CA1092863A - Iron-boron solid solution alloys having high saturation magnetization - Google Patents

Abstract

ABSTRACT OF THE INVENTION

Ferromagnetic substitutional solid solution alloys characterized by high saturation magnetization and having a bcc structure are provided. The alloys consist esentially of about 4 to 12 atom percent boron, balance essentially iron plus inciden-tal impurities.

Description

3f~8~;3 IRON~BORON SOLID SOLUTION ALLOYS
~AVING HIGH SATURATION MAGNETIZATION
BACKGROUND OF THE INVENTION
1. Field of the Invention This invention relates to ferromagnetic alloys char-acterized by a high saturation magnetization, and, in particular, to iron-boron solid solution alloys having a body centered cubic (bcc) structure.

2. Description of the Prior Art The equilibrium solid solubilities of boron in -Fe (ferrite) and y-Fe (austenite) are quite small, being less than 0.05 and 0.11 atom percent, respectively; see M. Hansen et al., Constitution of Binary Alloys, pp. 249-252, McGraw-Hill Book Co., Inc. (1958). Attempts have been made to increase the solubility of boron in iron by a splat-quenching technique, without success;
see, e.g., R. C. Ruhl et al., Vol. 245, Transactions of the Metal-lurgical 50ciety of AIME, pp. 253-257 (1969). The splat-quenching employed gun techniques and resulted only in the formation of ferrite and Fe3B, with no changes in the amount of austenitic phase. Compositio~s containing 1.6 and 3.2 wt.% (7.7 and 14.5 at.%, respectively) boron were prepared. These splat-quenched materials, as well as equilibrium alloys which contain two phases, are very brittle and cannot easily be processed into thin ribbons or strips for use in commercial applications.
SUMMARY OF THE INVENTION
In accordance with the invention, iron-boron solid solution alloys having high saturation magnetization are provided which consist essentially of about 4 to 12 atom percent boron, balance essentially iron plus incidental impurities~ The alloys of the invention possess a bcc structure and are totally substi-tutional across the range of about 4 to 12 atom percent of boron.

~O~Z8~3 The alloys of the invention are advantageously easilyfabricated as continuous filament with good bend ductility by a process which comprises (a) forming a melt of the material;
(b) depositing the melt on a rapidly rotating quench surface; and (c) quenching the melt at a rate of about 104 to 106C/-sec to form the continuous filament.
The alloys of the invention possess moderately high hardness and strength, good corrosion resistance, high saturation magnetization and hiyh thermal stability. The alloys in the invention find use in, for example, magnetic cores requiring high saturation magnetization.

DETAILED DESCRIPTION OF THE INVENTION
The compositions of alloys within the scope of the invention are listed in Table I, together with their equilibrium structures and the phases retained upon rapid quenching to room temperature. X-ray difraction analysis reveals that a single metastable phase ~-Fe(B) with bcc structure is retained in the chill cast ribbons. Table I also summarizes the change of lattice parameter and density with respect to boron concentration~ It is clear that the lattice contracts with the addition of boron, thus indicating a predominant dissolution of small boron atoms on the substitutional sites of the ~-Fe lattice. This is further supported by the number of atoms in the unit cell (calculated from the density and lattice parameters) in the solid solution as listed in Table I. The number of atoms per cell remains essen-tially constant at 2 (within experimental error) irrespective of the solute concentration. As is well-known, this is character-istic of a substitutional solid solution. For comparison, pureFe exists in the ~-phase (equilibrium) at room temperature and has ``` 1~3!9Z81~3 an average density of 7.87 g/cm3, a lattice parameter of 2.8664 and 2.0 atoms per unit cell. It should be noted that neither the mixture of the equilibrium phases of a-Fe and Fe2B expected from the Fe-B phase diagram nor the orthorhombic Fe3B phase pre-viously obtained by splat-quenching are formed by the alloys of the invention.

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,l~g~63 The amount of boron in the compositions of the invention is constrained by two considerations. The upper limit of about 12 atom percent is dictated by the cooling rate~ At the cooling rates employed herein of about 104 to 106C~sec, compositions con-taining more than about 12 atom percent boron are formed in a sub-stantially glassy phase, rather than the bcc solid solution phase obtained for compositions of the invention. The lower limit of about 4 atom percent is dictated by the fluidity of the molten composition. Compositions containing less than about 4 atom per-cent boron do not have the requisite fluidity for melt spinninginto filaments. The presence of boron increases the fluidity of the melt and hence the fabricability of filaments.
Table II lists the hardness, the ultimate tensile strength and the temperature at which the metastable alloy transforms into a stable crystalline state. Over the range of 4 to 12 atom percent boron, the hardness ranges from 425 to 919 kg/mm2, the ultimate tensile strength ranges from 206 to 360 ksi and the transformation temperature ranges from 880 to 770 K.
Table II

