Kappa-carbide
From Wikipedia, the free encyclopedia
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
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κ-Carbides crystallise in the perovskite structure type with the space group Pm3m (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 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.[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]
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.[1] Typical alloying elements are iron, manganese, aluminium, carbon, and silicon.[2][6]
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.[3] DFT-simulations confirmed these findings and indicated that other κ-carbides behave similarly.[7]
Occurrence
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-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.[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-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.[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]