The effect of B and Si additions on the structural and magnetic behaviour of Fe-Co-Ni alloy prepared by high-energy mechanical milling

: We investigate the structural and magnetic properties of nancrystalline Fe 50 Co 25 Ni 15 X 10 (X=B amorphous , B crystalline and Si) alloy powders prepared by mechanical alloying process. Morphological, microstructural and structural characterizations of the powders milled several times were investigated by scanning electron microscopy and X-ray diffraction. The final metallurgical state strongly depends on the chemical composition and the grinding time; it can be single-phase or two-phase. The crystallite size reduction down the nanometer scale is accompanied by the introduction of high level of lattice strains. The dissolution of Co, Ni, B (amorphous and crystalline) and Si into the α-Fe lattice leads to the formation of highly disordered Fe-based solid solutions. Coercivity (Hc) and the saturation magnetization (Ms) of alloyed powders were measured at room temperature by a vibration sample magnetization. The magnetic measurements show a contrasting Ms and (Hc) in all alloy compositions. Conclusively, soft magnetic properties of nanocrystalline alloys are related to various factors such as metalloids addition, formed phases and chemical compositions.


Introduction
Significant efforts were made to improve the strength and resistivity of iron through development of nanomaterials and alloying additions. Mechanosynthesis is among the techniques that favors the formation of nanocrystalline and/or amorphous iron alloys after a sufficient milling time [1][2]. Further, the release of residual stress generated during milling process, tend to lead to better magnetic properties, including higher saturation magnetization (Ms) and lower coercivity (Hc)..Fe-Co alloys are also known as soft magnetic materials [3,4].
The great effect of the addition of a third element (Ni, Cu, V, Mo, Sn) on the mechanical and magnetic properties of the Fe-Co system are also examined in detail [5][6][7][8][9][10]. On the other hand, the addition of certain amount of metalloids, in particular C, B, Si and P favors thermal stability and the soft magnetic properties of Fe-based alloys [11]. Moreover, the addition of small amounts of boron to alloys can modify the fracture mode from intergranular failure to transgranular fracture by being segregated to grain boundaries. Studies on the microstructure of these alloys gave evidence that the addition of boron can also result in the formation of Fe2B precipitates and a tetragonal phase which is likely coherent with the matrix [12]. Boron has been also found to facilitate the development of amorphous and nanocrystalline structures in the bcc-Fe phase [13]. Alternatively, Liu et al. [14] pointed out that the addition of 6 wt % Si to Fe-30Mn alloy rendered a material with shape memory effect and better corrosion performance. The addition of Si increases the resistivity leading to decrease in core losses in Fe-Si alloy [15,16]. In the present work we have selected the Fe50Co25Ni25 compound as the starting alloy, and we have explored what changes are induced on the microstructure by the addition of 10 at. % Bamorphous, 10 at. % Bcristalline and 10 at. % Si. Besides, the variation of the room temperature magnetic properties with the B and Si contents are reported.

Materials and methods
The mixtures of Fe50Co25Ni15X10 (X=Bamorphous, Bcrystalline and Si) (at.%) powders were prepared in proportions corresponding to the nominal composition from the elemental powders of Fe (99.97% purity, mean particle size  10 m), Co (99.9% purity, mean particle size  2-5 m), Ni (99.7 % purity, mean particle size  10 m), Bcristalline (purity> 99%), Bamorphous (purity> 99.5) and Si (99.5 % purity, mean particle size  10 m) by using a highenergy planetary ball-mill (Type P7) under Ar atmosphere. Ball milling experiments were carried out in a hardened steel container. The ball-to-powder weight ratio (Q) is 2:1 and the milling speed () was adjusted to 600 rpm. Different milling times ranging from 0 to 100 h were used. The milling sequence was selected such as 10 minutes of milling followed by 5 minutes of idle period, to prevent sticking of the powder to container walls and the balls, and powder agglomeration during milling.X-ray diffraction (XRD) measurements were done using a D-500 Siemens equipment with CuKα radiation. The size of the crystallites, the lattice strains and lattice parameter were calculated based on the Rietveld method using the Maud program [17]. In all refined XRD patterns, refinement parameter Rexp is lower than 10.5% and GOF parameter lower than 1.6. The morphology and the composition of mechanically alloyed powders were examined by scanning electron microscopy (SEM) in a DSM960A ZEISS microscope in secondary electron mode operating at a voltage of 15 kV. The SEM was equipped with a Vega_Tescan energy dispersive X-ray spectrometry (EDS) analyzer. The magnetic characterization was carried out by Superconducting Quantum Interference Device from Quantum Design SQUID MPMS-XL at 300 K (about 150 mg of powder in each experiment).

