Method Of Manufacturing A Magnetically Anisotropic Magnet Body

Abstract

A method of manufacturing anisotropic permanent magnets having a composition 28 - 42 percent Co, 10 - 20 percent Ni, 6 - 10 percent Al, 2 - 8 percent Cu, 4 - 10 percent Ti, remainder mainly Fe. The anisotropy is induced in the alloy, possibly together with the treatment in a magnetic field, by means of an elastic deformation of the alloy below the segregation temperature T.sub.o. The result, at least when T.sub.o is at least 25.degree. C higher than T.sub.c, is a higher energy product of the magnet than could be achieved so far.

Description

The invention relates to a method of manufacturing a magnet body having anisotropic permanent magnetic properties of which the part which is essential to said properties is an alloy on the basis of Al-Ni-Co-Fe which, after homogenization is subjected to a thermal treatment in which the homogeneous phase is split up into a phase (.alpha.') which is rich in Fe-Co and a phase (.alpha.) which is rich in Ni-Al, during which the alloy is elastically deformed mechanically.

Such a method is known from British Pat. specification No. 817,702: by elastically deforming by means of mechanical means during the cooling process it is found possible to induce a magnetic anisotropy in a given group of alloys on the basis of Al-Ni-Co-Fe. The requirement is imposed upon such alloys that the Co-content be less than 13 percent by weight.

The deformation in question is produced by means of mechanical pressure or tensile forces. The resulting preferential direction for the magnetic properties then is in the direction of the tensile stress induced in the alloy.

As is known, an important advantage of magnetically anisotropic magnet bodies with respect to magnetically anisotropic bodies is that the maximum energy product, (BH).sub.max, of the permanent magnet ultimately to be manufactured therefrom can be considerably higher.

It has now been found experimentally that an above-mentioned elastic deformation surprisingly also has the desirable result in an important group of alloys which have a much higher Co-content than the 13 percent by weight mentioned in the above British Pat. specification No. 817,702, namely in alloys of the following composition: 28-42 percent Co; 10-20 percent Ni; 6 - 10 percent Al; 2-8 percent Cu, 4-10 percent Ti, and the remainder mainly Fe, not counting the additions of elements such as Ta, Nb, S, Sn, and so on, normally used in said type of alloys.

Such alloys have been described, for example, in the Dutch Pat. specification No. 97,469. It appears from this patent specification that in magnet bodies consisting of such an alloy. a magnetic anisotropy can be induced by subjecting the alloy at a temperature below the Curie temperature (T.sub.c) to a thermal treatment in a magnetic field. T.sub.c is always to be understood to mean the T.sub.c of the .alpha.'-phase. Before being able to carry out said thermal treatment, the alloy must first be cooled from the temperature at which it was homogenized (for example, 1,250.degree. C) to a temperature below said T.sub.c which is, for example, approximately 850.degree. C. In this temperature range often lies the temperature which is dependent upon the composition of the alloy and below which segregation takes place of the homogeneous phase into a phase .alpha.' which is rich in Fe - Co and a phase .alpha. which is rich in Ni-Al. This is the segregation temperature T.sub.o. This segregation which, when To > T.sub.c, begins before an anisotropy can be induced in the alloy by means of a thermal treatment in a magnetic field is, in as far as it takes place above T.sub.c, undesirable for obtaining good magnetic properties and can be prevented in known manner for the greater part by causing the alloy to traverse the temperature range from T.sub.o to T.sub.c rapidly.

By means of the above elastic deformation, however, a magnetic anisotropy can be induced in the alloy, both at temperatures between T.sub.o and T.sub.c (T.sub.o > T.sub.c) and below T.sub.c. The result of this is that the (BH).sub.max product of a permanent magnet manufactured from such a magnetic body is higher than the (BH).sub.max product measured in a permanent magnet manufactured from the same alloy in which the anisotropic magnetic properties have been induced only by means of a thermal treatment in a magnetic field and hence at a temperature below T.sub.c. Essential for obtaining a higher (BH).sub.max product in this group of alloys hence is that T.sub.o to be higher than T.sub.c. When T.sub.o coincides with T.sub.c, or when T.sub.o < T.sub.c, this higher (BH).sub.max product is reached already by means of the above-mentioned thermal treatment in a magnetic field. In particular an elastic deformation as mentioned above is found only to be of advantage if T.sub.o is at least 25.degree. C higher than T.sub.c.

In agreement with the above, the method according to the invention is characterized in that it is applied to alloys of the composition: 28 - 42 percent Co; 10 - 20 percent Ni; 6 - 10 percent Al; 2 - 8 percent Cu, 4 - 10 percent Ti, the remainder mainly Fe, not ocounting the additions of elements such as Ta, Nb, S, Sn, and so on, normally used in said type of alloys, while the temperature at which the homogeneous phase is split up into the phases .alpha.' and .alpha. is at least 25.degree. C higher than the Curie temperature of .alpha.' phase.

