Note: Descriptions are shown in the official language in which they were submitted.
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D-7,804 C-3,517
IRON-RARE EARTH-BORON PERMANENT
MAGNETS BY HOT WORKING
This invention relates to high temperature
strain-anneal pro~`essing of extremely rapidly
solidified compositions comprising iron, one or more
rare earth metals and boron to produce useful permanent
magnets. More particularly, this invention relates to
the hot consolidation and hot working of overquenched
compositions comprising iron, neodymium and/or
praseodymium, and boron to form useful, magnetically
aligned permanent magnets.
Background
High energy product, high coercivity
permanent magnet compositions comprising, for example,
iron, neodymium and/or praseodymium, and boron and
methods of making them are disclosed in European
patent application 0 108 474 published May 16, 1984 by
John J. Croat and assigned to the assignee of this
application. An illustrative composition, expressed in
atomic proportions, is Ndo.13(Fe0 95B0.o5)o.87.
substantially the composition of a specific stable
intermetallic phase that possesses high coercivity when
formed as fine crystallites about 20 to 400 or 500
nanometers in largest dimension.
Melts of the above family of compositions can
be very rapidly quenched, such as by melt spinning, to
produce a solid material, e.g., a thin ribbon. When
the rate of cooling has been controlled to produce a
suitable fine crystalline microstructure (20 nm to 400
or 500 nm), the material has excellent permanent magnet
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properties. On the other hand, faster cooling
(overquenching) produces a material with smaller
crystallites and lower coercivity. However, as
disclosed, such overquenched material can be annealed
to form the suitable crystal size with the associated
high coercivity and high energy product.
An interesting and useful property of this
neodymium-iron-boron composition (for example) is that
it is magnetically isotropic. A fine grain, melt spun
ribbon can be broken up into flat particles. The
particles can be pressed in a die at room temperature
to form a unitary body of about 85% of the material's
density. Bonding agents can be employed before or
after the compaction. It is surprising to find that
such bonded magnets displayed no preferred magnetic
direction. Values of intrinsic coercivity or maximum
energy product were not dependent upon the direction of
the applied magnetic field. There was no advantage in
grinding the ribbon to very fine particles and
magnetically aligning the particles before compaction.
Such magnetically isotropic materials are
very useful because they can be easily pressed (without
magnetic alignment) into bonded shapes. The shapes can
be magnetized in the most convenient direction.
It is recognized that the iron-neodymium-
boron type compositions might provide still higher
energy products if at least a portion of the grains
or crystallites in their microstructure could be
physically aligned and if such alignment produced at
least partial magnetic domain alignment. The material
would then have a preferred direction of magneti2ation.
The material would be magnetically anisotropic and
would have higher residual magnetization and higher
energy product in the preferred direction. I have
now accomplished this using overquenched melt spun
material by hot working the material to consolidate
it to full density and to effect plastic flow that
yields magnetic alignment. The same improvement can
be accomplished on finely crystalline, high coercivity
material (e.g., HCi > 1000 Oe) if the hot work is
performed rapidly before excessive grain growth occurs
and coercivity decreases.
It is an object of my invention to provide
a fully densified fine grain, anisotropic, permanent
magnet formed by hot working a suitable material com-
prising iron, neodymium and/or praseodymiuml andboron. This anisotropic magnet has higher residual
magnetization and energy product than isotropic
magnets of like composition.
It is an object of my invention to provide
a method of treating overquenched compositions con-
taining suitable proportions of iron, neodymium and/or
praseodymium, and boron at suitable temperatures and
pressures to fully densify the material into a solid
mass, to effect the growth of fine, high coercivity
crystallites and to cause a flow and orientation of
the material sufficient to produce macroscopic mag-
netic anisotropy.
It is another object of my invention to
treat suitable transition metal-rare earth metal-boron
compositions that do not have permanent magnet proper-
ties because their microstructure is amorphous or too
finely crystalline. The treatment is by a hot working
process, such as hot pressing, hot die-upsetting, ex-
trusion, forging, rolling or the like, to fully consolidate
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pieces of the material, to effect suitable grain
growth and to proauce a plastic flow therein that
results in a body having magnetic anisotropy. It
is found that the maximum magnetic properties in
such a hot worked body are oriented parallel to the
direction of pressing (perpendicular to the direction
of flow. In the direction of preferred magnetic
alignment, energy products are obtainable that are
significantly greater than those in isotropic magnets
of like composition.
