SOFT MAGNETIC ALLOYS

Abstract: The invention provides a soft magnetic alloy having a composition represented by the general formula: Fe alpha Cu beta B gamma SixMyAlz, wherein M is Mo or Nb, and wherein 0.5</= beta </=1.5; 6</= gamma </=10; 12</=x</=18; 2</=y</=5; 1.5</=z</=9.5; and alpha =100-( beta + gamma +x+y+z), all quantities being expressed in at.%. The addition of at least 1.5 at.% of Al to the known Fe-Cu-B-Si soft magnetic compositions results in lower coercivity and lower magnetostriction. The most preferred composition is Fe65.5Si13.5B9Mo3Cu1Al8.

Description: SOFT MAGNETIC ALLOYS This invention relates to soft magnetic alloys exhibiting very low magnetostriction and very low coercive force. A need exists for soft magnetic alloys for use in transformer cores and the like. For optimum performance m these applications such alloys should exhibit very low magnetostriction and very low coercive force. Such alloys should preferably be iron-based rather than cobalt or nickel based, in order to minimise their cost. EP-A-0271657 and EP-A-0302355 describe iron-based soft magnetic alloys having low core loss, high permeability and low magnetostriction. At least 50% of the alloy structure consists of fine crystalline particles having an average particle size of lOOnm or less. The composition of the alloys is represented by the general formula: t Fe 1 a Ma]l00 χ .y-z- β -/ } -, CU x Si y B z M '« M -' wherein: M is Co and/or Ni; __ ' is at least one element selected from the group consisting of Nb, W, Ta, Zr, Hf, Ti and Mo; M ' is at least one element selected from the group consisting of V, Cr, Mn, Al, elements in the platinum group, Sc, Y, rare earth elements, Au, Zn, Sn and Re; X is at least one element selected from the group consisting of C, Ge, P, Ga, Sb, In, Be and As, and a, x, y, z, α, β and respectively satisfy 0_<a<x_<3, 0.1< . y<30, 0<z<25, 5< . y+z<30, 0.1<α<30, _<10 and 7 <10. The optional component I. " in the above alloys is included for the purpose of improving corrosion resistance or magnetic properties and of adjusting magnetostriction. However, no compositions containing more than one atomic percent of aluminium are disclosed in EP-A-0271657 or EP- A-0302355. EP-A-0342922 describes iron-based soft magnetic alloys having a controlled particle area ratio and a composition defined by the formula: Fe i OO -a-b- c CU a M b Z c < U > wherein M is at least one element selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Co, Ni, Al and the Platinum group; Z is at least one element selected from Si, B, P, and C; and a, b, and c, expressed in atomic % , are as follows: 3<a<8; 0. l_tb_ 8 ; 3. l<a+b_<12 ; and 15<c<28. The relatively high content of copper in these alloys is alleged to give reduced core losses at high frequency. No examples of soft magnetic alloy compositions containing aluminium are given in the specification of EP-A-0342922. It has now been found that the inclusion of relatively large amounts of aluminium in the soft magnetic Fe-Cu-B-Si alloy compositions similar to those described above results in lower coercivity and lower magnetostriction than in the basic alloys. Accordingly, the present invention provides a soft magnetic alloy having a composition represented by the general formula: Fe α Cu-B„Si χ M y Al z . wherein M is Mo or Nb, and wherein 0.5<J3<1.5; 6<_ 7 < . 10; 12<x<18; 2<y 5; 1.5<z<9.5; and , all quantities being expressed in at.