Two Fe-Zr-B-Cu Nanocrystalline Magnetic Alloys Produced by Mechanical Alloying Technique

1. Introduction

In magnetic alloys, it is important to check the thermal stability of the magnetic phase and their soft-hard behaviour. It is also known that the magnetic properties change if the alloy is amorphous or crystalline. Mechanical alloying is a production technique applied for the development of powdered Fe rich nanocrystalline alloys [1,2]. Mechanical alloying is a powder metallurgy technology applied before sintering or consolidation [3,4]. The thermal stability of these alloys front crystalline growth is determinant due to the loss of soft behaviour as increasing the crystallite size. A key parameter to determine this thermal stability is the apparent activation energy of crystallization [5,6].

Soft ferromagnetic alloys are characterized by low coercive field, H_c, high saturation magnetic flux density, B_S, and high permeability, μ [7]. Low coercivity and high permeability favours low core losses in applications under alternate current magnetic fields. Thus, the control of these parameters is associated to the optimization of the energy savings [8]. Regarding the saturation magnetic flux density, higher values favour the application in low dimensional systems, as the consequent miniaturization [9]. Likewise, the thermal behaviour of these alloys allows for the establishing of the working temperature limits; the Curie temperature that marks the transition from ferromagnetic to paramagnetic and the loss of soft magnetism. Magnetic thermogravimetry has been applied to determine this limiting temperature [10].

In the last decades, several families of nanocrystalline soft magnetic alloys have been developed as alternatives to traditional ferrites such as Finemet [11], Nanoperm [12], Hitperm [13] or Nanomet [14]. Figure 1 show the typical values of these nanocrystalline alloys representing the initial permeability as a function of the saturation magnetic flux density. For comparison, information about traditional ferrites, sendust, permalloy, Si steels, Fe-based amorphous and Co-based amorphous is also provided in this graph.

In this work, two nanocrystalline Fe-Zr-B-Cu Nanoperm-type alloys have been produced by mechanical alloying. The thermal stability has been determined by thermal analysis by checking the apparent activation energy of the main crystalline growth process and the Curie temperature.

2. Materials and Methods

In this work, two Fe rich soft magnetic alloys were produced by mechanical alloying (MA). The initial compositions were Fe₈₅Zr₆B₈Cu₁ (at.%) and Fe₈₀Zr₅B₁₃Cu₁ (at.%); both Nanoperm-type. These alloys are labelled as A and B respectively. The precursors were elemental Fe (6-8 µm, 99.9% purity), Zr (5 µm, 98 % purity), B (20 µm, 99.7% purity) and Cu (45 µm, 99 % purity) powders. The milling was performed in in a planetary ball mill device (Fritsch P7 model). The milling process was achieved up to 50 h, under argon atmosphere, with hardened steel vials and balls as milling media, a ball-to-powder weight ratio (BPR) of about 10:1, and a rotation speed of 700 r.p.m. To prevent an excess in the internal vial temperature the milling was performed in cycles (10 minutes on followed by 5 minutes off). The experimental conditions have been chosen to optimize the alloys production by the mechanical alloying technique.

The particles powder morphology and distribution size were checked by scanning electron microscopy (SEM) in a DSM960A Zeiss apparatus. The final composition of the alloys was checked by inductive coupled plasma (ICP) in a Liberty-RL ICP Varian equipment. The nanocrystalline state (bcc Fe rich phase) was confirmed by X-Ray diffraction (XRD) patterns collected using a D-500 Siemens (Bruker, Billerica, MA, USA) equipment with CuKα radiation (λ = 0.15406 nm). The thermal stability of the mechanically alloyed powders was studied by differential scanning calorimetry (DSC) using a LabSys Evo 1600°C apparatus. The DSC curves were measured in the temperature range of 350–950 K at different heating rates: 5, 10, 20, 30 and 40 K/min under argon flow (20 ml/min). The thermogravimetry measurements were performed in a TGA Star^e Mettler Toledo model under Ar atmosphere at a heating rate of 10 K/min. The magnetic parameters such as intrinsic coercivity, Hc, saturation magnetization, Ms, and saturation to remanence ratio, Mr/Ms, were determined by analysing the magnetic hysteresis loops collected in a Lakeshore 7404 vibrating sample magnetometer (VSM) at room temperature, under an applied magnetic field of 15 kOe.

