Amorphous Melt-spun Research Articles

Laser powder bed fusion of a nanocrystalline Finemet Fe-based alloy for soft magnetic applications

The aim of this work is to explore the laser powder bed fusion (LPBF) processability window of the nanocrystalline soft magnetic Finemet alloy. With that purpose, several laser power and scan speed values and a meander scanning strategy were probed to process simple geometry specimens. Good dimensional accuracy was obtained within the entire processing window investigated. Relative densities as high as 89% were achieved for processing conditions including high laser power and low scan speeds. The fraction of amorphous phase, which peaked at 49%, was found to be mostly dependent on the scan speed and only slightly influenced by the laser power. The microstructure of the crystalline domains is formed by ultrafine, equiaxed grains with random orientations. Irrespective of the processing conditions, the LPBF-processed samples exhibit a similar saturation magnetization, lower permeability, and higher coercivity than fully amorphous melt-spun ribbons of the same composition. The coercive field of the additively manufactured specimens is fairly independent of the relative density and exhibits a moderate inverse variation with the amorphous fraction. Consistent with earlier works, this study suggests that the average grain size is an important contributor to coercivity.

Enhanced specific heat of the Sc79Fe21 nanoglass compared to the Sc79Fe21 amorphous melt-spun ribbon in a temperature range of 150–300 K

The specific heat of the Sc79Fe21 amorphous materials was measured in a temperature range of 150 to 300 K. The specific heat of the Sc79Fe21 nanoglass prepared by consolidation of glassy nanoparticles was approximately 2.5% higher than that of the Sc79Fe21 amorphous melt-spun ribbon. The enhanced specific heat was primarily ascribed to the interfaces within the nanoglass of heterogeneous structure, since both the reduced packing density and interfacial enrichment of the Sc increased the specific heat of the interfaces, while adding Fe to the cores reduced the specific heat.

The microstructure and thermal stability of the two-phase amorphous melt-spun alloys ejected from a double chamber crucible.

This work presents the microstructure and properties of two-phase amorphous melt-spun alloys ejected from the crucible with partition between liquids. The microstructure was studied by scanning electron microscopy and transmission electron microscopy and the phase composition was studied by X-ray diffraction. The thermal stability of the alloys was determined using differential scanning calorimetry. The microstructure study proves that the composite alloys are heterogeneous because of the existence of the two amorphous phases obtained due to the use of a partition between the liquids. This microstructure correlates with complex thermal characteristics not found in homogeneous alloys of the same nominal composition. The layered structure of these composites influences the formation of fractures during tensile tests. This article is protected by copyright. All rights reserved.

Microstructural evaluation and mechanical properties of bulk nanostructured Al86Cu6Y6La2 (at.%) alloy produced by hot consolidation of amorphous melt-spun flakes

Amorphous melt- spun Al86Cu6Y6La2 (at.%) alloy is hot consolidated at T = 723 K and P = 700 MPa. The glassy ribbons show three devitrification steps which fcc-Al phase is precipitated during the first crystallization stage, and formation of intermetallic compounds (Al3Y, AlCu, AlCu3 and Al3Y5) take place in the next steps. The calculated Avrami exponent (n ∼ 2.6) shows the first crystallization reaction is governed by the diffusional growth process and increasing nucleation rate. Moreover, the activation energy calculated with different methods show that Ea for the primary crystallization are in the range of about 254- 267 KJ.mol−1. Amorphous ribbons exhibit a remarkably tensile strength of 910 ± 7 MPa. Combined rapid solidification and hot consolidation produce a bulk sample with high hardness (345.1 ± 12.3 HV), compressive strength (460 ± 10 MPa) and 4 pct plastic strain. These high mechanical properties are due to ultrafine microstructure induced by the rapid solidification process.

Influence of heating on structure and mechanical characteristics of amorphous Al86Ni9Gd5 ribbons

Changes of structure and mechanical characteristics of amorphous Al86Ni9Gd5 melt-spun ribbons subjected to linear heating at a rate of 10 K/min were investigated by differential scanning calorimetry, X-ray diffraction, transmission electron microscopy, three-point bending testing and microhardness measurements. Non-monotonic changes of all mechanical characteristics were related to the structural variations occurring at three-stage transformation process. It was established that nanocrystallization of amorphous phase led to drop of all properties measured by three-point bend testing (the modulus of elasticity, bending limit at fracture and plasticity) while formation of the intermetallic compounds and coarsening of structure during subsequent transformation stages resulted in their partial recovery. On the contrary, microhardness had a maximum (about 4.5 GPa) in the samples with the ultra-fine grained partially crystallized structure. The potential for practical applications of the established effect of recovery of plasticity from zero in nanocomposite structure to about 1% in the fully crystallized sample with microhardness about 3 GPa was discussed.

