Alloy, magnetic core and process for the production of a tape from an alloy

Abstract

An alloy is provided which consists of Fe.sub.100-a-b-c-d-x-y-zCu.sub.aNb.sub.bM.sub.cT.sub.dSi.sub.xB.sub.yZ.sub.z and up to 1 at % impurities, M being one or more of the elements Mo, Ta and Zr, T being one or more of the elements V, Mn, Cr, Co and Ni, Z being one or more of the elements C, P and Ge, 0 at %≦a<1.5 at %, 0 at %≦b<2 at %, 0 at %≦(b+c)<2 at %, 0 at %≦d<5 at %, 10 at %<x<18 at %, 5 at %<y<11 at % and 0 at %≦z<2 at %. The alloy is configured in tape form and has a nanocrystalline structure in which at least 50 vol % of the grains have an average size of less than 100 nm, a hysteresis loop with a central linear region, a remanence ratio Jr/Js of <0.1 and a coercive field strength H.sub.c to anisotropic field strength H.sub.a ratio of <10%.

Claims

1. A process for producing a nanocrystalline alloy tape, comprising the steps: providing a tape made of an amorphous alloy with a composition consisting of Fe.sub.100-a-b-c-d-x-y-zCu.sub.aNb.sub.bM.sub.cT.sub.dSi.sub.xB.sub.yZ.sub.z and up to 1 at % impurities, wherein M is at least one element selected from the group consisting of Mo, Ta and Zr, wherein T is at least one element selected from the group consisting of V, Mn, Cr, Co and Ni, wherein Z is at least one element selected from the group consisting of C, P and Ge, and wherein 0 at %≦a<1.5 at %, 0 at %≦b≦1.5 at %, 0 at %≦(b+c)<2 at %, 0 at %≦d<5 at %, 10 at %<x<18 at %, 5 at %<y<11 at % and 0 at %≦z<2 at %, and heat treating the amorphous alloy tape under tensile stress in a continuous furnace at a temperature T.sub.a such that 450° C. ≦T.sub.a≦750° C. wherein the tape is passed through the continuous furnace at a speed such that a period of time which the tape spends in a temperature zone of the continuous furnace is between 2 seconds and 2 minutes, wherein the temperature zone is T.sub.a±5% T.sub.a, determining magnetic properties comprising determining a desired permeability, anisotropic field value, a maximum remanence ratio Jr/Js value of less than 0.1, wherein Jr is remanent magnetization and Js is saturation polarization, determining a maximum value of the ratio of coercive field strength to anisotropic field strength H.sub.c/H.sub.a of less than 10%, and determining a permitted deviation range for each of these values, continuously measuring the magnetic properties of the tape as it leaves the continuous furnace, and where deviations from the permitted magnetic properties deviation are observed, adjusting the tensile stress at the tape accordingly to bring the measured magnetic property values back within the permitted deviation range.

2. The process in accordance with claim 1, wherein the tape is passed through the continuous furnace under a tensile stress of 5 MPa to 800 MPa.

3. The process in accordance with claim 1, wherein the temperature T.sub.a is selected dependent on the niobium content b of the amorphous alloy tape according to the relationship (T.sub.x1+50° C.)≦T.sub.a≦(T.sub.x2+30° C.), wherein T.sub.a is the heat treating temperature, and T.sub.x1 and T.sub.x2 correspond to crystallization temperatures of the amorphous alloy tape defined by maximum transformation heat.

4. The process according to claim 1, wherein the nanocrystalline alloy tape has a nanocrystalline structure having grains in which at least 50% vol of the grains have an average size of less than 100 nm, wherein the nanocrystalline alloy tape exhibits a J-H hysteresis loop having a central linear part, wherein the nanocrystalline alloy tape exhibits a remanence ratio Jr/Js <0.1, and wherein the nanocrystalline alloy tape exhibits a ratio of coercive field strength Hc to anisotropic field strength Ha of less than 10%.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Embodiments are explained in greater detail below with reference to the following figures, which are intended to illustrate certain features of certain embodiments of the appended claims, and not to limit them.

(2) FIG. 1 shows a diagram of hysteresis loops for control examples of nanocrystalline Fe.sub.77-xCu.sub.1Nb.sub.xSi.sub.15.5B.sub.6.5 with different niobium contents after heat treatment in a magnetic field perpendicular to the length of the tape.

(3) FIG. 2 shows a diagram of hysteresis loops for nanocrystalline Fe.sub.77-xCu.sub.1Nb.sub.xSi.sub.15.5B.sub.6.5 after heat treatment under tensile stress applied along the length of the tape for different niobium contents.

(4) FIG. 3 shows a diagram of the remanence ratio of nanocrystalline Fe.sub.77-xCu.sub.1Nb.sub.xSi.sub.15.5B.sub.6.5 after heat treatment in a magnetic field and after heat treatment under tensile stress as a function of the Nb content.

(5) FIG. 4 shows a diagram of the saturation polarisation of Fe.sub.77-xCu.sub.1Nb.sub.xSi.sub.15.5B.sub.6.5 as a function of the Nb content.

(6) FIG. 5 shows a diagram of the saturation magnetostriction λ.sub.s, anisotropic field H.sub.a, coercive field strength H.sub.c, remanence ratio J.sub.r/J.sub.s and non-linearity factor NL of Fe.sub.75.5Cu.sub.1Nb.sub.1.5Si.sub.15.5B.sub.6.5 after heat treatment under tensile stress at different annealing temperatures.

