Fe-Pt-Nb permanent magnet with an ultra-high coercive force and a large maximum energy product

The disclosed permanent magnet has a coercive force of larger than 500 Oe, a residual magnetic flux density of larger than 5 kG, and a maximum energy product of larger than 2 MGOe, and it consisting essentially of 48.about.66.9 Atm % of iron, 33.about.47 Atm % of platinum, and 0.1.about.10 Atm % of niobium. It includes a crystal structure of an incomplete single .gamma..sub.1 phase of a face-centered tetragonal system due to either the composition thereof or heat treatment applied thereto. The permanent magnet is made by heating an alloy of the above main composition at 900.degree..about.400.degree. for one minute to ten hours and quenching the alloy at a high speed of faster than 30.degree. C./minute but slower than 2,000.degree. C./sec.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a permanent magnet consisting of major 
ingredients of iron, platinum and niobium, with less than 0.5 atomic (Atm) 
% of impurities, which permanent magnet has an ultra-high coercive force 
and a very large maximum energy product. 
2. Description of the Prior Art 
As to conventional permanent magnets which use the order-disorder lattice 
phase transformation, Co-Pt alloys of even content in terms of number of 
atoms are known. With the Co-Pt alloys, an ultra-high coercive force and a 
very large maximum energy product can be obtained in the initial stage of 
transformation from a disordered .alpha. phase lattice into an ordered 
.gamma..sub.1 phase lattice, which transformation can be caused either by 
cooling of the alloy of .alpha. phase at a high temperature of about 
1,000.degree. C. with a constant cooling speed followed by reheating at 
about 600.degree. C., or by water quenching followed by reheating. 
The conventional Co-Pt alloy demonstrates better magnetic properties as 
compared with other alloys, but it has a shortcoming in that, since its 
ferromagnetic atom is cobalt whose magnetic moment is smaller than that of 
iron, there are limits in its magnetic properties; namely, its residual 
magnetic flux density is limited to 7.2 kG (kilo.multidot.Gauss) and its 
maximum energy product is limited to 12 MGOe 
(Mega.multidot.Gauss.multidot.Oersted). 
To overcome the above small magnetic moment, one may think of replacing 
cobalt in the alloy composition with iron having a large magnetic moment. 
However, with conventional Fe--50Pt (50 Atm % of Pt) alloys, the 
transformation temperature from ordered the lattice of .gamma..sub.1 phase 
to the disordered .gamma. phase is very high, being about 1,320.degree. 
C., and even a quick cooling, such as water quenching, results in a fairly 
well ordered lattice in an over-aged state. Thus, good magnetic properties 
cannot be produced by mere replacing of cobalt with iron. 
SUMMARY OF THE INVENTION 
Therefore, an object of the invention is to obviate the above-mentioned 
shortcoming of the Fe--50Pt alloys of the prior art by providing an 
excellent permanent magnet of Fe--Pt alloy system while improving the 
reproducibility of its magnetic properties. 
Another object of the invention is to provide a method for producing the 
above permanent alloy of Fe--Pt system. 
As a result of research efforts to solve the above shortcoming of the prior 
art, the inventors have found that an increase in the concentration of 
iron in the Fe--Pt alloy tends to reduce the order-disorder transformation 
point to about 800.degree. C. and to facilitate fairly easy formation of 
disordered lattice of .gamma. phase. More particularly, the inventors have 
succeeded in developing a method in which quenching of an alloy of 
specific composition prevents quick growth of ordered lattice, so as to 
provide a laerge maximum energy product and an ultra-high coercive force 
by using an initial stage of the transformation to the ordered lattice of 
.gamma..sub.1 phase Or by using homogeneous fine precipitation of 
.gamma..sub.1 phase in the matrix of .gamma. phase. 
The invention is based on the above finding of the inventors, and it 
further improves the fairly good magnetic properties of Fe--Pt alloys and 
ensures a highly dependable reproducibility of such improved magnetic 
properties. 
A preferred embodiment of the permanent magnet of the invention consists of 
48.about.66.9 Atm % of iron, 33.about.47 Atm % of platinum, 0.1.about.10 
Atm % of niobium, and less than 0.5 Atm % of impurities. The crystal 
structure of the permanent magnet may include an incomplete .gamma..sub.1 
single phase of a face-centered tetragonal system due to either the 
composition thereof or heat treatment applied thereto. Instead of the 
above single phase, the permanent magnet may have a two-phase crystal 
structure formed of a .gamma..sub.1 phase matrix of face-centered cubic 
system and homogeneously dispersed fine precipitate of .gamma..sub.1 
phase. The permanent magnet of the invention has a coercive force of 
larger than 500 Oe (Oersted), a residual magnetic flux density of larger 
than 5 kG, and a maximum energy product of larger than 2 MGOe. 
