Note: Descriptions are shown in the official language in which they were submitted.
20~632
G-7339 C-4240
HOT WORKED RARE EARTH-IRON-CARBON MAGNETS
This invention relates to permanent magnets
based on rare earth elements and iron. More
particularly, this invention relates to hot worked, fine
grain permanent magnets based on iron, neodymium and/or
praseodymium and carbon.
Background of the Invention
Permanent magnets based on the RE2Fe14B-type
structure have gained wide commercial acceptance. Such
magnets can be made by a sintering practice, and they
can be made by rapidly solidifying a melt of suitable
composition and producing bonded magnets or hot pressed
magnets or hot pressed and hot worked magnets from the
quenched material.
Recently, rare earth-iron-carbon compositions
have been formed in the RE2Fe14C structure which is
analogous to the above-mentioned iron-rare earth-boron
structure. Stadelmaier and Liu, US 4,849,035, cast
iron-dysprosium-carbon compositions and iron-dysprosium-
neodymium-carbon-boron compositions in the form of
ingots and through a prolonged annealing cycle at 900C
produced the magnetically hard tetragonal 2-14-1
structure. The casting displayed permanent magnet
properties as did comminuted particles produced from the
casting. The comminuted particles were disclosed as
suitable for use in a bonded magnet. While such
materials displayed appreciable coercivity, they
displayed relatively low remanence.
Coehoorn et al, "Permanent Magnetic Materials
Based on Nd2Fe14C Prepared by Melt Spinning", Journal of
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Applied Physics, Vol. 62, No. 2, 15 January 1989,
pp. 704-709, produced melt-spun ribbon particles of
neodymium, iron and carbon which, when annealed at a
suitable temperature, produced a permanent magnet of the
2-14-1 structure. Such particles could also be used to
make a resin-bonded magnet.
It is an object of our invention to provide
hot worked magnets, e.g., hot pressed or hot pressed and
die upset magnets, of the Nd2Fel4C-type structure that
have very fine grains, have permanent magnet
characteristics and are magnetically anisotropic. It is
another object of our invention to provide a method of
making such hot worked magnets.
Brief Summary of the Invention
In accordance with the preferred embodiment of
our invention, these and other objects and advantages
are accomplished as follows.
we prepare a melt comprising neodymium and/or
praseodymium, iron and carbon, or carbon and boron, that
is suitable, upon hot working, for forming the 2-14-1
type structure with a minor portion of one or more
second phases. This molten composition is very rapidly
solidified such as by melt ~pinning to produce an
amorphous composition or a composition of very fine
grain size, for example, no greater than about 40 nm in
average grain size. The melt-spun material is initially
in the form of friable, magnetically isotropic ribbon
fragments which may be readily broken into a powder
suitable for hot pressing and/or other hot working in a
die cavity.
2034~32
Such powder particles are amorphous or contain
many very fine grains. The particles are magnetically
isotropic. They are hot pressed at a suitable elevated
temperature of about, e.g., 700C to 900C for a period
of 20 to 30 seconds to a few minutes to form a fully
dense, fine grain Nd2Fel4C-type tetragonal crystal
structure. The hot pressed body may then be further hot
worked at an elevated temperature, e.g., 750C to 900C,
to promote the growth of platelet-like grains and to
plastically deform the body to align the platelets such
that their c-axes are generally parallel and the
resultant body is magnetically anisotropic. The body is
still fine grained although the grains are flattened and
aligned and its preferred direction of magnetization is
in the direction of pressing, i.e., perpendicular to the
direction of material flow during hot working. In
general, we prefer that the largest average dimension of
the flat grains be no more than about 1000 nm and that
they be no more than 200 nm thick. The microstructure
of the hot worked material is characterized by a
predominance of these flattened 2-14-1 grains with one
or more minor phases of intergranular material that is
typically composed of iron and the rare earth
element(s).
