Note: Descriptions are shown in the official language in which they were submitted.
Sm2Col7 ALLOYS SU I TAB LE FOR US E
AS PERMANENT MAGNETS
This invention relates to Sm~Col7 alloys suit-
able for use as permanent magnets.
~ he advantages of rare earth cobalt alloy mag-
nets are now well known. Such magnets are specially
suitable for use in electric motors, such as DC servo-
motors. It is also known that Sm2Col7 alloys have
potentlal advantages for use as permanent magnets over
SmCos alloys(l). For example, DC motors using Sm2Col7
alloy magnets have lower weight and inertia and
increased torque and acceleration compared to the use of
SmCo5 alloy magnets.
Various attempts have been made to provide
Sm2Col7 alloys which can form permanent magnets having
a high energy product (sH)max and a high intrinsic
coercivity iH~. Typical prior art is shown for example
in United States patent No. 4,172,717 issued October 30,
1979 to Tokunaga et al(2), United States patent No.
4,213,803 issued July 22, 1980 to Yoneyama et al(3),
United States patent No. ~,221,613 issued September 9,
1980 to Imaizumi et al(4) and United States patent
No. 4,375,996 issued ~larch 8, 1983 to Tawara et
al(5). Other prior art is shown in the published
literature(~,7,8,9,10) .
-- 2 --
As disclosed in the above-mentiGned prior art,
Sm2Col7 alloys are known which can form magnets having an
energy product (BH)maX in the range of 22 to 30 MGOe and
an intrinsic coercivity iHC in the range of 5.8 to 6.3
kOe(6~7)~ Later developments have resulted in the pro-
duction of Sm2Col7 alloys which can produce magnets with
higher coercivity, but this advantage has been offset by
loss in energy product. ~or example, one Sm2Col7 alloy
is now known which can produce magnets having an energy
product (BH)maX of 26 ~IGOe and an intrinsic coercivity
iHC of 15.0 kOe(7). Another Sm2Col7 alloy now known
has an energy product (BH)maX of 27 MGOe and an intrinsic
coercivity iHC of 10.0 kOe, see United States patent No.
4,375,996 mentioned above(5).
It is also known that, because of different
magnetic hardening mechanisms, Sm2Col7 alloys are harder
to magnetize from an unmagnetized state than SmCos
alloys. For example, in the construction of electric
motors, it is the preferred practice to construct the
field or stator assembly with unmagnetized magnets, and
then magnetize the finished assembly as a single unit.
This preferred industrial practice imposes an upper limit
of about 25 kOe on the intensity of the magnetizing field
which can be applied to the unmagnetized magnets of a
typical assembly. Thus, in order to be useful in
practice, an unmagnetized magnet must be capable of
attaining its specified properties in a magnetizing field
of 25 kOe. To date, it has not been possible to achieve
- this requirement with Sm2Col7 alloys with an energy
product greater than 30 MGOe(6).
Thus, although it is acknowledged that Sm2Col7
alloys have po-tential advantages over other rare
earth/transition metal alloys such as SmCo5 alloys,
Sm2Col7 alloys have not yet become practically useful
because improved coercivity has only been obtainable at
~L2~3:~
-- 3 --
the expense of energy product and also because such
alloys have not been capable of attaining their speci-
fied properties in a magnetizing field up to about 25
kOe.
It is therefore an object of the invention to
provide an Sm2Col7 alloy which overcomes these disad-
vantages.
According to the present invention, an Sm2Col7
alloy contains by weight:
22.5 to 23.5% Sm as an effective amount,
20.0 to 25.0~ Fe,
3.0 to 5.0% C~,
1.4 to 2.0~ Zr as an effective amount.
Further, in order to compensate for minor
amounts of oxygen and carbon which are inevitably present
in practice, it has been found that an additional amount
of Sm should be provided to compensate for the oxygen
content and that an additional amount of Zr should be
provided to compensate for the carbon content. Thus, an
alloy in accordance with the invention also includes an
additional amount of Sm in the range of from about 4 to
about 9 times the oxygen content of the alloy, preferably
6.2~5 times the oxygen content, and an additional amount
of Zr in the range of from about 5 to about 10 times the
carbon content of the alloy, preferably 7.595 times the
carbon content. The remainder of the alloy content is
cobalt.
The function of the "effective samarium" is to
develop the desired crystallographic structure consisting
of cells of the 2-17 Sm-Co rhornbohedral phase surrounded
by a continuous network of the 1-5 Sm-Co hexagonal
phase(11~12~13). It is necessary that the 1-5 network
be continuous to develop the desired second quadrant loop
squareness, that is to say a maximum value of HK, and
this is dependent upon the "effective samarium" present.