3. Results

The alloys are produced in powder form. Figure 2 shows two micrographs of alloys A and B milled for 50 hours. The rounded shape with smooth contours of the particles and a micrometric size of these is verified. Also, a relatively wide distribution in particle sizes was found. In order to check the particle size and its distribution, the particle size of five micrographs of each sample have been measured. The results are shown in Figure 3.

The contamination was measured by ICP. Mechanical alloying process favours contamination from the milling tools. Likewise, the high surface/volume ratio of the particles induces surface oxidation. Nevertheless, the results show only slight contamination from the milling tools (basically Fe from container and walls) and oxygen in both alloys after 50 h of milling. Similar results were previously reported [15]. ICP results show that the Fe content increase over the nominal composition was 1.7±0.3, and 1.6±0.4 at.% for alloys A and B, respectively. Likewise, the oxygen content was 2.1±0.6 and 1.9±0.5 at.% for alloys A and B, respectively; probably a decrease in the iron percentage is associated to a reduction in the oxygen level.

Regarding microstructural analysis, XRD patterns confirms the formation of the cubic bcc structure with nanocrystalline size. The analysis applying the Williamson-Hall method permits to determine both, the crystallite size and the microstrain. For alloy A. the crystalline size is 26 ± 2 nm and the microstrain 0.58 ± 0.06; whereas for alloy B the values are 15 ± 2 nm (crystalline size) and 0.61 ± 0.08 (microstrain). Thus, the increase of boron content favours the reduction of the crystalline size.

Figure 5 shows the DSC curves of the two alloys milled for 50h (heating rate: 10K/min). The general shape of the curves is similar in both alloys. Around 400 K, a large exothermic process begins. This process is typically found in samples produced by mechanical alloying [16]. Its origin is the microstructural relaxation of the high density of crystallographic defects induced during grinding/milling. The existence of small exothermic processes of the same character at higher temperatures could be an indicator of a not completely homogeneous sample. The weak exothermic process around 750 or 780 K in both samples is typical of a partial recrystallization of the material. The main exothermic process (in the form of a peak, with temperatures around 850 and 880 K in each sample) is due to the crystalline growth of the Fe-rich bcc phase. The development of high performance soft magnetic alloys is associated to amorphous and low crystalline size nanocrystalline alloys. It is necessary that the crystallite size remains smaller than 10 nm. In this case, by applying the random anisotropy model, Hc depending on D⁶ because the domain wall effect diminishes and each grain behaves as a single domain. Thus, crystalline growth should be avoided and the crystallization temperature is a limiting temperature for application of these alloys [17].

One of the most characteristic parameters to characterize a crystallization process is its activation energy. The apparent activation energy of the main crystallization process has been calculated by applying the Kissinger method [18], whose typical representation (for both analysed alloys) is shown in Figure 6. This method is based on the determination of the peak temperature of the crystallization process in the experiments carried out at different heating rates. The values are 282 ± 26 and 299 ± 25 kJ/mol for alloys A and B respectively. These values are consistent with those of the crystallization of the bcc-Fe rich phase.

The calculated energies of this work are compared with some of those of the scientific literature (applying Kissinger method) in Table 1.

This work’s values are in the same relatively wide range [19,20,21,22,23,24,25]. The shift in the activation energies can be related to different composition and microstructure (including crystallographic defects). This dispersion is also found in research of the sane alloy system. Liu et al. [26] found values ranged between 138 and 356 kJ/mol in Fe-Ni-Zr-B alloys produced by mechanical alloying.

Likewise, there are other linear methods (normally based on the determination of the peak temperature). However, the activation energy was similar to those calculated by Kissinger, and the differences are based more on the linear relationships established between the parameters than on real differences in the crystallization process [20]. For example, it is not the same to represent ln(β/T_p²) than ln (β/T_p) (in both cases as a function of the inverse of the peak temperature).

