Study of The Inuence of Chromium on A Cobalt Iron Alloy

The present work comes within the framework of research of new materials, with improved properties, which could be an important key for innovative applications. For this purpose, two types of alloys, a binary (Fe, Co) and a ternary (Fe, Co, Cr), were rst synthesized by mechanical grinding at high energy, varying the grinding time. In a second step, all the samples were subjected to various characterizations, a structural study (X-ray diffraction), a morphological study (scanning electron microscopy "SEM"), a magnetic characterization (the "VSM" vibrating sample magnetometer And nally, an electrical study (eddy currents). Numerous and valuable information was then deduced to know the variations in the average lens size, the internal micro deformation, the cell parameter, the saturation magnetization, the remnant eld, the coercive eld as well as the Z impedance, according to a only parameter, the grinding time.


Introduction
The study of magnetic materials is the basis of the extraordinary development recorded at the end of the nineteenth century. All recent research converges on the transition to a nanoscale in which the properties of these materials are signi cantly improved over those known.
We chose the mechanical grinding method; inexpensive and easy to implement for solid state materials, without melting and by grinding various basic elements in the form of powders.
The Fe-Co system is initially the focus of our attention because it is the basis of permanent magnets, which can be obtained in a wide range. We are also interested in a ternary Fe-Co-Cr system, with the aim of improving the properties for some speci c applications of these.
The structural behavior will be studied as a function of the grinding time by analyzing the graphs obtained and from the average lens size, the internal microdeformations, as well as the parameter of the cell.
Finally, we will pay particular attention to the magnetic behavior of these nano-alloys which will be studied according to different grinding times. From the obtained magnetization curves, we will determine the most relevant magnetic quantities, such as the coercive eld Hc, the remanent induction and the saturation moment. These characteristics will make it possible to evaluate the magnetic behavior of these nano-alloys.

Materials And Methods
The basic elements, namely iron, cobalt and chromium initially were the subject of a weighing in a highprecision analytical balance. Note that the starting powders were of high purity; namely for 99% iron, 99.8% cobalt and 99.5% chromium.
For our study, we used a Fritish Pulverisette 7 type planetary mill, consisting of two jars that rotate around a vertical axis (its own axis), in the opposite direction of the rotational movement animating the plate (disk) that carries it.
Choosing a mass of 2.25 G for binary and ternary one, we were brought to prepare alloys whose nominal compositions are of the Fe 65 Co 35 type for the binary one and (Fe 65 Co 35 ) 90 Cr 10 for the ternary one. Is result from it that the two types of alloys of the mixture differ by the percentage of each component (Fe, Co and Cr) with respect to the total weight of the sample and this, as follows:

Fe65Co35
(Fe65Co35)90Cr10 In order to obtain a homogeneous distribution of the starting elements, we carried out a manual mixing in a mortar during 10 min. the mixture of powders obtained was introduced into the two jars, each containing six stainless steel balls, sealed with a lid provided with a Te on ring seal which makes it possible to maintain the tightness of the jar during grinding and avoids the contact between the outside atmosphere and the powder during the grinding operation.
The grindings were carried out at different times: 0, 1.3, 6, 8, 12, 24 and 36 h; in order to prevent any excessive rise in the temperature inside the jars, grinding is interrupted for 15 minutes every half hour of grinding.
For a better characterization of our samples, we used several techniques to know diffraction of x-rays (DRX), magnetometry with vibrating sample (VSM) and the eddy currents.

STRUCTURAL STUDY
The use of adequate ASTM data sheets allowed the indexing of all existing lines, in particular revealing Fe (CC), Co (HC), Co (CFC) and Cr (CC), in addition to phases binary (Fe, Co) and ternary (Fe, Co, Cr), all the two cubic ones centered.
These graphs characterize the structural and the quantitative evaluation of each of these elements.
One can easily admit that that the primary action of grinding is to introduce a rather violent plastic deformation which is necessarily accompanied by a high population of speci c defects, linear and plans, whose respective movements are likely to change a number of characteristics of the systems studied, like the average size of crystalline, internal micro deformations or the cell parameter.
At rst approximation, the highlighting of this state of affairs is related to the height and the width at half height of the different diffraction peaks obtained the rst decreasing and the second increasing with the grinding time.
Moreover, it appears that the CFC phase of the Co disappears in the binary (Fe, Co) after 1 hour of grinding, while it does not even exist in the ternary alloy (Fe, Co, Cr).
On the other hand, for the same grinding time, the compact hexagonal phase of Co remains present in two alloys, this is probably due to the fact that the CFC phase of Co is metastable at ambient temperature, becomes unstable and is transformed nally into HC [1].
In addition, the compact hexagonal phase of the Co completely disappears from the two alloys, dice 8 hours of grinding.
Concerning the alloy (Fe 65 Co 35 ) 90 Cr 10 , it appears that only the solid solution (Fe, Co, Cr) remains present from 6 hours of grinding. The formation process of the ternary alloy is thus started until its nalization with time.

VARIATIONS IN THE AVERAGE SIZE OF CRYSTALLITES
Two theoretical methods have been used, that of Williamson-Hall and that of Scherrer [2; 3].
It rst appears that the average size of crystalline is a function that decreases with time.
The curve for the binary alloy (Fe Co) can be approximated by a negative straight line, implying that it is at the high grinding times that the difference between the two methods fades. Concerning the ternary (Fe 65 Co 35 ) 90 Cr 10 , the shape of variation as a function of the grinding time is more random. It seems that the smallest gap appears from the rst hour of grinding and then, slightly increases, before almost stabilizing. The effect of Cr most probably is part of this evolution. These graphs can be divided into four parts, a decrease of a between 0 and 1 hour of grinding, an increase of up to 6 hours of grinding, a second decrease of up to 12 hours of grinding and nally an increase of up to 36 hours of grinding.

