B2 long-range order in mechanically alloyed Fe 53.3-0.6x Co 46.7-0.4x Sn x (2 ≤ x ≤ 26) annealed at moderate temperatures
Résumé
Recent interest in Fe-Co-Sn alloys stems from Heusler alloys. A Co2FeSn Heusler alloy is found to be unstable relative to other phases from ab initio calculations. However, it may be synthesized by non-equilibrium techniques. The present work focuses similarly on metastable ordered phases in ternary Fe-Co-Sn alloys. First, disordered phases of Sn in near-equiatomic Fe-Co are prepared by high-energy ball-milling. As-milled powders are then annealed to possibly produce long-range ordered phases. As-milled powders, of overall composition Fe53.3-0.6xCo46.7-0.4xSnx (2 ≤ x ≤ 34), consist of a single supersaturated bcc phase for x<~15 at.%. For larger values of x, they are primarily composed of a bcc phase, whose tin content still increases up to ~26 at.%, and of extra phases, which include hexagonal (Fe,Co)3Sn2. Neutron diffraction patterns and 119Sn Mössbauer spectra prove that bcc phases of as-milled alloys order to CsCl (B2) type structures for x ≤ 26 when annealed at 673K. In addition, 119Sn Mössbauer spectra reveal that a Sn content of ~15 at.% marks the separation between domains with different short-range orders. From previous and present results, metastable B2 ordering of Sn-rich Co-Fe-Sn alloys is concluded to occur over a significant concentration range.
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