Azwar Manaf

New nanocrystalline high-remanence Nd-Fe-B alloys by rapid solidification

Abstract A new type of permanent magnet alloy based on Nd-Fe-B, containing 8–9 at% Nd and processed by melt spinning, is described. Remanences of greater than 1 T are obtained in isotropic ribbon samples, with energy products, (BH)max in excess of 160 kJ m−3 and intrinsic coercivity up to about 485 kA m−1. Microstructurally, the alloys consist of two phase, a matrix of magnetically hard Nd2Fe14B with numerous particles of α-iron on grain boundaries. Both the mean grain size of the Nd2Fe14B (

Enhanced magnetic properties in rapidly solidified Nd-Fe-B based alloys

Abstract The paper presents the results of a systematic investigation of the influence of melt-spinning processing conditions on the grain size in the nanocrystalline range and on the magnetic properties of Nd-Fe-B alloys with small silicon additions. The relationships between remanence, coercivity and grain size are established and discussed. The enhanced remanence observed, M r / M s > 0.5, has been shown to be due to ultra fine grain size below 20 nm.

Microstructure analysis of nanocrystalline Fe-Nd-B ribbons with enhanced hard magnetic properties

Abstract The structure of melt-spun, crystallographically isotropic Fe 84 Nd 10 B 6 alloy ribbon has been studied by high-resolution transmission electron microscopy. The ribbon was spun so as to give a nanocrystalline structure, which led to enhancement of remanence and energy product but, owing to the low Nd content and consequently high Fe concentration, the alloy contained magnetically soft α-Fe particles, in addition to the stoichiometric Fe 14 Nd 2 B phase. The identification of the phases by lattice imaging and energy-dispersive microanalysis is presented and the relationship between the microstructure and the enhanced magnetic properties is briefly discussed.

Magnetic properties and microstructural characterisation of isotropic nanocrystalline Fe-Nd-B based alloys

The effects of a nanocrystalline structure on the magnetic properties of isotropic melt spun Fe-Nd-B alloy ribbon are discussed. Three classes of alloys are considered: (a) low Nd (8-10 at.%) which also contains alpha -Fe; (b) near-stoichiometric Nd contents (11-13 at.%) which are single phase Fe/sub 14/Nd/sub 2/B; (c) high Nd (16-20 at.%) which also contains a Nd-rich phase. For types (a) and (b), as the scale of the nanostructive decreases, the remanence J/sub r/ is increasingly above J/sub s//2, while the coercivity H/sub cj/ is decreased, with a linear J/sub r/-H/sub cj/ relationship. No J/sub r/ enhancement occurs for type (c) alloys, even for a nanocrystalline structure, but rather a decrease due to volume dilution of the Fe/sub 14/Nd/sub 2/B phase. These effects are discussed in terms of inter-grain exchange coupling and decoupling, and the implications with respect to permanent magnets with improved properties are considered. It is demonstrated that the benefits of enhanced J/sub r/ in terms of improved energy product are limited by corresponding attenuation of the coercivity. >

Effect of grain size and microstructure on magnetic properties of rapidly solidified Fe82.4Nd13.1B4.5 alloy

The effect of grain size on the magnetic properties of a rapidly solidified low boron Fe‐Nd‐B alloy, both in directly quenched and in the overquenched and annealed conditions, has been studied. The magnetic properties in the as‐quenched crystalline state and changes occurring on annealing of the overquenched ribbons are discussed in relation to the microstructure. High values of the remanent magnetization, above 0.9 T (9 kG), in combination with coercivities of 1100 kA m−1 (13.7 kOe), have been observed for the directly quenched, isotropic single‐phase ribbons, for certain combinations of process conditions. The mean‐grain size for these ribbons measured on the roll contact side was found in some cases to be <20 nm, representing an essentially nanocrystalline structure. The enhanced remanence is considered to result from intergranular magnetic exchange interactions between fine adjacent grains. The overquenched ribbons were aged at temperatures between 500 and 800 °C for various times and these exhibited ...

