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Mechanical alloying was used to prepare Fe 57 Co 21 Nb 7 B 15 (7Nb) and Fe 61 Co 21 Nb 3 B 15 (3Nb) powder mixtures in a high-energy planetary ball mill under argon atmosphere. Thermal stability, structural and magnetic properties were followed by differential scanning calorimetry, X-ray diffraction, vibrating sample magnetometry and Mossbauer spectrometry. For the Fe 57 Co 21 Nb 7 B 15 powders, an amorphous matrix (∼69%) where nanocrystalline bcc FeCo, bcc Nb(B), tetragonal Fe 2 B and orthorhombic Fe 3 B type phases were embedded is reached after 96 h of milling. However, for the Fe 61 Co 21 Nb 3 B 15 mixture, the partially amorphous structure (∼75%) is achieved within 48 h of milling. Further milling time (up to 96 h) gives rise to the primary crystallization of α-Fe in addition to bcc FeCo, orthorhombic FeB, tetragonal Fe 2 B and tetragonal Fe 3 B type borides. Lower Nb content powders (3Nb) exhibit higher saturation magnetization, Ms, higher coercive field, Hc, and lower glass transition temperature values than those of the 7Nb powders.

L'alliage Fe 61 Co 21 Nb 3 B 15 a ete prepare a partir d'un melange de poudres par broyage mecanique haute energie. Les changements structuraux et microstructuraux ont ete suivis par diffraction des rayons X (DRX), spectrometrie Mossbauer, et calorimetrie differentielle programmee (DSC). Une structure amorphe, hautement desordonnee dans laquelle sont loges des grains de taille nanometrique de borure de fer, a ete decelee, apres un broyage de 48 h, par DRX et par spectrometrie Mossbauer. Un processus de recristallisation mecanique mene a la formation de nano-grains de α-Fe et α-FeCo, apres un temps de broyage prolonge. Le desordre dans les poudres broyees a ete confirme par DSC ou plusieurs courbes exothermiques se chevauchent sur un large domaine de temperature. Ce comportement provient de la relaxation des contraintes et du grossissement des grains.

Fe 61 Co 21 Nb 3 B 15 powder mixture was prepared by mechanical alloying process in a high energy planetary ball mill. Structural and thermal changes of the milled powders were followed by X-ray diffraction (XRD), Mossbauer spectrometry and differential scanning calorimetry (DSC). Both XRD and Mossbauer spectrometry results reveal the formation, after 48 h of milling, of a highly disordered amorphous-like structure where nanometer-sized iron borides were embedded. A mechanical recrystallisation process gives rise to the formation of α-Fe and α-FeCo nanograins on further milling. The occurrence of structural disorder in the milled powders might be confirmed by broad exothermic reaction in the DSC scans which consists of several overlapping exothermic peaks. Such behaviour originates from recovery, strain relaxation, grain growth and crystallisation.

X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were used to investigate the phase identification and the thermal behaviour of the mechanically alloyed Fe57Co21Nb7B15 powders. The diffusion of B into the Nb lattice leads, after 1 h of milling, to the formation of a bcc Nb(B) solid solution with a lattice parameter close to a = 0.3425 nm. The solid state reaction between Fe and B gives rise to the formation of Fe23B6 and Fe2B boride phases after 3 and 6 h of milling, respectively. On further milling (96 h), an amorphous matrix (∼80%), where nanocrystalline bcc α-Fe, bcc Nb(B), Fe2B and Fe3B phases were embedded, is obtained. The broad exothermic reaction in the DSC scans consists of several exothermic peaks and spreads over the entire temperature range 100-700°C. The enthalpy release at temperatures below 300°C can be attributed to recovery and strain relaxation. Crystallisation and grain growth are the dominating processes at high temperatures.