BACKGROUND OF THE INVENTION
[0001] The present invention relates to a rare earth-based alloy for permanent magnet having
excellent magnetic properties and suitable as a component of various kinds of electric
and electronic instruments.
[0002] Various kinds of rare earth-based permanent magnet alloys have been developed hitherto
and are under production in large quantities including the samarium-cobalt magnet
alloys of the chemical composition SmCo₅. The magnetic properties of the permanent
magnets of this type are so excellent that the maximum energy product (BH)
max thereof exceeds 20 MGOe in the magnets manufactured under experimental conditions
or is constantly in the range from 16 to 18 MGOe in the magnets manufactured as industrial
products. Accordingly, these permanent magnets are widely used in a variety of applications
such as speakers, electric motors, metering instruments and the like in which the
permanent magnets are required to exhibit high performance. One of the problems in
the samarium-based permanent magnets is the high production costs as a consequence
of the high content of relatively expensive cobalt metal therein up to 60% by weight
or more. It would therefore be a desirable measure to replace the cobalt metal with
a less expensive metal such as iron and some attempts have been made hitherto in this
direction. No fruitful results, however, have yet been obtained in such an attempt
presumably due to the absence of possible solid-solution formation of iron with the
SmCo₅ intermetallic compound.
[0003] On the other hand, several binary intermetallic compounds of rare earths and iron
are known including the compounds of the formulas RFe₂, RFe₃ and R₂Fe₁₇, R being a
rare earth element. These rare earth-iron intermetallic compounds, however, are not
utilized as a permanent magnet due to the low value of either one of the magnetic
parameters of Curie point T
c, saturation magnetization 4πM
s and crystalline magnetic anisotropy constant K
u, In contrast to the series of rare earth-cobalt intermetallic compounds including
the RCo₅-type compounds having the crystalline structure of CaCu₅, of which the above
mentioned SmCo₅ alloy is practically utilized as a material of permanent magnets,
no RFe₅ type compounds were known for long in the series of rare earth-iron compounds,
at least, in the form of a bulk body. It was only in 1984 that Cadieu, et al. reported
in Journal of Applied Physics, volume 55, page 2611 (1984) that thin films of SmFe₅
and (SmTi)
xFE₁₀₀
-x' in which the atomic ratio of Ti:Fe was 1:9 or 1:19, could be formed by the method
of sputtering. These intermetallic compounds in the form of a thin film reportedly
have a hexagonal crystalline structure of CaCu₅. These thin films formed by the sputtering
method, however, were in the state of a metastable phase and it was generally understood
that such an intermetallic compound could not exist as a bulk body. Therefore, the
only permanent magnet based on a rare earth-iron binary compound so far reported
is the magnet in a metastable phase prepared by the quenched thin-film method disclosed
by Croat, et al. in IEEE Transactions on Magnetics, volume MAG 18, page 1442 (November,
1982). The quenched thin-film magnet prepared by this method is isotropic and based
on a metastable phase so that the magnet is not free from the problem of low stability
so that the magnets of this type are not in practical use.
[0004] Turning now to the recently highlighted neodymium-iron-boron magnets formed of a
ternary compound of a chemical composition of the formula R₂F₁₄B, they are promising
as a high-performance permanent magnet since the base components are inexpensive
neodymium and iron and the magnetic properties thereof are even better than those
of the samarium-cobalt magnets. These neodymium-iron-boron magnets are, however, not
free from a very serious problem that they are highly susceptible to rusting so that
the magnets cannot be used practically without providing a protective coating. This
disadvantage can hardly be overcome and no practical solution of the problem has yet
been obtained to give a possibility of industrial production of the magnets of this
type.
SUMMARY OF THE INVENTION
[0005] An object of the present invention is therefore to provide a rare earth-based permanent
magnet having magnetic properties equivalent to or even better than those of the samarium-cobalt
permanent magnets without using or by decreasing the amount of expensive cobalt as
well as to provide a rare earth-based alloy as a base material of such a permanent
magnet.
[0006] Thus, the permanent magnet alloy of the present invention consists essentially of:
(a) form 12 to 45% by weight of a rare earth element or a combination of rare earth
elements;
(b) from 0.1 to 10% by weight of titanium; and
(c) the balance of iron or a combination of iron and cobalt, the amount of iron being
at least 40% by weight thereof, including unavoidable impurities.
[0007] Further, the permanent magnet of the invention is a sintered body of a powder of
the above defined rare earth-based alloy having magnetic anisotropy.
BRIEF DESCRIPTION OF THE DRAWING
[0008]
FIGURE 1 is a graph showing the magnetization of inventive and conventional samarium-based
alloys as a function of temperature.
