Nanocrystalline Fe Zr Research Articles

Grain size stabilization of mechanically alloyed nanocrystalline Fe-Zr alloys by forming highly dispersed coherent Fe-Zr-O nanoclusters

Grain size stabilization is crucial for the production and application of nanocrystalline (NC) materials. The mechanically alloyed (MA) NC Fe-Zr system is known as a very successful NC system as it exhibits excellent thermal stability at elevated temperatures. The grain size stabilization of this system has been previously ascribed to its reduced grain boundary (GB) energy by Zr segregation and Zener pinning of Zr-rich intermetallic precipitates. In this work, we report a different mechanism that significantly contributes to grain size stabilization of this NC alloy system using two MA-produced NC Fe-Zr alloys (Fe-1 at.% Zr and Fe-5 at.% Zr) as examples. We show by using atom probe tomography and Cs-corrected transmission electron microscopy that highly dispersed coherent Fe-Zr-O nanoclusters, with a number density up to 1024 m−3, form in ferrite matrix after annealing at certain temperatures. Our first-principles calculations indicate that the formation of these nanoclusters is caused by the ordering of Zr and O-impurity in ferrite matrix. We analyzed the underlying mechanism of grain size stabilization in terms of the experimental results and the Zener pinning theory, and suggest that the pinning effect exerted by these nanoclusters significantly contributes to grain size stabilization of the NC Fe-Zr alloys.

Microstructural evolution of nanocrystalline Fe–Zr alloys upon annealing treatment

Nanocrystalline Fe–Zr alloys exhibit an extraordinary thermal stability at elevated temperatures, which enables their potential applications in various fields. However, there remain concerns regarding the controlling stabilization mechanisms responsible for their thermal stability. In this work, two nanocrystalline Fe–Zr alloys containing 1at.% Zr and 5at.% Zr were annealed at various temperatures (Tann) up to 900°C. Microstructural evolution of the alloys upon annealing was investigated by means of an X-ray diffractometer equipped with a 2-dimensional detector and transmission electron microscopy. Below 600°C, microstructures of the two alloys consist of single nanocrystalline ferrite whose grain size is rather stable upon annealing treatments. Above 600°C, accompanying the precipitation of Fe3Zr phase, an apparent grain coarsening of ferrite is observed, whereas the thermal stability of the alloys is still considerably higher than that of nanocrystalline pure Fe. Based on the experimental results, it was claimed that stabilization of the nanocrystalline Fe–Zr alloys should not be totally ascribed to the thermodynamic stabilization mechanism due to the reduction in grain boundary energy as suggested in earlier investigations [K.A. Darling et al., Scr. Mater. 59 (2008) 530 and K.A. Darling et al., Mater. Sci. Eng. A527 (2010) 3572]; when Tann is higher than 600°C, along with the precipitation of Fe3Zr, the effect of thermodynamic stabilization is weakened, the kinetic effect arising from Zener pinning of Fe3Zr precipitates turns to be an important mechanism contributing to the stabilization of the nanoscale grain size.

Hyperfine fields in nanocrystalline Fe–Zr–B probed by F57e nuclear magnetic resonance spectroscopy

In nanocrystalline Fe90Zr7B3, hyperfine fields belonging to the amorphous residual matrix are distinguished from those of bcc-Fe nanograins by nuclear magnetic resonance spectroscopy on F57e nuclei. With this technique, the nanograins located in magnetic domains can be distinguished from those positioned in domain walls. Structural features of core and surface regions of both types of nanograins are described. Presence of small (∼0.2%) inclusions of Zr was identified in the core of nanograins.

Thermal stability of nanocrystalline Fe–Zr alloys

Fe–Zr nanocrystalline alloys with an as-milled grain size less than 10 nm were synthesized by ball milling. The microstructure changes due to annealing were characterized using X-ray line broadening, microhardness, focused ion beam channeling contrast imaging, and transmission electron microscopy (TEM). Additions of 1/3 to 4 at.% Zr stabilized nanocrystalline grain sizes at elevated annealing temperatures compared to pure Fe. With 4 at.% Zr, a fully nanocrystalline microstructure with a TEM grain size of 52 nm was retained at temperatures in excess of 900 °C. Alloys with lower Zr contents showed less stability, but still significant compared to pure Fe. Bimodal nano–micro grain size microstructures were also observed.