3.
Results and discussion Figure 1 shows the microstructure of the Fe50Co25Ni25, Fe50Co25Ni15B10(amorphous), Fe50Co25Ni15B10(crystalline) and Fe50Co25Ni15Si10(amorphous) before milling ( Fig. (1 a-d)) and after mechanical milling for 50 h (Fig.(1 a1-1d1)) and 100 h ( Fig.(1 a2-1d2)). It can be seen that increasing the milling time from 0 to 100 h results in a considerable reduction in the particle size of all the powder. After an intermediate time of 50 hours milling, the shape distribution of all the compositions and powders shows larger particles of irregular shape and size due to cold welding ( Fig. 1 a1-d1). Depending on the dominant compressive forces, the particle size either may become smaller (relatively fine particles) due to fracturing, or larger (agglomerated particles) by cold welding during the grinding process. Indeed, the work-hardening due to these compressive forces of the milling causes a hardening of the powder particles and consequently leads to their fracture. The atomically clean surfaces created during fracture allow the particles to be welded again, which increases the size of the particles. In addition, it can be noted that the size of the particles depends on the nature of added metalloid; the size increases from the sample free of metalloid to the rich in amorphous boron, the crystallized boron and the silicon. This may be related partly to the nature of the starting particles because the presence of B and Si increases the hardness and brittleness of the powders. As a result, relatively hard particles tend to resist attrition and compressive forces and therefore,may remain less deformed. Si-rich powder has the largest particle size. Finally, as shown in Fig.   1a2-d2, the morphology of the particles obtained after milling for 100 h becomes finer and more homogeneous. In fact, increasing deformation and work hardening disintegrate the agglomerated powders into fragments, giving rise to a fine particle size distribution. On the other hand, at this more advanced stage of milling, B and Si can help in refining the morphology. This effect could be due to the preferential diffusion of B and Si atoms to the interstitial sites of the bcc phase through grain and particle boundaries enhancing the precipitation of Fe-B and Fe-Si compounds there. This phenomenon has a beneficial effect on the fracture mode of FeCoNi alloys, which changes from intra-granular to inter-granular fracture mode. These observations suggest that the presence of the fracture surfaces can be associated with the presence of borides and silicon-rich phases in grain boundaries.

Structural properties
As the milling time increases, the X-ray diffractograms show an expansion of the different diffraction peaks and a decrease in their intensities. This behavior is a characteristic common to powders prepared by high-energy mechanical milling [1,2]. The broadening of the diffraction peaks can be attributed to (i) the finite dimension of the diffracting domains in a coherent way and/or (ii) the structural imperfections able to distort the crystal lattice and then to cause a variation of inter-reticular distances around a mean value. Thus, a slight displacement is observed which is not the same for all the diffraction peaks and milling times.
The displacement of a peak comes from the formation of a solid solution by mechanical milling and the introduction during the milling process of the first-order stresses acting on the macroscopic scale by modifying the lattice parameter of the material. Figure 2 shows the XRD patterns obtained of the milled samples with different metalloids addition as function of milling time. In the starting powders, all the major XRD peaks were found to correspond to that of bcc-Fe (SG Im-3m; a = 2.866 Å), hcp-Co (SG P63/mmc; a = 2.500 Å and c = 4.140 Å) and fcc-Ni (SG Fm-3m; a=3.523 Å) and fcc-Si (SG Fd-3m; a=5.4309 Å) elements as the B used was amorphous or crystalline with a low atomic scattering factor and did not therefore contribute to any significant peaks in the diffraction patterns. For the sample without metalloid addition, the small shift of the main diffraction peak (110) (Fig. 2b), but it disappears after 50 h. The boron doped samples (in its two states) showed that its dissolution in the iron lattice led to the appearance of new phases of boride type such as cubic-Fe23B6, tetragonal-FeB, tetragonal-Fe2B and orthorhombic-Fe3B. Indeed, in the case of sample with amorphous boron (Fig. 2b), one can notice the rapid formation of cubic-Fe23B6 phase. This phase adopts the Cr23C6 prototype structure with the Fm3m space group. This metastable phase can easily decompose into Fe3B and Fe2B. However, when the Fe atom site is partially substituted by another transition metal atom such as Co, or Ni, the phase is stabilized and more easily retained [21]. contraction. Therefore, the line profiles become symmetric (Fig. 2c).
In the case of the FeCoNiSi sample, the fcc-Ni(Si) solid solution which nucleated after 17 h with a proportion of 33%, persists up to a milling of 50 h (Fig. 2d). While the appearance of Fe(Co) solid solution was at 6 h milling and its disappearance was noted at 50 h. After 100h milling FeCoNiSi sample showed the complete dissolution of Co, Ni and Si in Fe lattice and the obtained powder consists of a highly disordered bcc-Fe(Co, Ni, Si) solid solution.
There was a slight shift in the XRD peaks of Fe towards lower or higher Bragg angle reflecting a level of increase or decrease in the lattice parameter with increasing milling time. the solution of point defects in the crystal lattice will disrupt its structure around the vacant positions ultimately leading to a distorted crystal lattice. The latter becomes more significant with smaller crystallites and promotes a higher solubility of the vacancies [23]. On the other hand, it is interesting to note that in the final milling products the lattice parameter increases from the FeCoNi to FeCoNiSi, FeCoNiBamorpous and FeCoNiBcrystalline.
The dependence of the calculated crystallites size and microstrains on milling time of the as-milled powders is given in Figure 4. It is clear that the crystallite size decreased with an increase in the milling time for all alloys ( Fig. 4a; Table 1). However, the crystallite size of field associated with the multiplication of the dislocations. Micro-stress in crystallites is generated by defects. It is worth to note that the reduction in crystallite size is accompanied by an increase in the lattice strain level as the MA time increases. This is a common behavior for all metallic systems prepared by MA.

