Nanoscale Phase Separation Induced by Mechanical Alloying in the Iron-Erbium-Nitrogen System
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NANOSCALE PHASE SEPARATION INDUCED BY MECHANICAL ALLOYING IN THE IRON-ERBIUM-NITROGEN SYSTEM Z. Fu, H. J. Fecht and W. L. Johnson Department of Materials Science, Keck Lab., California Institute of Technology, Pasadena, CA 91125 ABSTRACT 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 (Fe 2Er 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 FeEr 3 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. INTRODUCTION Mechanical alloying was developed by Benjamin['] as a new technique for synthesizing dispersion-strengthened alloy powders. The mechanical alloying method can create true alloys of metals or metal / non-metal composites that are difficult or impossible to combine by other methods. The synthesis of an amorphous phase by mechanical alloying was first reported by Koch et al.[21 for a mixture of elemental powders of Ni and Nb with composition Ni6 0Nb 4 0. Investigations of mechanical alloying have been mainly focused on amorphous phase formation[3] within the last few years. It is now known that amorphous phases can be synthesized by ball milling of either an intermetallic compound or a mixture of elemental metal powders. In addition to metallic glass formation, standard ball mill processes have recently been applied to produce metastable intermetallic compounds and nanocrystalline metals and alloys [4.5.61. In these experiments, powder samples of the Fe 2Er Laves phase as well as a mixture of elemental Fe and Er powders have been subjected to the ball milling process. Interest in the Fe-Er compounds and alloys was inspired by the recent discovery of destabilization and glass formation of the Fe2 Er compound by hydrogen absorption [71.The current studies find the steel vial for ball milling not remaining hermetically sealed during the milling process. They reveal that equilibrium alloy phases can be easily destabilized by nitride phase formation if a catalyst for the dissociation of nitrogen molecules is present. EXPERIMENTAL
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