(7) FIG. 6 shows a diagram of the remanence ratio J.sub.t/J.sub.s and coercive field strength H.sub.c of the alloy Fe.sub.77Cu.sub.1Si.sub.15.5B.sub.6.5 after heat treatment under tensile stress.

(8) FIG. 7 shows the crystalline behaviour measured using Differential Scanning calorimetry (DSC) at a heating rate of 10 K/min of the alloy Fe.sub.77Cu.sub.1Si.sub.15.5B.sub.6.5 and the definition of the crystallisation temperatures T.sub.x1 and T.sub.x2.

(9) FIG. 8 shows the X-ray diffraction diagram for the alloy Fe.sub.77Cu.sub.1Si.sub.15.5B.sub.6.5 in its amorphous starting state and after heat treatment under stress at different annealing temperatures in different crystallisation stages.

(10) FIG. 9 shows a diagram of the permeability μ, anisotropic field H.sub.a, coercive field strength H.sub.c, remanence ratio J.sub.r/J.sub.s and non-linearity factor NL of nanocrystalline Fe.sub.75.5Cu.sub.1Nb.sub.1.5Si.sub.15.5B.sub.6.5 after heat treatment under the specified tensile stress σ.sub.a.

(11) FIG. 10 shows the lower and upper optimum annealing temperatures T.sub.a1 and T.sub.a2 for different alloy compositions as a function of the crystallisation temperatures T.sub.x1 and T.sub.x2.

(12) FIG. 11 shows a diagram of the coercive field strength H.sub.c and remanence ratio J.sub.r/J.sub.s of the alloy Fe.sub.80Si.sub.11B.sub.9 and a control composition Fe.sub.78.5Si.sub.10B.sub.11.5 after heat treatment under tensile stress.

(13) FIG. 12 shows a diagram of hysteresis loops for an alloy Fe.sub.80Si.sub.11B.sub.9 and a control composition Fe.sub.78.5Si.sub.10B.sub.11.5 after heat treatment under different tensile stresses.

(14) FIG. 13 shows a schematic view of a continuous furnace.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

(15) Features of particular embodiments of alloy disclosed herein are shown in the tables, which are summarized below.

(16) Table 1 shows the non-linearity factor NL for different Nb contents of the alloy Fe.sub.77-xCu.sub.1Nb.sub.xSi.sub.15.5B.sub.6.5 after heat treatment in the magnetic field (control example) and after heat treatment under a mechanical tensile stress (process according to the invention).

(17) Table 2 shows measured crystallisation temperatures and suitable annealing temperatures T.sub.a for annealing times of approximately 2 s to 10 s for different Nb contents of the alloy Fe.sub.77-xCu.sub.1Nb.sub.xSi.sub.15.5B.sub.6.5.

(18) Table 3 shows magnetic properties of an alloy Fe.sub.76Cu.sub.1Nb.sub.1.5Si.sub.13.5B.sub.8 after heat treatment in a continuous furnace at 610° C. under a tensile stress of approximately 120 MPa as a function of the annealing time t.sub.a.

(19) Table 4 shows magnetic properties of an alloy Fe.sub.76Cu.sub.0.5Nb.sub.1.5Si.sub.15.5B.sub.6.5 after heat treatment with the specified tensile stress σ.sub.a.

(20) Table 5 shows a saturation polarisation level J.sub.s measured in the manufactured state, and non-linearity NL, remanence ratio J.sub.r/J.sub.s, coercive field strength H.sub.c, anisotropic field strength H.sub.a and relative permeability μ values measured at different annealing temperatures T.sub.a after heat treatment of different alloy compositions.

(21) Table 6 shows a saturation polarisation level J.sub.s measured in the manufactured state and non-linearity NL, remanence ratio J.sub.r/J.sub.s, coercive field strength H.sub.c, anisotropic field strength H.sub.a and relative permeability μ values measured after heat treatment of different alloy compositions.

(22) Table 7 shows the saturation magnetostriction λ.sub.s of different alloy compositions measured in the manufactured state and after heat treatment under stress at the specified annealing temperature T.sub.a.

(23) The features of the alloy, magnetic cores and applications therefore disclosed herein can be more clearly understood by reference to the following specific embodiments, which are intended to be illustrative, and not limiting, of the appended claims.

(24) FIG. 1 shows a diagram of hysteresis loops for a particular embodiment of nanocrystalline alloys in the form of a tape.

(25) The tests were carried out by way of example on metal tapes 6 mm and 10 mm wide and typically 17 μm to 25 μm thick. However, the inventive idea is not restricted to these dimensions.

(26) The exemplary tapes have a composition of Fe.sub.77-xCu.sub.1Nb.sub.xSi.sub.15.5B.sub.6.5. The hysteresis loops are measured after heat treatment in the magnetic field, heat treatment being carried out for 0.5 h at 540° C. in a magnetic field of H=200 kA/m perpendicular to the length of the tape. FIG. 1 shows that the hysteresis loops become more non-linear as the Nb content falls. This non-linear hysteresis loop is undesirable in some magnetic core applications as losses due to hysteresis are increased.