Brief Summary
In accordance with a preferred embodiment
of my invention, these and other objects and advantages
are accomplished as follows:
A molten composition comprising iron,
neodymium and/or praseodymium, and boron is prepared.
Other constituents may be present, as will be dis-
closed below. An example of a preferred composition,
expressed in terms of atomic proportions, is
0.13(FeO. 95Bo . 05) o 87- The molten material is
cooled extremely rapidly, as by melt spinning, to
form a thin ribbon of solid material that does not
have permanent magnet properties. Typically, the
material is amorphous in microstructure. It will not
produce an x-ray pattern containing many discrete
diffraction maxima like that obtained from diffraction
in crystalline substances. When highly magnified, as
in a scanning electron microscope micrograph, no
discrete grains (or crystallites) will be apparent.
The ribbon or other thin, solid form may be
broken, if necessary, into particles of convenient
size for an intended hot working operation. The par-
ticles ore heated under argon to a suitable elevated
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temperature, preferably 700C or higher, and subjected
to short term hot working under pressure, preferably
at least 10,000 psi. Such processing may be accom-
plished by any of a number of known hot working
practices. The material may be hot pressed in a die.
It may be extruded, or rolled, or die-upset, or
hammered. Whatever the particular form of hot working
employed, the several individual particles are pressed
and flowed together until the mass achieves full
density for the composition. In addition, the hot
mass is caused to undergo plastic flow. During the
exposure at high temperature the nonpermanent magnet
microstructure is converted to a suitable fine grain
crystalline material. The flow of the hot, fine
grain material produces a body, that upon cooling
below its Curie temperature, has preferred direction
of magnetization and provides excellent permanent
magnet properties.
As suitably practiced, the high temperature
working produces a finely crystalline or granular
microstructure (for example, up to about 0.4 to 0.5
micrometers in greatest dimension). Care is taken to
cool the material before excessive grain growth and
loss of coercivity occurs. The preferred direction
of magnetization of my hot worked product is typically
parallel to the direction of pressing and transverse
to the direction of plastic flow. A significantly
higher ene-gy product is obtained when the body is
magnetized transverse to the direction of plastic flow.
As previously stated, material of like
composition and similar microstructure has been made
without hot working. Such materials have been mag-
netically isotropic and had lower maximum energy
product.
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In another embodiment of my invention the
starting material may be a high coercivity (> 1000 Oe)
isotropic material. Suitable hot working of the
material will fully densify it and effect plastic
flow to orient the fine crystallites in a magnPtically
anisotropic structure. However the duration of the
hot working must be short so that the crystallites
do not grow so large that the desirable magnetic
properties are lost.
'rhese and other objects and advantages of
the invention will become more apparent from a de-
tailed description thereof, which follows. Reference
will be made to the drawings, in which
Figure 1 is a cross-sectional view of a hot
pressing die for practicing one embodiment of my
invention;
Figure 2 is a second quadrant, room temper-
ature, 4~M versus H plot of a sample produced by hot
pressing;
Figure 3a is a photomicrograph at 6QOX
magnification of a sample compacted to 85% of
theoretical density in accordance with earlier work;
Figure 3b is a photomicrograph at 600X
magnification of a sample hot pressed in accordance
with my method;
Figure 3c is a photomicrograph at 600X
magnification of a sample extruded it accordance
with my method;
Figure 4 is a second quadrant, room temper-
ature, 4~M versus H plot of a sample produced byextrusion;
Figure 5 is a Scanning Electron Microscope
micrograph at 43,600X magnification, illustrating
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the texture of the fracture surface of an extruded
sample prepared in accordance with my method;
Figure 6 is a second quadrant, room tem-
perature, 4~ versus H plot of a sample produced by
die upsetting in accordance with my method; and
Figure 7 is a second quadrant, room tem-
perature, 4~M versus H plot of a sample produced by
a different die upsetting practice in accordance with
my method.