In a method for producing a permanent magnet according to the invention, an 
alloy consisting of 48.about.66.9 Atm % of iron, 33.about.47 Atm % of 
platinum, 0.1.about.10 Atm % of niobium, and less than 0.5 Atm % of 
impurities is heated at 900.degree..about.1,400.degree. C. for one minute 
to ten hours so as to apply a homogenizing solid solution treatment 
thereto, and the heated alloy is quenched at a high speed cooling rate of 
faster than 30.degree. C./minute but slower than 2,000.degree. C./second, 
so that the thus produced permanent magnet has a large maximum energy 
product and an ultra-high coercive force. 
In another method for producing a permanent magnet having a large maximum 
energy product and an ultra-high coercive force according to the 
invention, an alloy consisting of 48.about.66.9 Atm % of iron, 33.about.47 
Atm % of platinum, 0.1.about.10 Atm % of niobium, and less than 0.5 Atm % 
of impurities is heated at 900.degree..about.1,400.degree. C. for one 
minute to ten hours so as to apply a homogenizing solid solution treatment 
thereto, and the heated alloy is quenched at a high speed of faster than 
30.degree. C./minute but slower than 2,000.degree. C./second, and then the 
quenched alloy is reheated at 450.degree..about.800.degree. C. for one 
minute to 500 hours, which reheating is followed by cooling. 
In an embodiment of the method of the invention for producing a permanent 
magnet having a large maximum energy product and an ultra-high coercive 
force, the above alloy consisting of 48.about.66.9 Atm % of iron, 
33.about.47 Atm % of platinum, 0.1.about.10 Atm % of niobium, and less 
than 0.5 Atm % of impurities is heated at 900.degree..about.1,400.degree. 
C. for one minute to ten hours so as to apply a homogenizing solid 
solution treatment thereto, and the heated alloy is quenched at a high 
speed of faster than 30.degree. C./minute but slower than 2,000.degree. 
C./second. Plastic working is applied to the quenched alloy at a reduction 
ratio of larger than 80%, for instance by wire-drawing or rolling, and the 
worked alloy is reheated at 450.degree..about.800.degree. C. for one 
minute to 500 hours and cooled thereafter. 
What is meant by the above incomplete .gamma..sub.1 single phase due to 
either the alloy composition or heat treatment applied thereto is as 
follows: namely, while the Fe--Pt binary alloy has a completely ordered 
lattice when its composition is Fe:Pt=50:50 in terms of the number of 
atoms, in the invention the iron content of the alloy slightly increases 
so as to produce the incompletely ordered lattice of the .gamma..sub.1 
phase. The incomplete .gamma..sub.1 phase can be also obtained by a heat 
treatment comprising either quenching alone or a combination of quenching 
and reheating thereafter, which heat treatment brings about the initial 
stage of transformation from .gamma. phase to .gamma..sub.1 phase of 
ordered lattice. 
When a permanent magnet is formed by using the alloy of the above-mentioned 
composition through a method to be described hereinafter, the crystal 
structure of the alloy magnet is either one of the following single phase 
and two-phases; namely, the incomplete .gamma..sub.1 single phase of a 
face-centered tetragonal system due to either the alloy composition or 
heat treatment applied thereto, and two-phases formed of a .gamma. phase 
matrix of a face-centered cubic system and homogeneously dispersed fine 
precipitate of .gamma..sub.1 phase. Regardless of the single or two phases 
structure, the permanent magnet of the invention has the desired magnetic 
properties, namely, a coercive force of larger than 500 Oe, a residual 
magnetic flux density of larger than 5 kG, and a maximum energy product of 
larger than 2 MGOe. 
The details of the method of the invention for producing the 
above-mentioned permanent magnet will be described now step by step. 
(A) Starting materials are measured so as to form a metallic mixture with a 
composition consisting of 48.about.66.9 Atm % of iron, 33.about.47 Atm % 
of platinum, 0.1.about.10 Atm % of niobium, and less than 0.5 Atm % of 
impurities. The metallic mixture is melted by a suitable furnace and 
thoroughly stirred so as to produce a molten alloy with a homogeneous 
composition. An alloy body is formed by using a suitable mold, and it may 
be further processed into a desired shape by wire-drawing, forging or 
rolling. The alloy body is heated at 900.degree..about.1,400.degree. C. 
for one minute to ten hours for homogenization and solid solution 
treatment, and quenched at a high speed of faster than 30.degree. 
