R.A. Buckley

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.

Remanence enhancement in nanostructured melt spun Sm(Fe/sub 0.209/Cu/sub 0.061/Zr/sub 0.025/Co/sub 0.704/)/sub 7.61/

Ribbons of an industrial, high coercivity grade of Sm/sub 2/Co/sub 17/ alloy have been produced by melt spinning at very high substrate velocities. X-ray diffraction has shown the resultant material to be single phase and nanostructured, with grain sizes ranging to below 30 nm. The remanence, J/sub r/, is consequently enhanced due to ferromagnetic exchange coupling, with values up to 0.75 T being observed. This corresponds to J/sub r//J/sub s/ (where J/sub s/ is the saturation polarisation) up to 0.7. Although the intrinsic coercivity is only moderate (up to 500 kA/m), it is still sufficiently high to give maximum energy product values up to 70 kJ/m/sup 3/. >

Structure, properties and response to heat treatment of melt-spun Al-Y and Al-La alloys

Al-Y and Al-La binary alloys containing 0.7–18 wt% (0.2–6.3 at%) Y and 0.9–18 wt% (0.2–4.2 at%) La, were rapidly solidified by chill-block melt-spinning to produce ribbons between 35 and 70 μm thick. Microstructures were of the classical zone A/zone B type with a notable increase in αAl lattice parameter for the Al-6.3 at% Y composition, which exhibited a Knoop hardness of 430±30 kg mm−2 as-spun. Isochronal ageing for 2 h at 200–500 °C gave significant hardening at 200 and/or 300 °C for all of the more concentrated alloys, the largest responses being produced by Al-6.3 at % Y and Al-4.2 at % La at 200 °C. X-ray diffraction asspun indicated the presence of only αAl and equilibrium Al11La3 in the Al-La alloy ribbons and αAl and a non-equilibrium Al4Y/Al11Y3 in the Al-Y ribbons. This non-equilibrium Al-Y phase was identified by X-ray diffraction as isomorphous with orthorhombic or tetragonal Al11 La3 with lattice parameters determined as ao = 0.42 ± 0.02 nm (bo = 1.26 ± 0.06 nm) and co = 0.97 ± 0.05 nm. TEM showed that it was present as an intercellular network with Energy dispersive spectroscopic analysis indicating an average composition Al-46 wt% Y consistent with the Al4Y/Al11Y3 stoichiometry and diffraction patterns consistent with an orthorhombic or tetragonal cell with these lattice parameters. While no significant change in phase constitution of the Al-La ribbons was detected by X-ray diffraction as a result of heat treatment, the Al11Y3 in Al-Y ribbons was seen to be replaced by βAI3Y on heat treatment at 400 and 500 °C.

The effect of Nd content on the structure and properties of melt spun Nd-rich NdFeB alloys

Abstract The influence of Nd content and cooling rate on the microstructure and extrinsic magnetic properties of nanostructured melt spun Nd x Fe 94- x B 6 ( x = 13–19 at%) alloys is reported. For a constant roll speed, the coercivity, j H c increases rapidly between x = 13 and 15 (from ∼ 0.9 to ∼ 1.56 MA m −1 ) while, in spite of the nanoscaled Nd 2 Fe 14 B grains, the remanence ( J r ) decreases to below 0.8 T. Further addition of Nd up to 19 at% leads to a more gradual enhancement of j H c up to ∼ 1.8 MA/m and to a further decrease of J r . A more detailed study of the effects of processing parameters was performed for the Nd 18 at% alloy. The magnetic properties are discussed briefly in terms of phase constitution and microstructure.

Spray deposition of an iron aluminide

High velocity oxyfuel (HVOF) spray forming of an iron aluminide [Fe–12.5 Al–2.93 Ni–0.02 B (wt%), containing 300 p.p.m. oxygen], followed by heat treatment for 24 h at 500°C, 18 h at 600°C and 20 min at 800°C, and multipass hot rolling at 800°C has been studied. Three different thicknesses (0.43, 0.93 and 1.33 mm) of sprayed deposit were produced by spraying for different times (approximately 10, 20 and 30 min). The spray-deposited layers exhibited some oxide and some porosity. This porosity was reduced by heat treatment. The as deposited layer had a high degree of B2 order, and a B2 antiphase domain size of 4.5 nm. On hot rolling this material to a reduction of 38%, it was found to be more susceptible to edge cracking than similar material processed by an ingot–extrusion–hot rolling route. In heat treatment, the aluminide-sprayed layer formed a non-protective Fe2O3 oxide, rather than the usual Al2O3 that forms on the binary alloy. This is attributable to the Ni content of the iron aluminide powder employed.