(27) Table 1 shows the non-linearity factors NL for the hysteresis loops shown in FIGS. 1 and 2 for different heat treatments and different Nb contents. In particular, Table 1 shows the non-linearity factor for nanocrystalline Fe.sub.77-xCu.sub.1Nb.sub.xSi.sub.15.5B.sub.6.5 after heat treatment in the magnetic field for 0.5 h at a temperature of 540° C. and after heat treatment under a tensile stress of 100 MPa for 4 s at 600° C. for different Nb contents.

(28) TABLE-US-00001 TABLE 1 Non-linearity factor NL (%) 0.5 h 540° C. 4 s 600° C. Nb (at %) in the magnetic field under stress (100 MPa) 0.5 16.sup.(1) 1.8.sup.(2) 1.5 10.sup.(1) 0.4.sup.(2) 3   0.4.sup.(1) 0.1.sup.(1) .sup.(1)Control example .sup.(2)Example according to the invention

(29) FIG. 3 shows a diagram of the remanence ratio J.sub.r/J.sub.s of heat treated samples as a function of the Nb content. In particular, FIG. 3 shows the remanence ratio of nanocrystalline Fe.sub.77-xCu.sub.1Nb.sub.1Si.sub.15.5B.sub.6.5 after heat treatment in the magnetic field for 0.5 h at temperatures of 480° C. to 540° C. and after heat treatment under tensile stress of at temperatures of between 520° C. and 700° C. as a function of the Nb content.

(30) In case of heat treatment in the magnetic field, as indicated by white circles in FIG. 3, particularly linear loops with a remanence ratio of less than 0.1 and a non-linearity factor of less than 3% are reliably obtained only with Nb contents greater than 2 at %. In case of heat treatment under tensile stress, by contrast, linear loops with a remanence ratio of less than 0.1 and a non-linearity factor of less than 3% can be reliably achieved with Nb contents of less than 2 at % and even for compositions without niobium.

(31) The results illustrated in FIGS. 1 and 3 show that, if the heat treatment is carried out in a magnetic field, a minimum Nb content of preferably more than 2 at % is required to produce a tape with magnetic properties suitable for use as a magnetic core.

(32) Tables 1 to 6 and FIGS. 2 to 12 show that, if the heat treatment takes place under mechanical tensile stress along the tape, linear loops with small remanence ratios can be achieved in compositions with a niobium content of less than 2 at %. Since niobium is a relatively expensive element, these compositions have the advantage of reduced raw materials costs.

(33) FIG. 2 shows a diagram of hysteresis loops for tapes after heat treatment in a continuous furnace with an effective annealing time of 4 s at a temperature of 600° C. and under a tensile stress of approximately 100 MPa.

(34) For purposes of this application, annealing time in the continuous furnace is defined as the period during which the tape passes through the temperature zone in which the temperature is within 5% of the annealing temperature specified here. The length of time required to heat the tape to the annealing temperature is typically of an order of magnitude comparable to that of the length of the heat treatment itself.

(35) FIG. 2 shows that it is possible to obtain hysteresis loops with a central linear region and a small remanence ratio for Nb contents of less than 2 at %. The composition comprising 3 at % Nb is a control example and the compositions with Nb<2 at % are the examples according to the invention. The arrow shows the definition of the anisotropic field strength H.sub.a by way of example.

(36) FIG. 3 shows a diagram of a comparison between the remanence ratios of samples tempered under tensile stresses, such as those indicated by black diamonds in FIG. 3, and those of samples tempered in a magnetic field, as indicated by white circles, as a function of the Nb content. Alloys with Nb contents of less than 2 at % have small remanence ratios of less than 0.05 only when they are heat treated under tensile stress. If these compositions are tempered in a magnetic field, however, the remanence ratio is significantly higher and such alloys are therefore unsuitable for some magnetic core applications. Even the alloy Fe.sub.77Cu.sub.1Si.sub.15.5B.sub.6.5, i.e. containing no added Nb, produces a largely linear loop with a remanence ratio of less than 0.05 if heat treated under tensile stress.

(37) FIG. 4 shows a diagram of the saturation polarisation of alloys with a composition of Fe.sub.77-xCu.sub.1Nb.sub.xSi.sub.15.5B.sub.6.5 as a function of the Nb content. Alloys with a reduced Nb content have a significantly higher saturation polarisation. This can advantageously be used to reduce both weight and production costs. In addition to reduced raw materials costs it also provides a further advantage in that the device containing the magnetic core can be made smaller.

(38) FIG. 5 shows a diagram of the saturation magnetostriction λ.sub.s, anisotropic field H.sub.a, coercive field strength H.sub.c, remanence ratio J.sub.r/J.sub.s and non-linearity factor NL of a composition Fe.sub.75.5Cu.sub.1Nb.sub.1.5Si.sub.15.5B.sub.6.5 after heat treatment for approximately 4 seconds under a tensile stress of approximately 50 MPa as a function of the annealing temperature. As shown in FIG. 2, the anisotropic field H.sub.a corresponds to the field in which the linear region of the hysteresis loop becomes saturated.

(39) As illustrated by hatching in the diagram, the annealing temperatures between which the desired properties can be achieved lie in the range of approximately 535° C. to 670° C.

(40) The hatched area shows the region of linear loops with low saturation magnetostriction, high anisotropic field and low remanence ratio. This is also the region in which the alloys have particularly linear loops. Thus in the embodiment disclosed in FIG. 5 the most suitable annealing temperature lies between 535° C. and 670° C.

(41) These temperature limits are largely independent of the level of tensile stress. They are, however, dependent on the length of heat treatment and Nb content. Thus, for example, as shown in FIG. 6 and Table 2, they fall as the Nb content falls or the length of heat treatment increases.