Detailed Des`cription
My method is applicable to compositions
comprising a suitable transition metal component, a
suitable rare earth component, and boron.
The transition metal component is iron or
iron and (one or more of) cobalt, nickel, chromium or
manganese. Cobalt is interchangeable with iron up to
about 40 atomic percent. Chromium, manganese and
nickel are interchangeable in lower amounts, prefer-
ably less than about 10 atomic percent. Zirconium
and/or titanium in small amounts (up to about 2 atomic
percent of the iron) can be substituted for iron.
Very small amounts of carbon and silicon can be
tolerated where low carbon steel is the source of
iron for the composition. The composition preferably
comprises about 50 atomic percent to about 90 atomic
percent transition metal component -- largely iron.
The composition also comprises prom about
10 atomic percent to about 50 atomic percent rare
earth component. Neodymium and/or praseodymium are
the essential rare earth constituents. As indicated,-
they may be used interchangeably. Relatively small
amounts of other rare earth elements, such as samarium,
lanthanum, cerium, terbium and dysprosium, may be
mixed with neodymium and praseodymium without substan-
tial loss of the desirable magnetic properties. Pref-
erably, they make up no more than about 40 atomic
percent of the rare earth component. It is expected
that there will be small amounts of impurity elements
with the rare earth component.
The overquenched composition contains about
1 to 10 atomic percent boron.
The overall composition may be expressed
by the formula REl X(TMl yBy)x. The rare earth (RE)
component makes up 10 to 50 atomic percent of the
composition (x = 0.5 to 0.9), with at least 60 atomic
percent of the rare earth component being neodymium
and/or praseodymium. The transition metal (TM) as
used herein makes up about 50 to 90 atomic percent
of the overall composition, with iron representing
about 80 atomic percent of the transition metal con-
tent. The other constituents, such as cobalt, nickel,
chromium or manganese, are called "transition metals"
insofar as the above empirical formula is concerned.
Boron is present in an amount of about 1
to 10 atomic percent (y = about 0.01 to 0.11) of the
total composition.
For convenience, the compositions have been
expressed in terms of atomic proportions. Obviously
these specifications can be readily converted to
weight proportions for preparing the composition
mixtures.
For purposes of illustration, my invention
will be described using compositions of approximatelv
the following atomic proportions:
0.13( 0.95 0.05)0.87
However, it is to be understood that my method is
applicable to a family of compositions as described
above.
Depending on the rate of cooling, molten
transition mekal-rare earth-boron compositions can be
solidified to have microstructures ranging from:
(a) amorphous (glassy) and extremely fine
grained microstructures (e.g., Tess than
20 nm in largest dimension) through
(b) very fine (micro) grained
microstructures (e.g., 20 nm to about
400 or 500 nm) to
(c) larger grained microstructures.
Thus far, large grained microstructure melt spun
materials have not been produced with useful permanent
magnet properties. Fine grain microstructures, where
the grains have a maximum dimension of about 20 to 400
or 500 nm, have useful permanent magnet properties.
Amorphous materials do not. However, some of the
glassy microstructure materials can be annealed to
i convert them to fine grain permanent magnets having
I isotropic magnetic properties. My invention is
1 20 applicable to such overquenched, glassy materials. It
¦ is also applicable to "as-quenched" high coercivity,
fine grain materials provided the materials are exposed
only for short times, c less than five minutes, at
high temperatures, over 700C, during the hot working.
Suitable overquenched compositions can be
made by melt spinning. In my melt spinning experiments
the material is contained in a suitable vessel, such
as a quartz crucible. The composition is melted by
induction or resistance heating in the crucible under
argon. At the bottom of the crucible is provided a
small, circular ejection orifice about 500 microns
in diameter. Provision is made to close the
I' ' : ' . ,
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top of the crucible so that the argon can be
pressurized to eject the melt from the vessel in a
very fine stream.
The molten stream is directed onto a mov-
ing chill surface located about one-quarter inch
below the ejection orifice. In examples described
herein the chi]l surface is a 25 cm diameter, 1.3 cm
thick copper wheel. The circumferential surface is
chrome plated. The crucible and wheel are contained
in a box that is evacuated of air and backfilled with
argon. In my experiments the wheel is not cooled.