Sufficient samarium must be present for this purpose but
3 3L~
too much samarium results in the breakdown of the 2-17
rhombohedral phase and loss of remanent induction Br.
The function of the "effectiYe samarium" present is
therefore to develop the 2-17 Sm-Co rhombohedral phase
having high remanent induction Br and to develop a com-
plete 1 5 Sm-Co hexagonal phase network to develop the
coercivity or magnetic hardening. Too little samarium
results in an incomplete 1-5 Sm~Co network and incomplete
hardening, that is to say a low HK, and too much
samarium resul~s in breakdown of the 2-17 Sm-Co phase and
loss of remanent induction Br and energy product
(sH)max Precise control of the "effective samarium"
content is necessary to obtain optimum properties and
this can be achieved by the present invention.
The function of the "effective zirconium" is to
facilitate the dissolution oE all the desired constitu-
ents into one single phase solid solution in the solution
heat treatment stage of the processing. Only when this
is achieved is it possible to establish complete homo-
genei-ty as the necessary starting point to develop in the
subsequent aging heat treatment the desired structure
consisting of 2-17 Sm-Co cells surrounded by a continuous
network of the 1-5 Sm-Co boundary phase. The presence of
zirconium distorts the Sm-Co lattice so as to reduce the
c/a ratio of the hexagonal unit cell(14) and this facil-
itates the accommodation of the desired elements in single
phase solid solution during the solution heat treatment at
1140-1170C. At elevated temperatures the 2-17 Sm-Co
composition has a hexagonal crystal lattice but at room
temperature it transforms to a rhombohedral crystal
lattice. These two crystal systems are closely related
and the rhombohedral lattice can be reg~rded as an
imperfect hexagonal lattice containing stacking faults.
The desired equilibrium structure at room temperature
consists of cells of the 2-17 Sm-Co rhombohedral phase
:~25~
surrounded by a continuous network of the 1-5 Sm-Co
hexagonal phase. It has also been observed that for a
fixed copper content the coercivity increases with the
zirconium content. Thus to achieve the objective of
easier magnetization it is necessary to keep the
zirconium content to a minimum commensurate with the
above stated requirements regarding the single phase
solid solution. The necessary precise control of the
"effective zirconium" content can be achieved by the0 present invention.
sy using an Sm2Col7 alloy in accordance with
the invention, it is possible to produce a permanent
magnet which attains its specified properties in a
magnetizing field of about 25 kOe, has an energy product
(BH)maX f at least 30 MGOe and has a satisfactory
coercivity iHC of 14-16 kOe. A magnet in accordance with
the present invention can also have a satisEactory
remanent induction Br oE at least about 11.5 kG, and a
better loop squareness in the second quadrant, i.e. HK
of approximately 9.0 kOe.
Preferably, the oxygen content of the alloy is
not greater than about 0.6% by weight, and the carbon
content of the alloy is not greater than about 0.1% by
weight.
Also, the alloy preferably contains:
23.0~ Sm as an effective amount,
22.0~ Fe,
4.6~ Cu,
1.5% Zr as an effective amount,
minor amounts of oxygen and carbon,
additional amounts of Sm and Zr as specified
above.
and the balance being cobalt.
The invention is at least partly based upon the
realization that it is possible to compensate for small
-- 6 --
traces of carbon present in many of the elements that are
used in the production of the alloy and which have an
adverse effect on the magnetic properties of the alloy.
In accordance with the invention, compensation is made
for the carbon content by providing an additional amount
of zirconium as specified above.
The additional zirconium may be incorporated in
the alloy by adding zirconium in the form of a master
alloy to an Sm2Col7 base alloy at a convenient stage in
the processing, for example prior to compactinq and
sintering the alloy powder. The master alloy may be of a
simple form such as ferrozirconium, which is a low
melting point (about 935C) eutectic containing 83% Zr
and 17% Fe by weight. Ferrozirconium may be successfully
used when only a small additional amount of zirconium is
required. In other words, only up to about 2%
ferrozirconium by weight should be added.
If a larger additional amount of zirconium has
to be added, it is preferable to utilize a master alloy
with the same composition as the base alloy with the
exception that the master alloy should contain a larger
amount of zirconium, for example from about 5 to 10% by
weight, the increase in the zirconium content being
achieved at the expense of the cobalt content.
The following table illustrates the compensa-
tion of zirconium content for carbon present and the
optimum zirconium level at about 1.5%.