Nanocrystalline structures and the enhancement of remanence and energy product in melt spun iron-rare earth-boron alloys for permanent magnets

Abstract The influence of alloy composition and processing conditions on the magnetic properties of melt spun hard magnetic iron-rare earth-boron-based alloys is discussed. It is shown that, under carefully controlled conditions, a nanocrystalline, randomly oriented grain structure with mean grain size dg≤30 nm can be obtained in the as-spun state and that as dg is decreased below ∼30 nm the remanence Jr is progressively increased above the value of 0.8 T expected from the Stoner-Wohlfarth theory. This results from exchange coupling between Fe14Nd2B grains. The effect is shown to occur not only in alloys having Si additions but also in ternary FeNdB, including alloys containing finely dispersed αFe phase and in Fe-Mischmetal-B alloys with remanence enhancement up to ∼1.2 T and 0.78 T, respectively. This leads to substantial improvement in maximum energy product, for instance up to > 160 kJ m−3 (20 MGOe) for some ternary FeNdB alloys. A roughly linear relation is shown to exist between Jr and intrinsic coercivity in the nanocrystalline regime.

Nanocrystalline Fe-Nd-B type permanent magnet materials with enhanced remanence

Abstract High values of the remanent magnetisation, above 0.9 T, in combination with high coercivities, have been observed for certain combinations of process conditions for rapidly quenched FeNdB-based alloy. This enhanced remanence has been obtained for both ternary and Si-containing materials and is considered to result from magnetic exchange interactions between adjacent grains.

Nanocrystallinity and magnetic property enhancement in melt-spun iron-rare earth-base hard magnetic alloys

Refinement of the grain size below ~35 nm mean diameter in melt-spun FeNdB-base alloys leads to en-hancement of remanent polarization,Jr, above the level predicted by the Stoner-Wohlfarth theory for an aggregate of independent, randomly oriented, and uniaxial magnetic particles. This article summarizes the results of the recent systematic research on this phenomenon, including the influence of alloy compo-sition and processing conditions on the crystallite size, degree of enhancement ofJr, and maximum en-ergy product(BH)max. It has been shown that the effect can also occur in ternary FeNdB alloys, without the addition of silicon or aluminum, which was originally thought necessary, providing the nanocrystal-lites are not magnetically decoupled by a paramagnetic second phase. Values of(BH)max above 160 kJ m-3 have been achieved. The relationship between grain size,Jr, intrinsic coercivity,JHc, and(BH)max are discussed in terms of magnetic exchange coupling, anisotropy, and other parameters. Recent exten-sion of this work to the enhancement of properties in Fe-Mischmetal-Boron-base alloys and to bonded magnets with a nanocrystalline structure is also described.

The HDDR behaviour of crystalline and amorphous rapidly quenched NdFeB

Abstract The disproportionation and recombination reactions in which Nd 2 Fe 14 B decomposes into αFe, Nd hydride and Fe 2 B in hydrogen and then reforms under vacuum, have been investigated in melt-spun ribbon with the compositions Nd 13.1 Fe 82.4 B 4.5 and Nd 18 Fe 76 B 6 . TPA and DTA investigations have shown that hydrogen absorption and desorption are more rapid in the amorphous Nd 13.1 Fe 82.4 B 4.5 ribbon cast at 30 m s −1 than in the fully crystalline ribbon of this composition, spun at 10 m s −1 . Disproportionation was found to occur in two stages in 30 m s −1 ribbon, the first stage in the range 400–500 °C associated with the amorphous phase, and the second stage in the range 500–700 °C due to the presence of crystalline Nd 2 Fe 14 B phase. By selecting ribbons with the smallest thicknesses, it was possible to investigate totally amorphous material on a Faraday balance, and these studies showed that, in this material, the disproportionation reaction was very rapid, and finished at 500 °C. This is in agreement with the observations on the two-phase material. In addition, DTA desorption measurements showed that recombination was more rapid and took place at a lower temperature in 30 m s −1 ribbon disproportionated at 500 °C compared with that disproportionated at 800 °C, suggesting a coarsening of the disproportionated microstructure at the higher temperature.

High‐energy‐product rapidly solidified Fe–Nd–B alloys with additions of Nb and Si

The ternary Fe82.4Nd13.1B4.5 alloy, which exhibits enhanced remanence (0.92 T) in the rapidly solidified state, was further modified by partial substitution of Nb or Si, or both Nb and Si, for Fe. The latter alloys containing Nb and Si, while maintaining remanence and coercivity values comparable with those obtained for ternary and quaternary compositions, exhibit higher (BH)max of ∼150 kJ m−3. This results from improved squareness of the demagnetization curve in the second quadrant of the B‐H loop and is due to a more homogeneous distribution of very small (∼20–30 nm) crystallites through the ribbon thickness. This can be attributed to the effect of increased undercooling of the liquid alloys containing Nb and Si on quenching.