(42) FIG. 6 shows the annealing behaviour of a niobium-free alloy variant for which the optimum annealing temperature lies in the range of approximately 500° C. to 570° C., i.e. significantly below that of the composition shown in FIG. 5. In particular, FIG. 6 shows a diagram of the remanence ratio J.sub.t/J.sub.s and the coercive field strength H.sub.c of the alloy Fe.sub.77Cu.sub.1Si.sub.15.5B.sub.6.5 after heat treatment for 4 seconds at T.sub.a=613° C. under a tensile stress of approximately 50 MPa. Here the optimum annealing temperatures disclosed lie within the range of approximately 500° C. to 570° C. As shown schematically in the inset, this gives a flat linear hysteresis loop with a remanence ratio of less than 0.1.

(43) FIG. 7 shows crystallisation behaviour measured by Differential Scanning calorimetry (DSC) at a heating rate of 10 K/min using the example of the alloy Fe.sub.77Cu.sub.1Si.sub.15.5B.sub.6.5. It shows two crystallisation stages characterised by crystallisation temperatures T.sub.x1 and T.sub.x2. Here the temperature range delimited by T.sub.x1 and T.sub.x2 in the DSC measurement corresponds to the optimum annealing temperature range which lies between 500° C. and 570° C. for this alloy as shown in FIG. 6.

(44) FIG. 8 shows the X-ray diffraction diagram for the alloy Fe.sub.77Cu.sub.1Si.sub.15.5B.sub.6.5 in its amorphous original state and after heat treatment under stress at different annealing temperatures corresponding to the different crystallisation stages defined by T.sub.x1 and T.sub.x2. In particular, FIG. 8 shows the X-ray diffraction diagram after heat treatment under stress for 4 s at 515° C., i.e. in the annealing range in which the magnetic properties disclosed in the invention are achieved, and at 680° C., i.e. in the unfavourable annealing range in which linear hysteresis loops with low remanence ratios are no longer produced.

(45) Analysis of the maximum diffraction values reveals that at annealing temperatures producing linear hysteresis loops with low remanence ratios the only crystallites to form in the crystalline phase are essentially cubic Fe—Si crystallites embedded in an amorphous minority matrix. In the case of the alloy Fe.sub.77Cu.sub.1Si.sub.15.5B.sub.6.5 the average size of these crystallites lies in a range of approximately 38 to 44 nm. If the same analysis is carried out with the alloy composition Fe.sub.75.5Cu.sub.1Nb.sub.1Si.sub.15.5B.sub.6.5 the average crystallite size achieved with the corresponding optimum annealing temperatures lies in the range of 20 to 25 nm.

(46) In the second stage of crystallisation, boride phases, which have an unfavourable effect on magnetic properties and lead to a non-linear loop with a high remanence ratio and high coercive field strength, crystallise out of the amorphous residual matrix.

(47) Table 2 shows further examples and additional data in the form of the crystallisation temperatures T.sub.x1 and T.sub.x2 measured at 25 10K/min by means of Differential Scanning calorimetry (DSC) which correspond to the crystallisation of bcc-FeSi and borides respectively. The suitable annealing temperature lies approximately between T.sub.x1 and T.sub.x2 and results in a structure of nanocrystalline grains with an average grain size of less than 50 nm embedded in an amorphous matrix and the desired magnetic properties.

(48) TABLE-US-00002 TABLE 2 Nb (at %) T.sub.x1 (° C.) T.sub.x2 (° C.) optimum annealing temperature T.sub.a 0 450 544 500° C. to 570° C. 0.5 457 578 510° C. to 620° C. 1.5 486 653 535° C. to 670° C. 3.0 527 707 580° C. to 720° C. (Control example)

(49) However, T.sub.x1 and T.sub.x2 and the annealing temperatures T.sub.a are dependent on the heating rate and length of the heat treatment. For this reason the optimum annealing temperatures for heat treatments of less than 10 seconds are higher than the crystallisation temperatures T.sub.x1 and T.sub.x2 measured using Differential Scanning calorimetry (DSC) at 10K/min shown in Table 2. Accordingly, the optimum annealing temperatures T.sub.a for longer annealing times of 10 min to 60 min, for example, are typically 50° C. to 100° C. lower than the T.sub.a values listed in Table 2 for a heat treatment of a few seconds.

(50) Accordingly, the annealing temperatures T.sub.a can be adapted to the composition and length of the heat treatment as required according to the teaching of FIG. 5 and using the crystallisation temperatures measured using Differential Scanning calorimetry as per Table 2.

(51) Table 3 shows the influence of annealing time using the example of an alloy of composition Fe.sub.76Cu.sub.1Nb.sub.1.5Si.sub.13.5B.sub.8. Annealing times in the range of a few seconds to a few minutes show no significant influence on the resulting magnetic properties. This applies as long as the annealing temperature T.sub.a lies between the limit temperatures discussed in Table 2. In this embodiment they are Tx.sub.1=489° C. and Tx.sub.2=630° C. measured using Differential Scanning Calorimetry at 10 K/min or Ta.sub.1=540° C. and Ta.sub.2=640° C. for heat treatment lasting 4 seconds.