C./minute but slower than 2,000.degree. C./second. The quenching process 
is to stabilize one of the following structures at room temperature; 
namely, a structure corresponding to the initial stage of the 
transformation from .gamma. phase of a face-centered cubic system to the 
.gamma..sub.1 phase of face-centered tetragonal system, or a structure 
formed of fine precipitate of .gamma..sub.1 phase of ordered lattice 
homogeneously dispersed in the .gamma. phase matrix of disordered lattice. 
(B) After the quenching of the above step (A), the alloy body is reheated 
at 450.degree..about.800.degree. C., preferably 
550.degree..about.750.degree. C., for one minute to 500 hours, preferably 
5 minutes to 100 hours, so as to produce local strain in the solid 
solution representing the initial stage of the transformation from the 
disordered .gamma. phase to the ordered lattice of .gamma..sub.1 phase, 
which transformation takes place at the high temperature. Thus, magnetic 
domains in the alloy body are prevented from dislocation, and a permanent 
magnet having both an ultra-high coercive force and a very high maximum 
energy product is produced. 
(C) Alternatively, after the quenching of the step (A), plastic working 
with a reduction ratio of larger than 80% may be applied to the alloy 
body, for instance by wire-drawing or rolling. 
(D) After the plastic working of the above step (C), the alloy body is 
tempered by applying the reheating of the above step (B). In this 
tempering, the internal strain produced during the plastic working of 
above step (C) acts to produce suitable local strain and crystal aggregate 
structures in the course of the transformation into the .gamma..sub.1 
phase. Whereby, a tendency toward a rectangular magnetic hysteresis curve 
is enhanced, resulting in a permanent magnet with excellent magnetic 
properties. 
The reasons for selecting the above alloy composition in the present 
invention will now be explained. 
Fe: 48.about.66.9 Atm % 
Basically, the present invention improves the magnetic properties of binary 
Fe--Pt alloy of even atomic fraction by increasing the iron content 
therein. If the iron content is less than 48 Atm %, the ratio of Fe and Pt 
in the alloy composition in terms of Atm % comes close to 50:50, and 
magnetic properties of the alloy becomes inferior. On the other hand, if 
the iron content exceeds 66.9 Atm %, the alloy tends to lose its magnetic 
properties. Thus, 48.about.66.9 Atm % is chosen. 
Pt: 33.about.47 Atm % 
If the platinum content is less than 33 Atm %, the alloy loses its magnetic 
properties. On the other hand, if the platinum content exceeds 47 Atm %, 
the ratio of Fe and Pt in the alloy composition in terms of Atm % comes 
close to 50:50, and magnetic properties of the alloy become inferior. 
Thus, 33.about.47 Atm % is chosen. 
Nb: 0.1.about.10 Atm % 
Niobium improves the reproducibility of the magnetic properties. If the 
niobium content is less than 0.1 Atm %, the desired reproducibility cannot 
be achieved. On the other hand, if the niobium content exceeds 10 Atm %, 
magnetic properties of the alloy become inferior. Thus, 0.1.about.10 Atm % 
is chosen. 
It is noted that a preferable content of platinum is 34.about.43 Atm % in 
combination with a preferably niobium content of 0.3.about.5 Atm %. 
The conditions for the homogenizing solid solution treatment according to 
the present invention will be now explained. 
As to the temperature for the homogenizing solid solution treatment, the 
order-disorder transformation point of the alloy with the composition of 
the invention is 800.degree..about.900.degree. C., depending on the 
composition, and its melting point is about 1,550.degree. C. If the 
temperature for the homogenizing solid solution treatment is below 
900.degree. C., the .gamma..sub.1 phase of ordered lattice remains, and 
single .gamma. phase of disordered lattice cannot be obtained. On the 
other hand, if the temeprature for the treatment is above 1,400.degree. 
C., which is close to its melting point, the alloy melts. Thus, the range 
of 900.about.1,400.degree. C. is chosen for the homogenizing solid 
solution treatment. 
If the duration of the homogenizing solid solution treatment is shorter 
than one minute, satisfactory homogeneity cannot be achieved even when the 
temperature for the treatment is 1,400.degree. C. On the other hand, ten 
hours of homogenizing heat-treatment results in satisfactory homogeneity 
even when the temperature for the treatment is 900.degree. C., and 
treatment longer than ten hours does not produce any meaningful 
improvement. Thus, the duration of one minute to ten hours is chosen for 
the homogenizing heat-treatment. 