Structure of AlNiGd nanocomposites with enhanced ductility produced by high pressure torsion processing

The fully consolidated bilayer (consisting of two amorphous Al86Ni9Gd5 melt-spun ribbons) disks with a diameter of 5 mm were produced using unconstrained high pressure torsion (HPT) processing subjecting for 2–5 turns at applied pressures of 2 and 4 GPa. The structure and mechanical properties of the disks as well as those of the as-prepared and heated up to 479 K ribbon samples were studied by X-ray diffraction analysis, transmission and scanning electron microscopy, three-point bending tests and measurements of microhardness. It was established that the heat treatment and consolidation resulted in formation of nanocomposite structures consisting of Al nanocrystals 21 and 6 nm in size, respectively, with enhanced microhardness. It was shown that the heat treatment of the ribbon led to a loss of the plastic strain (from 2 to 0.4%) while the consolidated disks retained the same level of plasticity as that in the as-cast amorphous ribbon. The possible reason of the enhanced plasticity of the nanophase composites obtained by deformation-induced crystallization was discussed.

High-performance Ce-substituted (Nd0.7Ce0.3)-Fe-B hot-deformed magnets fabricated from amorphous melt-spun powders

We report a method to produce high-performance and cost-effective Ce-substituted Nd-Fe-B hot-deformed magnets with 30% Nd reduction. To suppress the formation of the Ce-dissolved REFe2 phase, which primarily limits [001] texture development and RE-rich grain boundary phase formation, amorphous melt-spun precursors were used to fabricate 30%-Ce-substituted hot-deformed magnets. By suppressing the formation of REFe2 phase from the precursors, the c-axis alignment of 2:14:1 platelets in the magnets due to grain boundary sliding and rotation improved, and the Nd/Ce-concentration of grain boundary phases in the magnets increased. As a result, both coercivity (Hcj) and remanence (4πMr) of the magnets noticeably increased to 15 kOe and 13 kG, respectively, in relation to those of the magnets fabricated from crystalline precursors containing an REFe2 phase (Hcj = 13.4 kOe, 4πMr = 12.1 kG). The magnetic performance of the 30%-Ce-substituted magnets developed in this work is sufficient to replace the N42M-graded commercial Ce-free magnets.

Breaking Down SERS Detection Limit: Engineering of a Nanoporous Platform for High Sensing and Technology.

In this study, nanoporous gold (NPG) was synthesized by free corrosion dealloying of an amorphous precursor, Au20Cu48Ag7Pd5Si20 (at. %), in a mixture of nitric and hydrofluoric acid, starting from amorphous melt-spun ribbons. NPG revealed a 3D nanoporous structure composed of pores and multigrain ligaments of an average size of 60 nm. NPG was further anodized in oxalic acid at 8 V vs. Ag/AgCl reference electrode to obtain a bimodal morphology composed of ligaments disrupted in finer features. Both NPG and anodized samples (A-NPG) were found to be mechanically stable to bending and active for surface-enhanced Raman scattering (SERS). SERS activity of samples was investigated using 4,4′-bipyridine as a probe molecule. A detection limit of 10−16 M was found for both samples, but in A-NPG, the signal was strongly enhanced. The extremely high enhancement obtained for A-NPG is attributed both to the small size of ligaments and crystals of which they are made, as well as to the nanometric features resulting from anodization treatment. Such a microstructure showed homogenous SERS response in terms of average enhancement all across the surface, as demonstrated by mapping measurements. Furthermore, NPG and A-NPG were tested as electrodes for electrocatalytic applications, showing good properties. The engineering steps from the amorphous precursor to A-NPG led us to obtain a high-sensing platform, with extremely low detection limit and intrinsic properties, that might significantly contribute to the cutting-edge technology of the future.