%. The preferred alloy compositions are those in which β is about 1, 7 is about 9, x is between 13 and 14, y is about 3 and z is in the range 2-5 or 6-5-8.5. A particularly preferred alloy composition is Fe 65 5 Si 13 j B o o-C^Al g . The soft magnetic alloy according to the present invention is preferably microcrystalline, such that at least 50% of the alloy structure is occupied by fine crystalline particles having an average particle dimension of less than lOOnm. Preferably, the average particle dimension of the fine crystalline particles is less than 30nm and more preferably around lOnm. The soft magnetic alloy according to the present invention is preferably provided in the form of ribbon which can be wound or worked into the final required shape before heat treatment. The present invention also provides a dust core comprising a powder of the soft magnetic alloy according to the present invention as specified above, and further comprising a binder. The soft magnetic alloy according to the present ' invention can be manufactured by methods known in the art, such as those described in detail in EP-A-0271657 or EP-A- 0342922. Briefly, the preferred methods include quenching the molten alloy to form a substantially amorphous solid alloy, followed by annealing the amorphous alloy under controlled conditions to produce the desired microcrystalline structure. The alloy powders can be formed into a dust core by ordinary press forming and sintering with an inorganic binder such as a metallic alkoxide, water glass, or low melting point glass. Specific embodiments of the present invention will now be described further, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a graph showing the coercive force of alloys of the present invention and of comparative compositions from Example 1 below, when heat treated at 520°C; Figure 2 is a graph showing the coercive force of alloys of the present invention and of comparative compositions from Example 1 below, when heat treated at 540°C; Figure 3 is a graph showing the coercive force of an example (alloy 3 of Example 1 below) of the present invention, when heat treated at difference temperatures within the range 480°C to 600°C; Figure - is a graph showing the coercive force of an example (alloy 10 of Example 1 below) of the present invention, when heat treated at different temperatures within the range 480°C to 600°C; Figure 5 is a graph showing the magnetostriction of alloys of the present invention and of comparative compositions from Example 1 below, when heat treated at 520°C; Figure 6 is a graph showing the magnetostriction of alloys of the present invention and of comparative compositions from Example 1 below, when heat treated at 540°C; Figure 7 is a graph showing the magnetostriction of an example (alloy 3 of Example 1 below) of the present invention, when heat treated at difference temperatures within the range 500°C to 600°C; Figure 8 is a graph showing the magnetostriction of an example (alloy 10 of Example 1 below) of the present invention, when heat treated at different temperatures within the range 480°C to 600°C. Figure 9 is a graph showing the coercive force of a number of alloys containing 3% Mo and having different aluminium contents from Example 2 below, when heat treated at 520°C; and Figure 10 is a graph showing the coercive force of an alloy containing 3% Mo and 8% Al from Example 2 as a function of the heat treatment temperature. Example 1 A number of Niobium-containing alloys having the compositions set forth in Table 1 are fabricated into ribbon, 1cm wide by 20μm thick, using the free jet melt spinning technique under an argon atmosphere. The structure of these ribbons is found to be almost completely amorphous by X-ray examination. Flat 100mm lengths of the ribbon from each of the alloys are heat treated for a period of 1 hour at temperatures of 520 α C and 540°C. The temperature of 540°C is found to be optimum to achieve the lowest coercive force in the comparative non- aluminium containing alloy. The optimum temperature to achieve the lowest coercive force in the aluminium containing alloys of the invention is found to be approximately 15°C lower. The structure of the alloys following heat treatment at 520 C C/540°C is examined by X-ray diffraction and transmission electron microscopy. A nano-crystallme structure is observed, with a crystal size of between 7.5 and 10.5nm. Referring to the drawings, Figures 1 and 2 show the values of coercive force obtained