A Cluster-based Model Of The γ-(Fe,C) Interstitial Solid Solution

The short-range carbon distribution in carbon solid solution γ-(Fe,C) is crucial to the understanding of the correlations between carbon contents and alloy behaviors. There are mainly two ways describing the behavior of carbon atoms in γ-(Fe,C) at present, the random behavior and the repulsive behavior. Mossbauer experiments on γ-(Fe,C) point to the latter mode, which has also been supported by a Monte Carlo Simulation. In correspondence to the repulsive theory, the carbon atoms situating in octahedral sites are separated by at least a distance of the3rd octahedral neighbors.A geometrical model based on cluster-plus-glue-atom model is proposed in this paper to describe the possible short-range-order atomic configurations in y-(Fe.C). In this model, each C atom is situated in an octahedral interstice in the FCC austenite and is nearest-neighbored by six Fe atoms, forming an octahedral cluster [C-Fe6]. These clusters are homogeneously distributed in the FCC lattice, separated by glue atoms Fe that are located between the clusters, so that a y-(Fe,C) solid solution is always described by a cluster formula [C-Fe6]Fex. After imposing the shortest and the longest inter-cluster distances,96configurations conforming to cluster formulas [C-Fe6]Fex (x=0～20,26) are obtained, each being presented by three vectors of C-C pairs that fits the repulsive behavior.The configuration with the largest atom density is [C-Fee]Fe2. The distributions of C atoms in the configurations [C-Fe6]Fex(x=1,2.3,6,15.20,26) fall close to the Friedel minima and maxima among all the configurations, which means that these configurations are specially stable. The [C-Fe6]Fe2model, made up of vectors1/2-(0.2,2),1/2(2.0,2),1/2(2,2.0) forms an FCC structure, expresses the same long-range-ordered Fe8C model proposed by Bauer et al., but the [C-Fe6]Fe2model identifies easily the types of Fe1.0atoms (the Fe atoms having one lst-neighbor carbon and zero2nd-neighbor carbon) and Fe0.4atoms(the Fe atoms having zero1st-neighbor carbon and four2nd-neighbor carbon atoms). The cluster formulas with1or3glue atoms explain the compositions of the eutectic and the eutectoid points in the Fe-Fe3C phase diagram:The eutectic point is well understood by a combination of [C-Fe6]Fe3for γ-(Fe,C) and [C-Fe9]C2Fe for cementite Fe3C, and the eutectoid point is interpreted by [Fe-Fe14]Fe3from ferrite a-(Fe.C) and [C-Fe9]Fe from cementite Fe3C. Thus the theoretical compositions of the eutectic point and the eutectoid point are4.33wt.%C and0.76wt.%C respectively, while the corresponding experimental values are4.30wt.%C and0.76wt.%C.