(52) TABLE-US-00003 TABLE 3 Coercive Annealing Non- Remanence field Anisotropic Per- time linearity ratio strength field meability t.sub.a (sec) NL (%) J.sub.r/J.sub.s H.sub.c (A/m) H.sub.a (A/m) μ 3 0.03 <0.001 3 2970 363 4 0.04 <0.001 4 2860 377 6 0.04 <0.001 4 2870 376 13 0.04 <0.001 5 2950 365 32 0.08 <0.001 4 2970 363

(53) In this embodiment the annealing temperature is T.sub.a=610° C. and thus falls between the upper and lower values of the two limit temperature defined. The crystallisation temperatures measured at a heating rate of 10 K/min correspond approximately to the optimum annealing range for isothermal heat treatment lasting a few minutes.

(54) FIG. 9 shows the dependence of permeability, anisotropic field, coercive field strength, remanence ratio and non-linearity factor on the tensile stress applied during heat treatment. In particular, FIG. 9 shows a diagram the permeability, anisotropic field, coercive field strength, remanence ratio and non-linearity factor of nanocrystalline Fe.sub.75.5Cu.sub.1Nb.sub.1.5Si.sub.15.5B.sub.6.5 after heat treatment for 4 seconds at 613° under the specified tensile stress σ.sub.a. In all cases this produced a remanence ratio of typically less than J.sub.r/J.sub.s<0.04 and a non-linearity factor of less than 2%.

(55) Table 4 shows a further example of the dependence of permeability, anisotropic field, coercive field strength, remanence ratio and non-linearity factor on the tensile stress applied during heat treatment. In particular, the table shows the permeability, anisotropic field, coercive field strength, remanence ratio and non-linearity factor of nanocrystalline Fe.sub.76Cu.sub.0.5Nb.sub.1.5Si.sub.15.5B.sub.6.5 after heat treatment for 4 seconds at 605° C. under the specified tensile stress σ.sub.a. In all cases, this produced a remanence ratio of typically less than J.sub.r/J.sub.s<0.1 and a non-linearity factor of less than 3%.

(56) TABLE-US-00004 TABLE 4 Coercive Annealing Non- Remanence field Anisotropic Per- time linearity ratio strength field meability σ.sub.a (sec) NL (%) J.sub.r/J.sub.s H.sub.c (A/m) H.sub.a (A/m) μ 4.5 2.8 0.09 10 122 8730 7.2 1.7 0.05 8 168 6350 16 0.6 0.02 9 405 2630 27 0.3 0.01 9 781 1370 52 0.2 0.008 11 1490 715 105 0.07 0.004 12 3110 343 155 0.08 0.004 16 4560 234

(57) FIG. 9 and Table 4 show that anisotropic field strength H.sub.a and permeability μ can be set accurately by adjusting tensile stress σ.sub.a. Achieving a predetermined anisotropic field strength H.sub.a or permeability μ value requires a tensile stress σ.sub.a≈αμ.sub.0H.sub.a/J.sub.s or σ.sub.a≈α/μ during heat treatment, where μ.sub.0=(4π 10.sup.−7 Vs/(Am)) is the magnetic field constant. Here α indicates a material parameter which depends primarily on the alloy composition but can also depend on annealing temperature and annealing time. Typical values lie within the range α≈30000 MPa 10 to α≈70000 MPa. In particular, the example shown in FIG. 9 results in a value of α≈48000 MPa and that shown in Table 3 in a value of α≈36000 MPa.

(58) The embodiments in FIG. 9 and Table 3 also illustrate that the lower the permeability set, the greater the linearity of the loops. Thus permeabilities of less than approximately μ=3000 result in particularly linear loops with a non-linearity of less than 2% and a remanence ratio of J.sub.r/J.sub.s<0.05.

(59) The tapes in the preceding embodiments comprise an alloy with the composition (in at %) Fe.sub.100-a-b-c-d-x-y-zCu.sub.aNb.sub.bM.sub.cT.sub.dSi.sub.xB.sub.yZ.sub.z, where Cu 0≦a<1.5, Nb 0≦b<2, M is one or more of the elements Mo, Ta, or Zr with 0≦b+c<2, T is one or more of the elements V, Mn, Cr, Co or Ni with 0≦d<5, Si 10<x<18 B 5<y<11 Z is one or more of the elements C, P or Ge with 0≦z<2, With the alloy containing up to 1 at % impurities. Typical impurities are C, P, S, Ti, Mn, Cr, Mo, Ni and Ta.

(60) Under certain heat treatments composition can exert an influence on magnetic properties. It is possible to adjust the heat treatment, and in particular the tensile stress, in order to achieve the desired magnetic properties of a given composition.

(61) Table 5 shows examples of alloys which have been heat treated for approximately 4 seconds under a tensile stress of 50 MPa at an optimum annealing temperature T.sub.a for the composition in question and a control example with a composition containing a niobium content of over 2 at %. The other examples, numbered consecutively 1 to 10, represent compositions disclosed in the invention with a Nb content of less than 2 at %. In addition, FIG. 10 shows the optimum annealing and crystallisation temperatures of alloy examples 1 to 10. In particular, FIG. 10 shows the upper and lower optimum annealing temperatures T.sub.a1 and T.sub.a2 for an annealing time of 4 s as a function of the crystallisation temperatures T.sub.x1 and T.sub.x2 measured using DSC at 10 K/min.

