Note: Descriptions are shown in the official language in which they were submitted.
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SPECIFICATION
TITLE OF THE I~VENTION
RARE EARTH MAGNET ~AVING EXCELLENT
CORROS ION RES IS TANCE
BACKG ROUND OF THE INVENTIO~
This invention relates to an Fe-B-R type rare earth
permanent magnet having high magnetic properties. (In the
present invention, R represents the rare earth elements inclusive
of Y) More particularly, it is concerned with a permanent magnet
based on rare earth element ~R) ~ boron (B) and iron (Fe), with
its corrosion resistant property being improved significantly by
the particular compositional ratios of the constituent elements.
There was previously proposed by three of the present
inventors, as an improved permanent magnet of high performance
which exceeded the highest magnetic properties of the
conventional rare earth-cobalt magnet, an Fe-B-R type permanent
magnet which was composed of as the principal components iron
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(Fe), boron (a) and light rare earth elements such as neodymium
(Nd) and praseodymium (Pr) abundantly available in the natural
resources, but not using samarium (Sm) and cobalt (Co) which are
scarcely available in the natural resources or uncertain in the
commercial availability, hence expensive (Japanese Patent Rokai
Publications No. 59-46008 and No. 59-89401 or EPA 101S52).
Said inventors also succeeded in obtaining another Fe-B-R
type permanent magnet having a higher range of the Curie
temperature than that of the abovementioned magnetic alloy which
ranges, in general, from 300 C to 370 C, by substituting cobalt
(Co) for a part of iron (Fe) (Japanese Patent Rokai Publications
No. 59-64733 and No. 59-132104 or EPA 106948).
~ ith a view to improving the temperature characteristics
~in particular the coercivity "iHcn)t while retaining the Curie
temperature equal to, or higher than that, and a higher (EO max
than that, of the above-mentioned Co-containing Fe-B-R type
(i.e., more precisely (Fe,Co)-6-R type) rare earth permanent
magnet, the said inventors further proposed still another
Co-containing Fe-B-R type rare earth permanent magnet with much
more improved iHc, while still retaining a very high (~ max of
25 MGOe or above, which could be realized by including at least
one kind of heavy rare earth elements such as dysprosium (Dy),
terbium (Tb)t etc. as a part of R of the Co-containing Fe-~-R
type rare earth permanent magnet, R mainly containing light rare
earth elements such as Nd and/or Pr (Japanese Patent Kokai
Publication No. 60-34005 or EPA).
~ owever, the permanent magnets having the abovementioned
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excellent magnetic properties and being composed of the Fe-~-R
type magnetically anisotropic sintered body contain, as its
principal constituents, those rare earth elements and iron which
are apt to be oxidized in the air and tend to gradually form
stable oxides. On account of this, when such permanent magnet is
assembled in the magnetic circuit, various problems and
inconveniences wo~ d be brought about by the oxides formed on the
surface of the magnet: such as decrease in output of the magnetic
circuit; irregular functioning among the magnetic circuits; and,
in other aspect, contamination of various peripheral devices
around the magnetic circuits due to scaling off of the resultant
oxides from the -surface of the magnet.
In order therefore to improve the corrosion resistant
property of the abovementioned Fe-~-R type permanent magnet,
there was already proposed a permanent magnet with an
anti-corrosive metal layer having been plated on its surface by
the electroless plating method or the electrolytic plating method,
and another permanent magnet with an anti-corrosive resin layer
having been coated on its surface by the spraying method or the
dipping method.
With this plating method, however, there still remained
problem such that, since the permanent magnet is a sintered,
somewhat porous body, an acidic or alkaline solution used for its
pre-treatment before the plating procedure stays in the pores of
the sintered magnet body, which is apprehensively liable to corrode
the magnet with lapse of time; and further, since the
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1 336866
magnet body is inferior in its chemical-resistant property, the
surface of the magnet is corroded during the plating procedure to
deteriorate its adhesion property and corrosion-resistant
property.
Further, as to the latter spraying method, since the resin
coating by this method has directionality, a great deal of
working steps and time are required for applying the uniform
resin coating over the entire surface of the workpiece to be
treated; in particular, coating of a magnetic body having a
complicated configuration with the coating film of a uniform
thickness is all the more difficult. Furthermore, with the
dipping method, thickness of the resin coating becomes
non-uniform with the consequence that the finished product has a
poor dimensional precision.
Furthermore, as the Fe-B-R type permanent magnet which
could successfully solve the disadvantages inherent in the
abovementioned plating method, spraying method and dipping
method, and provide stabilized-corrosion resistant property over
a long period of time, there were also proposed improved
permanent magnets provided on its surface with a vapor-deposited
corrosion-resistant layer composed of various metals or alloys
By this vapor-deposition method, oxidation of the surface of the
magnet body is suppressed, so that the magnetic property is pre-
vented from deterioration. Also, since there is no necessity for
se of corrosive chemicals, etc., hence no apprehension whatsoever of
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.
its remaining in the magnet body as is the case with the plating
method, the permanent magnet as treated by this method is capable
of retaining its stability over a long period of time.