As to the cooling speed from the high temperature for the homogenizing 
solid solution treatment, the faster the better. When the cooling speed is 
slower than 30.degree. C./minute, the dispersed fine precipitates of 
.gamma..sub.1 phase of ordered lattice tend to grow into excessively large 
.gamma..sub.1 phase crystals so as to hamper the improvement of the 
magnetic properties. The upper limit of the cooling speed is selected at 
2,000.degree. C./second because this is about the technical limit of the 
quenching and no improvement is expected from cooling faster than this 
upper limit. Thus, the cooling speed of 30.degree. C./minute to 
2,000.degree. C./second is chosen for the cooling speed from the high 
temperature of the homogenizing solid solution treatment. 
The conditions for the reheating for tempering after the quenching will now 
be described. If the reheating temperature is below 450.degree. C., the 
reheating time which is necessary for achieving the desired tempering 
effects become too long, i.e., more than 500 hours. Such long heating is 
uneconomical and any meaningful improvement of magnetic properties cannot 
be expected from it. On the other hand, reheating at a temperature higher 
than 800.degree. C. tends to accelerate the formation of ordered lattice, 
resulting in an inferior magnetic properties. Thus, the range of 
450.degree..about.800.degree. C. is chosen for the tempering. The 
inventors have found that a more preferable range for the tempering is 
550.degree..about.750.degree. C. 
If the reheating is shorter than one minute, satisfactory tempering for 
improving the magnetic properties cannot be achieved even when the 
temperature for the reheating is 800.degree. C. On the other hand, 
reheating of longer than 500 hours tends to accelerate the formation of 
ordered lattice and hamper the improvement of the magnetic properties. 
Thus, the duration of one minute to 500 hours is chosen for the reheating 
for the tempering treatment. 
When plastic working, such as wire-drawing or rolling, is applied before 
the tempering, if the reduction ratio is less than 80%, the internal 
strain which is expected to be caused by such plastic working is too small 
to improve the magnetic properties. thus, the reduction ratio in the 
plastic working is selected to be more than 80%. 
The cooling at the end of the reheating for tempering can be either quick 
or slow, but quick cooling is preferable.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Specimens of alloys with compositions of Table 1 were prepared in the 
following manner by using electrolytic iron with a purity of 99.9%, 
platinum and niobium. Namely, the starting materials of 10 g in total with 
a desired composition were measured and loaded in an alumina Tammann tube, 
and the materials were melted in a Tammann furnace while blowing argon gas 
therein. The melt was thoroughly agitated so as to produce a homogeneous 
molten alloy, and the molten alloy was sucked into a quartz tube with a 
diameter of 2.0.about.3.8 mm so as to form a round alloy rod. Similar 
round alloy rods were prepared for different alloy compositions as shown 
in Table 1. Specimens for different alloys were made by cutting the round 
alloy rods at a length of 25 mm. 
The Specimens were homogenized by heating at 
900.degree..about.1,400.degree. C. for about one hour, and homogenized 
Specimens were quenched either by water quenching or by cooling in air. 
Some of the Specimens were tested after the quenching without being 
tempered, while other Specimens were tempered under the coditions of Table 
1 before testing. 
The Specimens thus treated were tested to check their magnetic properties. 
Specimens No. 2, No. 3and No. 14 of Table 1 were drawn into wires after 
the quenching, and then they were tempered and tested. The result of the 
test is also shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Tempering 
Spec- 
Composition Temper- Mag. Properties** 
imen 
(Atm %) ature 
Time Hc Br (BH)max 
No. Fe Pt Nb Quench* 
(.degree.C.) 