(62) TABLE-US-00005 TABLE 5 Composition J.sub.s T.sub.a NL H.sub.c H.sub.a (at %) (T) (° C.) (%) J.sub.r/J.sub.s (A/m) (A/m) μ (a) Fe.sub.74Cu.sub.1Nb.sub.3Si.sub.15.5B.sub.6.5 1.21 690 0.3 0.004 3 850 1130 1 Fe.sub.76Cu.sub.1Nb.sub.1.5Si.sub.13.5B.sub.8 1.35 610 0.5 0.005 5 950 1140 2 Fe.sub.75.5Cu.sub.1Nb.sub.1.5Si.sub.15.5B.sub.6.5 1.34 610 0.6 0.01 13 1240 780 3 Fe.sub.72.5Co.sub.3Cu.sub.1Nb.sub.1.5Si.sub.15.5B.sub.6.5 1.33 600 1.2 0.016 11 680 1550 4 Fe.sub.74.5Cu.sub.1Nb.sub.1.5Si.sub.16.5B.sub.6.5 1.31 630 0.4 0.007 6 950 1100 5 Fe.sub.75.5Cu.sub.0.5Nb.sub.1.5Si.sub.17.5B.sub.5.5 1.31 645 1 0.02 22 1050 990 6 Fe.sub.76.5Cu.sub.1Nb.sub.0.5Si.sub.15.5B.sub.6.5 1.41 600 0.9 0.013 14 1020 1100 7 Fe.sub.75.5Cu.sub.1Nb.sub.0.5Si.sub.16.5B.sub.6.5 1.40 575 0.5 0.008 8 970 1150 8 Fe.sub.77Cu.sub.1Si.sub.15.5B.sub.6.5 1.46 525 1 0.016 17 1070 1080 9 Fe.sub.75Cu.sub.1Si.sub.17.5B.sub.6.5 1.41 510 1.5 0.017 23 1400 800 10  Fe.sub.80Si.sub.11B.sub.9 1.54 565 0.5 0.013 12 925 1320 (a) Control example 1-10 examples according to the invention

(63) These examples demonstrate that the composition of the alloys disclosed in the invention can be varied within certain limits. Within the limits indicated above (1), elements such as Mo, Ta and/or Zr can be added to the alloy in place of Nb, (2) transition metals such as V, Mn, Cr, Co and/or Ni can be added to the alloy in place of Fe and/or (3) elements such as C, P and/or Ge can be added to the alloy without changing the properties significantly. To corroborate this finding, in a further embodiment the alloy composition
Fe.sub.71.5Co.sub.2.5Ni.sub.0.5Cr.sub.0.5V.sub.0.5Mn.sub.0.2Cu.sub.0.7Nb.sub.0.5Mo.sub.0.5Ta.sub.0.4Si.sub.15.5B.sub.6.5C.sub.0.2
was produced in a tape 20 μm thick and 10 mm wide. The alloy has a saturation polarisation of J.sub.s=1.25 T and reacts to heat treatment under tensile stress in a similar way to example alloys 2 to 5 in Table 3 for example. Thus heat treatment lasting approximately 4 s at 600° C. under a tensile stress of 50 MPa results in a non-linearity factor of 0.4%, a remanence ratio of J.sub.r/Js=0.01, a coercive field strength of H.sub.c=6 A/m, an anisotropic field of H.sub.a=855 A/m and a permeability value of μ=1160.

(64) Table 5 shows that desirable magnetic properties are also achieved without the addition of Cu.

(65) Table 6 therefore shows further example alloys in which the Cu content is systematically varied and heat treatment is carried out for approximately 7 seconds at 600° C. under a tensile stress of approximately 15 MPa. In particular, in Table 6 the element Fe was replaced step by step with Cu while the other alloy components remained unchanged.

(66) TABLE-US-00006 TABLE 6 Composition (at %) J.sub.s (T) NL (%) J.sub.r/J.sub.s H.sub.c (A/m) H.sub.a (A/m) μ 11 Fe.sub.76.5Nb.sub.1.5Si.sub.15.5B.sub.6.5 1.35 0.2 0.02 5 332 2990 12 Fe.sub.76.3Cu.sub.0.2Nb.sub.1.5Si.sub.15.5B.sub.6.5 1.35 0.3 0.02 6 371 2890 13 Fe.sub.76Cu.sub.0.5Nb.sub.1.5Si.sub.15.5B.sub.6.5 1.34 0.8 0.03 10 374 2850 14 Fe.sub.75.1Cu.sub.1.4Nb.sub.1.5Si.sub.15.5B.sub.6.5 1.33 1.2 0.03 10 375 2820 15 Fe.sub.74.5Cu.sub.2Nb.sub.1.5Si.sub.15.5B.sub.6.5 1.32 Critical for production and processing

(67) Table 6 shows no significant influence of the Cu content on the magnetic properties for Cu contents below 1.5 at %. However, the addition of Cu promotes the tendency of the tapes to brittleness during production. In particular, alloys with Cu contents greater than 1.5 at % (such as alloy no. 15 in Table 6, for example) show high brittleness in the manufactured state. For example, a 20 μm thick tape of the alloy Fe.sub.74.5Cu.sub.2Nb.sub.1.5Si.sub.15.5B.sub.6.5 can crack at a bending diameter of approximately 1 mm.

(68) Due to the high tape speeds reached during production (25 to 30 m/s), it is impossible or very difficult to catch a tape this brittle during the casting process and wind it immediately as it leaves the cooling roller. This makes the production of the tape uneconomical. In addition, many such tapes (being brittle from the outset) crack during heat treatment, in particular before they reach the higher temperature zone. When such cracks occur, the heat treatments process is interrupted and the tape has to be passed through the oven again.