Mechanical Properties of Melt Spun Fe(B) bcc Solid Solution Ribbon Alloy Hardne~s Ultimate Transformation Composition (kg/mm ) Tensile Temperature (at. %) Strength (K) (ksi) Feg4B6 557 242 860 At the transformation temperature, a progressive transformation to a mixture of stable phases, substantially pure ~-Fe and tetra-gonal Fe2B, occurs. The high transformation temperatures of the ~092863 alloys of the invention are indicative of their high thermal stability.
The room temperature saturation magnetization (Bs) of these alloys ranges from 16.6 kGauss for Fe88B12 to 20.0 kGauss for Feg6B4. Further magnetic properties of the alloys of the invention are listed in Table III. These include the saturation moments in Bohr magneton per Fe atom and the Curie temperatures.
For comparison, the saturation moment of pure iron (a -Fe) is 2.22 ~B and its Curie temperature is 1043 K.
Table III
Results of Magnetic Measurements on Crystalline Fe10O XBx Alloys of the Invention.

Boron Saturation Curie Content Moment Temperature x (at.~ B/Fe atom) (K) 4 2.19 978 6 2.17 964 2.13 916 12 2.10 878 Alloys consisting essentially of about 4 to 6 atom percent boron, balance iron, have Bs values comparable to the grain-oriented Fe-Si transformer alloys (BS = 19.7 kGauss).
Further, alloys in this range are ductile. Thus, these alloys are useful in transformer cores and are accordingly preferred.
The alloys of the invention are advantageously fabri-cated as continuous filaments. The term "filament~ as used herein includes any slender body whose transverse dimensions are much smaller than its length, examples of which include ribbon, wire, strip, sheet and the like having a regular or irregular cross-section.

The alloys of the invention are formed by cooling an - alloy melt of the appropriate composition at a rate of about - - - - . ~ . . ~:

10928~;3 104 to 106C/sec. Cooling rates less than about 104C/sec result in mixtures of well-known equilibrium phases of ~-Fe and F~ B.
Cooling rates greater than about 106C/sec result in the meta-stable orthorhombic Fe3B phase and/or glassy phases. Cooling rates of at least about 105C/sec easily provide the bcc solid solution phase and are accordingly preferred. A variety of tech-niques are available for fabricating rapidly quenched continuous ribbon, wire, sheet, etc. Typically, a particular composition is selected, powders of the requisite elements in the desired propor-tions are melted and homogenized and the molten alloy is rapidly quenched by depositing the melt on a chill surface such as a rapidly rotating cylinder. The melt may be deposited by a variety of methods, exemplary of which include melt spinning processes, such as taught in U.S.~. 3,862,658, melt drag processes, such as taught in U.S.P. 3,522,836, and melt extraction processes, such as taught in U.S.P. 3,863,700, and the like. The alloys may be formed in air or in moderate vacuum. Other atmospheric conditions such as inert gases may also be employed.
~ EXAMPLES
Alloys were prepared from constituent elements (purity higher than 99.9%) and were rapidly quenched from the melt in the form of continuous ribbons. Typical cross-sectional dimensions of the ribbons were 1.5 mm by 40 ~m. ~ensities were determined by comparing the specimen weight in air and bromoform (CBr4, p = 2.865 g/cm3) at room temperature. X-ray diffraction patterns were taken with filtered copper radiation in a Norelco diffracto-meter. The spectrometer was calibrated to a silicon standard with the maximum error in lattice parameter estimated to be +0.001 A.
The thermomagnetization data were taken by a vibrating sample magnetometer in the temperature range between 4.2 and 1050 K.
Hardness was measured by the diamond pyramid technique, using a 10~863 Vickers-type indenter consisting of a diamond in the form of a square-based pyramid with an included angle of 136 between oppo-site faces. Loads of 100 g were applied. The results of the measurements are summarized in Tables I, II and III.

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Claims (6)

What is claimed is:

1. A ferromagnetic material, characterized by a high saturation magnetization and having a body centered cubic struc-ture, consisting essentially of about 4 to 12 atom percent boron, balance essentially iron plus incidental impurities.

2. The ferromagnetic material of claim 1 consisting essentially of about 4 to 6 atom percent boron, balance essentially iron plus incidental impurities.

3. The ferromagnetic material of claim 1 in the form of substantially continuous filaments.

4. A process for fabricating substantially continuous filaments of a ferromagnetic material, characterized by a high saturation magnetization and having a body centered cubic struc-ture, consisting essentially of about 4 to 12 atom percent boron, balance essentially iron plus incidental impurities, which com-prises (a) forming a melt of the material;
(b) depositing the melt on a rapidly rotating quench surface; and (c) quenching the melt at a rate of about 104 to 106°C/-sec to form the continuous filament.

5. The process of claim 4 in which the quench rate is at least about 105°C/sec.

6. The process of claim 4 in which the ferromagnetic material consists essentially of about 4 to 6 atom percent boron, balance essentially iron plus incidental impurities.