Its mass is so much greater than the amount of melt
impinging on it in any run that its temperature does
not appreciably change. When the melt hits the
turning wheel, it flattens, almost instantaneously
solidifies and is thrown off as a ribbon. The thick-
ness of the ribbon and the rate of coolin,~ are
largely determined by the circumferential speed of
the wheel. In this work, the speed can be varied to
produce an amorphous ribbon, a fine grained ribbon
or a large grained ribbon.
In the practice of my method, the cooling
rate or speed of the chill wheel preferably is such
that an amorphous or extremely fine crystal structure
is produced. Such a structure will be amorphous or
will have finer crystals than that which produces a
permanent magnet as is, for example, less than about
20 nanometers in largest dimension. As a practical
matter, the distinction between an amorphous micro-
structure and such an extremely fine crystallinemicrostructure is probably not discernible. What
is desired is an overquenched material that has less
than optimum permanent magnetic properties but that
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can be annealed to produce improved permanent magnet
properties. However, in accordance with my practice,
the material is not separately annealed. It is, in
effect, annealed while it is hot worked to produce a
magnetic microstructure that has anistropic magnetic
properties.
A few examples will further illustrate the
practice of my invention.
Example 1
10 An overquenched, melt spun ribbon was
prepared. A molten mixture was prepared in accordance
with the following formula: Ndo.13(FeO 95Bo 05)0.87
About 40 grams of the mixture was melted in a quartz
tube that was about 10 cm long and ~,54 em in diameter.
The quartz tube had an ejection orifice in the bottom,
which was round and about 600 em in diameter. The
top of the tube was sealed and adapted to supply
pressurized argon gas to the tube above the molten
alloy. The alloy was actually melted in the tube
using induction heating. When the melt was at 1400C,
an argon ejection pressure of about 3 psig was applied.
An extremely fine stream of the molten metal
was ejected down onto the rim of the above described
wheel. The wheel was made of copper and the peri-
meter surface was slated with chromium. The wheelwas initially at room temperature and was neither
heated nor cooled during the experiment, except from
contact with the molten metal ejected onto it. The
wheel was rotated at a rim velocity of about 35 meters
per second (m/s).
A solidified melt spun ribbon came off the
wheel. It was about 30 em thick and about one mm wide.
This material was cooled too rapidly to
have useful permanent magnet properties. In other
words, it was overquenched. Tad the wheel been rotated
slightly slower, the ribbon could have been produced
to have a microstructure affording useful hard magnetic
properties
The ribbon was broken into short pieces and
they were placed into the cylindrical cavity 12 of a
round die 10 like that depicted in Figure 1. The
cavity was 3/8 inch in diameter and the material was
contained by upper and lower punches 14. The die was
made of a high temperature nickel alloy with a tool
steel liner, and the punches were tungsten carbide.
The die and the contents were rapidly hea~e~
under argon with an induction coil 16 to a maximum
temperature of 750C. The temperature was measured
using a thermocouple (not shown) in the die adjacent
the cavity. The upper punch was then actuated to
exert a maximum pressure of 32,000 psi on the broken-
up ribbon particles. Heating and pressure were stopped.
The workpiece was cooled to room temperature on the
die. However, the total time that the workpiece was
at a temperature above 700~C was only about five
minutes. The consolidated workpiece was removed from
the die. The resulting cylinder was hard and strong.
It had a density of about 7.5 grams per cubic centi-
meter, which is substantially its full density.
The magnetic properties of the material weredetermined by cutting a piece from the cylinder and
grinding a small sphere, about 2 mm in diameter, from
the cut off piece. The sphere was magnetized in a
known direction by subjecting it to a pulsed magnetic
field having a strength of about forty kilo gauss-. -
The sphere was then placed in a vibrating sample
magnetometer with-the positive magnetic pole of the
sphere aligned with the positive pole of the magne-
tome~er. The sample was subjected to a gradually
decreasing magnetic field from ~lOkOe to -20kOe that
produced corresponding decreasing sample magnetization
(4~1). In this manner the second quadrant demagneti-
zation plot (4~ versus H) was obtained for the par-tic-
ular direction magnet;zat;on.
