S. F. H. Parker

Precipitation in NdFeB-type magnet materials

The microstructure of sintered magnets and melt-spun samples made from Nb-containing (Nd,Dy)FeB material has been investigated using transmission and scanning electron microscopy, X-ray microanalysis and thermogravimetric analysis. Comparison of the sintered specimens with NdDyFeB and NdFeB magnets revealed the existence of two additional phases. One of these is an Fe 2 Nb Laves compound. The second additional phase takes the form of small Nb-containing coherent precipitates within the main Nd 2 Fe 14 B grains. In the sintered magnet these particles ranged in size from 20-50 nm. Melt-spun ribbon contained a dispersion of smaller inclusions, typically 5-10 nm in diameter. The presence of these precipitates may be related to the enhanced coercivity found in the magnet and melt-spun samples.

Crystallization and Domain Properties of Thinned Amorphous FeBSi Ribbon.

A transmission electron microscope study has been made of the crystallization and associated domain behavior of an amorphous Fe82B15Si3 alloy. The first obvious signs of crystallization in microscope‐annealed foils occur at a temperature of about 345 °C with the appearance of α‐iron nuclei. These crystallites grow with annealing time but the grain size at complete crystallization is still small (∼0.1 μm). At a temperature of 500 °C a bct Fe2B phase is formed. Furnace annealed specimens differ in that the iron nuclei have a dendritic morphology. The as‐quenched material shows an isotropic domain structure with no magnetization ripple. The onset of crystallization is accompanied by the formation of this magnetization dispersion and, in some cases, stripe domains which reflect a perpendicular anisotropy. The results of in‐situ magnetization observations of the domain structure clearly indicate the perturbing effects of the crystallization products.

Effect of Zr additions on the microstructural and magnetic properties of NdFeB based magnets

The microstructure of a sintered magnet made from Zr-containing NdFeB material was investigated using optical microscopy and scanning and transmission electron microscopy. The authors observed three phases in addition to those found in the ternary alloy. Two of these phases were Zr-rich, one having a lamellar structure and containing some iron and neodymium, and a second Zr-rich phase containing a small amount of iron. A substantial amount of Zr was found in the NdFe/sub 4/B/sub 4/ phase with Zr occupying some of the iron sites. A third phase took the form of small coherent precipitates within grains of the hard magnetic phase and was shown to be richer in Zr than the surrounding matrix. Magnetic measurements show an enhanced coercivity over the ternary alloy, which is attributed to the presence of coherent precipitates within the hard magnetic phase. >

The microstructure and magnetic properties of a permanent magnet alloy in the FeAlC system

Specimens of the alloy FeAl 7.74 C 1.77 have been quenched from 1200°C at different rates and annealed at 350°C for various times to optimise their magnetic properties. The remanent magnetisation and saturation magnetisation were found to increase at the expense of coercivity as the quench rate was reduced. A similar effect was observed during heat treatment at 350°C. X-ray diffractometry suggests that the transformation of austenite into ferrite and K-carbide is responsible for the increase in saturation magnetisation observed during heat treatment. Transmission electron microscopy revealed a finely divided micro-structure consisting of Widmanstatten type ferrite regions in a matrix of austenite and K-carbide. Preliminary studies of the domain structure indicate that domain wall impedance at the boundaries of the ferrite and K-carbide phases is a contributing factor to the coercivity of this material.

Property changes in neutron irradiated magnetic glasses

This paper presents some results of experiments attempting to produce irradiation induced defects in metallic glasses. After neutron irradiation, transition metal-metalloid glass specimens were investigated by differential scanning calorimetry and transmission electron microscopy. Changes in crystallization temperatures, crystallization behaviour and Curie temperatures were found. These findings are discussed in terms of the possible effects of structural and chemical changes in irradiated metallic glasses.

Some microstructurally dependent magnetic properties of amorphous FeBSi alloys

Amorphous ribbons of two Fe82(B, Si)18 alloys have been furnace‐annealed at temperatures ∼400 °C for times up to 25 h. The coercivity increases sharply at first, passes through a maximum, and then reaches a plateau after about 4 h anneal time. The saturation magnetization of the ribbons also increases slightly. These changes have been correlated with the microstructure observed with high voltage transmission electron microscopy. It is clear that the major factor is the formation of iron dendrites whose size and number density increase with time up to a saturation value. The volume fraction of crystallized material is directly related to coercivity. The interactions of dendrites with moving domain walls have also been studied: direct evidence of pinning effects when the dendrite size is of the order of the wall width is presented.

Lorentz microscopy of some amorphous and annealed cobalt-based melt-spun alloys

The transmission electron microscope has been used to study the microstructural changes accompanying the crystallization of some near zero magnetostriction amorphous cobalt-based transition metal-metalloid alloys. The domain structure of the specimens was also monitored both in the initial state and as a function of the annealing treatment. The results obtained are compared with those obtained previously on highly magnetostrictive materials such as Fe-B alloys.

Domain structure of the single crystalline hexagonal ferrite Co/sub 2/X

Domain patterns on both basal and axial surfaces of single crystals of the hexagonal ferrite Ba/sub 2/Co/sub 2/Fe/sub 28/O/sub 46/ have been studied with the Bitter technique. Patterns on the basal sections show a well-defined six-fold symmetry with 60 degrees , 120 degrees , and 180 degrees walls. Surfaces containing the c-axis show unusual features. It is suggested that the overall domain behavior is best explained by assuming that the ferrite has an easy cone with a large semiangle. >

A study of the effects of surface crystallization on the properties of metglas 2605 CO (Fe 67 Co 18 B 14 Si 1 ).

Samples of Metglas 2605 CO have been investigated with X-ray diffractometry and transmission electron microscopy. The studies show the existence of a crystalline layer on the wheelside of the as-quenched ribbon although the degree of crystallization varies between batches. The microscope reveals different forms of crystallization products (e.g. dendrites and spherulites) as well as different phases including α-iron, β-cobalt and Co 3 B. The Crystallization kinetics also vary between batches but do not appear to be influenced by the surface layer. On the other hand the DC coercivity of as-quenched ribbons is definitely influenced by the surface crystalline layer. Evidence for domain wall interaction with surface grains suggests that a pinning mechanism could be partly responsible for the elevated coercivity.

Defects and property changes in ion and neutron irradiated metallic glasses

Abstract The possible existence of structural defects in non-crystalline atomic arrangements and the resistance of amorphous metals to radiation damage are two important and related subjects in the study of metallic glasses. This paper presents some results of experiments attempting to produce irradiation induced defects in metallic glasses. After irradiation with neutrons and ions, Fe 80 B 20 and Fe 78 Mo 2 B 20 specimens were investigated by differential scanning calorimetry and transmission electron microscopy. Irradiation induced gas bubbles are observed in ion irradiated specimens but no obvious localised defects were found in the neutron irradiated specimens examined. However, changes in crystallization temperature and Curie point were found in the neutron irradiated materials. These findings are briefly discussed in terms of the possible effects of structural and chemical changes in irradiated metallic glasses.