A permanent magnet manufactured by means of the method according to the invention is found to have a higher remanence B.sub.r, and hence a higher (BH).sub.max product. In addition it often has also a higher coercive force H.sub.c than when no deformation has taken place during the manufacture.

It is to be noted that the thermal treatment during which elastic deformation is carried out can extend over a part of the temperature range between T.sub.o and T.sub.r, throughout the range T.sub.o - T.sub.c and over a range which terminates below T.sub.c. (T.sub.O > T.sub.c) For example, it may also be an isothermal treatment at a temperature below T.sub.o.

When the alloy has cooled to a temperature below T.sub.c, the elastic deformation and the thermal treatment known per se in a magnetic field can take place together to obtain a magnetic anisotropy. It is also possible that the elastic deformation is succeeded by a thermal treatment in a magnetic field.

An embodiment of the method according to the invention is therefore characterized in that below the Curie temperature the deformation is accompanied by or succeeded by a treatment in a magnetic field.

An embodiment of the method according to the invention in particular is characterized in that the deformation takes place between the segregation temperature and the Curie temperature of the .alpha.'-phase. Actually, in this temperature range it is not possible to attain magnetic anisotropy by means of a magnetic field.

It is to be noted that the use of the method according to the invention particularly in alloys having a crystal orientation results in a considerable gain in (BH).sub.max product. The direction of the mechanical forces producing the deformation should then be either perpendicular to -- a pressure force -- or parallel to -- a tensile force -- the easy axis.

In order that the invention may be readily carried into effect, a few examples of methods according to the invention will now be described in detail with reference to the accompanying drawing.

EXAMPLE 1

An alloy of the composition:

32.0 percent Co, 17.5 percent Ni, 8.0 percent Al, 8.0 percent Ti, 2.5 percent Cu, remainder Fe, was homogenized at a temperature of 1,250.degree. C for 20 minutes. It was then blown cold by means of compressed air (30 l. per minute) to approximately 650.degree. C and then heated again to 835.degree. C. At this temperature the resulting rod (diameter 20 mm, length 25 mm) of the alloy was exposed to a magnetic field of 3,000 Oe for 20 minutes and then cooled to room temperature at a rate of 1.5.degree. C/second. After an annealing treatment at 650.degree. C for 16 hours, at 585.degree. C for 9 hours and at 550.degree. C for 15 hours, the following magnetic values were measured:

(BH).sub.max =5.2.sup. . 10.sup. 6 GOe H.sub.c =1910 Oe B.sub.r =7400 G.

A rod of the same alloy as mentioned above, after homogenization, was subjected during the passage through the temperature range of 925.degree. C to 650.degree. C, to a pressure on all sides of approximately 1,000 kg/sq.cm. The rod was then subjected to a treatment identical to that to which the above-mentioned non-deformed rod was subjected. The segregation temperature of the alloy was 900.degree. C; the Curie temperature 860.degree. C. The following magnetic properties were measured:

(BH).sub.max =5.8.sup. . 10.sup. 6 GOe H.sub.c =2020 Oe B.sub.r =7530 G.

EXAMPLE 2

Starting material was an alloy of the same composition as that described in example I. However, the rod of diameter 20 mm, length 25 mm showed a crystal orientation.

The alloy was homogenized at 120.degree. C for 20 minutes, then cooled to 650.degree. C by means of compressed air, exposed to a magnetic field of 3,000 Oe at 825.degree. C for 15 minutes and finally annealed (at 650.degree. C for 2 hours and at 585.degree. C for 20 hours).

The magnetic properties measured in the rod were:

B.sub.r =8720 G H.sub.c= 1905 Oe (BH).sub.max =8.2.sup. . 10.sup. 6 GOe.

A rod of the same alloy which also had crystal orientation was subjected between 925.degree. C and 650.degree. C to a pressure on all sides of approximately 1,000 kg/sq.cm, after which it was subjected to the same treatment as above.

The measured magnetic properties were:

Br=9080 G H.sub.c =1960 Oe (BH).sub.max =9.3.sup. . 10.sup. 6 GOe.

EXAMPLE 3

Starting alloy: 34.5 percent Co; 14.5 percent Ni; 8.0 percent Al; 6.0 percent Ti; 2.5 percent Cu, remainder Fe. T.sub.o = 875.degree. C; T.sub.c = 865.degree. C.

