H.A. Davies

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.

Prediction of glass-forming ability for metallic systems

Abstract The relative glass-forming ability (GFA) of metallic alloys is considered in terms of a parameter ΔT ∗ = (T liq mix − T liq )/T liq mix , which represents the departure of the alloy liquids temperature, Tliq, from that of the simple rule of mixtures liquids temperature, Tliqmix. For values of ΔT ∗ > 0.20 a metallic system is likely to form a glass by melt-quenching in useful thicknesses (i.e. > 20 μm) at a cooling rate of 105−107 K s−1. Hence, a rapid assessment of the GFA of novel compositions may in general be obtained simply from a knowledge of the melting points of the pure components and the liquidust emperatures of the alloys.

Nanocrystalline exchange-enhanced hard magnetic alloys

Abstract The effects of microstructural refinement into the nanocrystalline range on the magnetic properties of iron-rare earth alloys are briefly reviewed and discussed. The influence of composition, including departures from the stoichiometric ratio and alloying additions, and of melt spinning process variables on the microstructure and properties are identified and the commercial prospects for nanophase bonded magnets considered.

Magnetic properties and glass formability of Fe61Co10Zr5W4B20 bulk metallic glassy alloy

Amorphous melt spun ribbon samples of Fe61Co10Zr5W4B20 alloy of various thicknesses up to ∼350 μm and suction cast 1 mm diameter rod and 2, 3, and 4 mm outside diameter tube samples of the same alloy were produced. The experimental results for the rod samples indicate a relatively high saturation polarization μ0Ms=0.8 T and a Curie temperature TC=615 K, as well as a very low coercive field Hc=1.4±0.5 A/m with high anisotropy that lowers the permeability of the samples. Thermal analysis by differential scanning calorimetry carried out on ribbon and rod samples revealed a large supercooled liquid region ΔTx of ∼60 K for this alloy.

Magnetic hardening of FeSiBCuNb ribbons and wires during the first stage of crystallization to a nanophase structure

The structural and magnetic properties of Fe73.5Si13.5B9Cu1Nb3 alloy wire and ribbon have been studied after thermal treatments up to temperatures at which partial (75%) devitrification to a nanocrystalline structure occurs. Nanocrystals are detected by x‐ray diffraction only after treatments around 500 °C, while differential scanning calorimetry studies suggest that substantial structural change is inititiated at a much lower temperature (about 400°). A clear magnetic hardening is observed in samples heated within the temperature range 400–480 °C. This phenomenon is also accompanied by an increase of the linear magnetostriction. These effects are discussed in terms of the local structural rearrangements produced during the first stages of the crystallization process. The study is also extended partly to FeSiB, FeSiBCu, and FeSiBNb alloys.

Cooling Rate and Size Effect on the Microstructure and Mechanical Properties of AlCoCrFeNi High Entropy Alloy

High entropy alloys are usually defined as the kind of alloys with at least five principle components, each component has the equi-atomic ratio or near equi-atomic ratio, and the high entropy alloys can have very high entropy of mixing, forming simple solid solution rather than many complex intermediate phases. In this paper, the size effects on the microstructure and mechanical behaviors of a high entropy alloy of AlCoCrFeNi was studied by preparing as-cast rod samples with different diameters. The alloy independent of cast diameter samples has the same phase of body centered cubic solid solution. With decreasing casting diameter, both the strength and the plasticity are increased slightly.

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.

Glass formability of Fe–Co–Pr–Dy–Zr–B alloys and magnetic properties following devitrification

Abstract Melt spun ribbon samples of Fe 61 Co 13.5 Zr 1 Pr 4.5− x Dy x B 20 ( x =0, 1) alloys of various thicknesses and suction cast 1 mm diameter rod and 1 and 3 mm outside diameter tube samples of the same alloy have been investigated. The nanocrystalline structure of the samples was derived by devitrification annealing at various temperatures for 30 min. Magnetic measurements revealed useful magnetic hardening and modest remanence enhancement for the (FeCo) 3 RE 14 B nanocrystalline structure.

Soft magnetic properties and structures of nanocrystalline Fe–Al–Si–B–Cu–Nb alloy ribbons

The effects of Al on the magnetic properties of nanocrystalline Fe73.5−XAlXSi13.5B9Cu1Nb3 alloy ribbons, where 0⩽X⩽10, are reported for the first time. The evolution of the structure and magnetic properties of the ribbons, which were initially cast into the amorphous state in an inert gas environment at subatmospheric pressure, were studied as a function of annealing temperature Tann. The minimum dc coercivity developed during annealing, Hcmin, was found to decrease significantly with increasing Al content from 0.5 A/m at X=0 to 0.3 A/m at X=2 and to remain at approximately this level over the range 2<X⩽8 before rising to 0.4 A/m at X=10. The saturation polarization, Js, was, however, found to decrease linearly over this range from Js=1.5 T at X=0 to Js=0.9 T at X=10 for samples exhibiting Hcmin. As there was little significant reduction in the mean crystallite size, dg, at Hcmin with increasing X, this decrease in coercivity was considered to result from a reduction of the magnetocrystalline anisotropy, ...