Melt-spun Ribbons Research Articles

Origin and inhibition on quasi-periodic coarse grain regions in hot-deformed Nd-Fe-B magnets

Hot-deformed Nd-Fe-B magnets with nanocrystalline grains are promising contenders for high-performance permanent magnets. However, the quasi-periodic coarse grain regions at ribbon interfaces counteract the positive effects on magnetic properties to some extent. Up to now, the ambiguous formation mechanism of coarse grain regions impedes the development of advanced process to obtain uniform microstructure and excellent properties in hot-deformed Nd-Fe-B magnets. To address this issue, systematical studies have been carried out on the grain growth behaviors from melt-spun ribbons to bulk hot-deformed magnets. In this work, we have confirmed that abnormal grain growth occurred in the wheel-side of melt-spun ribbons under high temperature of hot deformation process, due to the ultra-fine grain size with high activation and lack of isolation by Nd-rich phase. Correspondingly, low-temperature heat treatment was applied on melt-spun ribbons to optimize the distribution of grain size, resulting in the significant inhibition on coarse grain regions in hot-deformed magnets.

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.

Influence of γ phase on the magnetostructural transition and magnetocaloric properties of Ni38Co12Mn41Sn9 melt-spun ribbons

In the present work, Ni38Co12Mn41Sn9 melt-spun ribbons were annealed under different conditions to make samples free of secondary γ phase precipitates or get them with various average grain sizes. A comparative study was carried out to demonstrate how γ phase formation affects the martensitic transformation (MT) and the magnetocaloric properties linked to the first-order phase transition. The γ phase significantly reduces the structural transformation temperatures, while slightly increases the thermal hysteresis. The γ phase also decreases the maximum entropy change (|ΔS|max) and the effect is more significant for smaller γ phase precipitates. As compared with those with coarse γ phase grains, the ribbons containing smaller ones possess a lower |ΔS|max but a smaller maximum magnetic hysteresis loss.

Rapid solidified ductile Cu-Al-Mn ribbon and its elastocaloric potential

Cu-Al-Mn alloys display martensitic transformation over a wide range of temperatures. In addition to low cost, this alloy is known for its low transformation stress with reasonable latent heat favoring elastocaloric applications. However, the ductility of Cu-Al-Mn can be limited owing to ordering and intergranular fracture. Through rapid solidification by melt spinning, we show that Cu-Al-Mn ribbon can be made highly ductile (greater than 8% tensile strain in the as-spun state and 10% tensile strain after heat treatment). The ductility of the melt-spun ribbon is related to the suppression of L21 ordering that is characterized through magnetic property measurement. Heat treatment of the ribbon promotes bamboo grain formation, and the latent heat is increased to 6.4 J g−1. Under tensile conditions, we show that the ribbon exhibited about 4 °C temperature change (4.4 °C on heating and 4.2 °C on cooling from 6.3% strain).

Improvement of the microstructure and magnetic properties of (La,Ce)–Fe–B nanocrystalline ribbons

The obstacles of inferior microstructure and poor magnetic properties of (La,Ce)–Fe–B type magnets stand in the way of expanding their applications. In this work, (La,Ce)–Fe–B melt-spun ribbons were prepared with different La/Ce ratios and rare earth contents. The microstructure and magnetic properties of these ribbons were investigated. With the La/Ce ratio increased, the fraction of the CeFe2 phase decreased and the β-La phase segregated among matrix grains, which deteriorated the coercivity of the ribbons. Reducing the rare earth content effectively suppressed the β-La phase and improved the proportion of the matrix phase in the ribbons. Here, the optimal alloy composite (La15Ce85)14Fe80B6 has been obtained with modified microstructure and inhibited secondary phases. Good ribbon performance was tested, with H cj = 4.51 kOe, B r = 6.29 kG and (BH)max = 6.81 MGOe.

