As-quenched Amorphous Samples Research Articles

Origin of the separated α-Al nanocrystallization with Si added to Al86Ni9La5 amorphous alloy

An understanding of nanocrystallization is an important precondition for controlling the microstructure and properties of nanocrystal-reinforced amorphous alloys. In the present work, the microstructures of Al86Ni9La5 and (Al86Ni9La5)98Si2 amorphous alloys and their annealed products were investigated systematically to reveal why the nanocrystallization behavior of primary α-Al was considerably changed with Si addition. Upon heating, α-Al nucleates in the Al-rich regions of two amorphous alloys. The addition of Si significantly shrinks the size of single Al-rich region in the as-quenched amorphous sample, but raises the local maximum Al concentration and slows down the rate of diffusion of solute atoms. As a result, the primary crystallization of α-Al first takes place at a higher nucleation rate in the Al-rich regions at lower temperatures. However, the residual amorphous phase enriched with solute elements does not crystallize until the alloy is heated to a high enough temperature for massive atomic diffusion to occur, with further crystallization of α-Al occurring through epitaxial growth of the existing α-Al crystals. The high bonding strength between Si with Ni, and especially La, is responsible for the splitting of α-Al nanocrystallization into two stages.

Anisotropy field distribution in soft magnetic Hitperm alloys submitted to different field annealing processes

The magnetic anisotropy field distribution is discussed for Hitperm alloys annealed under different field conditions leading to different induced magnetic anisotropies: zero (ZF), transversal (TF), and longitudinal (LF) field annealing and compared to that of as-quenched (AQ) melt-spun amorphous ribbon. In order to accurately use the present method, the demagnetizing factor has been obtained by analyzing the field dependence of the inverse of the field derivative of the magnetization. The coherence of the analysis is supported by testing the normalization of the complete distribution of anisotropy fields. Independently of the composition, two groups can be distinguished among the studied samples: those with mainly perpendicular anisotropy field contributions (ZF and TF samples) and those with mainly longitudinal anisotropy field contributions (LF and AQ samples). Behavior of TF samples is well reproduced using Stoner–Wohlfarth model and, in the case of as-quenched amorphous samples, the anisotropy field depends almost linearly on the thickness of the ribbon.

Structural and magnetic study of Mo-doped FINEMET

In this paper, a study of the structural and the magnetic correlation of the crystalline and amorphous phases in the nanocrystalline system Fe 73.5Si 13.5B 9Nb 3− x Mo x Cu 1 ( x=0, 1.5, 2, 3) was made. By means of Mössbauer spectroscopy, simple mass balance considerations and density measurements, both phases fractions and chemical compositions were calculated (in at%, wt% and vol%). Then, quasistatic magnetic measurements and ab initio calculations were used in a magnetic balance model in order to estimate the magnetic contribution of the remaining amorphous phase, which was compared to that of as-quenched amorphous samples of the same composition. The difference in both magnitudes showed the influence of penetrating fields and that these became more important for higher crystalline fractions.

Crystallization of amorphous bismuth titanate

We studied the transformation of amorphous bismuth titanate by heat treatments. After an as-quenched amorphous sample was annealed at 500 °C, the lowest Raman peak became intense like a boson peak for glass. This fact indicates the formation of intermediate range order. The medium range correlation length of 7 nm calculated from the boson peak frequency is in agreement with the mean cluster size measured by atomic force microscopy (AFM). The differential thermal analysis (DTA) shows the two-step crystallization at 608 and 830 °C on heating. It suggests the existence of a metastable state. The samples annealed at 770 and 1000 °C are identified as the pyroclore and the layered perovskite structures, respectively. It strongly suggests that at first a three-dimensional (3D) crystalline state appears by nucleation process from a 3D amorphous state and secondly it transforms into the pseudo-2D layered perovskite structure.

Transverse domain structure related giant magnetoimpedance in nanocrystalline Fe73.5Cu1Nb3Si13.5B9 ribbons

A sensitive field- and frequency-dependent magnetoimpedance (MI) has been observed in nanocrystalline Fe73.5Cu1Nb3Si13.5B9 ribbons. A maximum value for the MI ration ΔZ/Z=[Z(H)−Z(Hmax)]/Z(Hmax), of more than 400% was obtained in these nanocrystalline ribbons (annealed for 3–5 h at 550 °C). A peak in the field dependence of the MI ratio ΔZ/Z was observed in nanocrystalline samples, but not in the as-quenched amorphous samples. The sensitivity attained a value larger than 60% Oe−1 in the field range 3–7 Oe at 800 kHz for the nanocrystalline ribbons. Domain observation experiments show that the transverse domain structure observed in nanocrystalline samples is responsible for their giant MI effects, and the domain wall movements in the transverse domain pattern region are dominant during the initial magnetization processes in both longitudinal and transverse directions of the applied field. The correlation between the magnetization processes and the giant MI effects is discussed briefly based on the view of a modified skin effect.

The sensitive magnetoimpedance effect in Fe-based soft ferromagnetic ribbons

A comprehensive analysis of magnetoimpedance (MI) phenomena in Fe-based nanocrystalline ribbons is presented in this paper. Giant MI responses have been observed in nanocrystalline samples annealed over the temperature range 490 and . More than 400% increases of the MI ratio were obtained both in and in ribbons. The sensitivity can reach a value larger than in the field range 3 - 7.2 Oe at 800 kHz for ribbons. No MI effect was observed in an as-quenched amorphous sample, because of its very low permeability. A mostly resistive feature was found in an as-quenched amorphous sample. The field dependences of the effective permeability show a typical transverse anisotropy property, and the development of the transverse anisotropy in naturally annealed Fe-based ribbons was analysed.

