Site Occupation and Defect Structure of Fe2Nb Laves Phase in Fe-Nb-M Ternary Systems at Elevated Temperatures

Abstract

AbstractFor the Fe2Nb Laves phase with C14 structure in the Fe-Nb-M (M : Cr, Mn, Co, Ni) systems, the site occupation of M in Fe2Nb has been examined in terms of XRD Rietveld analysis, particularly paying attention to the two Fe sublattice sites of Fe1 (36-net in the triple layer : t) and Fe2 (kagome-net of the single layer : s) with the fraction of 0.25 and 0.75, respectively. In any these four ternary systems, the Fe2Nb Laves phase region largely extends along the equi-Nb concentration direction; for Mn complete solid solubility exists, and the solubility of Cr and Co in Fe2Nb is more than 50 at.% and that of Ni is 44 at.%. Thus, at least two thirds of all Fe sublattices in Fe2Nb are occupied by M in all cases. Rietveld analysis revealed that Cr and Mn with which have a larger atomic size than Fe prefer to occupy the Fe1 sublattice site when the amount in solution is less than 0.25 fraction of Fe in Fe2Nb and the preferred occupation site changes to the Fe2 sublattice site when the amount in solution increases beyond 0.25. In contrast, Co and Ni whose atomic size is smaller than Fe preferentially occupy the Fe2 sublattice site, regardless of the amount. The c/a ratio of stoichiometoric Fe2Nb increases and becomes closer to the ideal value (1.633) of the cubic C15 structure when the Fe1 sublattice site is occupied by Cr and Mn. However, the degree of symmetries of both tetrahedron and kagome-net formed by Fe atoms become better when Fe2 sublattice site is occupied by a certain amount of Ni.