Note: Descriptions are shown in the official language in which they were submitted.
9 2 4
C-4187
G-4133
ALLOYING LOW-LEVEL ADDITIVES INTO
HOT-WORRED Nd-Fe-B MAGNETS
.
Field of the Invention
This invention relates to permanent magnetic
alloys and a method for making these alloys.
Particularly, this invention relates to permanent
magnet alloys having high room temperature coercivity
and to a method for forming such magnetic alloys
wherein a powdered metal additive is added to rapidly
solidified powders of neodymium, iron and boron.
Background of the Invention
Rapidly solidified neodymium, iron, boron
(Nd-Fe-B) alloys yield high performance, essentially
isotropic, permanent magnet materials whose principal
component is the tetragonal Nd2Fe14B phase. The
ribbons or flakes produced by rapid solidification,
i.e., melt-spinning, may be hot-worked by isostatically
pressing at elevated temperatures to produce fully
dense, or hot-pressed, magnets with essentially the
same magnetic properties as the original ribbons. With
further processing, specifically die-upsetting,
magnetically aligned magnets are produced with
approximately 50 percent higher remanences (Br) and
approximately 200 percent higher energy products
[(BH)maX~ compared to the hot-pressed precursor
material.
The process of magnetic alignment achieved
during die-upsetting has been described as a diffusion
slip mechanism which requires small grain sizes,
approximately 50 nanometers, and a ductile grain
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boundary phase. The combination of small grain size
and a ductile grain boundary phase allows an
orientation of the c-axis of the grains to take place
along the press direction during plastic deformation.
Since the c-axis is also the preferred orientation of
the magnetization, the magnetic properties are enhanced
along the pressed direction of the die-upset magnets.
Larger grains are deleterious to the alloy
since they do not respond as well as small grains to
the strains induced during die-upsetting, and
accordingly remain randomly oriented, lowering the
remanence and energy product of the alloy. In
addition, whether aligned or not, larger grains are
also associated with lower coercivities in these
materials. It is therefore desirable to use lower
processing temperatures and shorter times at those
temperatures to limit grain growth within the alloy
during the hot-working steps.
Another approach to limiting grain growth is
to introduce into the alloy impurities or additives
which collect in the grain boundaries. If the additive
is foreign to the 2-14-1 phase inside the grain it must
migrate with the boundary as the grain grows, resulting
in slower grain boundary movement, and thereby slowing
grain growth.
Although relatively large concentrations,
i.e., approximately 10 atomic percent, of a substituent
are typically required in order to have a measurable
effect on the intrinsic properties of the Nd2Fel4B
phase, much smaller additive levels, i.e.,
approximately 1 atomic percent, may have a substantial
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impact on the hard magnetic properties of a magnet.
This is because the grain boundary phase, which plays a
vital role in grain growth and domain wall pinning
mechanisms, may be preferentially occupied by the
additive creating a locally high concentration of that
additive within the alloy.
Previous work has been performed on the effect
of low-level additives in die-upset Nd-Fe-B magnets,
where the composition of the magnets was given as
Nd14Fe77B8Ml. This previous work concluded that
gallium, wherein M ~ Ga, provided the largest
enhancement of the coercivity, approximately 21.1
kiloOersteds, as compared to the additive-free
composition, wherein M = Fe, which had the lowest
coercivity of approximately 7.6 kiloOersteds. Other
additives have also enhanced the coercivity but to
lesser degrees. However, the remanences reported for
all these magnets were lower than that of the
additive-free magnet, by as much as 15 percent.
At present, the state-of-the-art concludes
that additives in the Nd2Fe14B-type magnets must be
added into the alloy at the initial melting and casting
of the ingot, prior to melt-spinning and hot-working.
However, it would be desirable to introduce the
additive into the magnetic alloy during the
hot-pressing phase, therefore permitting the additive
and its concentration to be adjusted during this final
step. The relatively low temperatures used in
hot-working compared to either melt-spinning or
sintering, probably would help limit the additive to
the neodymium-rich grain boundaries where they would
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most likely affect grain growth and therefore
coercivity.
Thus, what is needed is a method for making
permanent magnetic alloys wherein the additive is
introduced into the alloy prior to the hot-working
steps.