We prefer the use of iron as the transition
metal element although mixtures of iron and cobalt may
be employed. we prefer the use of neodymium and/or
praseodymium as the rare earth element although up to 40
percent of the total rare earth content may include
other rare earth elements. We prefer carbon or mixtures
of carbon and boron for the third constituent of the
2-14-1 structure. In the practice of our invention, the
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-
proportions of iron (or iron and cobalt), rare earth
elements and carbon must be balanced so that the
predominant crystalline phase formed is the 2-14-1
tetragonal structure. If this crystal ~tructure is not
formed, the hot worked product will have low coercivity
or no permanent magnetic characteristics at all.
Further objects and advantages of our
invention will become apparent from a detailed
description of the preferred embodiments.
In this description, reference will be had to
the drawing figures in which:
Figure 1 consists of two scanning electron
microscope (SEM) photographs [Figure l(a) and Figure
l(b)] from the fracture surface of a die upset
Nd13 75Fe80 25C6 magnet. The press direction lies
vertically in the photographs. Two magnifications of
the same region are provided.
Figure 2 consists of three graphs of process
parameters measured during the hot pressing of melt-spun
ribbons with the composition Nd16Fe78Cg.
Figure 3 consists of three graphs of process
parameters measured during the die upsetting of a hot
pressed precursor with the composition Ndl6Fe78Cg.
Figure 4 consists of demagnetization curves
for hot pressed and die upset magnets. The compositions
are indicated in each panel.
Detailed Description
The product of our practice is a permanent
magnet. It has a coercivity greater than 1000 Oersteds.
2034~2
Example 1
We prepared an ingot whose composition on an
atomic percent basis was neodymium, 13.75 percent; iron,
80.25 percent; and carbon, 6 percent. This material was
remelted by induction melting in a quartz crucible under
argon atmosphere at a superatmospheric pressure of
1-3 psi and melt spun by ejecting the molten material
through a 0.65 mm orifice at the bottom of the crucible
onto the perimeter of a 10 inch diameter chromium-plated
copper wheel rotating at a speed of 28 meters per
second. The ejected molten stream was instantaneously
quenched as it hit the rim of the spinning wheel and
thrown off as ribbon fragments.
An X-ray diffraction analysis of the ribbon
particles confirmed that they were substantially
amorphous. The ribbon fragments were crushed to powder
to facilitate handling. A portion was then placed in
the cavity of a 0.5 inch diameter graphite die. They
were preheated therein in vacuum to 450C. The die
temperature was then rapidly increased to 750C. When
the die temperature exceeded 640C, pressure was applied
by boron nitride-lubricated tungsten carbide-titanium
carbide punches. A pressure cycle was initiated,
causing the load to ramp to a maximum load of 100 MPa.
The load was held at maximum load for 30 seconds to
ensure full compaction before the punches were withdrawn
and the sample ejected. The entire process was done in
a vacuum. A fully densified cylindrical body was
formed.
The resulting hot pressed body had a density
of about 7.74 g/cc and contained the Nd2Fe14C tetragonal
crystal phase with small amounts of intergranular phases
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of uncertain composition believed to be largely
neodymium and iron. The lattice parameters of this
tetragonal phase were determined to be a ~ 8.797
angstroms and c - 12.001 angstroms.
The magnetic properties of this hot pressed
body were derived from a demagnetization curve measured
with a hysteresisgraph. The body displayed magnetic
anisotropy. The relevant properties in the direction
parallel to pressing were as follows: sr ~ 7.7 kG,
HCi ~ 10.7 kOe and (BH)maX 8 11.4 MGOe. In the
direction perpendicular to pressing, the magnetic
properties were: Br ~ 6.8 kG, Hci ~ 11.3 kOe and
(BH)maX ~ 8.1 MGOe.
Example 2
A hot pressed cylinder from Example 1 was
pressed a second time in the same direction in vacuum
using an oversized (0.75 inch ID) graphite die that
permitted the magnet to plastically deform the magnet at
a die temperature of 750C to 800C to about 40 percent
of its original height. The resulting die upset, flat
cylindrical magnet was sectioned with a high speed
diamond saw to produce a 2 mm cube for measurement of
its magnetic properties in a vibrating sample
magnetometer. The cube was cut so that two opposite
faces were perpendicular to the direction of pressing
and die upsetting, and the other four faces were
parallel to the direction of pressing and die upsetting.