While the vapor-deposition method is highly effective for
improvement in the corrosion resistance of the permanent magnet,
it has its own disadvantage such that a special treating
apparatus is re~uired, and its~productivity is low, so that the
treatment by this method is consi~erably expensive.
USP 4,588,439 discloses an Fe-E-R type permanent magnet
alloy containing 6,000 to 35,000 ppm, (preferably g,000 to 30,000
ppm) oxygen in order to avoid disintegration of the sintered body
based on an autoclave test. ~owever, this alloy consumes much
rare earth elements as oxides. For complete suppression 9,000
ppm oxygen is necessary. Namely rare earth elements of 6 times
by weight of the oxygen amount is consumed to form oxides. Such
large amount of oxide is not preferred since the presence of
nonmagnetic oxides adversely affects the magnetic properties, and
valuable rare earth elements are consumed. For instance, 10,000
ppm oxygen will consume 6% by weight of rare earth elements as
oxides.
SUMMARY OF THE DISCLOSURE
Thus there is much to be desired in the art. Stillmore,
the producing procedure and raw materials and intermediate products
must be carefully handled to avoid oxidation, which further leads
to an increase in the production costs.
It is therefore an object of the present invention to
provide an Fe-B-R type permanent magnet material having improved
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corrosion resistant property.
It is another object of the present invention to provide
an Fe-6-R type permanent magnet C2 pable of exhibiting its
eY.cellent corrosion resistant property, not by its surface
treatment for improving the corrosion resistant property thereof,
but by specifying its composition.
It is still another object of the present invention to
provide an Fe-~-R type permanent magnet having excellent
durability, whiIe maintaining its high magnetic property.
It is a further object of the present invention to provide
an Fe-B-R type permanent magnet having higher -temperature
characteristic.
Still further objects will become apparent in the entire
disclosure.
The present invention is based on the finding, as the
result of conducting various studies and researches on the
compositional aspects of the Fe-~-R type permanent magnet, that,
by specifying Nd and Dy as the rare earth element (R)r and by
defining specific amounts of B, Co, Al and Fe and specific
limitation of the amount of C in the magnet (or material~
composition, improvement in the corrosion resistance of the
permanent magnet (or material) could be attained without
deteriorating its magnetic properties, which improvement was so
significant that could not be realized with the conventional
permanent magnets. Further improvement may be achieved by
including Ti and/or Nb in specific amounts.
That is to say, according to the present invention, in
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general aspect thereof, there is provided an (Fe,Co)-B-R tetragonal
type rare earth magnet (or material) having excellent corrosion
resistant property, which consists essentially of: 0.2 - 3 0 at~ Dy
12 - 14 5 at~ Nd and 12 5 - 15 at~ of the sum of Nd and Dy; 6 - 8
at~ B; 0 5 - 8 at~ Co; 0 5 - 3 at~ Al; and the balance being Fe,
the principal phase being of the tetragonal structure Fe should
be at least 68 at~, while the sum of Fe and Co is, preferably, at
least 75 at~
The foregoinq objects, other objects and the specific
composition o~ the (Fe,Co)-e-R type rare earth permanent magnet
(or material) according to the present invention will become more
apparent and understandable from the following detailed
description thereof, with reference to the preferred embodiments
of its production and magnetic properties, when read in
conjunction with the accompanying drawing.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
In the drawing:
Fig. 1 is a graphical representation of a result of the
Pressure Cooker Test, showing the length of time lapsed until the
surface coating blistered or the material surface produced oxide
powders;
Fig. 2 is a graphical representation of a result of the
corrosion-resistance test, shcwing a relationship between the
standing time and variations in weight of the samples Fer unit
surface area;
Figs. 3 and 4 are graphs showing the effect of Co addition
where Al is 2 and 0 at%, respectively, in weight change per unit
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surface area versus standing time at 80C x 90% R. H.;
Figs. 5 and 6 are graphs showing the effect of Al addition
where Co is 4 and 0 at%, respectively, in weight change per unit
surface area versus standing time at 80C x 90% R. ~.; and
Fig. 7 is graphs showing the effect of Co and Al at
different amounts of C in magnetic flux loss versus standing time
in a testing atmosphere of 80C x 90% R.~.
DETAiLED DESCRIPRTION OF PREFERRED E~IBODIMENTS
In the following, the present invention will be descri~ed
in specific detail.
The rare earth permanent magnet material according to the
present invention possesses (B~)max of 25 ~Oe or above and iHc
of 10 kOe or above (when made to an anisotropic sintered magnet),
and, as the result of the Pressure Cooker Test (P.C.T.) in an
atmosphere of a temperature of 125C and a relative humidity of
85% as well as a prolonged holding test in an atmosphere of a
temperature of 80C and a relative humidity of 9o%~ it exhibits
particularly superior corrosion resistant property in comparison
with the conventional Fe-B-R type rare earth permanent magnet
material which has been subjected to undercoating treatment with
aluminum and then to further chromate treatment.
