Influence of the order-disorder transition on the magnetic properties of Fe75Al25-xSix alloys
Introduction
The interest in iron-rich intermetallic compounds is based on a wide range of potential applications including high-temperature structural materials, functional materials for magnetic applications, diffusion barriers, contacts, and interconnections in microelectronics [1]. Fe aluminides and Fe silicides are considered for structural applications because they unite high strength with excellent corrosion resistance in oxidizing and sulfiding environment [2]. The Fe-rich compounds Fe3Al and Fe3Si possess a very high magnetic susceptibility which makes them useful as soft magnetic materials [3]. The relationship between structural and magnetic properties in Fe–Al and Fe–Si alloys has been extensively studied during the last years [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14].
The mechanical and magnetic properties of these binary alloys can be improved by mixing Al and Si. For example, an almost 2:3 mixture of these two alloys, called Sendust, has been used as magnetic head material in recorders [15]. Nowadays, FeSiAl alloys have attracted much interest due to their high permeability [16]. Some researchers prepared flaky FeSiAl metallic powders and investigated the relationship between the crystal structure and the microwave properties [17], [18], [19]. The novel powders have a good anti-EMI (Electromagnetic Interference) characteristic in communication and computer systems [20], [21].
The effect of Si introduction in the magnetic properties of stoichiometric Fe3Al alloy is twofold; on the one hand it contributes to the decrease of the lattice parameter which makes the magnetism weaker and, on the other hand, the introduction of an additional electron makes the hybridisation to increase and therefore the magnetic moment decreases [22]. However, as far as the structure is concerned the Fe-rich side of the phase diagram of the Fe–Al, Fe–Si and Fe–Al–Si systems are quite similar [23], [24]. Indeed, for a 75 at% Fe, the stable structure at room temperature is the D03 structure, which is stoichiometric for this Fe content.
Mechanical deformation is known to induce A2 disordered structure in the three systems [25], [26]. Moreover, in Fe–Al alloys there is a relationship between the enhancement of the magnetization and the structural order-disorder transition. This relationship has been explained in terms of the change of the local environment of Fe atoms, where the Fe magnetic moment depends on the number of Al atoms as nearest neighbours decreases [27], [28]. Furthermore, mechanical deformation causes an increase in volume [29], [30] and the influence of this increase on the magnetic moment has been proved both theoretically and experimentally in a few studies [31], [32], [33].
The relationship between the crystal structure and magnetic properties of Fe75Al25.xSix alloys prepared by different non-equilibrium techniques, as mechanical alloying [34], [35], and melt spinning [17], [36], has been reported. However, we are not aware of a systematic investigation of the influence of the order-disorder transition on the magnetic properties of Fe75Al25-xSix, and the relationship between the variation of the lattice parameter and the magnetic properties in the D03 range of the phase diagram. The present investigation, therefore, fills in this gap.
Section snippets
Experimental
The samples have been obtained by means of induction melting in alumina crucibles and casted into ingots under a 40 kPa pressure of high purity argon. Iron, Aluminium and Silicon with a purity of 99.99%, 99.99% and 99.999%, respectively were used. Afterwards, they were powdered by mechanical crushing for the same time in order to apply similar deformation energy (as-crushed samples). The as-crushed samples were then annealed to obtain large domains of ordered structures. Fe75Al25−xSix were
Results and discussion
Fig. 1 shows some of the XRD patterns of two of the studied samples deformed by different ways. XRD patterns of annealed (ordered) alloys present (1 1 1) and (2 0 0) superstructure peaks. The appearance of both superstructure peaks indicates the existence of D03 structure. In the XRD patterns of as-crushed alloys the superstructure peaks can be clearly distinguished.
On the other hand, after 1 h of milling (Fig. 1) the diffractogram of binary Fe75Si25 and the ternary alloys are different. For
Conclusions
For Fe75Al25−xSix series the ordered alloys have only D03 structure. The mechanical deformation produces the appearance of the disordered A2 structure and a variation of the lattice parameter. However, there is no evidence on the influence of the degree of disorder and/or the variation of lattice parameter in the magnetism of the alloys.
The magnetization measurements show two different behaviours depending on Si content. On the one hand, the saturation magnetization of the Fe75Al17.5Si7.5
Acknowledgements
Financial support from the Spanish CICYT and Basque Government under Grants MAT2012-37923 and IT-443-10 is appreciated.
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