Summary of the Invention
It is an object of the present invention to
provide a Nd2Fe14B-type magnet.
It is a further object of this invention that
such magnet be formed by a method wherein the metal
additive is introduced into the magnet prior to the
hot-working phase.
In accordance with a preferred embodiment of
this invention, these and other objects and advantages
are accomplished as follows.
We are the first to diffusion-alloy a metal
additive into a magnetic alloy during hot-working, thus
permitting the additive and its concentration, and
correspondingly the magnetic properties, to be adjusted
during this final processing step. The relatively low
temperatures used in hot-working, as compared to other
techniques, such as melt-spinning or sintering, helps
limit the additives to the neodymium-rich grain
boundaries where they are most likely to effect grain
growth and thus coercivity. The elemental additives
are introduced into the alloy by first stirring a fine
powder of the additive into the crushed rapidly
solidified ribbons prior to hot-pressing. Pure
elements were used, however it is foreseeable that
compounds may also be used, as well as other techniques
202~9~4
for adding the additive such as plating or spraying
techniques.
Eleven metal elemental additives have been
determined to diffuse thoroughly through the Nd-Fe-B
magnets thereby resulting in an alloy having
homogeneous magnetic properties throughout: cadmium,
copper, gold, iridium, magnesium, nickel, palladium,
platinum, ruthenium, silver and zinc. Other elemental
additives were also tested, however they tended to only
diffuse over short distances (approximately 100
micrometers) and/or react with the Nd-Fe-B matrix to
form intermetallic phases.
A primary inventive feature of this invention
is the diffusion alloying of zinc, in concentrations
ranging from approximately 0.1 weight percent to
approximately 10 weight percent, throughout the Nd-Fe-B
magnets. Two other powdered additives; copper and
nickel, both at approximately 0.5 weight percent, were
also successfully diffusion alloyed into the Nd-Fe-B
alloys with this technique. The resulting magnetic
alloys are characterized by enhanced magnetic
properties as compared to conventionally formed Nd-Fe-s
magnets. For instance, the addition of these
individual elements to the rapidly solidified ribbons
enhanced the coercivity of the alloy by as much as 100
percent when the magnetic alloys were die-upset.
Other objects and advantages of this invention
will be better appreciated from a detailed description
thereof, which follows.
Brief Description of the Drawings
The above and other advantages of this
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invention will become more apparent from the following
description taken in conjunction with the accompanying
drawings wherein:
Figure 1 illustrates various magnetic
S properties in relation to the weight percent zinc in
die-upset Nd-Fe-B magnets;
Figure 2 illustrates the demagnetization
curves for two die-upset magnets, (a) a Nd-Fe-3 alloy
containing approximately 0.5 weight percent zinc and
(b) an additive-free Nd-Fe-B alloy, measured parallel
and perpendicular to the press direction; and
Figure 3 illustrates ~he demagnetization
curves for three die-upset Nd-Fe-B magnets each
containing approximately 0.5 weight percent of an
additive, measured parallel to the press direction.
Detailed Description of the Invention
Crushed ribbon flakes of rapidly solidified
material having an approximate composition of
Nd13 7Fe81 oB5 3 were used as the starting material.
The rapidly solidified ribbons were formed using
conventional techniques wherein first a mixture is
formed of neodymium, iron and boron, then the
constituents are melted to form a homogeneous melt, and
lastly the homogeneous mixture is rapidly quenched at a
rate sufficient to form an alloy having a very fine
crystalline microstructure. Hot-pressed magnets were
formed from these ribbons by heating quickly to about
750-800C in a vacuum and pressing isostatically at
approximately 100 MegaPascals. Die-upset magnets were
produced by pressing these hot-pressed precursors in an
over-sized die at 750C until their original height was
g 2 4
reduced by approximately 60 percent. Graphite dies
were used in both hot-working steps, and boron nitride
was used as a die-wall lubricant.