Also, by inclusion of 0.1 - 1.0 at% of one of Ti and/or Nb
in addition to the abovementioned composition, the rare earth
permanent magnet according to the present invention is capable of
improving its magnetic properties (in paritcular, its
rectangularity in the demagnetization curve) and its (BH)max
without deteriorating the excellent corrosion resistant property.
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The grain boundary phase in this Fe-B-R type rare earth
permanent magnet, in the case where Co and Al are not contained
in the alloy, lS composed of: an R-rich phase which does not
substantially contain B, but a few atomic percents of Fe, and is
composed mostly of the rare earth element; and an Rl+EFe4B4
phase with a high content of B (about 40 at% or more). On
account of this, the deterioration in the corrosion-resistance of
the Fe-6-R type rare earth permanent magnet is considered
primarily ascribable to the presence of the abovementioned R-rich
phase which contains the chemically active rare earth elements as
the principal constituent.
In the case of the Fe-B-R type permanent magnet according
to the present invention, it is presumed that the Co and Al
existing in the grain boundary phase enter into the
abovementioned R-rich phase to form a multi-phase which, based on
the specific control in quantity of Co and Al, and without
impairing the magnetic properties, contributes to significant
improvement in the corrosion resistance of the grain boundary
phase.
The magnetic properties of the Fe-B-R type magnet (or
magnet material) are primarily attributable to the Fe-B-R
tetragonal type intermetallic compound expressed in terms of the
chemical formula R2Fel4B. Generally, in order to provide a
magnetically anisotropic, sintered permanent magnet of the
practically high magnetic properties, the magnet composition
should be carefully selected within a region where the
composition is R-richer and B-richer than the stoichiometric
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composition of R2Fel4B. ~Particularly in a resion where R is
not sufficient, ~ -iron precipitates in the alloy and/or sintered
magnet which causes a ready invertion of magnetization resulting
in a low coercivity.)
In the region of the R-rich and B-rich side, an R-rich
phase composed almost of metallic R and a B-rich phase expressed
by R1~Fe4B4 occur, which serve to improve the sintering
characteristics and coercivity, particularly, the R-rich phase
smoothes the grain boundary of the tetragonal crystal grains
through the sintering (and further aginsj.
It has been revealed that the corrosion resistance is
primarily related with this R-rich boundary phase. The "R" in
the R-rich phase is very apt to be oxidized by oxygen and/or
moisture in the ambient stmosphere. Further, if carbon (C~
and/or chlorine (Cl) are included as impurities, they are present
as carbide or chloride of R, which will readily react with
moisture in the atmosphere to decompose. (Thus, generally
speaking, C and Cl should be maintained at a low leve~
R becomes oxide of R ~e.g., R2O3) which is nonmasnetic
and causes the magnetic properties to decrease as the amount of
the oxide increases (particularly, Br and (~O max will gradually
decrease). ~owever, if there is still a certain amount of R
(i.e., more than that to be present as R-oxide) re~uisite for
sintering to make a magnet. That is, if the amount of R is
large, oxygen may be allowed in a correspondingly large amount.
~owever, if the amounts of R and oxygen increases, it results in
occurrence of a large amount of the nonmagnetic phase, which
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leads to lowering in Br and (BH)max. So far as the amount of R
is limited (as is usual in the practice), the amount of R will
short when a large amount of oxysen is present, which finally
results in a compolete loss of coercivity.
According to the present invention, such problems
ascribable to the oxidation of the R-rich phase (or generally the
boundary phase) can be eliminated by incorporation of a certain
amount of Co and Al in the composition. Particularly, the ratio
of the sum of Co and Al to the amount of rare earth elements (R')
contained (or to be contained) in the boundary phase: SCo +
Al)/R' is important. By controlling this ratio, the rare earth
elements contained in the boundary phase can be stabilized. A
considerable amount of Co and Al forms stable intermetallic
compounds with R (e.g., NdCo3, Nd3Co7, etc.; there occur
certain compounds containing Al as solid-solution) which
contribute to the corrosion resistance.
Note that a certain amount thereof forms the
R2(Fe,Co)l4B tetragonal type phase. (It is presumed that
some part of Al also assumes the site of Fe in this tetragonal
type crystal structure to form R2(Fe,Co,Al)l4B.) These
compounds have improved corrosion resistance over the base
R2Fel4B phase.
Preferably, (Co+Al)/R' ranges about 0.5 to about 10 (more
preferably 0.7 to 5). Below 0.5 the improvement in the corrosion
resistance would be not sufficient, while above 10 the sintering
characteristics will deteriorate leading to a lowering in iHc.
As a guideline for control, the amount of R' can be
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roughly calcurated by the following equation:
R' i total R ~ (I7 A + 5 RO) (by at~)
where A is the total amount of the elements contained in the
tetragonal type phase and RO is the amount (by at%) of the
R-oxide (R2O3) in the magnet or material.
