Mechanically Driven Decomposition of Intermetallics

Abstract

Mechanically driven decomposition of intermetallics during mechanical milling(MM) was investigated. This process for Fe-Ge and Fe-Sn system was studied using conventional XRD, DSC, magnetization and alternative current susceptibility measurements.Mechanical alloying and milling form products of the following composition (in sequence of increasing Ge content): α(α1) bcc solid solution, α + β-phase (Fe2-xGe), β-phase, β + FeGe(B20), FeGe(B20), FeGe(B20) + FeGe2, FeGe2, FeGe2 + Ge, Ge. Incongruently melting intermetallics Fe6Ge5 and Fe2Ge3 decompose under milling. Fe6Ge5 produces mixture of a-phase and FeGe(B20), Fe2Ge3 produces mixture of FeGe(B20) and FeGe2 phases. These facts are in good agreement with the model that implies local melting as a mechanism of new phase formation during mechanical alloying. Stability of FeGe(B20) phase, which is also incongruently melting compound, is explained as a result of highest density of this phase in Fe-Ge system.Under mechanical milling (MM) in planetary ball mill, FeSn intermetallic decomposes with formation Fe5Sn3 and FeSn2 phases, which have the biggest density among the phases of Fe-Sn system. If decomposition degree of FeSn is relatively small (<60%), milled powder shows superparamagnetic behavior at room temperature. For this case, magnetization curves can be fitted by superposition of two Langevin functions. Particle sizes for ferromagnetic Fe5Sn3 phase determined from fitting parameters are in good agreement with crystalline sizes determined from XRD data and remain approximately changeless during MM. The decomposition of FeSn is attributed to the effects of local temperature and local pressure produced by ball collisions.