Site-occupation and structure relaxation behavior of C14-type stoichiometric and iron-rich Fe2Nb Laves phases with addition of aluminum or chromium

Powder X-ray diffraction Rietveld refinement for Al-or Cr-added stoichiometric and Fe-rich off-stoichiometric C14-type Fe2Nb Laves phases were examined to elucidate the site-occupation behavior of Al and Cr, and the resulting changes in the lattice parameters and the c/a axial ratio in the Fe2Nb Laves phases. In the case of the stoichiometric composition of the Fe2Nb Laves phase, no anti-site atoms were presented at the Fe and Nb both sites Al or Cr addition to the Fe-33.3at%Nb Laves phase by replacing Fe with Al or Cr did not generate anti-site Fe atoms, and Al and Cr preferentially occupied the Fe sites. Al and Cr atoms can replace Fe atoms at both the 6 h and 2a Fe sites in the case of Fe-rich alloy, and they also can occupy the Nb site (4f) by replacing anti-site Fe atoms. However, Al and Cr had no preferential occupation of any of these sites. The accuracy of Rietveld refinement was confirmed by observation through high-angle annular dark-field scanning transmission electron microscopy. The lattice parameters of the a and c axes increased almost linearly with increasing the added Al or Cr concentrations, but the c/a ratio showed different behavior. Therefore, the change in c/a cannot be explained solely in terms of the concentration of the added element; however, it is proposed that the change in c/a is interpreted by using parameters consistent with site-occupancy values.

Automated summary

This study examined the site-occupation behavior of Al and Cr in Fe2Nb Laves phases and their effects on lattice parameters and the c/a axial ratio. The results showed that Al and Cr preferentially occupied Fe sites in stoichiometric Fe2Nb, while they could occupy both Fe and Nb sites in Fe-rich alloys. The change in lattice parameters could not be solely explained by the concentration of the added element.