.. .
Total Zr C Cx6.265 ~ffective Zr Br iHc HK
(%) (%) (%) (~) (kG) (kOe) (kOe)
. _ _
2.24 0.07 0.53 1.71 11.6 19.1 8.3
2.1 0.072 0.55 1.55 11.7 16~9 9.8
2.05 0.068 0.52 1.53 11.6 15.2 ~.9
2.0 0.195 1.48 0.52 1.6 0.2 0.
:~S~3;~
The present invention is also at least partly
based on the realization that it is possible to
compensate for the small traces of oxygen which are
pic~ed up by the alloy during its manufacture and which
have an adverse effect on the magnetic properties of the
alloy. In accordance with the invention, the small
traces of oxygen are compensated for by addition of an
additional amount of samarium as specified above. The
amount of oxygen in the final product can be estimated
from the oxygen content of the starting material or more
preferably determined by analyzing a sample product.
The samarium addition may be accomplished by
adding a samarium rich alloy to a Sm2Col7 base alloy at a
convenient stage in the processing, for example prior to
compacting and sintering the alloy powder. It is not
practicable to add elemental samarium because of its high
rate of oxidation. The samarium rich alloy preferably
has the same composition as the base alloy except that
the samarium content would be about 1 to 3~ higher than
in the base alloy, the higher samarium content being
achieved at the expense of the cobalt content. A simple
binary master alloy (such as 60% Sm, 40% Co) can also be
used to add Sm.
The following table illustrates the compensa-
tion of samarium content for oxygen present and theoptimum effective samarium in the range 22.5-23.5%.
Total Sm 2 02x6.2~5 Effective Sm 3r iHc HK
(%) (~) (%) (~) (kG) (kOe) (kOe)
24.6 0.44 2.7621.8 9.62 4.0 1~85
25.1 0.42 2.6322.5 11.64 19.45 6.25
25.3 0.42 2.6322.7 11.73 17.9 g.l
26.0 0.55 3.4522.6 11.65 16.9 9.8
26.1 0.56 3.5122.6 11.54 17.7 8.9
26.0 0.38 2.3823.6 11.3 15.9 6.3
~ y adding additional zirconium and samarium to
compensate for the inevitable presence of small traces of
carbon and oxygen in the alloy, it has been found to be
possible to define more precisely the alloy composition
in order to produce the preferred magnetic properties.
Thus, the preferred samarium range is 22.5 to
23.5% with the preferred samarium value being 23.0% Sm.
This is the effective amount, as compared to the
additional amount provided as specified to compensate for
oxygen content. The range of effective samarium content
is considerably narrower than has been specified in the
prior art.
The effective zirconium range has been
specified to be from 1.4 to 2.0% with the preferred value
being 1O5~. Thus, with the present invention, it has
been possible to specify a zirconium range which is
considerably narrower than that taught by the prior art.
It has also been found possible to optimize the
iron and copper contents. The composition limits of
iron, copper and zirconium are interrelated and each can
critically affect the existence of the single phase
structure. It is known that the addition of iron to the
2-17 Sm-Co system increases the remanent induction
~z~
provided that the structure can be maintained as a single
2-17 Sm-Co phase. If the optimum iron content is
exceeded the alloy breaks down into an Fe-rich eutectoid
structure having lower remanent induction. It has been
observed that the copper content acts to increase the
coercivity or magnetic hardness of the alloy. It is
believed that copper concentrates in the 1-5 Sm-Co phase
network and enhances the coherent nucleation of regions
of 2-17 Sm-Co phase within the 1-5 Sm-Co phase network
during cooling from the aging temperature, thereby
creating lattice strain and magnetic hardness or
coercivity(15).
The iron content has been specified to be 20.0
to 25.0%, preferably 22.0~, and the copper content has
15 been specified to be 3~0 to 5 0%, preferably 4.6~. The
iron content has also been defined within a much narrower
range than has been taught by the prior art. Similar
remarks apply to the copper content~ As previously
indicated, cobalt forms the balance of the composition.
