G. W. Johnson

Rapidly solidified Si-Fe alloys

Si-Fe ribbons produced by the chill-block melt-spinning method, with silicon contents of 5.59 and 6.06 wt%, were annealed at temperatures between 1000 and 1215° C, in vacuum, 5.33 to 13.13 Pa. During annealing for 1 h abnormal grain growth was observed to occur, with modification of the as-cast texture. Two main texture components were identified: a(100)[0kl] texture as the main texture component at all temperatures; and the (421)[−5 84] or a combination of (421) [−584] and (321) [−563] as the second component. The growth of the two main components is affected by the annealing temperature and the annealing time.d.c. coercivity and core loss are significantly reduced by annealing with minimum values being obtained for a 6.06 wt% Si ribbon annealed at 1085° C for 1 h.

The influence of molybdenum concentration on the magnetic permeability and kinetics of order formation in molybdenum permalloys

The initial magnetic permeabilities of molybdenum permalloys with a range of molybdenum concentrations have been measured as a function of isothermal annealing time in the temperature range 400 to 600° C. Annealing in the range from 400 to 460° C produces short-range ordering, leading to a maximum initial permeability when the magnetostriction is reduced to zero. From 460 to approximately 490 to 500° C it is suggested that microdomains of long-range order nucleate and grow in a short-range ordered matrix and these reduce the maximum initial permeability which can be obtained. Above about 490 to 500° C, long-range order does not form, but equilibrium values of short-range order are too small to produce initial magnetic permeabilities as large as in the lower temperature region. The rate of short-range order formation has been found to increase sharply at molybdenum concentrations of about 4 wt % (2.5 at. %) and the activation energy for short-range order formation falls sharply from about 180 kJ mol−1 to 140 kJ mol−1 at this molybdenum concentration. It is thought that different ordered structures and mechanisms of ordering occur below and above 4 wt% molybdenum. Quenching experiments suggest that quenched-in vacancies can increase the rates of short-range order production.

The effect of sheet thickness on the magnetic properties of 77 wt% Ni permalloys

The effect of sheet thickness,t, on the magnetic properties of some Ni-Fe alloys of approximate composition 77% Ni-14% Fe-5% Cu-4 wt% Mo for various heat treatments in the temperature range 300 to 1250° C has been investigated. The range of sheet thickness used was 50 to 375μm and was obtained by cold-rolling without interstage annealing. It was found that permeability increases with decreasing sheet thickness, attains a maximum att≃75μm and starts to fall as the thickness is further reduced. The increase in permeability with decreasing sheet thickness (t>75μm) is thought to be due to a decrease in eddy current losses, changes in texture formation and possibly the degree of short range order (SRO) developed in the material. The permeability obeys a 1/t2 relationship with sheet thickness in the range 75 to 375μm. Below a sheet thickness of 75μm permeability starts to decrease with decreasing sheet thickness. The reason for this fall in permeability is thought to be due to a thickness dependent magnetostatic energy contribution to the wall energy associated with free poles along the domain wall and also to the loss ratio,η, which increases rapidly at small thicknesses. This rapid increase inη is due to factors such as the ratio of domain wall spacing to sheet thickness, which varies with sheet thickness, the spin relaxation term, which has a 1/t2 dependence on sheet thickness, and the increase in the wall energy which increases the domain wall spacing. Finally a decrease in permeability could be caused by a different degree of SRO in the thinner specimens due to different cooling rates which could lead to an increase in first anisotropy constant,K1, and saturation magnetostriction,λs which then produces a decrease inμi att<75μm when the degree of SRO in the specimens is not optimum.

The influence of texture on the magnetic permeability of 77 wt% Ni permalloys

The effect of annealing on the initial magnetic permeability of cold-worked permalloys, of approximate wt % composition 77 Ni, 14 Fe, 5 Cu and 4 Mo, has been investigated and interpreted in terms of the development of crystallographic texture. Two competing recrystallization textures have been observed. One is a modification of the deformation texture, with a range of components between {110} 〈1¯12〉 and {112} 〈11¯1〉, and the other is the cube texture. The two textures develop approximately equally in the temperature range from about 950 to 1050° C, but the cube texture is dominant after annealing between about 1050 and 1100° C. In material annealed for 4 h the magnetic permeability increases to a maximum after annealing at about 900 to 950° C, due to primary recrystallization, and then decreases to a minimum at about 1050° C, before rising to a maximum at about 1100° C and decreasing at higher temperatures. It is thought that the retained deformation texture tends to decrease the permeability and this accounts for the decrease above about 900° C. However, the cube texture increases the permeability and when it becomes dominant above 1050° C, the permeability increases. Secondary recrystallization above about 1100° C destroys the texture and the permeability is reduced. Similar effects on the magnetostriction constant have been observed.

Thermal expansion and magnetostriction of Nd16Fe76B8 alloy

The spontaneous magnetization variation in the temperature range from room temperature to 500 °C was examined by correlation with thermal expansion measurement. Unaligned and aligned sintered specimens were examined to study the temperature dependencies of their dimensions resulting from spontaneous magnetostriction and thermal expansion. Above the Curie temperature (315 °C) all specimens showed the same normal thermal expansion, whilst below this temperature their thermal expansions were quite different and anomalous.

Microstructural development and sintering of NdFeB alloys

The reactions and phase changes occurring during sintering of NdFeB permanent magnet alloys were studied by differential thermal analysis and scanning electron microscopy. The powders were produced by hydrogen decrepitation and on heating, hydrogen evolution occurred in two stages: firstly from the matrix phase (∼170 °C) and then from the neodymium-rich phase (350–750 °C). The neodymium-rich phase melted at ∼630 °C and no significant microstructural changes occurred below this temperature. Above 630 °C the neodymium phase is mobile, wetting the matrix grains and leading to their magnetic isolation. Densification occurs by a solution/precipitation mechanism.