The demagnetization curves for the neodymium-
iron-carbon die upset magnet revealed a higher remanence
in the press direction (sr - 12.3 kG) than in the
direction perpendicular the press direction where Br
1.7 kG. This magnetic anisotropy is indicative of the
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alignment of the c-axis of the individual die upset
grains along the press direction. The coercivity of the
sample in the press direction was 2.8 kOe.
Figures l(a) and l(b) are two SEM photographs
at different magnifications of the same region of a
fracture surface of this die upset specimen. The grains
of the Nd2Fe14s tetragonal crystals are ~een to be
aligned flat platelets. The grains are about 100 nm
thick and up to about 700 to 800 nm in their largest
dimension. The short dimension of the grains, the
c-axis, the preferred direction of magnetization lies
along the direction of applied stress.
Example 3
A family of four alloys was prepared so as to
be composed as follows: Ndl3.75Fe8o.25(Bl-xcx)6 w
in the four samples was respectively 0.2, 0.4, 0.6 and
0.8.
The several samples were individually melt
spun to form amorphous ribbon fragments as in Example 1.
The four lots of ribbon fragments were pulverized and
hot pressed into cylindrical bodies in accordance with
the practice of Example 1. They contain fine grains of
the tetragonal phase Nd2Fe14CxBl x where the values of x
were as indicated above. The densities and the magnetic
properties of the cylindrical magnetic bodies were as
follows:
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Density Br Hci (BH)max
(g/cc) (kG) (kOe) (MGOe)
0.2 7.38 8.2 14.5 14.3
0.~ 7.39 8.2 14.0 14.4
0.6 7.20 8.1 13.6 14.2
0.8 7.35 8.2 12.9 14.5
Example 4
The relatively low coercivity and high
resistance to deformation of our die upset
Ndl3 75Fe80 25C6 magnets suggested to us the need for
higher neodymium concentrations. Several alloys were
prepared as described in Example 1 using the formula
13.75+x e80.25-xC6 The several respective
compositions were melt spun as described in Example 1
except that a wheel speed of 30 m/s was used. The
samples were hot pressed and most were die upset. These
hot working steps were carried out using graphite dies
and tungsten carbide-titanium carbide punches also as
described in Example 1.
Typical process parameters used for hot
pressing these Nd-Fe-C ribbons are shown in Figure 2.
The ribbons were heated to 650C in about 5.75 minutes,
at which point the pressure was applied (see panels A
and B of Figure 2). The time interval required to reach
full (or nearly full) density was between 1 and 2
minutes at maximum pressure (about 65 MPa), as the lower
two panels in Figure 2 show. The final hot press
temperature was around 850C for the hot pressed carbide
magnets, compared to about 800C for Nd-Fe-B maqnets.
203463~
The hot pressed magnets were removed from the
die and cooled to room temperature. Magnetic
measurements were then made as described below. The
data is reported in Table I below. Some of the hot
pressed magnets were then r~heated and die upset in a
larger die as described in Example 2.
The temperature reached 700C in about 8.25
minutes of heating. An initial die upsetting pre~sure
of about 15 MPa was applied at about 800C (see Figure
3). This pressure was maintained until the sample
height had decreased at least about 5 percent, at which
point the pressure was increased to 20 to 25 MPa.
Starting with 15 MPa ensured that deformation could be
induced without cracking the precursor; however, the
strain rate at 15 MPa was too slow. Increasing the
pressure to 20 to 25 MPa enhanced the strain rate to
levels comparable to those observed for Nd-Fe-B alloys
(about 1 min 1). Higher temperatures were required to
produce fully die upset carbide magnets; the final
temperature (about 900C) was 50 to 100 degrees higher
than that used for die upsetting boride magnets. All
die upset magnets discussed here were reduced to 45
percent of their original height (i.e., 55 percent die
upset).