~n Sm2Col7 alloy in accordance with the inven-
tion is preferably made in the following manner. The
alloy oE the desired composition was produced by pul-
veri~ing melted and cast alloy into particles of 3-8 m
size. Small additions of ferrozirconium and a samarium
rich alloy of similar composition to the parent alloy
were then blended in to compensate for the deleterious
effects of the trace amounts o~ carbon and oxygen present
according to the present invention. The blended powders
were aligned in a die under a transverse magnetic field
of 12 kOe and compacted under a pressure of ~0 kpsi. The
green compact was sintered in hydrogen at 1150C for 30
min. The atmosphere was then changed to argon and the
compact was heated to 1205C at a rate of 4-5C/min, held
at 1205QC for 10 min and then cooled to 1160C at
2C/min. The sample was then solution treated at
~2~
1140-llhOC for 2 hours, quenched from 1140 300C at
10C/s and air cooled from 800C to room temperature. It
was then reheated to 845~5C and held for 20 hours,
cooled at about 2C/min from 845C to about 600~C and at
about lDC/min from about 600C to 410C, held at 410C
for 10 hours and cooled to room temperature.
An Sm2Col7 alloy having the previously men-
tioned preferred composition in accordance with the
invention and produced in the above described manner
achieved the following properties:
(BH)max Br iHc Hc HK
MGOe kG kOe kOe kOe_ _
30.8 11.7 15.8 11.0 9.0
The advantage of the invention can readily be
seen from the above Table.
It has also been found that praseodymium can be
substituted in part for samarium in the alloy of the
present invention without decreasing the aforementioned
desirable properties. To preserve the required 2-17
Sm-Co rhombohedral crystal structure the substitution of
praseodymium must be made on an atomic basis, that is to
say since praseodymium has a lower atomic weight than
samarium, on a weight percent basis less praseodymium
will be required in the alloy than the weight percent of
samarium replaced. In the example illustrated below it
was found that whereas optimum properties were obtained
with 23.0% effective samarium in the standard alloy, when
a combination of samarium and praseodymium was used the
optimum properties were obtained with an effective amount
30 of 22.5% (Sm + Pr), comprising 2000% Sm -~ 2.5% Pr.
Furthermore, in calculating the effective amount of
praseodymium present with respect to that amount which
has been rendered ineffective by combination with oxygen,
account must be taken of the molecular weight of the
praseodymium oxide and the correction factor of 6.265
3~S~3~
-- 10 --
times the oxygen content for samarium must be changed to
5.871 for the praseodymium added. In a co-pending appli-
cation the process for the production of high strength
2-17 Sm-Co permanent magne~s of the present composition
is described. Of particular importance is the selection
of the solution treatment temperature which is marginally
below the liquid plus solid phase transformation tempera-
ture for the specific alloy composition. In the case
where praseodymium has been partially substituted for
samarium care must be taken since the liquid plus solid
phase transformation temperature will be lower than that
of the standard samarium alloy by an amount depending on
the level of praseodymium substituted. The following
example illustrates the partial replacement of samarium
15 by praseodymium, An alloy containing 20.3% Sm and 2017%
Pr as effective amounts was prepared as described earlier
with the exception that the solution treatment step was
carried out in the range 1130-1150C. The following
properties were obtained and are compared with those of
a similar alloy containing only samarium as the rare
earth element.
Eff. Sm Eff. Pr Fe Cu ~r Co Br iHc ~
(%) (%) (~) (%) (%) (%) (kG) (kOe) (kOe)
_ _
20.32.17 20.5 5.2 2.5 bal 11.6 22.65 8.7
23.8_ 20.5 5.2 2.5 bal 11.5 22.4 9.7
It has also been found that other group
Ivs or VB transition elements may be substituted
full or in part for zirconium in the alloy of the
present invention. Since the function oE the group
IVB or VB transition element is to reduce the
c/a ratio of the 2-17 Sm-Co hexagonal unit cell,
the replacement of zirconium by other elements must
5~
be made on an atomic basis. Furthermore in calculating
the effective amount of the transition metal present with
respect to that amount rendered ineffective by combina-
tion with carbon, account must be taken of the molecular
weight of the transition metal carbide and the correc-
tion factor of 7.595 times the carbon content of the
alloy must be adjusted accordingly. For example~ in the
case of hafnium the correction factor would be 14.862
times the carbon content of the alloy. In a co-pending
application the process for the production of high
strength 2-17 Sm-Co permanent magnets is described and it
is noted therein that the aging temperature is critically
dependent upon the zirconium content. In the case where
other group IVB or VB transition elements are substituted
for zirconium in the alloy of the present invention the
optimum aging temperature and time may be different. The
following example illustrates the substitution of hafnium
for zirconium in an alloy of the present invention. An
alloy was prepared as described earlier for the standard
alloy except that zirconium as an effective amount was
replaced by hafnium as an effective amount. In calculat-
ing the additional amount of hafnium required to arrive
at the effective amount the carbon content of the alloy
was multiplied by the factor 14.862. The alloy was pro-
cessed as described earlier for the standard alloy with
the exception that after ~uenching from the solution
temperature to room temperature the alloy was reheated to
an aging temperature of 845~5C and held there for 24
hours.