The sample was removed from the magnetometer
and magnetized in a pulsed field as before in a dif-
ferent direction. It was returned to the magnetometer
and a new demagnetization curve determined. This
process was again repeated and the respective curves
compared. The sample displayed magnetic anistropy.
Figure 2 contains four different second
quadrant plots of 4~M versus H. The second quadrant
portion of a hysteresis loop provides useful informa-
tion regarding permanent magnet properties. Threeof these plots in Figure 2 represent good properties.
The residual magnetization at zero field (H=0) is high
and the intrinsic coercivity, i.e., the reverse field
to demagnetize the sample (4~M = 0) is high. The
upper curve 18 represents a favorable direction of
magnetization obtained in the spherical sample. The
lowest curve 20 represents the data obtained from a
direction relatively far removed from the aligned
direction of the hot pressed compact. The middle line
22 is the demagnetization plot also generated in the
vibrating sample magnetometer of an isotropic array
of the same ribbon from which its hot compact was made.
These ribbon samples were heated (annealed) at a rate
of 160 per minute to a temperature of 727C, and
then cooled at the same rate to room temperature. The
data obtained was normalized to a sample density of
100%. Thus plot 22 is of an isotropic magnet of the
same composition as the anisotropic magnet produced
in this example.
13
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14
A hysteresis curve was also prepared from
a sample of the original overquenched ribbon. The
second quadrant portion is produced as curve 24 in
Figure 2. It has relatively low intrinsic coercivity
and residual magnetization.
Thus, the hot pressing operation produced
a fully densified body and also produced flow in the
material that oriented the microstructure so that it
became magnetically anisotropic. In the preferred
direction of magnetization represented by curve 18
the residual magnetization and energy product are
greater than in the isotropic material.
In addition to having excellent permanent
properties at room temperature the hot pressed body
retains its properties during exposure at high tem-
peratures in air. A hot pressed body of this example
was exposed at 160~C in air to a reverse field of
4kOe for 1507 hours. It suffered only minimal loss
in permanent magnet properties.
Figure 3a is a pho-tomicrograph of a cross-
section of a bonded magnet that was compacted at
room temperature to 85% of full density, The plate-
like sections of the original ribbon are seen to
line up and be preserved in the bonded magnet. Fig-
ure 3b is a photomicrograph at the same magnification
of a hot pressed specimen fully densified in accor-
dance with my invention. The flat ribbon fragments
are still perceptible at about the same size as in
the bonded magnet, but there are no voids in this
fully densified specimen.
Example 2
Another overquenched, melt spun ribbon was
prepared by the method described in Example l The
nominal composition of the ribbon was in accordance
with the empirical formula Ndo 13 Leo 94Bo 06~0 87.
14
The ribbons were produced by quenching the melt on a
chill wheel rotating at a velocity of 32 m/s. The
thickness of the ribbon was approximately 30~m and
the width approximately one millimeter. This cooling
rate produced a microstructure that could not be
magnetized to form a magnet having useful permanent
magnet properties.
Ribbon pieces were compacted at room
temperature in a die to form a precompacted body of
about 85% full density. The precompact was then
placed in the cavity of a high temperature alloy die
similar to that described in Example 1. However,
the die had a graphite liner. Carbide punches con-
fined the precompact in the die cavity. The die and
its contents were quickly heated under argon to 740C
and a ram pressure of 10 kpsi was applied in an attempt
to extrude the preform. An unexpected form of back-
ward extrusion was obtained as the precompacted material
flowed out from between the punches and displaced
graphite die liner to form a cup-like piece, After
cooling to room temperature this piece was removed
from the die and it was found that the extruded por-
tion of the sample was of sufficient dimensions to
allow density measurement as well as magnetic measure-
ment. The extruded portion was fully densified.