Measurement, e.g., by X-ray micro-analyser (XMA) etc. can
provide àefinite figure of R'~ Co and Al.
By the incorporation of Co and Al, the corrosion
resistance not only of the final sintered product but of the
alloy material (particularly powder) therefor can be
significantly increased. For instance the alloy powder obtained
by the direct reduction process from rare earth oxide through a
reduction agent, e.g., Ca can reduce the amount of oxygen through
the incorporation of Co and Al. Thus the present invention
provides significant improvement in the practical, industiral
production and utilization of the generally Fe-B-R type permanent
magnets.
In the present invention, the reason for limiting the
range of content for each of the constituent elements in the rare
earth permanent magnet is as follows.
With the content of Dy not reaching 0.2 at %, no increase
is seen in both iHc and (BH)max. On the contrary, with its
content exceeding 3.0 at%, improvement is seen in iHc. However,
since Dy is available only in small quantity in the natural
resources, it is very expensive and hence unfavorably pushes up
the production cost of the permanent magnet. On account of this,
its content is limited to a range of from 0.2 at% to 3.0 at%, or
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preferably from 0.2 at% to 2.0 at%. Dy also serves to improve
the temperature characteristics of the magnet paritcularly in
reversible loss of magnetic flux at a high temperature and
irreversible loss of magnetic flux after being subjected thereat.
When the total quantity of Nd and Dy (i.e., the total
quantity of the rare earth elements) is below 12 at%, ~-Fe would
precipitate in the metallic compound of the principal phase to
abruptly decrease iHc. On the other hand, above 17 at%, the
corrosion resistance of the basic Fe-B-R ternary composition is
deteriorated due to the occurrence of greater amounts of R-rich
phase if a large amount of Co and Al is not present ~such large
offers problem in the magnetic properties). For these reasons,
the total quantity of Nd and Dy is limited to a range of from 12
at% to 17 at%~ or preferably from 12.5 at% to 15 at% tfor
achieving 30 MGOe or more and good corrosion resistance). The
amount of Nd is preferably 11 - 16 at% (more preferably 12 - 14.5
at%). At least 11 at% Nd is preferred to provide sufficient
Nd-rich boundary phase, and generally to save Dy (the latter is
applied to also 16 at% Nd). However, Nd may be partly replaced
by Pr so far as the magnetic and anticorrosion properties are not
affected. Similarly, as a commertially available Nd materlal,
Didymium containing Nd, Pr and Ce may be partly employed.
With the content of B not reaching 5 at%, i~c unfavorably
drops down to 10 kOe or lower. On the other hand, with its
content exceeàing 10 at%, iHc increases, but Br drops down to
become unable to obtain (~)max of 25 ~Oe or higher. Besides,
above 10 at% ~, the nonmagnetic B-rich phase increases to a
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1 33686~
considerable amount. For these reasons, the content of B is
limited to a range of from 5 at% to 10 at% (preferably 6 - 8
at%).
Co is effective for increasing the Curie temperature,
improving the weather-resistance of the product and the oxidation
resistance of the raw material (alloy, particularly its powder),
as well as increasing Is. With the Co content below 0.~ at%, the
effect of increasing the Curie temperature and improving the
corrosion resistance of the product (or material) is small. On
the contrary, with its content exceeding 13 at~, Co is locally
con~entrated to be agglomerated in the grain boundary at a high
density with the consequence that a ferromagnetic R(Nd,Dy)-Co
compound containing therein 30 at% or more of Co is precipitated
to readily bring about reversal of magnetization in the Fe-B-R
type rare earth permanent magnet of the present invention,
resulting in a lowered iHc. For these reasons, the content of Co
is limited to a range of from 0.5 at% to 13 at%, or preferably
from 1 at% to 10 at% in view of these aspects. Besides, at 5 at&
Co or more, the temperature coefficient of 8r is 0.1 %/C or
less.
Al is effective for increasing iHc and, in particular,
improving the corrosion resistance of the product in cooperation
with Co by synergic effect therewith. It has an effect of
improving iHc which tends to decrease with increase in the adding
quantity of Co. With the Al content below 0.5 at%, the effect of
increasing i~c and improving the corrosion resistance of the
product (or material) is not satisfactory. On the contrary, with
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its content exceeding 5 at%, the effect is seen in the improved
- iHc, but Br lowers and tE~)max lowers below 25 MGOe. In
balancing these, the content of Al is limited to a range of from
0.5 at% to 5 at%~ or preferably from 0.5 at% to 3 at%.
