Structural and magnetic behavior of Fe(Nb,Zr) rich alloys produced by mechanical alloying

Fe80Nb7B12Cu1 and Fe80(NiZr)7B12Cu1, nanocrystalline alloys were synthesized in two high-energy ball milling devices (planetary, shaker). The microstructure, thermal and magnetic properties of the milled powders were characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and vibrating sample magnetometry (VSM); respectively. Milling device influences the microstructure and properties of final products. The results suggest more energetic milling in shaker mill. The main phase is always bcc Fe rich solid solution. Nevertheless, in Fe80Nb7B12Cu1 alloy minor Nb(B) phase is found after shaker milling and in Fe80(NiZr)7B12Cu1 alloy a low crystalline size Zr rich phase after planetary milling. Crystalline grain size ranges between 9.5 and 15.1 nm; lower values correspond to alloys with a second minor phase. Coercivity values ranges between 28.6 and 36.9 Oe.


INTRODUCTION
Fe-M-B alloys with M an early transition metal (so called Nanoperm) are known to be usually soft-magnetic materials due their high saturation magnetization, good permeability and low magnetocrystalline anisotropy, Makino et al. (1997). These alloys are generally obtained as amorphous by rapid solidification techniques and nanocrystalline structure was found after an ulterior annealing, Skorvanek et al. (1999). On the other hand, these compositions can be also obtained directly as nanocrystalline grains by mechanical alloying (MA), Blázquez et al. (2014). MA is a solid-state powder processing technique that can produce a variety of equilibrium and non-equilibrium alloy phases at a nanoscale level, Suñol et al. (2004). During MA of Fe rich alloys, an important fraction of grains boundaries is created leading, generally, to the apparition of novel physical properties, especially, magnetic ones. Likewise, in the last decades, many researchers have been carried out experimental studies to investigate the alloying effect of systems with three, four or even more elements, Miglierini et al. (2016), Hocine et al. (2017), Kong et al. (2011). One option is the addition of a minor amount of copper as in Finemet alloys. In this work we analyze the nanostructure, thermal stability and magnetic response of two Fe(Nb,Zr) rich alloys produced by mechanical alloying. The competition during alloying between different elements sometimes is not well understand and therefore needs more carefully investigations. Furthermore, alloying is also influenced by milling processing parameters and devices. For it, milling was performed in two milling devices for 80 hours to detect their influence on milled nanostructured samples.

EXPERIMENTAL
Mechanical alloying was carried out in two high-energy ball milling devices: a) planetary ball mill (Fritsch Pulverisette P7) and b) shaker mill (SPEX 8000 pure element and compound powders. Fe of 99.7% purity, with a particle size under 8 µm; Nb of 99.85% purity, with a particle size under 74 µm; Cu 99.8% purity, particle size under 45 µm, B of 99.6% purity, with a particle size 50 µm; and prealloyed Ni 70 Zr 30 powders (purity of 99.9% and particle size >150 µm). The nominal compositions produced and analyzed were Fe 80 Nb 7 B 12 Cu 1 and Fe 80 (NiZr) 7 B 12 Cu 1, labeled as A and B, respectively. In both devices, milling was performed during 80h in CrNi steel vials and the powder to balls mass ratio was fixed at 1:5. The containers were sealed in a glove box with a stationary argon atmosphere. The MA powders were thermally characterized by differential scanning calorimetry (DSC) in DSC30 Mettler-Toledo equipment. X-ray diffraction (XRD) structural analysis was carried out using D-500 Siemens equipment using Cu K α radiation. Magnetic measurements were performed by vibrating sample magnetometry (VMS) in a Lake-Shore device.