The following table illustrates the replacement
of zirconium by hafni~m as an effective amount.
- 12 -
Total Zr ¦ Cx7.595 Effective Zr Br(l)
~%) (~) (%) (%) (kG)
5 1.78 0.07 0.53 1.25 10.6
1.90 0.07 0.53 1.37 10.9
10Total Hf C Cx14.862Effective Hf Br(l)
(%) (%) (%) (%) (kG)
3.65 0.082 1.22 2.43 10.6
3.80 0.082 1.22 2.58 10.7
_
(1) The results in the above table are for parallel
aligned magnets. The residual induction (Br) for
the same magnets transversely aligned would be
approximately 1.0 kG higher, i.e. 11.6-11.9 kG.
The invention thus also provides an Sm2Col7
alloy permanent magnet containing also iron, copper and
zirconium or similar group IVB or VB transition metals,
said alloy containing: an effective amount of samarium,
in addition to that samarium combined with oxygen, such
that after the aging stage of the process the crystal
structure of the alloy consists of the single phase 2-17
Sm-Co rhombohedral structure containing a continuous
network of the 1-5 Sm-Co phase, an effective amount of
zirconium, in addition to that zirconium combined with
carbon, such that during the solution heat treatment
stage of the process the 2-17 Sm-Co crystal lattice is
distorted to facilitate the dissolution of all the con-
stituents of said alloy into a uniform single phase soli
solution, an amount of iron being as high as possible to
maximize the remanent induction of said alloy whilst
:~2~
- 12a -
still maintalning the single phase ~-17 Sm-Co uniform
solid solution in the solution heat treatment stage of
the process, an amount of copper such that during the
cooling stage from the aging temperature the coherent
nucleation of 2-17 Sm-Co phase within the 1-5 Sm-Co
phase network is enhanced to produce lattice strain and
coercivity, it being understood that both zirconium and
copper levels must be controlled to permit the iron
level to be optimized whilst still maintaining a uniform
solid solution in the solution heat treatment stage.
(continued on page 133
- 13 -
The optimi~ation of composition must be based
on the requirement that all the alloying elements are
first put into a uniform solid solution. In the studies
of composition variations in Fe, Cu and Zr it was found
that the optimum effective samarium content remained
constant at 23+0.5%. However, it was found that the
effective samarium content can be partly replaced by
praseodymium with the added observation that slightly
less (Sm+Pr) is required for optimum properties. From
an economic point of view this could be an attractive
alternative.
_ Other embodiments and examples of the invention
will be clearly apparent to a person skilled in the art,
the scope of the invention being defined in the appended
claims.
The process is more fully described in our co-
pending application Serial No. 474,045 filed on the same
date as -this application.
- 14 -
References
1. Wallace, W.E., "Rare Earth Intermetallics", Academic
Press, New York, 1973.
2. Tokunaga, M., Hagi, C. and Murayama, H.~ "Permanent
Magnet Alloy", U.S. Patent No. 4,172,717, October 30,
1979.
3. Yoneyama, T., Tomizawa, S., Hori, T. and Ojima, T.,
"R2Col7 Rare Type Earth Cobalt Permanent Magnet Material
and Process for Producing the Same", U.S. Patent No.
4,213,803, July 22, 19~0.
Imaizumi, N. and Wakana, K., "Rare Earth-Cobalt System
Permanent Magnetic Alloys and Method of Preparing Same",
~ U.S. Patent No. 4,221,613, September 9, 1980.
5. Tawara, Y., Chino, T. and Ohasi, K., "Rare Earth Metal
Containing Alloys for Permanent Magnets", U.S. Patent
No~ 4,375,996, March 8, 1983.
6. Semones, B.C., "High Energy Density Rare Earth-Cobalt
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8. Yoneyama, T., Tomizawa, S., Hori, T. and Ojima, T., "i~ew
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g. Yoneyama, T., Fukuno, A. and Ojima, T., "Sm2(Co,Cu,Fe,Zr)l7
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A~stria, 1982.
t~
- 15 -
11. Fidler, J. and Skalicky, P., I'Domain Wall Pinning in REPM",
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1982.
12. Kronmuller, l~., "llucleation and Propagation of Reversed
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1982.
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Alloys", J. Appl. Phys. 55 (6), 15 March 1984.