FIGURE 2 illustrates the X-ray diffraction diagrams of the same alloys using CuKα
line.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0009] In respect of the problem of possible existence of RFe₅ phase, the results of experiments
hitherto undertaken support that no such a 1:5 phase or, in particular, SmFe₅ phase
is formed even when samarium and iron are melted together and alloyed in a proportion
corresponding to the composition of SmFe₅ although three phases of Sm₂Fe₁₇, SmFe₂
and SmFe₃ could be found as is indicated by the results obtained in the measurement
with a vibration magnetometer and in the X-ray diffractometry as is shown in FIGURES
1 and 2.
[0010] With an object to obtain a rare earth-iron based permanent magnet having excellent
magnetic properties equivalent to those of the imaginary SmFe₅ magnet, the inventors
have conducted extensive investigations on a third additive element to be added to
the rare earth-iron binary magnet alloys and arrived at discoveries that addition
of titanium would give a quite satisfactory result and a hitherto unknown ternary
intermetallic compound of samarium, titanium and iron can exist in a bulky form by
the optimization of the amount of substitution of titanium for the rare earth element
or, in particular, samarium. Thus, a ternary alloy of samarium, titanium and iron
was prepared in such a proportion as to correspond to the formula of SmTiFe₁₀ and
the alloy was subjected to the measurement of the magnetization as a function of temperature
and X-ray diffractometry to give the results shown in FIGURES 1 and 2, respectively.
These figures indicate that, in clear difference from those of the SmCo₅-type crystalline
structure, crystallographic indices approximately corresponding to those of the tetragonal
crystalline structure can be allotted to the peaks in the X-ray diffractometric diagram
of the ternary ally and the temperature dependency of the magnetization thereof is
also close to that of a single-phase alloy leading to a conclusion that the ternary
compound of samarium, titanium and iron is imparted with stability as a result of
introduction of titanium into the samarium-iron binary alloy. The further continued
investigations have led to confirmation that the above described unique phenomenon
is held also for the rare earth elements in general other than samarium including
yttrium.
[0011] Thus, the present invention provides, as an embodiment, a ternary alloy composed
of (a) from 12 to 45% by weight of a rare earth element or a combination of rare earth
elements; (b) from 0.1 to 10% by wieght of titanium; and (c) the balance of iron including
unavoidable impurities. Namely, the magnet alloy can be obtained by melting the component
metals together and the alloy is finely pulverized followed by the powder metallurgical
processing of the powder by compression molding and sintering. When the amount of
the rare earth component in the alloy formulation is outside the above specified range,
the ternary compound would be less stable and, therefore, any amounts thereof smaller
than 12% by weight and larger than 45% may result in a disadvantageously rapid decrease
in the coercive force
iH
c and saturation magnetization 4πM
s, respective-ly. The above mentioned range for titanium is also critical because the
ternary compound is less stable when the amount of titanium is smaller than 0.1% by
weight while the fraction of the phase of the ternary compound is decreased when
the amount of titanium is larger than 10% by weight. The rare earth element here implied
include the so-called lanthanoid elements having atomic munbers of 57 to 71 and yttrium.
Any of these rare earth elements can be used either singly or as a combination of
two kinds or more according to need.
[0012] The rare earth-based permanent magnet of the invention prepared of the ternary alloy
contains the stable phase of the ternary compound as a result of the introduction
of titanium so that the Curie point thereof is about 310°C when the rare earth element
is samarium which is much higher than 120°C of the Sm₂Fe₁₇ phase. In addition, the
saturation magnetization is also greatly increased so that the thus obtained permanent
magnet has very high magnetic properties. Moreover, the rare earth-titanium-iron
permanent magnet of the invention can be imparted with magnetic anistropy by the powder
metallurgical method so that the overall magnetic performance of the inventive permanent
magnet can be almost equivalent to or even better than the samarium-cobalt based
magnets.
[0013] Despite the high content of iron in the alloy of the inventive permanent magnet,
which is formed mainly of a tetragonally crystalline phase, the inventive permanent
mgnet is highly corrosion-resistant and free from rusting in clear contrast to the
neodymium-iron based magnets. Accordingly, the inventive permanent magnets can be
used in practical applications without any prtective coating on the surface although
the corrosion resistance thereof can of course be further increased by a protective
coating or surface treatment by forming a resinous layer or a metallic layer formed
by electrolytic or electroless plating, vacuum vapor deposition, sputtering or ion
plating.
[0014] Further, the ternary alloy can be processed into a thin film having a high coercive
force by the quenched thin-film method and the thin film can be finely pulverized
into fine particles of which magnetically isotropic permanent magnets can be prepared.