A rod (20 mm diameter, 25 mm long(of this alloy with crystal orientation was homogenized at 1240.degree. C for 20 minutes, then blown cold by means of compressed air to approximately 700.degree. C and finally heated to 825.degree. C. At 825.degree. C the rod was exposed to a magnetic field of 3,000 Oe for 20 minutes, then cooled to room temperature and finally annealed (at 650.degree. C for 2 hours; at 585.degree. C for 20 hours).

The magnetic properties measured in the rod were:

B.sub.r =10,200 G H.sub.c =1,600 Oe (BH).sub.max =10.6.sup. . 10.sup. 6 GOe.

A rod of the same alloy which also showed crystal orientation was subjected to a tensile stress of 700 kg/sq.cm after homogenization during cooling in compressed air from 900.degree. C to room temperature.

It was then subjected to a treatment equal to that of the non-deformed rod as described above.

The measured magnetic properties were:

B.sub.r =10,050 G. H.sub.c =1,570 Oe. (BH).sub.max =10.4.sup. . 10.sup. 6 GOe.

So from this it appears that in an alloy of which T.sub.o is less than 25.degree. C higher than T.sub.c, the method according to the invention produces no effect.

EXAMPLE 4

Starting alloy: 38.0 percent Co; 15.0 percent Ni; 8 percent Al; 8.0 percent Ti; 30 percent Cu; 0.5 percent Sn; remainder Fe. T.sub.o = 925.degree. C; T.sub.c = 865.degree. C.

After homogenization at 1,240.degree. C for 20 minutes, cooling in compressed air (30 l. per minute) to approximately 750.degree. C, treatment in a magnetic field of 3,000 Oe for 20 minutes at a temperature of 820.degree. C, cooling to room temperature at a rate of approximately 0.8.degree. C/second and finally annealing (at 650.degree. C for 2 hours; at 585.degree. C for 20 hours), a non-crystal-oriented rod (20 mm diameter, 30 mm long) was obtained having the following magnetic properties:

B.sub.r =6720 G H.sub.c =1950 Oe (BH).sub.max =4.6.sup. . 10.sup. 6 GOe.

A rod of the same alloy which also showed no crystal orientation was subjected during the cooling after homogenization between 920.degree. C and 750.degree. C to a pressure on all sides of approximately 1,000 kg.sq.cm. The further treatment was identical to that described above.

The measured magnetic properties:

B.sub.r =7400 G H.sub.c =2065 Oe (BH).sub.max =5.9.sup. . 10.sup. 6 GOe.

EXAMPLE 5

Starting alloy: the same as in example 4. This time, however, a rod (20 mm diameter, 70 mm long) of the alloy showed a crystal orientation. The treatment was further identical to that described in example 4.

The measured magnetic properties without mechanical deformation having taken place were:

B.sub.r =9200 G H.sub.c =1960 Oe (BH).sub.max =9.1.sup. . 10.sup. 6 GOe.

A rod of the above alloys was cooled to room temperature in compressed air after homogenization. From 925.degree. C to room temperature, the rod was exposed to a tensile stress of 700 kg/sq.cm. The rod was then subjected to the same treatment as described in example 4.

The measured magnetic properties were:

B.sub.r =9750 G H.sub.c =1965 Oe (BH).sub.max =11.3.sup. . 10.sup. 6 GOe.

The curves in graph show the variation of the demagnetization of the two permanent magnets according to example 5.

Curve 1 is associated with the magnet which has not been deformed during manufacture. Curve 2 is associated with the magnet manufactured by means of the method according to the invention.

It is found that when during cooling deformation is carried out, the B.sub.r has increased, the demagnetization curve is "more convex" and that also the H.sub.c is higher. From the curve it may be concluded that the (BH).sub.max product will be larger than when deformation according to the invention has not been carried out.

Claims

What is claimed is:

1. A method of manufacturing a magnetically anisotropic body consisting essentially of an alloy of about 28-42 percent Co, about 10-20 percent Ni, about 6-10 percent Al, about 2-8 percent Cu, about 4-10 percent Ti, balance essentially Fe, comprising the steps of cooling said alloy from a temperature of about 1,250.degree. C at which the alloy is homogenized to a temperature T.sub.o, at least 25.degree. C above the Curie temperature of the .alpha.' phase and at which the alloy is split into two phases, .alpha.' and .alpha., and elastically deforming said alloy from said temperature T.sub.o to a temperature below said Curie temperature.

2. A method as claimed in claim 1, wherein the alloy is subjected to treatment in a magnetic field below the Curie temperature of .alpha.' phase.

3. A method as claimed in claim 1, the deformation takes place between temperature T.sub.o and the Curie temperature of the .alpha.' phase.

4. A method as claimed in claim 1, wherein the alloy consists of oriented crystals and the deformation force is directed so that a tensile stress is formed in the body in the easy axis of magnetization.