Isothermal martensitic transformation in Ti-Ni-Cu-Co shape memory alloy: Insight from a thermally activated kinetic model

Recently the isothermal martensitic transformation in shape memory alloys (SMAs) has been reported in many literatures, and several models have been proposed to interpret the isothermal and athermal kinetics. However, the underlying mechanisms remain inadequately understood. In this work, the isothermal transformation from B2 to B19 is confirmed in Ti-Ni-Cu-Co melt-spun ribbons at the temperature range between Ms and Mf. It reaches a saturation point at every isothermal temperature Tiso, and the saturation points correspond to the f−T curve at the cooling rate of 0.5 K/min. The experimental results indicate that the isothermal accumulation of martensite is a relaxation process from the transient state to the thermoelastic balance one. A thermally activated kinetic model is developed in this study to characterize the isothermal and athermal kinetics. The model is able to estimate the evolution of martensite volume fraction under any temperature path T(t) and it agrees with the experimental results well. According to the model, the effects of elastic energy, nucleation density, and activation energy on the kinetics are investigated. Among those, a small nucleation density ni as well as a large activation energy Q will result in a significant isothermal transition. In this work, the slighter isothermal effects originate from the higher value of ni. As for the non-stoichiometric SMAs, the higher value of Q is responsible for the accumulation of martensite at the isothermal process. Accordingly, the present work provides a novel view from a kinetic model to understand the isothermal martensitic transformation in SMAs.

Enhanced magnetocaloric properties and reduced annealing time of off-stoichiometric La1.2Fe11.6Si1.4B0.75 melt-spun ribbons

The off-stoichiometric La1.2Fe11.6Si1.4B0.75 alloys with different solidification rate were prepared by the arc-melting and melt-spinning methods, and the microstructure, magnetic and magnetocaloric properties of the annealed samples were investigated. It is found that the extra La and refined microstructure (induced by rapid cooling technique) shortened the annealing time to 30 min at 1273 K. The B atoms partially dissolved in lattice interstice of 1:13 phase and brings about the lattice expansion, which improves the TC and enhances the MCE of the La1.2Fe11.6Si1.4B0.75 alloy. The high cooling rate contributes to the depression of thermal and magnetic hysteresis. In the annealed La1.2Fe11.6Si1.4B0.75 ribbon with wheel speed of 10 m/s, the mass fraction of 1:13 phase is 86.8 wt%, and a large maximum magnetic entropy change of 20.2 J·kg−1K−1 with small thermal and magnetic hysteresis can be obtained at 193 K under field change of 2 T.

High cerium content (Nd-Pr-Ce)2Fe14B melt-spun ribbons with high coercivity

The effects of wheel speeds on the magnetic properties and microstructures of [(Nd,Pr)1–xCex]-Fe-B melt-spun ribbons were investigated. Compared to melt-spun ribbons with low cerium (Ce) content (x = 0.2), amorphous formation can be suppressed at high wheel speed in the ribbons with a relatively high Ce content (x = 0.8), and with the increase of the wheel speed, the magnetic properties of the ribbons with high Ce content raise continuously. At high wheel speed, the coercivity mechanism of melt-spun ribbons is mainly pinning field, and the high wheel speed introduces a large number of defects into the matrix as pinning sites. Furthermore, ribbons with high Ce content form a fine and uniform grain structure, so the grain boundary area enlarges, which further hinders the movement of domain walls. The combined effects of the above factors enable ribbons with high Ce content to have excellent magnetic properties (Hcj = 0.977 T, Mr = 64 m2 A/kg) at the wheel speed of 35 m/s.

Structure and Thermal-enhanced Magnetic Properties of Mn₄C Melt-spun Ribbons with Varied Stoichiometry

Cubic perovskite-type Mn4C is difficult to prepare for its metastable characteristics. In this work, we have obtained high-purity Mn₄C successfully by using melt-spinning method. The effects of stoichiometry on the structure and magnetic properties of the samples were studied systematically. We found that x = -0.1 is the optimum composition for the formation of the cubic perovskite phase in Mn4+xC during rapid quenching. Most MnSUB4+x/SUBC melt-spun ribbons with x other than -0.1 are composed of MnSUB23/SUBCSUB6/SUB, α-Mn, and Mn4C, while the fraction of different phase in MnSUB4+x/SUBC ribbons varies with x. The Curie temperature of MnSUB4+x/SUBC ribbons increases slightly with decreasing x, which may affect the lattice parameters of cubic Mn₄C and thus the Mn-Mn exchange interactions. The magnetization of MnSUB4+x/SUBC (x = -0.1 and 0) increases with increasing temperature in high-temperature region while the onset temperature for such behavior is dependent on the fraction of Mn₄C in the samples.

