Nanoscale Phase Separation Induced by Mechanical Alloying in the Iron-Erbium-Nitrogen System

Metastable phases, including nanocrystalline and amorphous structures, can be prepared by high energy cyclic deformation processes. In the present study, we compare the behavior of a stable congruent melting compound (Fe2Er Laves phase) with a mixture of pure elemental Fe and Er powders subjected to high energy ball milling. X-ray diffraction and transmission electron microscopy reveal similar results in both cases. In the early stages, a nanocrystalline fcc phase with lattice parameter a = 0.484 nm and a grain size of 6 nm is formed together with a bcc Fe-rich phase. Extended milling results in a nanoscale phase separation into Fe-rich and Er-rich crystallites with average grain sizes of 1.8-4 nm. Based on a lattice parameter analysis, the fcc phase was initially thought to be a metastable FeEr3 phase. Further studies revealed nitrogen gas in the milling vial had reacted with the powder during ball milling to produce the cubic ErN phase (“NaCl” structure with a lattice parameter of 0.4836 nm). Our experiments demonstrate that the steel vials for ball milling do not remain hermetically sealed during the milling process and a nitride phase can be formed easily if a catalyst for the dissociation of nitrogen molecules (such as Fe) exists in the system.