B2 long-range order in mechanically alloyed Fe53.3−0.6x Co46.7−0.4x Sn x (2 ≤ x ≤ 26) annealed at moderate temperatures

Abstract

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 Mossbauer spectra prove that bcc phases of as-milled alloys order to CsCl (B2)-type structures for x ≤ 26 when annealed at 673 K. In addition, 119Sn Mossbauer 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.