Magnetic properties and magnetocaloric effect (MCE) in the melt-spun Tm20Ho20Gd20Ni20Al20 amorphous ribbon

We fabricated the Tm20Ho20Gd20Ni20Al20 ribbons by melt-spun technology and determined its glass forming ability (GFA), magnetic properties, magnetic phase transition (MPT) character, and magnetocaloric performance. The Tm20Ho20Gd20Ni20Al20 melt-spun ribbon exhibits fully amorphous nature and excellent GFA. A second order (SO) type MPT from ferromagnetic to paramagnetic was observed around the Curie temperature of 30.3 K. Moreover, considerable MCE and magnetocaloric performance in Tm20Ho20Gd20Ni20Al20 amorphous ribbon were realized with the maximum magnetic entropy change, temperature-averaged entropy change of 10 K lift, and relative cooling powder/refrigerant capacity of 9.4 J/kgK, 9.0 J/kgK, and 527.2/422.1 J/kg under 0–5 T field change, respectively. These magnetocaloric parameters are close to most of the recently reported magnetocaloric materials, which made the Tm20Ho20Gd20Ni20Al20 amorphous HE ribbon amorphous ribbon also of potential for practical cryogenic MR applications.

Substantial thinning of melt-spun ribbons by an optimised and high-yield ball-milling process

Melt-spun Fe-based ribbons are widely used as the core of transformers and inductors due to their high flux density and low coercivity. However due to their high thickness (∼19 µm) these ribbons are prone to large eddy current losses at MHz frequencies. Despite low yield, ball milling has been widely used to break such ribbons down to thinner flakes to suppress the eddy current losses at high frequency. In this work, we demonstrated an optimized ball milling process with increased yield for flakes in the desired size range (2–4 µm). We have demonstrated that reducing pre milling annealing temperature from 450 to 350 °C increases the yield in desired size range from 2% to 5% and further increasing batch size from 10 to 20 g increases the yield to 21%. The coercivity of the milled flakes increases from 139 to 1352 A/m due to the ball milling process. A post-mill annealing at 350 °C in Ar atmosphere decreases the coercivity to 341 A/m. X-ray diffraction analysis showed no sign of crystallization during ball milling. The result presented here demonstrates an efficient approach to fabricate ultra-thin flakes out of soft magnetic ribbons for high-frequency applications.

Selection of solidification pathway in rapid solidification processes

Rapid solidification processing of alloys enables the formation of exotic nonequilibrium microstructures. However, the interrelationship between the processing parameters and the resulting microstructure is yet to be fully understood. In melt spinning (MS) and additive manufacturing (AM) of rapidly solidified alloys, opposite microstructure development sequences were observed. A fine-to-coarse microstructural transition is typically observed in melt-spun ribbons, whereas melt pools in AM exhibit a coarse-to-fine transition. In this paper, the microstructural evolutions during these two processes are investigated using phase-field modeling. The variation of all key variables of the solid-liquid interface (temperature, composition, and velocity) throughout the entire rapid solidification of AM and MS processes was acquired with high accuracy. It is found that the onset of nucleation determines the selection of the solidification pathway and, consequently, the evolution of temperature and velocity of the interface during the rapid solidification. The switching of control mechanisms of the solid-liquid interface, which happens in both processes but in opposite directions, is found to cause the velocity jump and disrupt the microstructure development.