A 2 mm cube was ground from a portion ofthe extruded metal and it was tested in a vibrating
sample magnetometer. By magnetizing and demagnetizing
the sample transverse to the cube faces it was ob- --
served that the specimen displayed magnetic anisotropy.Three orthogonal directions are displayed in Figure
by curves 26, 28 and 30. The separations of these
second quadrant plots from different directions of
magnetization results from physical alignment of
magnetic domains withi'n the''sample.' The greater the
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separation of the plots, the greater the degree of
magnetic alignment. It is seen that the alignment for
the extruded sample was even more pronounced khan for
the sample of Figure l. The demagnetization curves
for the annealed ribbon 22 and the overquenched ribbon
24 are also included in this figure as in Figure 2.
It is seen that the coercivity of the extruded sample
is even higher than that of the annealed ribbons pre-
sumably hecause a more appropriate crystallite size
was achieved during the extrusion. The magnetization
of the extruded sample in its most preferred direction
is higher and results in higher energy product than
that obtainable in isotropic annealed ribbons.
Figure 3c is a photomicrograph at 600X
magnification of a cross-section of the extruded
sample. It is seen that greater plastic flow occurred
in the extruded sample as evidenced by the reduction
in thickness of the original ribbon particles. It is
believed that this plastic flow is essential to align-
ment of the magnetic moments within the material andthat this alignment is genérally transverse to the
plastic flow. In other words, with respect to this
sample, the magnetic alignment is transverse to the
long dimension of the extruded ribbons (i.e., up and
down in Figure 3c).
Figure 5 is a scanning electron microscope
micrograph at nearly 44,000X magnification of a frac-
ture surface of the extruded sample. It shows the
wine grain texture.
Additional hot press tests, like Example 1,
and modified extrusion tests, like Example 2, were-
carried out at various die temperatures in the range
of 700 to 770C and pressures in the range of ln,000
to 30,000 psi. These tests showed that full densi-
fication could be realiæed even at the lower pressures
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and temperatures. However, the samples prepared at
the lower temperatures and pressures appeared to be
more brittle. Optical micrographs revealed the ribbon
pieces to have cracks similar to those present in
Figure 3a. Evidently, higher pressure is required at
temperatures of 750C and lower before such cracks
disappear as in Figure 3b. The preferred magnetiza-
tion direction for the hot pressed samples is parallel
to the press direction and perpendicular to the direc-
tion of plastic flow. Greater directional anisotropydevelops when more plastic flow is allowed, as in the
extrusion tests.
Example 3
This example illustrates a die upsetting
practice
Overquenched ribbon fragments of Example 2
wexe hot pressed under argon in a heated die, like
that in Figuxe 1, at a maximum die temperature of
770C and pressure of 15 kpsi. A 3/8" cylindrical
body, 100~ density, was formed. This hot pressed
cylinder was sanded to a smaller cylinder (diameter
less than 1 cm) with its cylindrical axis transverse
to the axis of the original cylinder. This cylinder
was re-hot pressed in the original diameter cavity
~5 along its axis (perpendicular to the original press
direction) so that it was free to deform to a shorter
cylinder o 3/8" dial~Rter (i.e., die upsetting). The
die upsetting operation was conducted at a maximum
temperature of 770C and a pressure of 16 kpsi. As
in previous examples the part was cooled in the die.
A cubic specimen was machined from the die upset body
and its magnetic properties measured parallel and
transverse to the press direction in a vibrating
sample magnetometer, as in the above Exa~,ples 1 and 2.
Second quadrant, room temperature 4~M versus H plots
17
18
for these two directions are depicted in Figure 60
Curve 32 was obtainer in the direction parallel to
the die upset press direction and curve 34 in the
direction transverse thereto and thus parallel to the
direction of material slow. It is seen that this die
upset practice produced greater anisotropy than the
single hot pressing operation or the extrusion test.
This translates to a Br of 9.2 kG and an energy produced
of 18 MGOe compared with isotropic rïbbon values of
Br = 8kG and energy product of about 12 MGOe.
Example 4
This example illustrates a die upsetting
practice similar to Example 3, except a fully dense,
hot pressed sample was die-upset with pressure applied
in the same direction as the original hot press
pressure.