The effect of α-Fe crystallites on the magnetic structures of Fe100−xZrx glasses

Neutron scattering experiments with polarization analysis have established that Fe100−xZrx glasses have non-collinear ferromagnetic structures. The magnitudes of the non-collinear components are enhanced by small fractions of α-Fe precipitated in these glasses. Mössbauer spectroscopy has therefore been used to measure the fractions of α-Fe in two samples each of Fe90Zr10 and Fe92Zr8 metallic glasses, which are confirmed to contain up to 6 atomic % α-Fe. The measured fractions and their influence on the magnetic structures are discussed in the context of the existing literature on nanocrystalline FeZr.

Grain-size stabilization in nanocrystalline FeZr alloys

Nanocrystalline Fe–Zr alloys with a nominal grain size of 10 nm were synthesized by mechanical alloying. The grain size in pure Fe was >200 nm after annealing for 1 h at T/TM = 0.5. Additions of 1 at.% Zr stabilized the grain size at 50 nm up to T/TM = 0.92. Particle pinning, solute drag and reduction in grain-boundary energy have been proposed as stabilization mechanisms. The stabilization in Fe–Zr alloys is attributed to a reduction in grain-boundary energy due to Zr segregation.

Microstructure investigations and magnetic after-effect in amorphous and nanocrystalline Fe–Zr–Ti–B–Cu alloy

The microstructure evolution and low field magnetic properties i.e. initial magnetic susceptibility, stabilization field and magnetic after-effect as disaccommodation of the amorphous and nanocrystalline Fe 80Zr 4Ti 3B 12Cu 1 alloy were investigated. The heat treatment of the as-quenched Fe 80Zr 4Ti 3B 12Cu 1 alloy at 773 K for 1 h leads to its nanocrystallization. It was stated that initial magnetic susceptibility increases and intensity of disaccommodation decreases with increasing of annealing temperature. The magnetic after-effect of the investigated nanocrystalline samples is connected with relaxation processes that occur in the amorphous matrix.

Mössbauer study of a Fe–Zr–B–Cu–(Ge, Co) nanocrystalline alloy series

Amorphous and nanocrystalline Fe–Zr–B–Cu alloys with partial substitution of Co for Fe and Ge for B have been studied by Mössbauer spectrometry (MS). The compositional and microstructural dependence of the different hyperfine parameters were related to the results obtained by X-ray diffraction (XRD) and saturation magnetization measurements. Combination of MS and XRD leads to estimate an interface region, of thickness ∼0.6 nm. The magnetic moment per transition metal of the crystalline phase is reduced with respect to binary crystalline alloys due to the existence of the interface.

The influence of the surface topography on the magnetization dynamics in soft magnetic thin films

In this work we study the influence of surface roughness on the magnetization dynamics of soft magnetic nanocrystalline Fe–Zr–N thin films deposited (under identical conditions) onto a Si oxide, a thin polymer layer, and a thin Cu layer. The substrate temperature during deposition was approximately −25°C ensuring a nanocrystalline state. The demagnetizing factors due to sample roughness were calculated based on atomic force microscopy (AFM) analysis of the surface topography. A clear correlation between sample roughness and the width of the high-frequency response is observed. The local random demagnetizing field created by the nanocrystalline structure and the surface topography is responsible for the positive shift of the ferromagnetic resonance frequency. In addition, a pronounced effect of line broadening is induced by the surface topography at large wavelengths. Finally, we show a good agreement between the values of the average demagnetizing field 4πNMS as calculated from the AFM scans, and the values calculated from the frequency-dependent complex permeability measurements.

Effect of intergranular magnetic coupling on soft magnetic and magnetotransport properties in nanocrystalline materials

The soft magnetic and magnetotransport properties of nanocrystalline Fe–Zr–B–Cu–Ge and Fe–Zr–Cu–Ru alloys have been studied. Our study demonstrates that modification of the chemistry of the residual amorphous phase is an effective approach to adjusting the stiffness of the intergranular magnetic coupling and thereby tailoring the soft magnetic and magnetotransport properties.

