US 3024142 A
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United States Patent O No Drawing. Filed Aug. 31, 1959, Ser. No. 836,937 Claims priority, application Great Britain Sept. 3, 1958 4 Claims. (Cl. 148-108) The present invention relates to magnetic alloys of the type having low coercivity, i.e. excluding magnetic alloys suitable for use as permanent magnets, and has for an object to provide a magnetic alloy of the type referred to, having high resistivity and the hysteresis loop of which is substantially rectangular. The rectangularity of a hysteresis loop can be represented by the ratio of the flux at remanence Br to the flux at maximum magnetic field Bm, which ratio in the ideal case would be unity. For the purpose of the present invention a hysteresis curve is considered to be rectangular when the said ratio exceeds 0.9.
It is known to employ nickel-iron alloys containing about 65% nickel, as magnetic alloys and to treat such a nickel-iron alloy to give a rectangular hysteresis loop by cooling the alloy while exposed to a magnetic field, from a temperature of 600 C. to 300 C., the rate of cooling being approximately 100 C. per hour.
It is also known to incorporate in such alloys a small proportion of another metal to increase the resistivity of the alloy. A suitable such metal is molybdenum but it is found that when the amount of molybdenum exceeds 1.5% to give a satisfactory increase in resistivity, the ratio of Br to Bm when the alloy is treated in the afore-mentioned manner falls below 0.9 and the alloy is unsuitable for use as a rectangular-loop alloy.
As will be seen from the following Table I, while the addition of amounts of molybdenum exceeding 1.5% gives very satisfactory increases in resistivity, the ratio of Br to Bm falls sharply away when the addition exceeds 1.5%.
it is also found that the addition of small amounts of other metals, such as for example copper, manganese, tungsten or chromiiun, instead of molybdenum, while suitably increasing the resistivity, renders the known heat treatment ineffective for producing a rectangular hysteresis loop, when the said other metals are added in an amount such as to give a resistivity exceeding 40 microhm-cm.
Table Ii gives the resistivity together with the ratio BrzBm obtained with the known magnetic annealing process for nickel-iron alloys having added thereto chromium, tungsten or manganese in varying proportions and it Will be evident that although by adding a suitable proportion of one or a combination of said additives, the requisite resistivity can be obtained, the alloy, upon being sub- 3,024,142 Patented Mar. 6, 1962 jected to the known magnetic annealing process, does not give a rectangular hysteresis loop.
Furthermore, it has been proposed to produce nickeliron alloys having a rectangular hysteresis loop by heat treatment at a constant temperature in a magnetic field for the predetermined time. This method has proved to be ineffective when other metals are added to increase the resistivity, due to the use of the high temperatures heretofore considered necessary for carrying out the method. It has been discovered that by suitable selecting temperature and time of heat treatment in a magnetic field, nickel-iron alloys which include additives to give a resistivity of more than 40 microhm-cm., can be made to have a substantially rectangular hysteresis loop.
According to the invention, therefore, there is provided a nickel-iron alloy containing nickel and iron in a proportion of between 60:40 and 72:28 together with a resistivity-increasing metal or metals, and having a substantially rectangular hysteresis loop and a resistivity exceeding 40 microhm-cm.
According to a further feature of the invention, a method of treating a nickel-iron alloy containing nickel and iron in the proportion of between 60:40 and 72:28 together with one or more resistivity-increasing metals in an amount suflicient to give a resistivity exceeding 40 microhm-cm., comprises subjecting the alloy to heat treatment in a magnetic field at a substantially constant temperature of between 400 C. and 490 C. for a period of time not less than 6 hours to produce an alloy having a substantially rectangular hysteresis loop.
According to a still further feature of the invention the said resistivity-increasing metal or metals includes molybdenum in an amount substantially in excess of 1.5%
The time required for the heat treatment according to the invention increases with decrease of the temperature to which the alloy is heated and as shown in the following table, at temperatures below 400 C. the time becomes so great as to render the process too slow for practical use.
Table III Temperature of heat treatment: Time, hours 480 C. 6 440 C. 24 405 C.
resistivity, the maximum temperatures which can be employed with varying amounts of molybdenum are as follows:
From these figures it will be seen that with temperatures above 490 C. the alloy will not have the required minimum resistivity while with temperatures below 400 C. the time required for the heat treatment becomes too great for practical purposes, as previously mentioned. Similar conditions apply in the case of the other additives to which reference has been made.
In carrying out the constant temperature treatment according to the invention, it is found that once a condition of stable equilibrium is reached, no further improvement in the ratio of Br/Bmi results from continuing the treatment.
The magnetic fields may be produced by means of a coil surrounding the material being treated and connected to a direct or an alternating current source. The strength of the field should be at least ten times greater than the coercive field for the material and in practice a field of a strength of about oersteds has been found satisfactory in most cases although fields of greater strength may be employed without deleterious efiect.
After the heat treatment at constant temperature the material is cooled, the rate of cooling being not critical.
1. A method of producing a nickel-iron alloy having a substantially rectangular hysteresis loop with a ratio of fiux at remanence to flux at maximum magnetic field equal to at least 0.9, which consists of the following steps: combining nickel and iron in the relative proportions of between 60:40 and 72:28 with the addition of resistivityincreasing metal selected from the group consisting of molybdenum, chromium, tungsten and manganese in an amount sutiicient to give the alloy a resistivity in excess of microhm-centimeters, and subjecting the alloy to heat treatment in a magnetic field at a substantially constant temperature of between 400 degrees C. and 490 degrees C. for a period of time, not less than six hours, suificient to impart thereto the said rectangular hysteresis loop.
2. A method as claimed in claim 1, in which the said resistivity increasing metal consists of molybdenum in an amount in excess of 1.5 percent and not in excess of 4 percent.
3. A method as claimed in claim 1, in which the strength of the magnetic field is at least 10 times greater than the coercive field of the alloy.
4. A method as claimed in claim 1, in which the strength of the magnetic field is about 10 oersteds.
References Cited in the file of this patent UNITED STATES PATENTS 1,757,178 Elmen May 6, 1930 2,002,689 Bozorth et al. May 28, 1935 2,891,883 Howe June 23,1959