Effect of quenching rate on the structural and hard magnetic properties of Nd-Fe-B melt-spun ribbons

The phase structure, microstructure, magnetic and thermomagnetic properties of nanostructured Nd–Fe–B melt-spun ribbons were investigated. The melt-spun ribbons have been prepared at different wheel speeds varying from 17 to 25 m/s. The hard magnetic Nd2Fe14B phase with (00l) texture, indicating preferred crystallographic orientation, was observed in all the ribbons with some α-Fe(Co) as the minor phase. Nd2Fe14B grains are uniformly distributed with grain sizes ranging from 50 to 150 nm. A decrease in the average grain size of Nd2Fe14B and fading away of texture formation in the ribbons were found with the increase in the wheel speeds. The best combination of magnetic properties with a coercivity of 14.5 kOe, the saturation magnetization of 132 emu/g, and the energy product of 16 MGOe was achieved at 23 m/s and these ribbons are suitable for the fabrication of hot deformation Nd–Fe–B magnets.

Magnetostructural transition and magnetocaloric effect in Mn0.5Fe0.5NiSi1−xAlx melt-spun ribbons (x = 0.055 and 0.060)

Melt-spun ribbons samples of the multicomponent alloy Mn0.5Fe0.5NiSi0.940Al0.060 were prepared and the magnetostructural transition (MST) and related magnetocaloric properties studied for as-solidified ribbons and ribbon samples annealed between 800 and 950 °C for 4 h. The results are compared with those reported in the literature for melt-spun ribbons with an Al content x = 0.055 and bulk alloys. It is shown that all samples undergo a first-order MST from a paramagnetic Ni2In-type hexagonal structure to a ferromagnetic TiNiSi-type orthorhombic one. Ribbons show broader isothermal entropy change ΔST(T) curves with moderate maximum values of |ΔST|max at 2 T (7.2–7.3 J kg−1 K−1) in comparison with the reported for bulk alloys. However, the average value of the magnetic hysteresis loss linked to the hexagonal-to-orthorhombic transition is low in comparison with the one reported for most magnetocaloric materials with first-order magnetostructural transitions. This work underlines the effectiveness of this rapid solidification technique to produce highly homogeneous ribbon samples of multicomponent alloys.

Enhanced magnetic refrigerant capacity power in Dy3Ni2 melt-spun ribbons by controlling the solidification rate

Both mechanical and magnetic properties of Dy3Ni2 ingots and melt-spun ribbons were investigated in the present work. The melt spinning rate has an important effect on their performance. The higher the melt spinning rate, the better the ductility, the larger the coercive force of ribbons. With increasing the spinning rate, the intensity of the diffraction peak suggesting the presence of polycrystalline becomes weaker and weaker, and eventually disappeared. The magnetic phase transition temperature is decreased from 66 K to 38 K with increasing the spinning rate. All samples undergo a second order paramagnetic-ferromagnetic phase transition. The full-width-at-half maximum of the temperature dependence of magnetic entropy change curve in Dy3Ni2 ribbons was much larger than that of Dy3Ni2 ingot. So a high refrigerant capacity power about 530 J.kg−1 of Dy3Ni2 ribbons was achieved.

Reduction of the mechanical anisotropy in hot-deformed Nd-Fe-B magnets by the construction of heterogeneous structure

Conventional hot-deformed Nd-Fe-B magnets exhibit significant mechanical anisotropy in the two directions perpendicular and parallel to c-axis, resulting in a complicated machining process. In this work, hydrogenation-disproportionation-desorption-recombination (HDDR) powders were added to reduce the mechanical anisotropy of hot-deformed magnets. The effects of the addition of HDDR powders on the compressive strength of hot-deformed magnets and their corresponding microstructures have been investigated. The mechanical anisotropy was maximally decreased by 68 % at 20 wt. % addition. Microstructure observation indicated that a heterogeneous structure containing two kinds of grains with different morphology was formed after the addition of HDDR powders. Further analyses revealed that the reduction of mechanical anisotropy was mainly attributed to the change of the shape anisotropy of grains and the disruption of the regular stacking structure of the melt-spun ribbons. This work provides a new insight for the preparation of high performance hot-deformed Nd-Fe-B magnet with outstanding comprehensive service capability.