Magnetic measurements of the hot pressed and
die upset magnets were made using a Walker Model MH-5020
hysteresisgraph; the results are summarized in Tables I
and II. X-ray (Cu Ka) diffraction patterns were
obtained for powdered ribbons after annealing for about
30 minutes at 700C.
Surprisingly, at neodymium concentrations
above 14.5 atomic percent with the carbon concentration
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at 6 atomic percent, the coercivity of the hot pressed
magnets decreased sharply compared to similar boride
compositions. The coercivity apparently vanishes
altogether at Nd16Fe78C6 due to the formation of the
phase Nd2Fel7. The major diffraction peaks are easily
accounted for when compared to the calculated pattern
for the 2-17 phase. It is quite possible that the
observed 2-17 phase contained di~solved carbon, as
reported by others studying annealed ingots.
To suppress the formation of the 2-17 phase,
higher concentrations of carbon were tried using the
composition formula Nd16Fe78 yC6+y. With increasing
carbon levels, the coercivity of hot pressed magnets
increased sharply, exceeding 12 kOe for concentrations
at or above 9 percent. Powder X-ray diffraction
patterns for annealed Ndl6Fe75Cg ribbons revealed strong
intensities from the tetragonal 2-14-1 phase with
lattice parameters of a=0.8803 nm and c~1.2010 nm.
Comparing the observed reflections to the calculated
pattern for Nd2Fe14C confirmed that the 2-14-1 phase was
the major phase, but it was still by no means the only
phase present. In addition to the possibility of small
amounts of 2-17, the presence of elemental iron (a-Fe)
was also indicated.
The presence of phases such as a-Fe and 2-17
in these alloys was made more apparent by adjusting the
neodymium concentration while maintaining high carbon
levels of 9 percent and 10 percent. Increasing the
neodymium levels above 16 percent (up to about 17
percent) reduced the coercivity in these hot pressed
magnets, and again the X-ray diffraction patterns of the
annealed ribbons revealed the presence of the 2-17
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phase. Reducing the neodymium levels below 16 percent
(to about 14 percent) also lowered the coercivity, but
this time the decrease can be attributed to -Fe.
The demagnetization properties of our
Nd13.75+XFe80.~5_xc6 and Nd16Fe78_yC~+y are fiummarized
in the following Table I.
ABLE I. The demagnetization properties of hot pressed neodymium-iron-carbon magnets. The
compositions are divided into three groups by carbon levels: 6, 9 and 10 atomicpercent. Neodymium levels ranged from a low of 13.75 atomic percent to a high of
17.5 atomic percent.
Neodymium Iron Carbon Remanence Coercivity En. Product
at% (wt%) at% (wt%) at% (wt%) (kG) (kOe) (MGOe)
13.75 (30-3) 80.25 (68.6) 6.0 (1.1) 7.9 9.0 11.6
14.50 (31.7) 79.50 (67.2) 6.0 (1.1) 6.3 8.6 4-9
15.25 (33.0) 78.75 (65.9) 6.0 (1.1) 4.9 2.8 1.6
16.00 (34.3) 78.00 (64.7) 6.0 (1.1) 3.0 0.2 0.1
13.75 (31.0) 77.25 (67.4) 9.0 (1.7) 6.4 5.2 5.7
14.50 (32.3) 76.50 (66.0) 9.0 (1.7) 7.3 7.8 9.7
15.25 (33.6) 75.75 (64.7) 9.0 (1.7) 7.2 9.2 10.3
16.00 (34.9) 75.00 (63.5) 9.0 (1.6) 7.1 12.0 10.4
16.75 (36.2) 74.25 (62.2) 9.0 (1.6) 4-9 7-5 2.7
17.50 (37.5) 73.50 (60.9) 9.0 (1.6) 3.1 0.7 0.4
13.75 (31.2) 76.25 (66.9) 10 (1.9) 5.9 1.7 2.2
14.50 (32.5) 75.50 (65.6) 10 (1.9) 7.2 7.9 9.3
15.25 (33.9) 74.75 (64.3) 10 (1.9) 7.1 8.9 9.8
16.00 (35.2) 74.00 (63.0) 10 (1.8) 6.6 12.3 8.8
16.75 (36.5) 73.25 (61.7) 10 (1.8) 6.7 13.7 9.0
I r~
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The three hot pressed magnets with the highest
coercivities (2 12 kOe) were die upset using the process
parameters already described (see Table II for
compositions). Demagnetization curves for the three die
upset magnets and their hot pressed precursors appear in
Figure 4; in each case, die upsetting increased the
remanence by just over 40 percent. More importantly,
the coercivity of these die upset magnets was sufficient
to permit much higher energy products (about 18 MGOe to
about 22 M&Oe) than those observed with lower neodymium
and carbon concentrations (see Example 2).