Ti or Nb has an ef~ect of compensating decrease in Br
and (B~)max due to addition of Al. With the content of Ti or Nb
not reaching 0.1 at%, no sufficient effect of increasing Br is
recognized. On the other hand, with the content thereof
exceeding 1.0 at%, Ti or Nb is combined with B in the magnetic
alloy to form borides of Ti or Nb, which invites decrease (thus
short) in B necessary for the magnetic alloy, entailing, at the
same time. decrease in i~c. For these reasons, the content o~ Ti
and/or Nb is limited to a range of from 0.1 at% to 1.0 at%, or
preferably from 0.2 at% to 0.7 at%. V, Mo, W, Ta, Hf and zr may
be present each in an amount 0.1 - 1.0 at%, which serve like Ti
or Nb.
C gives also great influence on the corrosion-resistance
of the permanent magnet. C may be contained as carbide of R
which will readily react with moisture in the atmosphere to be
caused to decompose. When its content exceeds 2,000 ppm, the
corrosion resistance abruptly decreases, which entails difficulty
in obtaining a practical permanent magnet. Therefore, its
content should be 2,000 ppm or below, or preferably 1,000 ppm or
below, or more preferably 700 ppm or below. C tends to come from
the starting materials such as iron, ferro-boron or rare earth
elements as an impurity, or sometimes through the production
process (e.g., from organic compacting aids or when solvents are
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used for pulverization etc.).
In the rare earth permanent magnet or alloy material
according to the present invention, the remainder of the
composition other than the abovementloned elements is Fe and
unavoidable impurities.
Fe should be present at least 65 at~ since below this
amount, it is difficult to achieve 25 MGOe or more. Fe is
preferably at most 81 at% since above this, ~-iron tends to
precipitate. Thus Fe of 68 - 81 at% is more preferred. It
should be noted that Co may replace some part of the Fe site in
the basic Fe-B-R tetragonal type crystal structure to form the
tFe,Co)-6-R tetragonal type crystal structure.
Oxygen is generally not preferred since valua~le R is
consumed as oxide which is nonmagnetic. Oxygen is believed to be
present almost as R-oxide (e.g., R203) in the magnet after
sintering at l,000C or higher since R is chemically active.
However, oxygen is inevitably contained as the impurity because
rare earth elements are generally very apt to be oxidized by
oxygen or H20, and it is not easy to maintain the raw
materials, production process, and intermediate and final
products free from oxygen or moisture (i.e., air). Therefore the
oxygen content should be maintained as low as possible in the
sense of the practically or industrially achievable level in
light of the magnetic properties and saving (or efficiency) of ~.
Thus oxygen should be kept at 10~000 ppm or below, or preferably
8,000 ppm or below (more preferably 6,000 ppm or below).
Further impurities may possibly be P, S, ~n, Ni, Si, Cu,
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Cr and so on, which might be unavoidably mixed into the alloy
components in the course of the industrial production. Such
impurities are-allowed to be present in the magnet or material of
the present invention so far as the re~uisite properties are
satisfied.
Chlorine (Cl) may be contained as an impurity, too, e.g.,
when the pulverization of alloy is effected by wet pulverization
using a solvent of organic chlorine compound (trichlorethylene
etc.). Then chlorine is contained as chloride of R which will be
readily decomposed by moisture in the air. Thus chlorine should
be, if contained, 1~500 ppm or less, preferably, 1,000 ppm or
.. . ..
less.
Nitrogen might be incorporated through the production
process, e.g., jet milling using ~2 as a pulverization medium
amounting to about 1,000 ppm while wet-milling by a ball mill
using a solvent provides very low amount of nitrogen, e.g., below
100 ppm. If nitrogen is present in the magnet, it may form
Nd-nitride which is, very apt to react with ~2 Therefore it
is preferred to control it to 2,000 ppm or below, more preferably
1,000 ppm or below.
According to a preferred aspect of the present invention,
there is provided a magnet consisting essentially of: 12 to 14.5
at% of Nd; 0.2 to 2.0 at% of Dy (the total quantity of Nd and Dy
being in a range of from 12.5 to 15 at%~; 6 to 8 at% of B; 1 to
10 at% of Co; 0.5 to 3 at% of Al; 1,000 ppm or below of C; and
remainder of Fe (68 - 81 at%) and unavoidable impurities, wherein
the principal phase (preferably at least 85 vol %) is the
1 336866
(Fe,Co)-e-R tetragonal type crystal structure, exhibits excellent
magnetic properties of (BH)max and iHc which are 30 MGOe or
higher and 13 kOe or higher, respectively, as anisotropic
sintered magnets and also exhibits very high corrosion-resistant
property.
Note, however, that by applying appropriate aging, the
magnet achieves still higher magnetic properties.
Further, the permanent magnet (or material) according to
the present invention exhibits its best corrosion resistance when
it contains, as the principal phase, R2(Fe,Co)l4B type
compound having the tetragonal crystal structure, and has ~a grain
boundary phase which contains from 5 to 30 at% Co and 5 at% or
less Al in the R-rich multi-phase. The R-rich multi-phase is
composed of an R-rich phase not containing therein Al but Co and
another R-rich phase containing therein both Al and Co. When the
crystal grain size of the magnet is about 1 ~m - 100 ~m (pref.
