5): 2421–2435. Bibcode:2014MMTA...45.2421B. doi:10.1007/s11661-014-2187-3.
  7. Seo, Seung-Wo. "First Principles Calculations on Thermodynamic Properties and Magnetism of k-carbide and Monte-Carlo Cell Gas Model" (PDF). Retrieved 14 July 2020.
  8. Gutierrez-Urrutia, I.; Raabe, D. (March 2013). "Influence of Al content and precipitation state on the mechanical behavior of austenitic high-Mn low-density steels". Scripta Materialia. 68 (6): 343–347. doi:10.1016/j.scriptamat.2012.08.038.
  9. ^ Frommeyer, Georg; Brüx, Udo (September 2006). "Microstructures and Mechanical Properties of High-Strength Fe-Mn-Al-C Light-Weight TRIPLEX Steels". Steel Research International. 77 (9–10): 627–633. doi:10.1002/srin.200606440.
  10. Rana, Radhakanta; Lahaye, Chris; Ray, Ranjit Kumar (29 August 2014). "Overview of Lightweight Ferrous Materials: Strategies and Promises". JOM. 66 (9): 1734–1746. Bibcode:2014JOM....66i1734R. doi:10.1007/s11837-014-1126-5.
  11. ^ Akdeniz, M. Vedat (26 August 2008). "Solidification Microstructures and Carbides Morphology in Rapidly Solidified Fe-Al-Cr-C Alloys". Metals and Materials International. 14 (4): 397–402. Bibcode:2008MMI....14..397A. doi:10.3365/met.mat.2008.08.397. S2CID 121181321.

External links

  • κ Carbide in Steels (Phase Transformations & Complex Properties Research Group, University of Cambridge)
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