EVOLUTION OF THE CRYSTALLINE PARAMETER
Moreover, the reduction phase of the parameter a, for the two alloys, can have different origins. Indeed, concerning the binary Fe 65 Co 35 , the decay of a can be explained by the possible substitution of Fe atoms by those of the Co in the sub-network Fe, and also by the formation of defects (gaps, dislocations, grain boundaries) [4][5].
Recall that the atomic radius of Co is signi cantly lower than that of Fe. Between 12 and 36 hours of milling, the increase in "a" is attributed to the migration of Co atoms out of the Fe sub-network, towards the grain boundaries. However, for the ternary (Fe 65 Co 35 ) 90 Cr 10 , the increase of "a" could be understood by a divisional competition between Co and Cr, in the sub-network Fe, it being understood that the diffusion coe cient of Co is by far, superior to that of Cr [5,6].
More generally, the process of decreasing the average crystallite size is the product of a competition between the generation of dislocations created by the grinding process and the crystallization dynamics of the materials, due to the local increase in temperature [7].

MAGNETIC PROPERTIES
The starting point of any study of magnetic properties of a material is the determination of the hysteresis cycle associated with it, under the operating conditions chosen. It is indeed from this graph that all the important parameters are extracted, such as the remnant eld Br, the saturation magnetization BS or the coercive eld Hc. Figures 7 and 8 shows the evolution of the magnetization of the samples as a function of the applied magnetic eld for different grinding times.
The coercivity Hc is often considered as an important parameter because it strongly contributes to the identi cation of the magnetic nature of the material. A high value of Hc indicates the existence of microdeformations, impurities and various defects that occur during the grinding operation [8].
In Fig. 9, it appears that between 0 and 8 hours of grinding, the behavior of his is quite erratic, in that for Fe 65 Co 35 , it decreases, grows and decrease successively, while for and (Fe 65 Co 35 ) 90 Cr 10 , it is the opposite phenomenon which appears, a decrease followed by an increase.
This last result is quite comparable to that found by S. Khosravi et al [1]. It is admitted that the drop of Hc during the rst crushing times is attributed to the decrease of the volume fraction of Co, when it passes from the cubic phase with centered faces (CFC) to compact hexagonal cell (HC).
Indeed, the H.C structures are known to have a higher magneto-crystalline anisotropy than those that are CFC [9]; however, graph analysis shows the progressive disappearance of compact hexagonal Co in favor of face-centered cubic Co, hence the behavior of his.
On the other hand, the fact that his increases between 8 and 24 hours of grinding, in the binary and ternary alloys, could be related to an accumulation of defects and to a greater contamination, with regard to the duration of the grinding which is higher. For a grinding time greater than 24 hours, Hc decreases due to other phenomena. Machine et al [10] showed that his decreases rapidly with decreasing mean crystalline size when the domain wall is larger than the size of the crystalline. In addition, during this grinding period, the particles change shape from a lamellar to a spherical shape, thus leading to the reduction of shape-related anisotropy. Moreover, the variations of saturation magnetization bias as a function of time are shown in Fig. 10.
It can rst be noted that the values of Bs, for the two samples, behave differently, so the addition of Cr appears to be disturbing. Moreover, contrary to the results of the literature [11], Bs is not constant in the case of Fe 65 Co 35 . In our case, Bs is sensitive to the microstructural changes of the material, ie the defects of all kinds that appear and their movement, especially at the beginning of grinding.
In a second step, the decrease in the average size of the crystalline, associated with a possible modi cation of their shape, from lamellar to spheroidal, induces a loss of anisotropy, therefore an increase of Bs. This trend is likely to be reinforced by the increase of the crystalline parameter and the nalization of the process of synthesis of the product.
The presence of Cr which is antimagnetic in the ternary explains the decrease of Bs; hence the total magnetic moment is affected by a modi cation of the con guration between neighboring atoms; therefore, and assuming that Fe and Co are ferromagnetic, the presence of Cr in their immediate environment will cause a sharp fall in Bs.
Starting from the observation that Br and BS behave in the same way, a cross analysis of all the results shows that the values of Br and BS are relatively high, implying that Fe 65 Co 35 and (Fe 65 Co 35 ) 90 Cr 10 has a magnetic energy from 12 hours of grinding, the binary alloy seems to form and reach a form of stability; this is due to the disappearance of cobalt. Moreover, the fact that Hc admits high values suggests a possible use of this alloy in the magnetic discs.
The addition of chromium in the ternary disturbs the formation of the alloy, which is made from 12 hours of grinding; this is characterized by instability observed during this period and is particularly evident in the variations of the remanent eld Br. nally, over a long time, Hc remains almost constant, with a similar evolution to that observed in binary; nevertheless Hc is magni ed around 24 hours, this con rms the in uence of chromium and therefore accredits the idea of the magnetic hardness of the ternary studied.

EDDY CURRENT
For the binary, we note in the rst, the curves Z = f (t), obtained for the two frequencies, con rm the results obtained by DRX and VSM. Indeed, as soon as the 12 hours of grinding, Z admits a quasi-constant, this result is to connect with Hc and with the appropriate curve of DRX. It has been mentioned above that this remarkable point corresponds to the beginning of the formation of the alloy, before stabilizing.
Note that a second remarkable point, corresponding to t = 8 hours, is also to be noted; some authors claim that it corresponds to a metallurgical instability, materialized here by a magnetic polarization [11].
For the ternary alloy, quite comparable results were obtained, as regards the beginning of the formation of the alloy and its stability whatever the frequency considered.