Abstract
The effect of mixed partial occupation of metal sites on the phase stability of the γ-Cr23−xFexC6 (x = 0–3) carbides is explored as function of composition and temperature. Ab initio calculations combined with statistical thermodynamics approaches reveal that the site occupation of the carbides may be incorrectly predicted when only the commonly used approach of full sublattice occupation is considered. We found that the γ-M23C6 structure can be understood as a familiar sodium chloride structure with positively charged rhombic dodecahedron (M(4a) M12(48h)) and negatively charged cubo-octahedron (M8(32f) C6(24e)) super-ion clusters, together with interstitial metal atoms at the 8c sites. The stability of the partially occupied phase can be easily rationalized on the basis of a super-ion analysis of the carbide phase. This new understanding of γ-M23C6 carbides may facilitate further development of high-chromium heat-resistant steels.
Highlights
The phase stability of carbide precipitates plays a crucial role in improving the creep lifetime of steels[1,2]
The partial/full occupation of a sublattice site refers to the fraction of the site that is occupied by a particular atomic species, averaged over many unit cells
We investigated the impact of site occupancy of Fe in the γ-Cr23–xFexC6 (x = 0–3) carbide phase in detail using the cluster-expansion method (CEM) and the cluster-variation method (CVM) based on first-principles as function of composition and temperature
Summary
The phase stability of carbide precipitates plays a crucial role in improving the creep lifetime of steels[1,2]. The partial/full occupation of a sublattice site refers to the fraction of the site that is occupied by a particular atomic species, averaged over many unit cells. Mixed occupancy was considered in the specific context of the coherent potential approximation[25], and for several configurations in the Fe-Cr σ structure[26] but an in-depth analysis including associated effects such as due to lattice vibrations remains lacking We address this issue by considering many partially occupied configurations of a non-trivial technically important carbide phase, Cr23C6, while including vibrational effects. In this structure, each 4a site is surrounded by 12 metal atoms in 48h sites, forming a cubo-octahedron with the composition Cr(4a)Cr12(48h). Ab initio approaches which properly account for many-body interactions are much preferred for studies of the occupancy sequence in M23C6
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