(69) In contrast, alloys with a Cu content of less than 1.5 at % can be bent to a bending diameter of twice the tape thickness, i.e. typically less than 0.06 mm, without breaking. This allows the tape to be wound up directly during casting. In addition, the heat treatment of such tapes, which are ductile from the outset, is considerably simpler. Alloys with a Cu content of less than 1.5 at % embrittle during heat treatment, but not until they have left the oven and cooled. The probability of a tape cracking during heat treatment is thus significantly lower. In addition, in most cases tape transport through the oven can continue despite the crack. Overall, tapes which are ductile from the outset can be both produced and heat treated with fewer problems and thus more economically.

(70) The compositions shown in Tables 5 and 6 are nominal compositions in at % which correspond to the concentrations of individual elements found in the chemical analysis to an accuracy of typically ±0.5 at %.

(71) Silicon and boron contents also exert an influence on the magnetic properties of this type of nanocrystalline alloy with a niobium content of less than 2 at % if they are produced under tensile stress.

(72) The examples given in Tables 3 to 6 have the following desired combinations of properties: a magnetisation loop with a linear central region, a remanence ratio J.sub.r/J.sub.s<0.1 and a low coercive field strength H.sub.c which typically represents only a few percent of the anisotropic field strength H.sub.a.

(73) FIGS. 11 and 12 compare the magnetic properties of the compositions Fe.sub.80Si.sub.11B.sub.9 and Fe.sub.78.5Si.sub.10B.sub.11.5. FIG. 11 shows a diagram of the coercive field strength H.sub.c and remanence ratio J.sub.r/J.sub.s curves for both alloys after heat treatment under a tensile stress of approximately 50 MPa as a function of the annealing temperature T.sub.a. The coercive field strength H.sub.c and remanence ratio J.sub.r/J.sub.s of the alloy Fe.sub.80Si.sub.11B.sub.9 disclosed in the invention, indicated by black circles, and of the control composition Fe.sub.78.5Si.sub.10B.sub.11.5, indicated by white triangles, are shown after heat treatment for 4 seconds at the annealing temperature T.sub.a under a tensile stress of approximately 50 MPa.

(74) FIG. 12 shows a diagram of hysteresis loops for the two alloys after heat treatment for 4 s at approximately 565° C. under tensile stresses of 50 MPa (broken line) and 220 MPa (continuous line). The hysteresis loop for the alloy Fe.sub.80Si.sub.11B.sub.9 disclosed in the invention is shown on the left and that of the control composition Fe.sub.78.5Si.sub.10B.sub.11.5 on the right.

(75) Although the alloys shown in FIGS. 11 and 12 differ only slightly in their chemical composition, there are significant differences in the magnetic properties of the two alloys.

(76) For example, after heat treatment at between approximately 530° C. and 570° C. the composition Fe.sub.80Si.sub.11B.sub.9 has a linear magnetisation loop with a low remanence ratio J.sub.r/J.sub.s<0.1 and a low coercive field strength which is significantly below 100 A/m and represents only a few percent of the anisotropic field strength H.sub.a.

(77) In contrast, the composition Fe.sub.78.5Si.sub.10B.sub.11.5 has a high remanence ratio over the entire heat treatment range. Even the lowest remanence ratio values, which are achieved at annealing temperatures of between 540° C. and 570° C., are around J.sub.r/J.sub.s<0.5 (cf. FIG. 11). In addition, at these lowest J.sub.r/J.sub.s values there is an unfavourably high coercive field strength of approximately H.sub.c≈800-1000 A/m. The central region of the magnetisation loop thus loses linearity and the significant divergence in the hysteresis loop leads to disadvantageously high hysteresis losses (cf. FIG. 12).

(78) These embodiments show that after heat treatment under tensile stress alloy compositions with a Si content of more than 10 at % and a B content of less than 11 at % produce a flat, largely linear hysteresis loop with a remanence ratio J.sub.r/J.sub.s<0.1 and a low coercive field strength which is significantly below 100 A/m and represents no more than 10% of the anisotropic field. Where the silicon content is lower and the boron content higher than these limit values, the desired magnetic properties are not achieved after such heat treatment under tensile stress.

(79) The upper Si content limit and the lower B content limit are also examined. While the alloy composition Fe.sub.75Cu.sub.0.5Nb.sub.1.5Si.sub.17.5B.sub.5.5 (see alloy no. 5 in Table 5) could be produced as an amorphous ductile tape without difficulty and had desirable properties following heat treatment, after heat treatment the alloy composition Fe.sub.75Cu.sub.0.5Nb.sub.1.5Si.sub.18B.sub.5 presented only borderline magnetic properties and the alloy composition Fe.sub.75Cu.sub.0.5Nb.sub.1.5Si.sub.18.5B.sub.4.5 could no longer be produced as a ductile amorphous tape.

(80) The embodiments show that after heat treatment under tensile stress alloy compositions with a Si content of less than 18 at % and a B content of more than 5 at % produce a flat, largely linear hysteresis loop with a remanence ratio Jr/Js<0.1 and a low coercive field strength which is significantly below 100 A/m and represents no more than 10% of the anisotropic field. Where the silicon content is greater than 18 at % and the boron content less than 5 at %, the desired magnetic properties are not achieved or an amorphous and ductile tape can no longer be produced with such heat treatment under tensile stress.