(hr) (kOe) 
(kG) 
(MGOe) 
__________________________________________________________________________ 
2 63.0 
36.5 
0.5 
a 630 100 2.8 11.0 
10 
c 630 70 3.2 11.5 
11 
3 62.5 
37.0 
0.5 
a 630 100 3.5 10.9 
14.5 
c 630 70 3.6 11.0 
15.5 
5 61.5 
38.0 
0.5 
a 620 100 4.0 10.6 
18 
b 600 50 3.5 10.6 
13 
6 61.0 
38.5 
0.5 
a 620 100 4.5 10.5 
19 
a not not 1.2 7.7 
3 
tempered 
tempered 
7 60.5 
39.0 
0.5 
a 610 100 4.8 10.7 
21 
a not not 1.5 8.5 
3.5 
tempered 
tempered 
b 600 50 4.0 10.0 
17 
8 60.0 
39.5 
0.5 
a 610 150 5.2 11.0 
22 
a not not 1.8 9.2 
4 
tempered 
tempered 
9 59.5 
40.0 
0.5 
a 610 200 5.4 10.0 
20 
a not not 2.5 10.0 
6 
tempered 
tempered 
10 58.5 
41.0 
0.5 
a 610 100 5.0 8.5 
13.5 
a not not 3.0 8.0 
4 
tempered 
tempered 
14 62 37 1 a 625 50 3.2 10.0 
13 
c 625 30 3.5 10.5 
15 
16 61 38 1 a 600 200 3.8 10.8 
17 
18 60 39 1 a 600 200 4.7 10.6 
21 
b 610 50 4.0 10.0 
16 
20 59 40 1 a 600 150 4.5 9.5 
18 
23 61 37 2 a 620 150 2.5 10.0 
9 
24 60 38 2 a 610 150 3.5 10.0 
16 
25 59 39 2 a 610 100 4.0 9.5 
17 
31 59 38 3 a 620 70 3.0 10.0 
10 
33 58 39 3 a 610 80 3.7 9.6 
12 
40 56 39 5 610 70 2.8 8.0 
6 
__________________________________________________________________________ 
*a: water quenched 
b: cooled in air 
c: wiredrawn after being water quenched 
**Hc: coercive force 
Br: residual magnetic flux density 
(BH)max: maximum energy product 
As can be seen from Table 1, those Specimens with compositions of the 
invention which were treated under the conditions of the invention proved 
to have an ultra-high coercive force, a high residual magnetic flux 
density, and a very large maximum energy product. 
FIG. 1 shows the effects of tempering on magnetic properties for three 
Specimens having different alloy compositions; namely, Specimen No. 3 
(Fe--37Pt--0.5Nb), No. 6 (Fe--38.5Pt--0.5Nb), and No. 9 (Fe--40Pt--0.5Nb). 
The three Specimens were tempered for the same duration of two hours at 
different temperatures in a range of 500.degree..about.750.degree. C. As 
can be seen from the figure, the tempering temperature for producing a 
high coercive force varied depending on the alloy composition. In the case 
of Specimens No. 6 and No. 9 whose platinum contents were 38.5 Atm % and 
40 Atm %, the quenching alone provided a fairly large coercive force, and 
the tempering proved to further improve their coercive forces to very 
large values of 3.5.about.5.2 kOe. When such very large coercive forces 
were provided, their residual magnetic flux densities were 10.9-10 kG and 
their maximum energy products were 14.5.about.20 MGOe. 
As shown in Table 1 and FIG. 2, among the Specimens tested, Specimen No. 8 
(Fe--39.5Pt--0.5Nb) showed the largest maximum energy product, which was 
22 MGOe. The inventors found that the Specimen No. 8 showed an extremely 
large maximum energy product of 26 MGOe when it was cooled to a very low 
temperature (-196.degree. C.) by using liquid nitrogen. 
It is noted that plastic working is possible in the case of the alloy of 
the invention. The tests showed that permanent magnets formed by plastic 
working had better magnetic properties than those without the plastic 
working. 
FIG. 2 shows the relation between the magnetic properties and the 
conditions for constant-temperature tempering, i.e., the heating 
temperature and duration, for Specimen No. 8 (Fe--39.5Pt--0.5Nb) which is 
a typical alloy of the invention. In the case of this Specimen, when the 
temperature for the tempering is low, a long duration of heating treatment 
is necessary to achieve good magnetic properties. 
FIG. 3 shows the relation between the compositions of the Fe--Pt--Nb 
ternary alloys and their coercive forces. FIG. 4 shows the relation 
between the compositions of the Fe--Pt--Nb ternary allows and their 
residual magnetic flux densities. FIG. 5 shows the relation between the 
compositions of the Fe--Pt--Nb ternary alloys and their maximum energy 
products. 
FIG. 6 illustrates the demagnetizing curve for Specimen No. 8 
(Fe--39.5Pt--0.5Nb) whose residual magnetic flux density and coercive 
force were high and whose maximum energy product proved to the largest 
among the tested Specimens. The alloy of Specimen No. 8 was easy to work, 
and it was found to be suitable for both small magnets with complicated 
shape and magnets to be used at a temperature considerably lower than room 
temperature. 
The shaded area of FIG. 7 summarizes the composition of the alloy for the 
permanent magnet according to the invention. 
As described in detail in the foregoing, the permanent magnet of the 
invention can be produced by very simple heat-treatment, and it has a high 
workability due to its composition consisting of iron, platinum and a 
small amount of niobium. Furthermore, the permanent magnet of the 
invention provides an ultra-high coercive force and the very large maximum 
energy produce which are of great value in various industries. 
Although the invention has been described with a certain degree of 
particularity, it is understood that the present disclosure has been made 
only by way of example and that numerous changes in details may be 
resorted to without departing from the scope of the invention as 
hereinafter claimed.