Soft magnetic properties of Ge-doped nanocrystalline Fe–Zr–B alloys

The effects of Ge doping on the coercivity and hyperfine interactions of nanocrystalline Fe 89Zr 7B 3Cu 1 have been investigated by means of DC magnetization measurements and 57Fe Mössbauer spectroscopy. The coercivity of Fe 89− x Zr 7B 3Cu 1Ge x annealed for 3.6 ks at 773 and 823 K exhibits a tendency to decrease with increasing Ge content while this effect is not evident at higher annealing temperatures. Our Mössbauer analysis suggests that Ge-induced magnetic softening in Fe 89− x Zr 7B 3Cu 1Ge x is due to the preferential enrichment of Ge into the residual amorphous phase which results in an enhancement of the exchange stiffness in the intergranular region.

Microstructure and magnetic properties of nanocrystalline Fe–Zr–B alloy

The microstructure and soft magnetic properties have been studied for the nanocrystalline Fe 86Zr 6B 8 alloy. It was stated that the ribbon of the Fe 86Zr 6B 8 alloy with 60% amount of the crystalline α-Fe phase exhibits good soft magnetic properties. The differences in the crystallization on the surface and in the bulk for the nanocrystalline Fe 86Zr 6B 8 alloy are studied using Mössbauer spectroscopy.

Microstructure of nanocrystalline FeZr(N)-films and their soft magnetic properties

The microstructure of nanocrystalline FeZr(N) films, deposited by DC sputtering in a Ar+N 2 atmosphere was studied in correlation with their soft magnetic properties. The micromagnetic properties of the films were investigated using the Fresnel mode of Lorentz microscopy.

Microstructure and properties of nanocrystalline Fe–Zr–Nb–B soft magnetic alloys with low magnetostriction

We have investigated the microstructure–property relationship of nanocrystalline Fe 85Zr 1.2Nb 5.8B 8 and Fe 85.5Zr 2Nb 4B 8.5 soft magnetic alloys in order to understand the origin of drastic change in the permeability regardless of the zero magnetostriction in these two alloy compositions. Plan-view and cross-section transmission electron microscopy (TEM) observations showed strongly textured α-Fe particles on the free surface of the Fe 85Zr 1.2Nb 5.8B 8 alloy ribbon, while uniform nanocrystalline microstructure was observed in the Fe 85.5Zr 2Nb 4B 8.5 alloy ribbon. The high Zr content of the latter improves the glass forming ability, thereby suppressing the surface crystallization, resulting in higher permeability. By adding Cu in the Fe–Zr–Nb–B alloy, uniform nanocrystalline microstructure was obtained, from which superior soft magnetic properties with zero magnetostriction was achieved.

Low core losses of nanocrystalline Fe–Zr–Nb–B soft magnetic alloys with high magnetic flux density

The soft magnetic properties of the nanocrystalline Fe–Zr–Nb–B alloys, which are mixed Fe–Zr–B alloy with negative magnetostriction and Fe–Nb–B alloy with positive one, have been investigated. The magnetostriction and the grain size of the Fe–Zr–Nb–B alloys show intermediate values between those of the Fe–Zr–B and Fe–Nb–B alloys. The soft magnetic properties are strongly affected by Zr+Nb amount and Zr/Nb ratio. The best soft magnetic properties have been obtained for the Fe 85.5Zr 2Nb 4B 8.5 alloy. The alloy shows a high permeability of 60 000 at 1 kHz, a high saturation magnetic flux density of 1.64 T and zero-magnetostriction, simultaneously. The alloy also exhibits a very low core loss of 0.09 W/kg at 1.4 T and 50 Hz, which is extremely lower than that of Fe–Si–B amorphous, and good thermal stability of the core loss. The nanocrystalline Fe–Zr–Nb–B alloy is therefore suitable for a core material for pole transformers.

Magnetic properties of zero-magnetostrictive nanocrystalline Fe–Zr–Nb–B soft magnetic alloys with high magnetic induction

The soft magnetic properties of the nanocrystalline Fe–Zr–Nb–B alloys have been investigated. The best soft magnetic properties have been obtained for the Fe 85.5Zr 2Nb 4B 8.5 alloy. The alloy shows a high permeability of 60,000 at 1 kHz, a high magnetic induction of 1.64 T and zero magnetostriction, simultaneously. The alloy also exhibits a very low core loss of 0.09 W/kg at 1.4 T and 50 Hz, which is extremely lower than that of Fe–Si–B amorphous. The nanocrystalline Fe–Zr–Nb–B alloy is therefore suitable for a core material for pole transformers.