2 - 30 ~m) the magnet provides significantly high magnetic
properties.
With a view to enabling those persons skilled in the art
to put the present invention into practice, the following
preferred examples are presented.
EXAMPLES
Example 1
As the starting material, use was made of electrclytic
iron of 99.9 % purity (by weight as to the purity); ferro-boron
alloy (20 % B); Nd (> 97 % the balance being Pr~; Dy, Co, Al and
Ti of > 99 %; ferro-niobium containing 67 % Nb; After these
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336866
ingredients were mixed at their various predetermined ratios,
each mixture was molten to form an alloy under high frequency
heating, after which the molten alloy was cast in a water-cooled
copper mold. As the result, there were obtained alloy ingots of
various compositions as shown in Table 1 below. Certain amounts
of Si, Mn, Cu and Cr were incorporated originating from the
ferro-boron. These elements improve iHc and rectangularity of
the demasnetization curves, which seems to be based on the
presence of 300 - 5,000 ppm Si and 200 - 3,000 ppm in total of
Mn, Cu and Cr in the magnet.
ThereafteE, the ingot was crushed coarsely by a stamping
mill, followed by wet pulverization in a ball mill using
trichloro-trifluoroethane, thereby obtaining pulverized powders
having an average particle size of 3 ~um.
Each of the pulverized powders was then charged in a metal
mold of a pressing device, subjected to alignment in a magnetic
field of 12 kOe, and compacted under a pressure of 1.5 tons/cm2
in the direction perpendicular to the magnetic field. The
resultant compact was then sintered at a temperature ranging from
1,040C to 1,120C, for two hours in an argon atmosphere, after
which it was allowed to cool. Thereafter, the sintered body was
further subjected to aging treatment at 600C. As the result,
there were obtained the permanent magnet material specimens
having a dimension of 20 mm x 10 mm x 8 mm, which were magnetized
by applying a magnetic field of at least 25 kOe.
The magnetic properties of the thus obtained permanent
magnets were measured, the results being shown in Table 1 below.
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The quantity of Co and Al were determined by use of an X-ray
micro-ar,alyzer, wherein the compositional analyses of the R-rich
phase in the grain boundary were carried out. The evaluation of
the analyses was given in terms of the average values of the
compositions in the grain boundary phase primarily at the triple
points.
The magnetic properties were measured after the
magnetization. As is apparent from Table 1, the Fe-B-R type
permanent magnet having the composition as specified in this
invention possesses magnetic properties which are equal to, or
higher than, that of the conventional Fe-B-R type permanent
magnet.
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. . . . .. .
o O O ~O O o
Z ~ _ ~ ~r~ ~ ~
~3
.
1-- o o~
uo F 1 ua~u I ~31 dwex
~uasa~ T~ dwo~
~ ~ 336866
Example 2
Some of the test specimens obtained from Example 1 above
were subjected to the undercoating treatment with Al followed by
surface-treatment with chromate to provide surface-treated
specimens; and, on the other hand, the remainder were left
untreated as the surface-untreated precimens. Each group of the
specimens was then subjected to the Pressure Cooker Test (P.C.T.)
in an atmosphere of a relative humidity of 85% at a temperature
of 125C under a pressure of 2 kgf/cm2. Through the P.C.T.
tetragonal grains will be isol2ted from the surface of the
specimen through the corrosion of the boundary phase to proauce a
grey colored powder. Thus the P.C.T. represents the evaluation
of the corrosion resistance primarily due to the stabil ~ ation of
the boundary phase.
The test result was ev~luated by the length of time taken
until the surface-treated film peeled off the surface of the
specimen to bring about blisters, or the length of time lapsed
until the surface of the specimen material produced powder.
Figure 1 indicates the test results.
As is apparent from Figure 1, the permanent magnets
according to the present invention which are in a state as
produced and have not undergone any surface-treatment exhibit
particularly excellent corrosion resistance in comparison with
that of the conventional permanent magnets which were subjected
to the surface-treatment for improving the corrosion-resistance.
The specimens which did not suffer disintegration exhibited
almost the same magnetic properties as those before testing while
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1 336866
those of the disintegrated specimens were not measured.
Example 3
The test specimens Nos. 2, 3, 6 and 7 in Table 1 as
obtained form Example 1 above and not subjected to the
surface-treatment were subjected to the corrosion-resistance
test,in which the specimens were held in an abmosphere of a
relative humidity of 90% at temperature of 80C over a long
period of time (accelerated weather-proof test). The test result
was evaluated by increase in quantity of the oxide per unit
surface area of each specimen versus the length of time, during
which the specimen was h~ld in the abovementioned atmosphere.
The test results are shown in Figure 2. The resultant specimens
after this test produce red rust. Thus this test is an
acceleration test representing the weather proofness (or
oxidation resistance) of the magnet surface under the usual
conditions of use thereof. Namely, the corrosion resistance of
the "tetragonal grains as well as the boundary phase of the magnet
surface is evaluated by this test. Therefore it is necessary to
apply also this test for complete evaluation of the corrosion
resistance of this type of magnets.