The scientific literature on the activation energy of crystallization processes shows methods in which two energies are determined: that of crystal nucleation and growth respectively [27], the second having a higher value. This approximation is not applicable when there is only crystal growth (for example in initially nanocrystalline alloys). In recent decades, a set of methods based on the calculation of the apparent activation energy has been extended to different transformation/crystallization fractions. These methods are defined as isoconversional [28,29]. Figure 7 shows the calculated values (for the main exothermic process) at fractions transformed from 0.1 to 0.9 [29]. A fairly stable value is found, except for high fractions where a slight decrease in activation energy is detected. In the zone of low-medium transformed fraction, the set of calculated values is similar to that determined by applying the Kissinger method, around 288 and 298 kJ/mol for alloys A and B, respectively. Being nanocrystalline alloys, it is normal for this value to be stable, since the crystal nucleation process is negligible [30]. Thus, the activation energy is that corresponding to the crystal growth mechanism. At high transformed fractions, the degree of transformation slows down and the local activation energy decreases [31]. There is an impingement between the crystal grains.

Regarding the magnetic behaviour, one of the most characteristic and fundamental thermomagnetic parameters of magnetically soft alloys is the Curie temperature. Temperature that marks the maximum value of applicability (magnetic) of the alloy, since it defines the change from a ferromagnetic to a paramagnetic behaviour. The Curie temperature can be determined from the DSC curves [32]. However, in the case of overlapping processes (as is often the case with alloys produced by mechanical alloying) their determination can be complex [33]. Therefore, on many occasions this temperature is determined directly by magnetic measurements (of variation of magnetization as a function of temperature). Another alternative based on thermal analysis techniques is magnetic thermogravimetry. By adding a small external magnet near the sample zone and performing the thermogravimetry experiment, an apparent variation of the sample mass during the transition from ferromagnetism to paramagnetism is detected. This method, previously applied [34], has been used in the present study. Figure 8 shows the thermogravimetric curves recorded. A variation of the apparent mass of the samples around 937 and 971 K for alloys A and B, respectively, is found. These temperatures are typical of ferromagnetic alloys with a high iron content.

As the interest of these alloys is based on their magnetic response, the hysteresis cycles of both alloys (after 50 h of milling) at room temperature have been obtained. Figure 9 shows both magnetic cycles (magnetization M as a function of external magnetic field H). Its analysis allows to determine the parameters that define its soft magnetic response. The determined values are shown in Table 2. Magnetization of saturation M_s 146 and 139 A·m²·Kg^-1, coercivity H_C 10.6·10^-4 T and 12.4 10^-4 T and remanence 0.60 and 0.71 A·m²·Kg^-1 for alloys A and B respectively.

The main influence will be related to the Fe atoms local environment and to Fe-Fe interatomic distance due to the magnetic behaviour of iron. For it, it was expected the detected diminution in the magnetization of saturation as decreasing the Fe/B ratio. Likewise, it is known that the magnetic properties depend strongly on the microstructure evolution, crystalline size, internal stress, particle shape anisotropy, magnetic anisotropy, and magnetostriction of the materials [35]. In our work, higher B content decreases crystallite size and this effect counteract partially (favouring soft behaviour) the effect of the reduction of the magnetic element, Fe. The other values of the magnetic properties are similar in both samples. The relatively low value of the squareness ratio is typical of alloys obtained in powder form by mechanical alloying [6].

Thus, we can conclude that the alloys produced are magnetically soft at room temperature and that thermal analysis is useful to determine the thermal stability (front crystallization and front magnetic transition) of this magnetic behaviour.

5. Conclusions

Two ferromagnetic nanocrystalline alloys of the Fe-Zr-B system have been produced by mechanical alloying (powder shape). Its thermal stability has been analysed using thermal analysis measurements. High apparent activation energy as well as high transition temperatures (crystallization, Curie) are needed to prevent the loss of the soft behaviour caused by the increase of the crystalline size or the ferro- to paramagnetic transition. The differential calorimetry allows to determine (by Kissinger and isoconversional methods) the apparent energy of activation of the main process of crystallization. Being the lowest value, 288 kJ/mol, the one corresponding to the alloy with higher iron content. Regarding the transition temperatures, both the peak crystallization temperature and the Curie temperature are higher in the alloy with lower Fe/B ratio.

Regarding the magnetic response, the characteristic parameters are similar in both alloys, being the magnetization of saturation higher in the alloy with more iron content due to the Fe-Fe magnetic moment atomic interactions.