Observation of magnetic domains in 6.06 % Si-Fe by Lorentz microscopy

Magnetic domains were observed by Lorentz microscopy in thin foils of a 6.06 wt% Si-Fe alloy. The foils were prepared from rapidly solidified Si-Fe ribbons, in the as-cast and the as-annealed conditions. A complex domain structure was observed in small grain-size specimens and a typical structure of anti-parallel domains was observed in large grain-size specimens. An asymmetric periodic structure formed above a certain critical thickness of foil.

Effect of grain size and orientation on the initial permeability of 36 wt % Ni-Fe alloys

The effect of grain size and grain orientation on the initial permeability of a 36 wt % Ni-Fe alloy with additions of molybdenum, chromium and copper is reported. The initial permeability was found to increase with annealing temperature between 600° C and approximately 900° C due to the formation of a (1 2 3) [4 1 ¯2] primary recrystallization texture. Increasing the annealing temperature in the range 900 to 1100° C led to progressively lower permeabilities due to the growth of randomly oriented abnormal grains within the textured matrix. It is suggested that an increase in the misorientation between adjacent grains gives rise to an increase in the local magnetostatic energy, leading to much stronger pinning of magnetic domain walls, with a consequent decrease in permeability. Annealing at temperatures above 1100° C tends to increase the permeability, because of the increase in grain size.

Texture mechanism in some fcc-type soft magnetic materials

The authors have studied the mechanism for recrystallization texture in some soft magnetic materials with fcc crystal structure. The alloys used were 77% Ni-14% Fe-5% Cu-4 wt% Mo permalloys. Thin foils selected area electron diffraction (SAD) and X-ray diffraction techniques were employed using a Philips 300 Electron Microscope (EM 300) and an X-ray diffractometer, respectively. Investigations were carried out on deformed, recovered and recrystallized states of the alloys. The various results show that the cold-rolled (deformed) and recovered states of the alloys possess copper-type of rolling texture with {110} 〈112〉 texture as the predominant deformation texture though other minor components such as {112} 〈111〉, {110} 〈001〉 and {123} 〈420〉 textures were detected. No cube texture, {100} 〈001〉 was detected in any of the deformed and recovered materials though the recrystallization texture in these alloys is the cube texture, {100} 〈001〉 which forms over 80% of the annealing texture in these alloys.It is concluded here that the detection of cube texture in the deformed and recovered materials is not a prerequisite for the detection of cube texture in these alloys. The present work is not conclusive about the mechanism for recrystallization texture, but it is proposed here that recrystallization texture, in these alloys is attributed to the growth-oriented mechanism based on the following model. (1) The lattice domains which form the recrystallization texture are present in the cold-rolled matrix. (2) The favoured site for nucleation are the grain boundaries and deformation band boundaries. (3) For the nucleus to be able to grow and form the recrystallization texture it must possess the necessary free energy. (4) Grains must be capable of growth into two or more orientations between which it forms, i.e. the nuclei which form the cube texture should have a [111] pole in common with the matrix in which they grow and a rotation of about 30° around this pole.

Dependence of magnetic properties on atomic order in 77% Ni−14% Fe−5% Cu−4wt% Mo alloys

We have studied the effect of atomic order on the lattice parameter,a, Curie temperature,Tθ, and the initial permeability,μi, of a series of molybdenum permalloys with varied Ni-Fe ratios and molybdenum concentrations. The ordering temperature range was between 380 and 600° C. The results of the effect of short-range order and long-range order on the lattice parameter after annealing for about 5 h between 420 and 470° C indicated a decrease of between 0.15 and 0.22 pm (0.04 to 0.06%) in the lattice parameter. No superlattice lines were detected. This may probably be due to the similarity in the atomic scattering factor of nickel and iron. It was also noted that both long-range and short-range ordering increased the Curie temperature of the ordered materials by about 1.4 to 6.6% due to the production of stronger and shorter Ni-Fe bonds. The number of Ni-Fe bonds, which controls the exchange force, was found to depend on the amount of order and molybdenum content in each material. The Curie temperature, which is a measure of the exchange force, is also an indirect means of measuring the degree of lattice ordering because the exchange integral is affected by metallurgical variables such as atomic ordering, composition, etc. Results of the isothermal annealing time on the initial permeability in the temperature range (380 to 600° C) indicated that between 380 and 460°C, maximum permeability was obtained at a critical degree of short-range ordering which is thought to correspond to a state when both the magnetostriction and anisotropy constants are close to zero. The maximum permeability was independent of ordering temperature in this range, although the time to reach this maximum decreases with increasing temperature. On annealing in the temperature range 460 to 500° C, the permeability reaches a maximum, the maximum permeability in this temperature range decreases with increasing temperature. As in the lower temperature range, the time to reach this maximum decreases with increasing temperature. Annealing between 500 and 600° C produced no maximum permeability. The permeability levelled off after an initial gradual increase. The activation energies for short-range order formation were found to be smaller (between 135 and 142 kJ mol-1) in alloys with molybdenum concentrations above 4 wt% Mo (>2.5 at.% Mo) and higher activation energies (between 196 and 210 kJ mol−1) in alloys with molybdenum concentrations below 4 wt% Mo (<2.5 at.% Mo) which suggests the formation of different ordered structures and mechanisms below and above 4 wt% Mo.