As is apparent from Figure 2, the permanent magnet
according to the present invention has a significantly superior
corrosion resistance of such a degree that could not be attained
by the conventional Fe-B-R type rare earth permanent magnet.
Example 4
Specimens having no surface treatment were prepared based
on the compositions as shown in Table 2 and pulverization was
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1 336866
carried out by jet-milling in N2 gas containing 1,0~0 ppm
oY.ygen, otherwise in the same manner as Example 1. In Table 2
Specimens 12 - 14 did not include Co and Al. These specimens
were tested by an autoclave under a saturated steam atmosphere at
180C for 16 hrs for the corrosion resistance. The magnetic
properties were measured before and after the corrosion
resistance test, while those before the test are shown in Table
3. The loss in weight of the specimens versus the lapse of time
was measured, too, and is shown in Table 3.
As apparent in Tables 2 and 3, specimen Nos. 9 - 11 which
inclu~e Co and Al did not suffer the loss in weight nor
disintegrated, whereas specimen Nos 12 - 14 were classified in
two groups depending upon the total amount of rare earth
elements, one group suffering loss and disintegration on the
surface portion and the other not.
The specimens which did not suffer disintègration
demonstrated the same level of the magnetic properties within the
measurement error even after the test in the autoclave.
Accordingly it is concluded that the corrosion resistance
of the Fe-B-R type magnets can be significantly improved by
incorporating specific amounts of Co and Al. Furthermore, the
corrosion resistance of the Fe-B-R type magnets is greatly
affected by the total amount of rare earth elements in the magnet
or material. Generally, the amount of the rare earth elements
which are present in the boundary phase of the Fe-~-R type
magnets will increase as the total amount of R increases. Such
abundant or excess presence of R adversely affects the corrosion
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' 1 336866
resistance, which, however, can be completely eliminated by the
incorporation of Co and Al. Co and Al are believed to stabilize
the boundary phase. It was further confirmed that the copresence
of Co and Al has an effect to reduce the amount of N in the
sintered magnet to a half to a third of that in the base magnet
not including Co an~ Al.
It is also concluded that even when Co and Al are not
included, the Fe-6-R type magnet does not suffer disintegration
if the total amount of R does not exceed about 1~ at% (and the
level of C is low). This is believed to be attributable to the
non-presence of the abundant R-rich phase in the boundary phase.
Furthermore, the absolute amount of oxygen appears to be
not definitive for the corrosion resistance ~or disintegration),
not only in the case where Co and Al are included but in the case
where these are not included. Rather, the definitive factor for
suppressing the corrosion is the control of the boundary- phase
either by stabilizing it by Co and Al or by eliminating the
presence of excess ~-rich boundary phase, i.e., more than the
minimum amount necessary to achieve the re~uisite high magnetic
properties. In light of this aspect, an Fe-~-R type magnet
composition containing 14 at% or less R in total in conjunction
with the allowable level of impurity tparticularly C etc.) will
also provide a stable base composition. (Note, however, the
presence of Co and Al further stabilize the base composition even
as the material.)
- 2S -
1 336866
Table 2
Composition (at%) Oxygen C
(ppm) (ppm)
No.Nd Dy Fe B Co Al
915.5 0.5 69 7 6 2 6800 170
1014.5 0.5 70 7 6 2 5500 220
1113.5 0.5 71 7 6 2 5200 1g0
1215.5 0.5 77 7 - - 7200 240
1314.5 0.5 78 7 _ _ 6400 220
,5 0,5 79 7 _ _ 5500 180
Table 3
- Br iHc (BH)max loss in weight (%)
911.9 17.1 34.2 0
1012.1 16.5 35.7 0
1112.5 15.7 36.9 0
1212.0 14.1 34.917 %
1312.5 12.8 37.61 %
1412.7 9.1 37.20
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1 336866
~xample 5
Based on the composition 2S shown in Table 4 and otherwise
in the same manner as in Example 1 magnet specimens were produced
and measured for the amounts of oxygen and carbon and the
magnetic properties to be shown in Table 4. The specimens were
tested in an atmosphere of a 90 % relative humidity (R.H.) at
80C and measured for the change in weight per unit surface of
the specimen. The result is shown in Figs. 3 - 6.
Fig. 3 represents the ~hange in weight in the case where 2
at% Al is present and the Co amount is changed from 0 - 6 at%.
When Co is not present, the corroding rate expressed in terms of
the change rate in weight is large, whereas the corroding rate
becomes to an extremely low level after the lapse of a certain
period of time as the Co amount increases.
Fig. 4 represents the change in weight in the case where
Al is not present and the Co amount is changed from 2 to 6 at%.
The changing rate in weight decreases with the lapse of time
while the decreasing tendency enhances with increase in the Co
amount. In comparison to Fig. 3, Fig. 4 where Al is not present
demonstrates greater change (increase) in weight than those in
Fig. 3. Such tendency is more significant in Figs. 5 and 6.