ABLE II. The d- a~netization properties of die upset neodymium-iron-carbon magnets with four
different compositions
Neodymium Iron Carbon Remanence Coercivity En. Product
atX (wt%) atX (wtX) atX (ut%) (kG) (kOe) (MGOe)
13.75 (30-3)80.25 (68.6) 6.0 (1.1) 9.9 4.4 12.7
16.00 (34.9)75.00 (63.5) 9.0 (1.6) 10.2 9.0 22.4
16.00 (35.2)74.00 (63.0) 10 (1.8) 9.4 11.0 18.3
16.75 (36.5) 73.25 (61.7) 10 (1.8) 9.4 9.5 19.0
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In accordance with the practice of our
invention, rapidly solidified compositions of rare earth
elements, iron (or iron and cobalt) and carbon (or
carbon and boron) are hot worked to form fully
densified, fine grained bodies in which the fine grains
are wrought into magnetic alignment such that the body
is magnetically anisotropic. By hot working we mean hot
pressing, hot die upsetting, extrusion, hot isostatic
compaction, rolling and the like so long as the
specified resultant hot worked microstructure is
attained. Generally, if the hot working practice
comprises more than one step, such as the combination of
hot pressing and die upsetting, all steps can be carried
out without an intervening cooling step.
The compositions selected, the rapid
solidification practice and the practice of rapid
solidification and hot working are controlled and
carried out so that the microstructure of the resultant
body consists essentially of the magnetic phase
Re2TM14CxB1 x together with a minor portion of
intergranular material. The hot working aligns the fine
platelet-like grains of the principal phase such that
the c-axes of the grains are aligned and the resultant
body is magnetically anisotropic. The melt spun
(rapidly solidified) material is preferably amorphous or
suitably extremely fine grained such that the average
grain size is no greater than about 40 nm. Following
severe hot working, flattened grains are obtained and it
is preferred that, on the average, their greatest
dimension be no greater than about 1000 nm.
We prefer that the overall composition of our
anisotropic magnets comprise on an atomic percent basis
-
203~63~
.
50 to 90 percent iron, 6 to 20 percent neodymium and/or
praseodymium, and 0.5 to 18 percent carbon or carbon and
boron. Neodymium and/or praseodymium content of 13 to
17 atomic percent and a carbon content of 6 to 12 atomic
percent are especially preferred. Consistent with these
ranges and referring to the formula for the tetragonal
crystal structure RE2TM14CXBl x' RE is neodymium and/or
praseodymium or mixtures of these rare earths with other
rare earths provided that the other rare earths make up
no more than about 40 percent of the total rare earth
content, TM is iron or mixtures of iron with cobalt, and
x has a value in the range of 0.2 to 1Ø Cobalt may
make up about half of the TM content of the alloy.
Our hot worked, anisotropic magnets can be
comminuted to an anisotropic magnetic powder for use in
bonded magnets. The pulverized powder is mixed with an
epoxy resin or other suitable bonding material,
magnetically aliqned, and pressed or molded. This resin
is cured by heating, if appropriate.
While our invention has been described in
terms of certain preferred embodiments thereof, it will
be appreciated that other forms could readily be adapted
by one skilled in the art. Accordingly, the scope of
our invention is intended to be limited only by the
following claims.
