Estudo de fases do sistema Fe-Co-Nb e correlações de suas propriedades estruturais e magnéticas

Abstract

The binary Fe-Co system, in the composition range 40-60at%Fe, and the ternary Fe-Co-Nb system, in nearly equiatomic FeCo alloys with 0-13.4at%Nb, have been studied here. Our leading purposes in this investigation were to describe the phases present in the quasi-equilibrium ternary diagram at room temperature and to correlate the micro-compositional alterations and the structural and magnetic properties. We used the following experimental techniques: transmission Mössbauer spectroscopy, X-ray diffraction, optical and electron scanning microscopy and microprobe analysis (EDX). Our data show unequivocally that the Fe-Co-Nb diagram exhibits two distinct phases in the range 0-70at%Co, above the Fe-Co edge, and up to 33at%Nb, namely: -(FeCo) = bcc or B2 superstructure, dissolving a low content of Nb and -(FeCoNb) = Laves hexagonal with MgZn2 type structure. We propose the existence of a continuous Laves domain in the neighborhood of (Fe1-xCox)2Nb ranging from the Fe-Nb to the Co-Nb edges. The same is true for a eutectic valley extending from 10.6at%Nb on the former and 14.5at% on the latter corner respectively. The alterations of the Co content in the -phase and the consequences on the magnetic (changes in Bint) and structural properties (changes in the structurally disordered volume fractions) were also investigated. The application of this disproportion of the equiatomic Fe-Co balance justifies the use of Nb as a beneficial ternary addition in order to retard the ordering process in the binary Fe-Co system.