(81) Table 7 shows the saturation magnetostriction constant λ.sub.s for different alloy compositions measured in the manufactured state and after 4 s heat treatment under a stress of 50 MPa at the specified annealing temperature T.sub.a. In particular, the annealing temperature selected was no more than 50° C. from the maximum possible annealing temperature Ta.sub.2 in order to obtain particularly small magnetostriction values for a given composition (cf. FIG. 5), these values ultimately being determined by the alloy composition. The effect of the Si content is shown.

(82) TABLE-US-00007 TABLE 7 λ.sub.s (ppm) λ.sub.s (ppm) after heat Composition Manufactured T.sub.a T.sub.a2-T.sub.a treatment at (at %) state (° C.) (° C.) T.sub.a Fe.sub.80Si.sub.11B.sub.9 39 565 10 16 Fe.sub.76Cu.sub.1Nb.sub.1.5Si.sub.13.5B.sub.8 29 610 40 3.5 Fe.sub.75.5Cu.sub.1Nb.sub.1.5Si.sub.15.5B.sub.6.5 29 635 35 0.6 Fe.sub.74.5Cu.sub.1Nb.sub.1.5Si.sub.16.5B.sub.6.5 30 630 50 0.2 Fe.sub.75Cu.sub.0.5Nb.sub.1.5Si.sub.17.5B.sub.5.5 29 645 15 −1.8

(83) As a complement to Table 7, FIG. 5 demonstrates that heat treatment under tensile stress results in a clear reduction in saturation magnetostriction which can in turn lead to reproducible magnetic properties. In particular, by low magnetostriction, mechanical stresses have no or only a minor influence on the hysteresis loop. Such mechanical stresses may occur if the heat treated tape is wound into a magnetic core or if in the course of further processing the magnetic core is embedded in a trough or plastic mass to protect it or is subsequently provided with wire coils. This can be used to devise particularly advantageous compositions, i.e. compositions with low magnetostriction.

(84) As demonstrated by the examples given in Table 7, particularly advantageous magnetostriction values in terms of amount of less than 5 ppm can be achieved if the Si content is greater than 13 at % and the heat treatment temperature is not more than 50° C. below the upper limit Ta.sub.2 of the optimum annealing range. Even smaller saturation magnetostriction values in terms of amount of less than 2 ppm can be achieved if the Si content is greater than 14 at % and less than 18 at % and the heat treatment temperature is not more than 50° C. below the upper limit Ta.sub.2 of the optimum annealing range. Even lower saturation magnetostriction values in terms of amount of less than 1 ppm can be achieved if the Si content is greater than 15 at % and the heat treatment temperature is not more than 50° C. below the upper limit Ta.sub.2 of the optimum annealing range.

(85) The higher the permeability, the more important a small magnetostriction value in terms of amount. For example, alloys with a permeability value greater than 500, or greater than 1000, have a comparatively low dependence on mechanical stresses if the saturation magnetostriction in terms of amount is less than 2 ppm or less than 1 ppm.

(86) The alloy can also have a saturation magnetostriction in terms of amount of less than 5 ppm. Alloys with a saturation magnetostriction below this limit value continue to have good soft magnetic properties even where there is internal stress if the permeability is less than 500.

(87) The saturation magnetostriction value may still depend to a small extent on the tensile stress σ.sub.a applied during heat treatment. For example, the following values are measured for the alloy Fe.sub.75.5Cu.sub.1Nb.sub.1.5Si.sub.15.5B.sub.6.5 after heat treatment of 4 s at 610° C. dependent on the annealing stress: λ.sub.s≈1 ppm at σ.sub.a≈50 MPa, λ.sub.s≈0.7 ppm at σ.sub.a≈260 MPa and λ.sub.s≈0.3 ppm at σ.sub.a≈500 MPa. This corresponds to a small reduction in magnetostriction von Δλ.sub.s≈−0.15 ppm/100 MPa. The other alloy compositions show comparable behaviour.

(88) FIG. 13 shows a schematic view of a device 1 suitable for producing an alloy with a composition in accordance with one of the preceding embodiments in tape form. The device 1 comprises a continuous furnace 2 with a temperature zone 3, this temperature zone being set such that the temperature in the oven in this zone is within 5° C. of the annealing temperature T.sub.a. The device 1 also comprises a coil 4 on which the amorphous alloy 5 is wound, and a take-up coil 6 which takes up the heated treated tape 7. The tape passes from the coil 4 through the continuous furnace 2 to the receiving coil 6 at a speed s. In the process the tape 7 is subject to a tensile stress σ.sub.a exerted in the direction of travel and in the region between tension device 9 and tensioning device 10.

(89) The device 1 also comprises a device 8 for the continuous measurement of the magnetic properties of the tape 6 after it has been heat treated and removed from the continuous furnace 2. The tape 7 is no longer under tensile stress in the area of this device 8. The measured magnetic properties can be used to adjust the tensile stress σ.sub.a under which the tape 7 is passed through the continuous furnace 2. This is shown schematically in FIG. 13 by means of the arrows 9 and 10. This measurement of the magnetic properties and continuous adjustment of the tensile stress can improve the regularity of the magnetic properties along the length of the tape.

(90) The invention having been thus described by reference to certain examples and specific embodiments, it will be recognized that these are intended to illustrate, but not limit, the scope of the appended claims.