Comparison and process study of hot-pressed and hot-deformed Nd-Fe-B magnets prepared by amorphous and nanocrystalline powders

Hot-pressed and hot-deformed Nd-Fe-B magnets are generally made from rapidly quenched powder after crystallization treatment. In this work, the amorphous melt-spun ribbons are also employed directly as the precursor. The amorphous and crystallized powders with the composition of Nd28.47Pr0.56FebalB0.97Co3.63Al0.13Nb0.56Zr0.44 were employed for fabricating hot-pressed and hot-deformed magnets. The influences of processing parameters on the microstructure and magnetic properties have been investigated. For the hot-pressed magnets produced from both crystallized and amorphous powders, the grain pre-orientation and magnetic properties including coercivity, remanence and maximum energy product increase with the increasing temperature, pressure and holding time in the ranges of 650–800 °C, 300–600 MPa and 30–150 min, respectively. The hard magnetic properties of the hot-pressed magnets obtained from amorphous powder are slightly higher than those from crystalline powder due to the smaller grain size of the former. The magnetic properties of hot-deformed magnets are mainly determined by the deformation temperature and not influenced by the state of the powder and hot-pressed precursors. The hot-pressed magnets and hot deformed magnets with different microstructures exhibit different magnetization mechanisms. The optimal hot-deformation temperature is found to be 750 °C for obtaining a high maximum energy product of 39 MGOe. The present results suggest that the rapidly quenched amorphous powder can be used to not only directly prepare hot-pressed magnets with higher magnetic properties, but also to reduce the process step for hot-deformed Nd-Fe-B magnets.

High-pressure torsion of Zr-based bulk metallic glasses and amorphous melt-spun ribbons

High-pressure torsion allows one to achieve high strains of bulk metal glasses without their destruction. The article presents the results of studies on the effect of high-pressure torsion on the amorphous alloys of the same composition but obtained in different ways: in a form of a bulk plate and a melt-spun ribbon. It is shown that the degree of nonequilibrium after high-pressure torsion, determined by the diffraction and calorimetry methods, increases for bulk metallic glass, approaching the values characteristic for melt-spun alloy.

Influence of Cu Content on Structure and Magnetic Properties in Fe86-xCuxB14 Alloys.

Influence of Cu content on thermodynamic parameters (configurational entropy, Gibbs free energy of mixing, Gibbs free energy of amorphous phase formation), crystallization kinetics, structure and magnetic properties of Fe86-xCuxB14 (x = 0, 0.4, 0.55, 0.7, 1) alloys is investigated. The chemical composition has been optimized using a thermodynamic approach to obtain a minimum of Gibbs free energy of amorphous phase formation (minimum at 0.55 at.% of Cu). By using differential scanning calorimetry method the crystallization kinetics of amorphous melt-spun ribbons was analyzed. It was found that the average activation energy of α-Fe phase crystallization is in the range from 201.8 to 228.74 kJ/mol for studied samples. In order to obtain the lowest power core loss values, the isothermal annealing process was optimized in the temperature range from 260 °C to 400 °C. Materials annealed at optimal temperature had power core losses at 1 T/50 Hz—0.13–0.25 W/kg, magnetic saturation—1.47–1.6 T and coercivity—9.71–13.1 A/m. These samples were characterized by the amorphous structure with small amount of α-Fe nanocrystallites. The studies of complex permeability allowed to determine a minimum of both permeability values at 0.55 at.% of Cu. At the end of this work a correlation between thermodynamic parameters and kinetics, structure and magnetic properties were described.

On the mechanisms limiting power loss in amorphous CoFeB-based melt-spun ribbons

The mechanisms that limit the power loss performance in melt-spun amorphous ribbons have been investigated through DC and AC magnetic characterization methods. The measured total power loss is resolved into hysteresis, eddy current, and anomalous losses. The anomalous loss is found to account for more than 90% of the total loss, which significantly reduced by annealing in a transverse magnetic field. This is attributed to the reorientation of preferred magnetisation axis perpendicular to the length of ribbons. Transverse magnetic annealing promotes the relative contribution of domain rotation over domain wall motion during magnetisation reversal process. Magnetic annealing also causes a measurable decrease in the domain width, which promotes pinning and inhibits domain wall motion, thus further favoring coherent domain rotation as the primary mechanism of magnetization. This combination accounts for a 75% decrease in the total power loss in the so-processed ribbons and renders them attractive for applications in mid-and high-frequency power supplies and inverters.