Ball-milling-induced crystallization and ball-milling effect on thermal crystallization kinetics in an amorphous FeMoSiB alloy

Microstructure evolution in a melt-spun amorphous Fe77.2Mo0.8Si9B13 alloy subjected to high-energy ball milling was investigated by means of X-ray diffraction (XRD), a transmission electron microscope (TEM), and a differential scanning calorimeter (DSC). It was found that during ball milling, crystallization occurs in the amorphous ribbon sample with precipitation of an α-Fe solid solution, and the amorphous sample crystallizes completely into a single α-Fe nanostructure (rather than α-Fe and borides as in the usual thermal crystallization products) when the milling time exceeds 135 hours. The volume fraction of material crystallized was found to be approximately proportional to the milling time. The fully crystallized sample with a single α-Fe nanophase exhibits an intrinsic thermal stability against phase separation upon annealing at high temperatures. The ball-milling effect on the subsequent thermal crystallization of the amorphous phase in an as-milled sample was studied by comparison of the crystallization products and kinetic parameters between the as-quenched amorphous sample and the as-milled partially crystallized samples. The crystallization temperatures and activation energies for the crystallization processes of the residual amorphous phase were considerably decreased due to ball milling, indicating that ball milling has a significant effect on the depression of thermal stability of the residual amorphous phase.

Experiments on nanostructured formed due to short-time annealing

For technical applications the commonly used thermal processing of the amorphous alloy consists of one-hour anneals at about . This procedure leads to very soft magnetic properties due to the formation of the nanocrystalline Fe(Si) phase in the remaining amorphous matrix. In order to study the onset and development of the nanocrystalline phase the as-quenched amorphous samples were annealed in an Ar atmosphere at temperatures of and for different annealing times: 15 s, 30 s, 60 s, 90 s, 120 s, 180 s and 300 s. For sample characterization we used the conventional Mössbauer effect technique and additionally Mössbauer measurements in radiofrequency magnetic fields, which are very sensitive to magnetostriction and magnetic anisotropies. The measurements were supplemented with electron transmission microscopy and electron diffraction studies. Our short-time annealing experiments clearly show that in order to form good magnetically soft properties the annealing times of 1 - 2 min at and of only 15 - 30 s at are sufficient.

Temperature dependence of the Mössbauer spectra of amorphous and nanocrystallized Fe86Zr7Cu1B6

Mossbauer measurements have been performed on amorphous and nanocrystalline alloy ribbons of nominal composition Fe86Zr7Cu1B6. The nanocrystalline samples were obtained by annealing the as-quenched alloy at different temperatures in the range between 650 and 870 K. Mossbauer spectra of the as-quenched amorphous sample have been recorded at 77 K, room temperature and above the Curie temperature (≈330 K) at 360 K. We have also performed Mossbauer measurements at room temperature in the nanocrystalline alloys to characterize the phases that appear after the annealing and their relative concentration. The as-quenched sample spectra reveal the existence of two inequivalent sites for Fe. Such a feature is also observed in the remaining amorphous phase of the annealed samples. In the first steps of crystallization, α-Fe precipitates and its concentration increases with the annealing temperature. The experimental results suggest that the composition of the whole amorphous phase does not suffer large changes during crystallization.

Microstructures of Fe 73.5Si 13.5B 9Nb 3Cu 1 nanocrystalline soft magnetic material investigated by APFIM and HRTEM

The microstructure of the Fe 73.5Si 13.5B 9Nb 3Cu 1 soft magnetic material with a nanocrystalline structure has been investigated by atom probe field ion microscopy and high resolution electron microscopy. In the as-quenched amorphous sample, all the alloying elements were found to distribute homogeneously as an amorphous solid solution. In the annealed sample with the optimum soft magnetic properties, we found that two phases were present. One was bcc FeSi solution and the other was the B and Nb enriched amorphous phase with little Si content. Although some uncertainty remains regarding the distribution of Cu, the present result is consistent with earlier speculations that the bcc phase would be the FeSi solid solution based on the lattice parameter measurement and the Curie temperature measurement.

Atom probe analysis of Fe73.5Si13.5B9Nb3Cu1 nanocrystalline soft magnetic material

The microstructure of Fe73.5Si13.5B9Nb3Cu1 soft magnetic material with a nanocrystalline structure has been investigated by atom probe field ion microscopy. In the as-quenched amorphous sample, all the alloying elements were found to distribute homogeneously as an amorphous solid solution. In the annealed sample with the optimum soft magnetic properties, we found that two phases were present. One was Fe-Si solid solution and the other was the B- and Nb-enriched region with less Si content. Although some uncertainty remains regarding Cu, the present result is consistent with an early speculation based on the lattice parameter measurement.

Anomalous X-ray scattering study of amorphous and icosahedral Al75Cu15V10 alloys

Atomic structures of amorphous and icosahedral Al75Cu15V10 alloys were precisely examined by the ordinary and anomalous X-ray diffraction techniques. Some distinct features of intensity profiles, and coordination numbers and interatomic distances determined from the RDF analysis, indicate the presence of the icosahedral short-range ordering clusters in the as-quenched amorphous sample and other annealed amorphous samples, which are similar to the basic atomic structure constructing the icosahedral phase. The intensity differences of the amorphous and icosahedral phases obtained in the anomalous X-ray scattering (AXS) at the CuK-absorption edge suggest that copper atoms are homogeneously dispersed in the icosahedral short-range ordering clusters.