As-quenched Amorphous State Research Articles

The prediction of upper limit of copper content in CoBCu glass-forming alloys

The soft magnetic properties of ferromagnetic nanocrystalline alloys can be enhanced considerably by addition of copper until the upper limit of Cu content which is unknown at present. This study explored the upper limit of Cu content in the Co81−XB19CuX (X = 2, 3 and 5) alloy system using ab initio molecular dynamics simulations and experiments. It was found that when the Cu content rose above 2 at%, the first-nearest-neighbor peaks of partial pair distribution function for Cu-Cu pair became stronger steadily with the decreasing temperature, demonstrating that Cu clusters were formed during the quenching processes from initial melt. As to the Cu content was less than or equal to 2 at%, the trace of such clusters was difficult to find. So the maximal Cu content in Co81−XB19CuX alloys in as-quenched amorphous state was predicted to be less than 2 at%. Co81−XB19CuX ribbons were prepared using the melt-spinning technique with as large cooling ability as possible. Structure analysis results from X-ray diffraction patterns of the three as-quenched alloys supported the above theoretical predictions. These results would provide a theoretical reference for the design of ferromagnetic nanocrystalline alloys.

Effect of thermally induced relaxation on passivity and corrosion of an amorphous Al–Co–Ce alloy

The corrosion behavior of Al87Co7Ce6 alloy was investigated in the amorphous as-quenched, amorphous thermally relaxed, and two partially devitrified conditions using AC and DC electrochemical methods combined with high resolution microscopy in the active dissolution and passivity-local breakdown regimes. The best corrosion resistance was observed after relaxation. Intermediate behavior was observed in a partially devitrified treatment that formed small fcc Al particles and intermetallic phases. Further devitrification decreased the corrosion resistance. The amorphous “relaxed condition”, characterized by EXAFS, is an Al–Co–Ce solid solution with short bond length, high coordination number and reduced free volume compared to the as-quenched amorphous state.

Segregation-controlled nanocrystallization in an Al–Ni–La metallic glass

The early stages of devitrification of Al89Ni6La5 metallic glass were investigated using three-dimensional atom probe. Even in the as-quenched amorphous state, nanometer-sized fluctuations of Ni and Al were observed. The Al-rich fluctuations act as nucleation sites for the crystallization at 443K in which α-Al clusters with a number density up to 5×1023m−3 nucleate and grow to nonspherical, 12nm large crystals. α-Al particles are almost pure in Al and are surrounded by a La-rich shell. It is concluded that La segregation controls the growth and limits the size of the α-Al nanocrystals.

Nanocrystallization behavior and magnetic properties of amorphous Fe 78Si 9B 13 ribbons

The amorphous state of ferromagnetic Fe 78Si 9B 13 ribbons and its nanocrystallization were investigated by in situ transmission electron microscope (TEM), X-ray diffraction (XRD), Mössbauer spectroscopy (MS), differential scanning calorimeters (DSC) and magnetic moment measurements. The Mössbauer spectrum exhibited an essentially symmetric hyperfine field pattern of 259 kOe in as-quenched amorphous state at room temperature. The Curie and crystallization temperature were determined to be T C=708 K and T x =803 K, respectively. The T x value was in good agreement with DSC measurement results. The occupied fraction of the nanocrystalline phases of α-Fe(Si) and Fe 2B at in situ optimum annealing temperature was about 57% and 43%, respectively. It is notable that the magnetization of the amorphous phase decreases more rapidly with increasing temperature than those of nanocrystalline ferromagnetism, suggesting the presence of the distribution of exchange interaction in the amorphous phase or high metalloid contents.

On determination of volume fraction of crystalline phase in partially crystallized amorphous and nanocrystalline materials

The method for determination of volume fraction of crystalline phase in amorphous-crystalline materials is proposed. The method is based on the analysis of X-ray patterns obtained under study of structure-phase changes in nanocrystalline Finemet- type alloys. Verification of the method was carried out with the use of X-ray diffraction data (including small-angle X-ray scattering) of specimens in as-quenched amorphous state as well as after annealing at various temperatures, which provided formation and growth of crystals in amorphous matrix with sizes in a range from 2 to 15 nm. The method is the most effective under nanocrystals' size exceeding 5–6 nm, when their further increase does not affect the height and width of diffraction reflexes.

Investigations On In Situ Nanocrystallization And Magnetic Properties For Amorphous Fe78Si9B13 Ribbons

ABSTRACTThe amorphous state of ferromagnetic Fe78Si9B13 (Metglas 2605S-2) and its nanocrystallization were investigated by in situ transmission electron microscope (TEM), Xray diffraction (XRD), Mossbauer spectroscopy (MS), differential scanning calorimeters (DSC) and magnetic moment measurements. The Mössbauer spectrum exhibited an essentially symmetric hyperfine field pattern of 259KOe in as-quenched amorphous state at room temperature. The Curie and crystallization temperature were determined to be Tc=708K and Tx= 803K, respectively. The Tx value was in good agreement with DSC measurement results. The occupied fraction of the nanocrystalline phases of α-Fe(Si) and Fe2B at in situ optimum annealing temperature was about 57% and 43%, respectively. It is notable that the magnetization of the amorphous phase decreases more rapidly with increasing temperature than those of nanocrystalline ferromagnetism, suggesting the presence of the distribution of exchange interaction in the amorphous phase or high metalloid contents.