High-Frequency soft magnetic properties of Fe-Si-B-P-Mo-Cu amorphous and nanocrystalline alloys

This paper deals with formation, thermal stability and magnetic properties of Fe-Si-B-P-Mo amorphous and nanocrystalline alloys with minor Cu alloying. Amorphous melt-spun Fe84-85B8P3.5-4Mo1-2Cu0.5-1 (at%) alloy ribbons show good bending plasticity and crystallize through two stages with each onset temperature of about 673 and 813 K. Good soft magnetic properties were obtained in both amorphous and nanocrystalline (bcc-Fe + amorphous) states. The highest saturation magnetization and the lowest coercivity are ∼1.4 T and ∼4 A/m, respectively, for the amorphous phase and 1.8 T and 2.5 A/m, respectively, for the bcc-Fe + amorphous phases. The frequency dependence of permeability for nanocrystalline Fe84B8P3.5Si1.5Mo2Cu1 alloy remains nearly constant in a wide frequency range up to 65 kHz. Besides, the quality factor is higher over the whole frequency range up to 10 MHz for the nanocrystalline than for the amorphous alloy. These characteristics are promising for future engineering of high-frequency type soft magnetic materials.

Comparative study on sintering behavior of Al86Ni6Y4.5Co2La1.5 mechanically alloyed amorphous powder and melt-spun ribbon

In this research work, the sintering characteristics of Al86Ni6Y4.5Co2La1.5 mechanically alloyed amorphous powders and milled melt spun ribbon have been compared. Mechanically alloyed amorphous powders were synthesized via 200 h high energy ball milling. Melt spun ribbons were synthesized by single roller melt spinning technique and grounded to powder form by ball milling. Mechanically induced partial crystallization occurred in the ribbons during milling. Significantly higher amount of contaminations such as carbon, oxygen and iron were observed in the mechanically alloyed amorphous powders compared to the milled ribbons. Both powders were consolidated via spark plasma sintering. Superior particle bonding was found in the sample consolidated from milled ribbons, ascribed to the lower amount of contamination that could not effectively restrict the viscous flow and diffusion of atoms. Various complex crystalline phases evolved in the sample consolidated from milled ribbon particles due to the presence of crystalline phases in the powders which acted as nucleation sites, whereas the amorphous phase was mostly retained in its counterpart. Vickers microhardness of the consolidated alloys from milled ribbon and mechanically alloyed amorphous powders were 3.60 ± 0.13 GPa and 2.53 ± 0.09 GPa, respectively. The higher hardness in the former case was attributed to the superior particle bonding and distribution of hard intermetallic phases in the amorphous matrix.

Effect of Sm on thermal and mechanical properties of Cu-Zr-Al bulk metallic glasses

The effect of rare-earth (Sm) microalloying on the thermal stability and phase selection along with the effect of nanocrystallization on the mechanical properties of amorphous melt-spun ribbons of Zr50Cu40Al10, Zr49Cu39.2Al9.8Sm2 and Zr48Cu38.4Al9.6Sm4 alloys were investigated using differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), Vickers and nanoindentation hardness tests and micropillar compression analysis. XRD and TEM analyses showed that all samples were fully amorphous in as-spun state; however, crystallization sequences for the Sm-free and the Sm micro-alloyed samples were different during devitrification. Combined study of XRD, DSC and TEM on melt-spun ribbons show that Zr48Cu38.4Al9.6Sm4 have nanocrystallization of Cu2Sm phase with an average diameter of 10 nm, which was absent in Zr50Cu40Al10, prior to crystallization of Cu10Zr7 phase. The nanoindentation and micropillar compression tests revealed Cu2Sm nanocrystals embedded in Zr48Cu38.4Al9.6Sm4 alloy improves strength and hardness. On the other hand, presence of these nanocrystals deteriorate shear band stability and thus result in a catastrophic brittle fracture through a single shear band burst.

