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]
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]
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]
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]
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]