Namely, Figs. S and 6 represent the effect of Al at a Co amount
of 4 at% and 0 % (not included). When Co is not included (Fig.
6), not remarkable effect on the weight change test is achieved
by incorporating Al, whereas when Co is included (Fig. 5) the
magnitude of the change in weight diminishes with increase in the
Al amount. Based on this fact it has turned out that the
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1 336866
presence of Al contributes to the improvement in the corrosion
resistance.
Furthermore, based on the results of Table 4, i~c is
significantly improved when a small amount of Al (e.g., 1 at%) is
contained, although i~c tends to decrease with increase of Co
when Al is not present.
As discussed hereinabove, the synergic effect of the
coprese~ce of Co and Al in the Fe-E-R type magnets is significant
in improving the corrosion resistance as well as in proviaing
high magnetic properties.
Example 6
Based on ingots having the compositions of Nos. 15 and 17
of Table 4, specimens containing different amounts of C were
prepared as follows; (1) jet-milling the ingot using N2-gas as
a pulverizing medium (or carrier), (2) fine pulverizaion by a
ball-mill using a solvent (organic fluorine solvent, e.g., flon)
as pulverizing medium, and/or (3) to certain specimens admixing a
paraffine wax to adjust the C amount.
The results including the measured magnetic properties are
shown in Table 5. The specimens were further magnetized by
application of an external magnetic field of at least 25 kOe and
thereafter tested for the weather corrosion resistance in an
atmosphere of 90 % R.~. at 80C to measure the change in the
magnetic flux by using a flux meter. The results are shown in
Fig. 7.
As is apparent in Fig. 7, the flux loss generally
increases with increase in C, however, the rate of flux loss
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-
1 336866
significantly diminishes at the presence of Al even when C
increases, particularly at about 500 ppm C or more.
As is apparent from the Examples, the present invention
can eliminate the surface treatment for improving the corrosion
resistance. A further surface treatment may be applied, too.
However the surface treatment can be quite simplified in order to
give a complete corrosion protection, e.g., resin impregnation
with epoxy or the like resin will be sufficient.
~ o far, the present invention has been described with
r-eference to particular embodiments thereof. It should, however,
be noted that changes and modifications may be made by those
persons skilled in the art within the gist of the present
invention or scope of the present invention as recited in the
appended claims.
- 29 -
Table 4
Impurities Magnetic
Composition (at %) (ppm) Properties
Br (Bll) m~ illc
No. Nd Dy Fe B CoAl a~ C (KG) (MGOe) (KOe)
1 5 14 0.5 70.5 7 6 2 2400 340 11.8 33.6 16.1
1 6 14 0.5 71.5 7 6 1 2900 360 12.2 35.6 14~5
1 7 14 0.5 72.5 7 6 0 2700 330 12.6 37.7 10.1
1 8 14 0.5 72.5 7 4 2 2700 290 11.7 33.0 16.6
w
1 9 14 0.5 73.5 7 4 1 2600 330 12.3 36.1 14.7
14 0.5 74.5 7 4 0 2900 300 12.7 38.1 12.2
21 14 0.5 74.5 7 2 2 2000 350 11.8 33.7 16.9 C~
22 14 0.5 75.5 7 ~ 1 2800 350 12.4 36.6 15.
23 14 0.5 76.5 7 2 0 3300 3~0 12.7 38.5 12.7
24 14 0.5 76.5 7 0 2 3000 330 12.0 34.2 17.2
14 0.5 77.5 7 0 1 2900 350 12.3 36. 1 16.2
26 14 0.5 78.5 7 0 0 3300 350 12.7 38.7 14.2
Table 5
Impurities Magnetic
Composition (at %)(ppm) Properties
Br (Bll) n~ lllc
No. Nd Dy Fe B Co~11 ~ C (KG) (MGOe) (KOe)
27 14 0,5 70.5 7 6 2 6500 170 12.1 34.9 16.3
28 14 0.5 70.5 7 6 2 2000 340 12.0 34.3 16.0
29 14 0.5 70.5 7 6 2 3400 610 12.0 34.4 15.7 - `
14 0~5 70~5 7 6 2 3700 790 12.0 34.8 15.4
~ 31 14 0.5 71.5 7 6 1 6000 170 12.5 34.8 16.0 cr~
i 32 14 0.5 71.5 7 6 1 2200 330 12.4 36.9 13.8
33 14 0.5 71.5 7 6 1 3600 620 12.5 37.3 14.0
34 14 0.5 7~.5 7 6 1 3400 830 12.4 37.1 13.5
14 0.5 7~.5 7 6 0 5800 240 12.9 39.9 11.8
36 14 0.5 72.5 7 6 0 2200 350 12.8 39.0 11.2
37 14 0.5 72.5 7 6 0 3700 550 12.9 39.4 ll.l
38 14 0.5 72.5 7 6 0 3500 760 12.9 39.8 10.6