Changes of chemical composition and structure of soft magnetic nanocrystalline Fe–Zr–N alloy under vacuum annealing

The amorphous soft magnetic alloys were produced by magnetron sputtering of Fe–8 at%Zr target in Ar and Ar+10%N 2 atmospheres as films 1.5–2.5 μm thick. X-ray diffraction analysis showed that annealing of the films resulted in formation of BCC Fe-based phase and additionally Fe 2Zr in the Fe–Zr films. Amorphous phase in the Fe–Zr–N films is more thermostable in comparison with that in the Fe–Zr ones. X-ray spectrum microanalysis showed that nitrogen content in the Fe–Zr–N films decreases significantly during vacuum annealing.

Compositional dependence of the soft magnetic properties of the nanocrystalline Fe–Zr–Nb–B alloys with high magnetic flux density

The compositional dependence of the soft magnetic properties of the nanocrystalline Fe–Zr–Nb–B alloys has been investigated. The magnetostriction (λs) and the grain size of the (Fe90Zr7B3)1−x(Fe84Nb7B9)x alloys, which are two typical ternary alloys mixed with the best soft magnetic properties, show intermediate values between those of the Fe90Zr7B3 with negative λs and the Fe84Nb7B9 with positive λs. However, the soft magnetic properties of the Fe–(Zr, Nb)7–B alloys are inferior to those of the Fe90Zr7B3 and the Fe84Nb7B9 alloys. The best soft magnetic properties have been obtained at Zr+Nb=6 at. %. The Fe85.5Zr2Nb4B8.5 alloy shows a high μe of 60 000 at 1 kHz, a high Bs of 1.64 T, and zero λs, simultaneously. The alloy also exhibits a very low core loss of 0.09 W/kg at 1.4 T and 50 Hz, which is extremely lower than that of Fe–Si–B amorphous alloys. The nanocrystalline Fe–Zr–Nb–B alloys with improved soft magnetic properties are therefore suitable for pole transformers.

Sodium borohydride reduction of aqueous iron–zirconium solutions: chemical routes to amorphous and nanocrystalline Fe–Zr–B alloys

Fine particle Fe–Zr–B alloys have been prepared by sodium borohydride reduction of aqueous solutions of iron(II) and zirconium(IV) sulfate. A comprehensive survey of reaction conditions was completed and optimum parameters found for the preparation of precipitated products. These included rapid stirring, controlled pH in the range 5–6, using degassed solvents, Fe∶Zr ratios between 4∶1 and 20∶1 and sodium borohydride concentrations between 0.1 and 1.2 M. Scanning electron microscopy showed that the products comprised spherical particles of 0.1 µm diameter with metal compositions, determined by atomic absorption, electron microprobe and EDAX, which mirrored the initial starting solutions. The products fell into two categories: amorphous Fe–Zr–B alloys when the Fe∶Zr ratio was near 9∶1, and nanocrystalline alloys, comprising a small amount of nanometre scale α-Fe crystallites embedded in amorphous Fe–Zr–B alloy, when the Fe∶Zr ratio was either higher or lower than 9∶1. Minor impurities in the form of Fe(II) complex and Fe(III) oxide phases were also detected in the products. Full structural and magnetic characterisations were performed on selected samples, including X-ray diffraction, Mossbauer spectroscopy, differential scanning calorimetry, magnetometry and Zr-edge EXAFS measurements.

Partitioning of Si in a Fe87Zr7Si4B2 nanocrystalline soft magnetic alloy

We have studied partitioning of Si in nanocrystalline Fe–Zr–Si–B soft magnetic alloy by atom probe field ion microscopy (APFIM). Unlike our expectation from our previous results in nanocrystalline Fe–Si–B–Nb–Cu alloy, we have found that Si atoms are rejected from α-Fe primary crystals and partitioned into the residual amorphous phase. This result suggests that Si addition does not cause reduction in inherent magnetostriction of α-Fe nanoparticles, but it brings the volume fraction of the α-Fe particles to an optimum value so that the average magnetostriction becomes zero. This conclusion has been strengthened by comparing the changes in magnetostriction constants as a function of Si content in Fe–Zr–Si–B and Fe–Si–B–Nb–Cu alloys.