Magnetic properties
The microstructures obtained after mechanical milling can strongly influence the magnetic properties of alloys [24]. The time dependence of the hysteresis loops at 300K, of the mechanically alloyed FeNiCo, FeNiCoBamorphous, FeNiCoBcrystalline and FeNiCoSi powder mixtures for selected milling times are shown in Fig.5. All hysteresis cycles exhibited a sigmoidal shape, which is usual in nanostructured samples with small magnetic domains [22].
This is due to the presence of structural distortions inside the grains. The small hysteresis losses are properties generally desired in soft magnetic materials. Fig. 6 presents the influence of metalloid additions on coercivity (Hc) and its dependence with milling time. It is noteworthy that the evolution of Hc can be divided into three stages for the doped alloys and in two stages for the undoped one : an ascending part for advanced milling times below of 6 h followed by a descent for times up to 50 h milling for all alloys, while for higher times there is an increase in the Hc value which intensifies from doped alloys by boron to that doped by of Hc can be attributed to the introduction of internal stresses and structural defects such as dislocations.Since the dislocations distort the surrounding material, stress field is always associated with dislocations and its interaction with moving magnetic domain walls would impede the wall motion [22,25]. This behavior tends to harden the nanocomposite system, because its magnetic anisotropy is greater than that of the bcc phase. However, materials with high magnetic anisotropy usually have high coercivity and hence they are hard to demagnetize. Since the magnetocrystalline anisotropy contributes strongly to the coercive field, it has a great influence on industrial uses of ferromagnetic materials. presence of superparamagnetic particles [24]. Therefore, the reduction of the coercive field, Hc, in nanocrystalline materials must be clearly distinguished from the superparamagnetic phenomenon where the reduction of Hc is well established in small isolated or weakly coupled particles following thermal excitation [26,27]. In addition, the variation in saturation magnetization (Ms) with milling time provides additional information on the evolution processes occurring during mechanical alloying. The variation in saturation magnetization Ms with milling time is given in Fig. 7 This fact can be explained by the interaction between the metallic atoms which are ferromagnetic with Si and B atoms that are non-ferromagnetic in nature [28,29]. In other way, the fragmentation of the magnetic particles during the milling process leads to a heterogeneous ferromagnetic system where the Fe-rich ferromagnetic grains are separated by Si-rich and B-rich phases. Thus, the coupling between the ferromagnetic grains becomes less effective, giving rise to the observed hardening. For the undoped alloy, Ms increases continuously to a value of 188 emu/g because of the progressive dissolution of the elements Co and Ni, then decreases strongly to regenerate the initial state with an Ms value of 124 emu/g because the grain refinement after 50 h milling.

Conclusion
Nanostructured Fe50Co25Ni15X10 (X=Bamorphous, Bcrystalline and Si) (at.%) powders with semihard and soft magnetic properties were prepared by mechanical alloying. The most interesting results of this research are as follows:


The formation of supersaturated Fe-based solid solutions in nanometer scale is accompanied by an allotropic hcp-Co  fcc-Co transformation. The structure of defects produced by high-energy mechanical milling was considered as responsible of these phases transformations.


The coercivity and magnetization values strongly depend on the structure and magnetically soft materials are only obtained for undoped FeCoNi samples at the end of mechanical milling. In fact, when Si and B are alloyed to FeCoNi, the coercivity increases and the magnetic behavior tends to be semi-hard. Fig. 1. Morphological evolution of the powder particles of the Fe50Co25Ni25, Fe50Co25Ni15B10(amorphous), Fe50Co25Ni15B10(crystalline) and Fe50Co25Ni15Si10(amorphous), before milling (a, b, c and d, repectivety) and after 50 h (a1, b1, c1 and d1, respectively) and 100h