It is of course that the magnetically anisotropic sintered magnet is pulverized into
fine particles of which anisotropic plastic magnets can be prepared.
[0015] As is mentioned above, the permanent magnet of the ternary alloy of samarium, titanium
and iron has a Curie point of about 310°C. Although this Curie point is well within
the practically acceptable range, it is of course desirable to have a higher Curie
point when comparison is made with the SmCo₅ permanent magnets having a Curie point
at about 740°C. In this regard, the inventors have further continued extensive investigations
and arrived at a discovery that a magnetic alloy suitable for the purpose can be
obtained when a solid solution is formed of the above described ternary compound of
rare earth, titanium and iron with cobalt. For example, an increase by about 40 to
100°C can be obtained in the Curie point of the ternary alloy when 10 atomic % of
iron in the alloy is replaced with cobalt although the increment depends on the kind
of the rare earth element. The Curie point T
c is increased approximately linearly with the increase in the amount of replacement
of iron with cobalt up to 50% replacement by weight but thereafter the increment in
the Curie point is relatively small with further increased replacement of iron with
cobalt to finally level off. In addition, the saturation magnetization of the magnet
is increased as a trend though dependent on the kind of the rare earth element by
the substitution of cobalt for a part of iron in the ternary magnet alloy of rare
earth, titanium and iron to level off with increase of the proportion of cobalt relative
to iron.
[0016] It should be noted, however, that replacement of iron with cobalt has some adverse
effect of decreasing the coercive force of the magnet. For example, more than 40%
replacement of iron with cobalt is undesirable due to the great decrease in the coercive
force of the magnet. This is the reason for the limitation that more than 60% by weight
or, preferably, more than 40% by weight of iron should not be replaced with cobalt.
[0017] In the following, the rare earth-based alloy for permanent magnets and the sintered
permanent magnet of the alloy accord ing to the invention are described in more detail
by way of examples.
Example 1.
[0018] Metals of samarium, titanium and iron each having a purity of 99.9% were taken by
weighing in the proportion indicated in Table 1 below and melted together in a high-frequency
induction furnace. The melt was cast into a water-cooled, copper-made casting mold
to form an ingot of the alloy. The ingot was crushed and then pulverized in a jet
mill using nitrogen gas as the ject gas to give a fine powder having an average particle
diameter in the range from 2 to 10 µm. The powder was compression-molded under a pressure
of 1.5 tons/cm² with the particles oriented in a static magnetic field of 15 KOe into
a green body, which was sintered by heating in an atmosphere of argon gas for 1 hour
at a temperature in the range form 1000 to 1200 °C and then subjected to thermal aging
for 4 hours at a temperature in the range from 500 to 900 °C followed by quenching.
[0019] The thus obtained magnetically anisotropic sintered body after the thermal aging
was subjected to the measurement of the density of residual magnetic flux B
r, coercive force
iH
c and maximum energy product (BH)
max to give the results shown in Table 1 for the three different formulations of the
alloys No. 1, No. 2 and No. 3. For comparison, Table 1 also includes the results of
the magnetic measurement of a sintered body of a samarium-iron al loy corresponding
to SmFe₅ (No. 4) prepared in the same manner as above. As is shown in the table, this
comparative sintered body had only negligibly small values of coercive force and maximum
energy product.

Example 2.
[0020] Metals of praseodymium, samarium, cerium, titanium and iron were taken by weighing
in the proportion indicated in Table 2 and magnetically anisotropic sintered bodies
were prepared each in the same manner as in Example 1. The coercive force
iH
c of these sintered bodies was measured to give the results shown in the table which
supports the conclusion that the coercive force is little affected by the replacement
of samarium with other rare earth elements.

Example 3.
[0021] Magnetically anisotropic sintered permanent magnets No. 1 to No. 4 were prepared
each in the same manner as in Example 1 except that the magnetic alloy was prepared
from metals of neodymium, titanium, iron and cobalt each having a purity of 99.9%
taken by weighing in the proportion indicated in Table 3.
[0022] These sintered permanent magnets were subjected to the measurement of the density
of residual magnetic flux B
r, coercive force
iH
c and maximum energy product (BH)
max as well as Curie point to give the results shown in the table, in which
ΔT
c means the increment of the Curie point T
c in °C obtained by the replacement of a part of iron with cobalt.

Example 4.
[0023] Magnetically anisotropic sintered permanent magnets No. 1 and No. 2 were prepared
in the same manner as in the preceding examples from metals of samarium, cerium, titanium,
iron and cobalt taken by weighing in the proportion indicated in Table 4 below. These
sintered permanent magnets were subjected to the measurement of the magnetic properties
to give the results shown in the table.
