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.[1]
Properties
Structure
κ-Carbides crystallise in the perovskitestructure type with the space groupPm3m (Nr. 221).[2] 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.[3] In steelalloys 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.[4] This is especially important for the role as hydrogen-traps in steels.[5]
Composition
A first glance at the composition of a steel alloy is achieved by analysing its surface with EDX-technique.[3]
κ-carbides are typically found as precipitates in high-performance steels.[8] 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 %).[9] It is a high-strength, low-densitysteel 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.[9] Other similar steels are known for their high ductility.[4] κ-carbides are usually formed from areas enriched in carbon through spinodal decomposition and are key determinants of the properties of these steels.[10] The low density is e.g. obtained after a hot rolling post-process.[1] Upon cooling, different domains of austenite and ferrite are formed and κ-carbides form at the boundaries of these domains.[11] 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.[11]
The κ-carbides can have an additional strengthening effect on steels[5] because they can function as a hydrogen trap to counteract hydrogen embrittlement.[3]Ab-initioDFT-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.[5] 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.[4]
^ abcdDierkes, 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. PMID28094520.
^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.
^ abFrommeyer, 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.