Investigations on in situ Nanocrystallization and Magnetic Properties for Amorphous Fe78Si9B13 Ribbons

ABSTRACTThe amorphous state of ferromagnetic Fe78Si9B13 (Metglas 2605S-2) and itsnanocrystallization were investigated by in situ transmission electron microscope (TEM), Xraydiffraction (XRD), Mossbauer spectroscopy (MS), differential scanning calorimeters(DSC) and magnetic moment measurements. The Mössbauer spectrum exhibited anessentially symmetric hyperfine field pattern of 259KOe in as-quenched amorphous state atroom temperature. The Curie and crystallization temperature were determined to beTc=708K and Tx.= 803K, respectively. The Tx value was in good agreement with DSCmeasurement results. The occupied fraction of the nanocrystalline phases of α-Fe(Si) andFe2 at in situ optimum annealing temperature was about 57% and 43%, respectively. It isnotable that the magnetization of the amorphous phase decreases more rapidly withincreasing temperature than those of nanocrystalline ferromagnetism, suggesting thepresence of the distribution of exchange interaction in the amorphous phase or highmetalloid contents.

Thermal behavior of the initial permeability in soft magnetic nanocrystalline alloys

Initial permeability thermograms for soft magnetic nanocrystalline alloys show two characteristic peaks: the first corresponds to the Curie temperature (T c ) of the remaining amorphous phase and the second to the as-precipitated bcc phase. The former one shows a broad distribution and in general shifts with annealing towards lower temperatures compared to the T c of the as-quenched amorphous state. The latter one is relatively sharp and shifts with annealing towards higher temperatures but disappears before reaching the T c of the equilibrium bcc phase. The first and second peaks are explained by the effect of the inhomogeneous chemical composition of the remaining amorphous phase and by the superparamagnetic transition of the dipolar (and/or exchange)-coupled bcc nanograins respectively, both changing with the evolution of annealing.

Behaviors of Ga during crystallization of amorphous Nd 4.5Fe 73Ga 1Co 3B 18.5 by fluorescence XAFS

Combined addition of Ga and Co into Fe 3B-based NdFeB hard magnetic materials improves squareness of hysteresis loop and remanence. Since Ga addition is believed to reduce the grain size of the crystallites precipitated from the as-quenched amorphous state, fluorescence XAFS measurements of the Ga K-edge are carried out. A small change of XAFS spectrum for the as-quenched (amorphous) state just after the Fe 3B precipitation suggests that Ga is excluded from Fe 3B crystallites to surrounding amorphous region. Good agreement of the observed XAFS spectra for the annealed at a higher temperature where Nd 2Fe 14B is precipitated with the calculated CsCl-type CoGa alloy indicates a preferential bonding between Ga and Co.

Soft Magnetic Fe–Zr–Si–B Alloys with Nanocrystalline Structure

Good soft magnetic properties and high saturation magnetization (Bs) were obtained for the Fe 87 Zr 7 Si 4 B 2 and Fe 86 Zr 7 Si 4 B 3 alloys with the partially crystallized structure consisting of nanoscale bcc grains surrounded by the remaining amorphous phase. These alloys consist only of an amorphous phase and have good bending ductility in the as-quenched state and the partial crystallization to the nanocrystalline structure by annealing for 3.6 ks at 873 K causes the simultaneous achievement of 1.6 T for Bs, 14000 for permeability at 1 kHz (μe) and < 1 x 10 -6 for magnetostriction (λs). No zero magnetostriction is obtained for nanoscale bcc Fe 90 Zr 10 and Fe 90 Zr 7 B 3 alloys and the addition of about 4 at% Si is essential for the achievement of the zero magnetostriction in the Fe-Zr base alloys. The glass formation at the new alloy compositions, the good ductility in the as-quenched amorphous state and the good soft magnetic properties in the partially crystallized state allow us to expect that the present alloys develop subsequently as a new type of soft magnetic material.

Diffusion and Absorption of Hydrogen in Liquid-Quenched and Annealed Ni81P91Alloy

Abstract Transport and absorption of hydrogen were studied in the Ni81P19 alloy in the as-quenched amorphous state and after annealing for 1 h at temperatures up to 873°K. The diffusivity and the amount of diffusible and trapped hydrogen were determined from electrochemical permeation measurements performed at 298°K. After annealing below the crystallization temperature (Tc ≃ 560°K), the hydrogen diffusivity and absorption rose by up to about twice, whereas at higher temperatures (formation of the Ni and Ni3P phases) they drastically decreased, with a drop in diffusivity being almost three orders of magnitude. Below Tc, the ratio of trapped hydrogen to total absorbed hydrogen almost did not change with the annealing temperature, being about 0.75. The rise in the hydrogen diffusivity and absorption below Tc may result from local ordering of atoms; it demonstrates that the behavior of hydrogen may be a sensitive measure of structural relaxation in the amorphous alloy.