Enhanced magnetostriction through dilute Ce doping of Fe-Ga

Doping of magnetostrictive galfenol (${\mathrm{Fe}}_{82}{\mathrm{Ga}}_{18}$, in at. %) with rare-earth elements significantly enhances magnetostriction, with the largest gains achieved in textured melt-spun ribbons. Here, it is demonstrated that even extremely dilute Ce, as little as 65 ppm, can double the magnetostrictive response of galfenol when coupled with an appropriate heat treatment. This improvement is correlated with a compression of the host lattice, both of which reach their maximum extent at the calculated solubility limit of Ce in in body-centered cubic (bcc) galfenol, $\ensuremath{\sim}50\phantom{\rule{0.16em}{0ex}}\mathrm{ppm}$. Beyond this point, excess Ce segregates into $\mathrm{Ce}{\mathrm{Ga}}_{2}$, which forms an interdendritic network throughout the sample at high Ce levels and cannot be resolutionized through heat treatments. These findings point to the importance of solubility limits (i.e., equilibrium thermodynamics) in determining appropriate doping levels or heat treatment couples to optimize magnetostrictive performance, confirming that overdoping is actively detrimental to both material properties and cost.

Mechanism of Ti-rich grain boundary phase formation and coercivity reinforcement in Sm(Fe0.8Co0.2)11TiBx melt-spun ribbons

The effects of boron doping on the microstructure and magnetic properties of Sm(Fe0.8Co0.2)11TiBx melt-spun ribbons were investigated. At a quenching speed of 25 m/s, the Ti-rich grain boundary (GB) phase and more percent of SmFe12 phase was formed in the sample with x = 0.25, which increase the coercivity from 3 kOe to 6 kOe and remanence from 61 emu/g to 75 emu/g. The Ti-rich grain boundary phase show TiFe2-type structure, in which the (02–1) plane spacing of the grain boundary phase is half that of the adjacent (020) plane spacing of ThMn12 phase. Electron holographic analysis showed that the Ti-rich GB phase can reduce the magnetization coupling between the SmFe12 main phase grains. This served as a guide for the further fabrication of the GB phase in SmFe12-based permanent magnetic materials with high magnetic properties.

Effect of Cr addition and heat treatment on the physical properties of Fe-Al melt-spun ribbons

Iron-based alloys with compositions close to Fe75Al25 (at.%) attract much attention due to their unique properties useful, e.g., for high-temperature or high damping engineering applications. Their mechanical, electron transport and magnetic properties strongly depend on the structural characteristics. In this paper we carried out a comparative study of the physical properties of the as spun ribbons Fe70Al30 and Fe70Al20Cr10 (at.%) alloys. The ribbons were produced using the melt spinning technique and were characterized by SEM, X-ray diffraction, differential scanning calorimetric, resistivity measurement and vibrating sample magnetometer. The addition of Cr to Fe70Al30 has a significant influence on atomic ordering, increases the electrical resistivity and a strong influence on the magnetic behavior.

Crystallization kinetics of Ti-doped Nd<sub>2</sub>Fe<sub>14</sub>B/α-Fe nanocomposite permanent magnets

Nanocomposite magnet consisting of a fine mixture of magnetically hard and soft phase has received much attention for potential permanent magnet development. One of the important requirements for alloys to exhibit excellent magnetic properties is a nanocrystalline grain size. The soft and hard magnetic phases can simultaneously achieve ideal nanoscale composites. The effect of Ti additions in the amorphous crystallization process of the exchange-coupled nanocomposite Nd<sub>2</sub>Fe<sub>14</sub>B/α-Fe magnet prepared by melt spinning is investigated. The results show that Ti can change the crystallization kinetics of the NdFeB melt-spun ribbons. The Ti can increase the activation energy of α-Fe and contrarily reduce the activation energy of a metastable 1∶7 phase, so the growth speed of α-Fe decreases and the metastable 1∶7 phase can stably precipitate from the amorphous phase. When the annealing temperature increases, a metastable 1∶7 phase is decomposed into the α-Fe phase and the Nd<sub>2</sub>Fe<sub>14</sub>B phase. The microstructure observation shows that the grains of the alloys doped with Ti are fine and uniform, with an average grain size of about 20 nm, and no particularly large α-Fe particles appear. The optimal magnetic property is (<i>BH</i>)<sub>max</sub> = 12 MG·Oe (1 G = 10<sup>–4</sup> T, 1 Oe = 79.57795 A/m) when Ti addition is 1.0%.