Formation, structure and magnetism of the γ-(Fe,M)₂₃C₆ (M = Cr, Ni) phases: A first-principles study

The γ-(Fe,M)₂₃C₆ phases constitute an important class of iron carbides. They occur both as precipitates in steels and iron alloys, thereby increasing their strength, and as common minerals in meteorites and in iron-rich parts of the Earth's mantle. Here we investigate the composition-dependent relative stability of these phases and the role of magnetism therein. The γ-(Fe,M)₂₃C₆ phases have mineral names isovite (M = Cr) and haxonite (M = Ni), and have a complex crystal structure (116 atoms in the cubic unit cell) in which the metal atoms have a rich variety of atomic coordination numbers, ranging from 12 to 16. First-principles calculations show a narrow formation range for γ-(Fe_1-xNi_x)₂₃C₆ (x = 0-0.043), while the formation range for γ-(Fe_1-xCr_x)₂₃C₆ is very broad (x = 0-0.85), in good agreement with available experimental data. The present study also shows the importance of magnetism on the formation and stability of these compounds. The conditions of formation and several factors enhancing or hampering the formation of γ-(Fe,M)₂₃C₆ in man-made steels and in meteorites are discussed.