RESULTS AND DISCUSSION
The formation of the nanocrystalline structure by mechanical alloying was checked by X-ray diffraction. Figure 1 show the XRD patterns corresponding to the alloy A after 80 h milling. In alloy milled with SPEX only bcc-Fe rich solid solution phase was found. This phase is the main phase
AIP Advances 8, 047704 (2018) in alloy milled in P7 device. Likewise, in this sample, a minor phase associated to Nb(B) rich solid solution is also found. In alloys with Fe and B milling sometimes favors the apparition of Fe-B compounds, but in Nb-B milling the Nb(B) rich phase here detected was also found in the literature, Izumi et al., 2006. The reaction between B and Nb is related to the negative enthalpy of mixing: ∼ -39 kJ/mol. This fact indicates that milling with SPEX allows to the formation of a unique phase whereas probably additional milling time is needed in P7 device, as it was obtained in Fe-Nb-B milled alloys by Alleg et al., 2010. This seems indicate a higher milling intensity and energy transfer to the sample in SPEX device. Figure 2 show the XRD patterns corresponding to the nanocrystalline alloy B obtained after 80 h milling. In this alloy, the bcc Fe based phase was also found. The main difference is that Zr rich nanocrystalline solid solution was found in sample milled with SPEX. This Zr phase was previously found in other FeZr based alloys, Pilar et al. (2008). In the FeNiZrBCu alloy, P7 milling is more effective to obtain a single nanocrystalline phase and milling in SPEX device produces the formation of a Zr phase coexisting with the nanocystalline main phase. Nb and Zr atoms have high atomic radius and this fact does not favor its introduction in the solid solution, Pilar et al. (2007).
The microstructural and structural parameters were obtained, from the Rietveld refinement of the XRD patterns, by applying the MAUD Program, Lutterotti et al. (1999). Results of the main bcc-Fe rich phase are given in Table I (2) 15.1(9) 0.6(1) 5.5(9) B (P7) 0.28706 (2) 13.4(5) 0.59 (6) 6.2(6) B (SPEX) 0.28717(3) 9.5(2) 0.40 (3) 5.9(4) (pure bcc Fe) indicating the formation of a solid solution. All alloys are nanocrystalline (crystalline size range between 9.5 and 15.1 nm). Lower values were found in samples with a minor phase. Thus, as increasing the content of Nb and Zr atoms inside the solid solution, a higher grain size was achieved. Microstrain values ranged between 0.40(3) and 0.66 (5), values similar to those found in the literature for Fe rich ball milled alloys, Alleg et al. (2013). Furthermore, the density of dislocations has been also calculated taking into account the expression given by Williamson and Smallman et al. (1956). The values are slightly lower than dislocation density limit in metal achieved by plastic deformation (10 16 /m 2 for edge dislocations), Eckert et al. (1992). The general character of the DSC traces for all samples is similar. Figure 3 show the results corresponding to alloy A milled for 80 h. The exothermic process at ∼200 K is due to structural relaxation of milled alloy. The broad exothermic process starting at ∼250-300 K might be due to early surface crystallization (particle surface) coupled with internal stress, Ipus et al. (2013). The main crystallization process (detected in alloy milled in SPEX at ∼ 500 K) is related to the crystalline growth of the bcc Fe phase. Figure 4 show DSC scans of alloy B. The broad exothermic processes detected by DSC correspond to different Fe environments probable favored by Ni addition in alloy B, Suñol et al. (2007). In sample milled in P7 device the small exothermic peak at ∼350 K is linked to crystalline growth of the bcc Fe rich phase. Likewise, in alloy B milled in SPEX a very intense crystallization peak (∼550 K) is found. As higher is the temperature of the crystalline growth exothermic peak, higher is the thermal stability of the nanocrystalline alloy front crystallization. The behavior here found is different than crystallization from amorphous phase. The crystallization of as-quenched amorphous ribbons is reported by Makino et al. (1994) in Fe-X-B (X = Zr, Hf or Nb) alloys. The amorphous phase crystallizes in a two-stage crystallization process in which a single bcc Fe nanocrystalline phase appears after the first stage, and coarse bcc Fe grains with intermetallic phases appear after the second stage. The reported temperature of the second crystallization stage (>1000 K) is higher than the maximum temperature achieved in our DSC equipment. Magnetic hysteresis cycles, at room temperature, were shown in figures 5 and 6 corresponding to alloys A and B, respectively. Such sigmoidal hysteresis cycles are usually observed in nanostructured samples with small magnetic domains. This is due not only to the presence of structural distortions inside grains, but also to a higher density of nanocrystalls which hinder the domain wall motion. Results are given in Table II. All the studied samples present in the nanocrystalline state ferromagnetism at room temperature and have low coercivity, H c , values (ranged between 28.6 and 36.9 Oe) which are the most important requirements for a soft magnetic material. Coercivity values are similar to those found in literature, Ipus et al. (2013). Lower coercivity was found in the two samples with only one crystalline phase (higher difference in alloy B). Thus, second phase is not as soft magnetic than Fe rich bcc main phase and provokes an increase of H c , There are other considerations to be take into account. As regarding boron element influence, sometimes boron inclusions were formed; Ipus et al. (2009). These inclusions can be not detected by XRD favoring the hardening of the alloy. Likewise, an amount of niobium/zirconium can remain in the grain boundaries.
The decrease of the saturation magnetization, M s , was measured in alloy A milled in SPEX. This decrease usually suggests remarkable change of the magnetic moment during the alloying due to the change of the nearest neighbor configuration of the magnetic element, Fe. This effect is coherent with the existence of non-magnetic niobium and boron atoms in the vicinity of iron sites reducing the FIG. 5. Hysteresis loops at room temperature of FeNbBCu alloy after 80h of milling.  magnetic moment, Alleg et al. (2013). In alloy A milled in P7 a big amount of minor elements are in Nb(B) phase detected by XRD. Minor differences were found in alloy B. Slight diminution in the magnetization of saturation is due to highly disordered Zr rich phase. It is known that the saturation of magnetization increases as increasing crystalline fraction, Ipus et al. (2013). Thus, milling device influences soft magnetic behavior of the final product. Furthermore, usually MA samples have always higher coercivity values than samples prepared by other routes, for example melt-spinning, Tsepelev et al., 2017. Nevertheless, these nanocrystalline materials (ribbon shape) have limited applications due to restriction on core winding process, Shyni et al. (2015). In MA alloys the decrease of the grain size to the nanoscale favors soft magnetic behavior. Nevertheless, MA provokes the increase of microstrain and, therefore, of coercivity. One option to reduce microstrain and coercivity is the annealing of the milled powders. It is necessary the optimization of the annealing process to reduce microstrain level (and coercivity) without a significant increase of crystalline size (and preventing the formation of undesired phases).The main problem is that coercivity values lower than 100 KA/m are not achieved. One option is the ball milling of ribbons, Torrens-Serra et al., 2009.

CONCLUSIONS
Two nanocrystalline Fe 80 Nb 7 B 12 Cu 1 (A) and Fe 80 (NiZr) 7 B 12 Cu 1 (B) nanocrystalline alloys were synthesized in two (planetary, shaker) high-energy ball milling devices. Nanocrystalline size ranged between 9.5 and 15.1 nm. Lower crystalline size was found in samples with a second minor phase. Milling on shaker mill favors the formation of a unique phase in alloy A, and the formation of minor Zr rich solid solution in alloy B. All samples are soft magnetic at room temperature. Lower coercivity values (28.6 and 29.2 Oe) were found in the two samples with only one crystalline phase. Thus, in soft ferromagnetic alloys produced by mechanical alloying, final product microstructure and the material properties depends of milling device. Results seem indicate that shaker mill is more energetic.