Influence of crystallization treatment on structure, magnetic properties and magnetocaloric effect of Gd71Ni29 melt-spun ribbons

The influence of crystallization treatment on the structure, magnetic properties and magnetocaloric effect of Gd71Ni29 melt-spun ribbons has been investigated in detail. Annealing of the melt-spun samples at 610 K for 30 min, a majority phase with a Fe3C-type orthorhombic structure (space group, Pnma) and a minority phase with a CrB-type orthorhombic structure (space group, Cmcm) were obtained in the amorphous matrix. The amorphous melt-spun ribbons undergo a second-order ferromagnetic to paramagnetic phase transition at 122 K. For the annealed samples, two magnetic phase transitions caused by amorphous matrix and Gd3Ni phases occur at 82 and 100 K, respectively. The maximum magnetic entropy change (–ΔSM)max is 9.0 J/(kgˑK) (5T) at 122 K for the melt-spun ribbons. The values of (–ΔSM)max in annealed ribbons are 1.0 and 5.7 J/(kgˑK), corresponding to the two adjacent magnetic transitions.

Sm5(Fe,Ti)17 melt-spun ribbons with high coercivity

It has previously been reported that annealing of amorphous Sm5Fe17 melt-spun ribbon resulted in the formation of the Sm5Fe17 phase and the resultant Sm5Fe17 melt-spun ribbon exhibited a high coercivity. However, the annealing condition of the amorphous Sm5Fe17 melt-spun ribbon was somewhat critical and it was not easy to obtain Sm5Fe17 grains with high coercivity. In the present study, it was found that the small substitution of Ti for Fe in the Sm5Fe17 melt-spun ribbon stabilized the Sm5Fe17 phase. Annealed Sm5Fe16.7Ti0.3 melt-spun ribbon consisted of small and homogeneous Sm5(Fe,Ti)17 grains and exhibited a higher coercivity than the annealed Sm5Fe17 melt-spun ribbon.

Structure, phase transformations and properties of the TiNi-TiCu alloys subjected to high pressure torsion

It was performed a complex study of stability, crystallization kinetics and martensitic transformations of the melt-spun (MS) Ti50Ni25Cu25 alloy subjected to high pressure torsion (HPT). The amorphous MS Ti50Ni25Cu25 alloy was subjected to high-pressure torsion (HPT) at temperature of 20°C 10 turnes. HPT processing of the melt-spun amorphous Ti-Ni-Cu alloy does not effect on intensive nanocrystallization. HPT processing leads to a decrease of the energy of structural relaxation during heating up to 400°C, in comparison with the initial MS ribbons. The temperature of crystallization of the MS Ti50Ni25Cu25 alloy decreases after HPT processing. In the MS Ti50Ni25Cu25 alloy HPT processing and subsequent annealing produces noticeably finer grains than annealing at 450°C of the initial MS alloy. Consequently, HPT processing alters the crystallization kinetics of the MS Ti50Ni25Cu25 alloy. The size effect on the stabilization of martensitic transformation in nanocrystalline TiNiCu alloy has been established.

Structure and magnetic properties of amorphous Fe-(Zr,Nb)-B melt spun alloys

In this work, we report the structure and magnetic behavior of an amorphous Fe81Zr5Nb4B10 melt-spun alloy. The radial distribution function (RDF) afforded the resolution of the nearest-neighbor configuration on the basis of the atom-pair distance information, for which the positions of each peak indicated the atom-to-atom separation involved for short-range ordering. The first peaks of RDF were attributed to the distances of B-B, Fe-Fe and Zr-Nb atomic pairs, indicating a glassy structure equivalent to a distorted bcc-Fe cluster. From magnetic measurements, a magnetic moment of 0.65 Bohr magneton per Fe atom was established, together with a Curie temperature of 334 K and an initial ac permeability of 550 for frequencies as high as 250 kHz. In addition, the magnetocaloric effect, quantified from isothermal magnetization measurements through the magnetic entropy variation, reached a maximum of 2.0 J/kgK for a magnetic field change of 2.0 T.

Structural aspects of magnetic softening in Fe-based metallic glass during annealing

Amorphous Fe81Si9B10 melt-spun ribbons were annealed at various temperatures to relieve the internal stress. Magnetic studies manifest the occurrence of magnetic softening. The structure evolution of samples was then investigated by synchrotron radiation X-ray diffraction. It was found that, from short-range order to medium-range order, a positive and negative sign for volumetric thermal strain alternates upon heating and the amplitude of strain decays. Moreover, extending to outer shells, thermal strain seems less temperature-dependent in terms of the rate of increment. Magnetic softening in Fe81Si9B10 metallic glasses upon annealing may result from the inhomogeneous thermal strains.