Overquenched ribbon fragments ox Example 2
were hot pressed under argon in a heated die, like
that depicted in Figure 1, at a maximum temperature
of 760C and pressure of 15 kpsi. A 3/8 inch cylin-
drical body, 100% density, was formed. This hot
pressed piece was sanded to a smaller diameter (less
than about 1 cm) and die upset in the same diameter
cavity in a direction parallel to.the first press
~5 direction. The die upset operation was conducted at
a maximum temperature of 750C and a pressure ox 12
kpsi. The sample was cooled in the die.
A cubic specimen was machined from the die
upset body and its magnetic properties measured in a
vibrating sample magnetometer parallel and transverse
to the d;e upset press direction as in the above
example. Second quadrant, room temperature, 4~M
versus plots for these two directions are depicted
in Figure 7. Curve 36 was obtained in the direction
parallel to the die upset press directions and curve 38
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19
in the direction txanSYerse thereto. It is seen that
this pxactice of hot pressing followed by die upsetting
in the same direction produced greater anisotropy than
was obtained in any of k preYïous samples. It is
seen in figure 7 that in the preferred d;rection of
magnetization (curve 36) toe remnant magnetization was
greater than 11 kG, while the intrinsic coercivity was
still greater than 7 kOe. The maximum energy product
of this sample was 27 MGOe.
It is believed that still greater alignment
can be obtained by a practice that provides greater
plastic flow at elevated temperature. One may define
an alignment factor by (Br)parallel/(Br)perpendicular~
where Br is residual induction (at H = 0) measured
parallel to and perpendicular to, respectively, the
press direction. An alignment factor of 2.46 was
obtained in Example 4. An alignment factor of 1.32
has been achieved by die upsettïng (like in Example
3). An alignment factor of 1.18 has been achieved
for extrusion (like in Example 2).
My practice of high temperature consolida-
tion and plastic flow can be viewed as a strain-anneal
process. This process produces magnetic alignment of
the grains of the workpiece and grain growth. However,
if the grain yrowth is excessive, coercivity is de-
creased. Therefore, consideration tand probably trial
and error testing) must be given to the grain size of
the starting material in conjunction with the time
that the matexial is at a temperature-at which grain
growth can occur. If, as is preferred, the starting
material is overquenched, the workpiece can be held at
a relatively high temperature for a longer time because
some grain growth is desired. It one starts with near
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optimal train size material, the hot working must be
rapid and subsequent cooling prompt to retard excessiYe
grain growth. For example, I have carried out hot
pressing experiments on enodymium-iron-boron-melt spun
compositions thaw haze teen optimally quenched to
produce optimal grain size for achieving the highest
magnetic product. During such hot pressing the
material was over 700C for more than five minutes.
The material was held too long at such temperature
because the coerci~ity was always reduced although not
completely eliminated. Therefore, optimal benefits
were not obtained.
I also conducted hot pressing experiments
on annealed ingot that had a homogenized, large grain
microstructure. When magneti2ed, such ingots con-
tained very low coercivity, less than 500 oersted. My
hot pressing strain-anneal practice produced a signif-
icant directional dependence of Br in the ingot
samples, but no coercivity increase. It had been
hoped that the strain-anneal practice would induce
recrystallization in the ingot which would allow for
development of the optimal grain size. The failure to
obtain a coercivity increase in these experiments
indicates that the strain-anneal practice is not bene-
ficially applicable to large grained materials.
Thus, my high temperature-high pressure
consolidation and hot working of suitable, transition
metal, rare earth metal, boron compositions yields
magnetically anisotropic product of excellent perman-
ent magnet properties. For purposes of illustration,the practice of my invention has been described, using
specific composition of neodymium, iron and boron.
However, other materials may be substituted or present
in suitably small amounts. Prafieodymium may be sub-
stituted for neodymium or used in combination with it.
~363~
Other rare earth metals may be used with neodymiumand/or praseodymium. Likewise, other metals, such as
cobalt, nickel, manganese and chromium, in suitably
small amounts, may be used in combination with iron.
The preferred compositional ranges are described above.
While my invention has been described in
terms of preferred embodiments thereof, it Jill be
appreciated that other embodiments could readily be
adapted by those skilled in the art. Accordingly,
the scope of my invention is to be considered limited
only by the following claims.
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