for all 12 experimental alloys following heat treatment at 520°C and 540°C, respectively. It can be seen that very low values have been achieved with alloy 3, containing 2% aluminium, and with alloy 10 which contains 6.5% aluminium. A surprising peak in the coercive force is observed near 6% Al content, and further investigation of this region is needed. Figures 3 and 4 show the coercivity values following heat treatment at temperatures in the range 480°C to 600°C for alloys 3 and 10 respectively. Figures 5 and 6 show the measured values for magnetostriction obtained for all of the 12 alloys following heat treatments at 520°C and 540°C respectively. Figures 7 and 8 show that very low magnetostriction values can be achieved by increasing the heat treatment temperatures to 600°C. Example 2 A number of molybdenum-containing alloys having the composition set forth in Table 2 are fabricated using the method described in Example 1. The crystal structure of the alloys is substantially identical to that of the alloys of Example 1. Figure 9 shows that the coercive force of the alloys is lowered for the compositions containing 2% and 4% Al, and is especially low for the alloy composition containing 8% Al. A surprising peak in the coercive force is seen once again at around 6% Al, but further experiments are needed in this composition region to confirm the peak. Figure 10 shows that, for the alloy composition containing 8% Al, the coercive force is a minimum (i.e. Optimised) for an annealing temperature of about 520°C. The above specific embodiments have been described by way of example only. Many other embodiments falling within the scope of the invention as defined by the accompanying claims will be apparent to the skilled reader. TABLE 1 Alloy Composition 1* Fe 73> -Si 13 _ 5 B 9 C 1 Nb- 2 * Fe 72.5 Si 13.5 B 9 Cu i Nb 3 Al l 3 Fe 7 ._ 5 Si 1 3 >5 B 9 Cu 1 Nb j Al- 4 Fe 70._ Si .3.. B 9 Cu l Nb 3 A1 3 5 Fe^^Si. j ^B-Cu^b j Al,. 6 Fe 69 Si 13 5 B 9 Cu,Nb 3 Al, 5 7 Fe 68.5 Si 13.5 B 9 Cu i Nb 3 A1 5 8 Fe 68 Si 13.5 B 9 Cu i Nb 3 Al 5.5 9 Fe 67.5 Si 13.5 B 9 CU 1 Nb 3 Al 6 11 Fe^Si. j ^Cu.Nb j Al- 12 Fe 66 Si 16 5 B 9 Cu.Nb-Al-_ 5 comparative example. TABLE 2 Alloy Composition 1* Fe 73.5 Sl 13. 5 B 9 Mo 3 Cu 1 Fe 71 5 Si 13 5 B 9 Mo 3 Cu 1 Al 2 3 Fe 69.5 Si 13.5 B 9 Mθ 3 CU 1 Al 4 4 Fe 67.5 Si 13.5 B 9 Mθ 3 CU 1 Al 6 5 Fe 65.5 Si .3.5 B 9 Mθ 3 Cu . A1 8 * comparative example.

Claims: CLAIMS 1. A soft magnetic alloy having a composition represented by the general formula: Fe α C U/3 B 7 Si χ M y Al 2 , wherein M is Mo or Nb, and wherein 0.5<__. l.5; 6< 7 <10; 12<x<18; 2 y<5 , * 1.5<z<9.5; and c.=100- (β+y +x+y+z) , all quantities being expressed in at.%. 2. A soft magnetic alloy according to claim 1, wherein β is about 1, is about 9, 13<;x<14, and y is about 3. 3. A soft magnetic alloy according to claim 1 or 2 , wherein "4. A soft magnetic alloy according to Claim 2, wherein M is Mo and z is about 8. 5. A soft magnetic alloy according to any preceding claim which is substantially amorphous. 6. A soft magnetic alloy according to any of claims 1 to 4 , wherein at least 50% of the alloy structure is occupied by fine crystalline particles with an average particle dimension of less than lOOnm. 7. A soft magnetic alloy according to claim 6, wherein at least 50% of the alloy structure is occupied by fine crystalline particles with an average particle dimension of less than 30nm. 8. A soft magnetic alloy according to any preceding claim in the form of a powder. 9. A dust core comprising a powder according to claim 8 and a binder. 10. A soft magnetic alloy substantially as hereinbefore described with reference to the examples.

Inventors: DAVIES HYWEL, MAJOR RODNEY, PARMAR BALJIT, DAVIES, HYWEL, MAJOR, RODNEY, PARMAR, BALJIT
Applicants: DAVIES HYWEL, MAJOR RODNEY, PARMAR BALJIT, TELCON LTD, DAVIES, HYWEL, MAJOR, RODNEY, PARMAR, BALJIT, TELCON LIMITED
Application year: 1996
Main classification: H01F1
IPC codes: H01F1