The magnets were sliced with a high speed
S diamond saw, yielding both (l) cross-sections for
microscopy analysis and (2) 50 milligram cubes for
demagnetization measurements on a vibrating sample
magnetometer (VSM). All samples were premagnetized in
a pulsed field of 120 kiloOersteds (kOe) and then
measured with the VSM in directions parallel and
perpendicular to the pressed direction. A
self-demagnetization factor of one-third was used to
correct for the geometry of the sample. Unless
otherwise indicated, the values given throughout this
specification for remanence (sr)/ coercivity (HCi) and
energy product l(BH)maX] of the magnetic alloy will
always refer to the direction parallel to the pressing.
Densities of the alloys were also measured using the
standard water displacement technique.
The powdered elemental additives used were
characterized by a fine particle size, i.e., less than
about 75 micrameters for zinc, less than about 45
micrometers for the copper and manganese, and less than
about 10 micrometers for the nickel. The powdered
elemental additives were individually added to the
rapidly solidified and crushed Nd-Fe-B ribbons by
weight. Therefore, for example, l weight percent zinc
additive corresponds to a mixture containing about 1
weight percent powdered zinc and 99 weight percent
crushed Nd-Fe-B ribbons.
Die-upset Nd-Fe-B magnets were formed from the
2~2~4
hot-pressed precursors containing the various elemental
additives as described by the method above. The
densities and magnetic properties of die-upset,
zinc-containing magnets are summarized in Table I.
Table I. The density and magnetic properties of
die-upset Nd-Fe-B magnets formed from
hot-pressed Nd-Fe-B precursors containing
diffusion-alloyed zinc. The magnetic
properties were measured parallel and
(perpendicular) to the press direction.
Zinc Density Br (BH)maX H
wt% g/cc kG MGOe kOe
0.0 7.5712.1 ~3.5)30.9 (2.3)7.9 (10.2)
0.1 7.6212.3 (3.4)34.1 (2.1)10.9 ( 9.8)
0.2 7.6012.2 (3.6)33.4 (2.5)14.0 (11.6)
0.5 7.5812.0 (3.6)32.4 (2.2)15.3 (11.2)
0.8 7.5711.9 (3.7)~1.4 (2.6)15.8 (12.6)
1.0 7.6011.7 (4.1)30.6 (3.2)13.6 ~12.8)
2.5 7.5811.5 (3.8)25.6 (2.6)7.4 ( 9.2)
5 0 7.5511.0 (4.2)22.4 (2.7)7.8 ( 7.7)
7.569.2 (3.9)9.7 (0.8)3.7 ( 2.1)
From the results tabulated in Table I, it is
apparent that the optimum amount of zinc additive
within the Nd-Fe-B precursors is about 0.5 to 0.8
weight percent, which corresponds to the results shown
in Figures 1 and 2. Figure 1 illustrates various
magnetic properties versus weight percent zinc in
die-upset Nd-Fe-B magnets. In particular, Figure l(a)
shows coercivity (HCi) vs. weight percent zinc; Figure
l(b) shows remanence (Br) vs. weight percent zinc; and
Figure ltc) shows energy product [(BH)maX] vs. weight
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percent zinc. For comparison purposes, the
corresponding magnetic properties of the zinc-free
Nd-Fe-B magnet are indicated with dashed lines in each
Figure.
As shown in Figures la and lb, for the Nd-Fe-B
magnets having approximately 0.5-0.8 weight percent
zinc, the coercivities of 15.3 and 15.8 kOe
respectively, were double that of the additive-free
magnet, 7.9 kOe. At higher concentrations the gain in
coercivity was reversed, and all magnetic properties
deteriorated markedly with additions of approximately
10 weight percent zinc. The 0.5 weight percent zinc
and zinc-free magnets have essentially the same
remanence, Br = 12 kG, and energy product, (BH)maX =
15 31-32 MGOe.
In addition, as shown in Figure 2, the knee of
the demagnetiæation curve occurred at proportionally
larger reverse fields in the zinc-containing magnets.
Figure 2 illustrates the demagnetization curves for
die-upset Nd-Fe-B magnets. Figure l(a) containing
about 0.5 weight percent zinc, and Figure l(b) being
zinc-free. Measurements were made parallel (par.) and
perpendicular (perp.) to the press direction. Again,
for comparative purposes, a vertical dashed line is
provided corresponding to the parallel direction
coercivity measurement of the 0.5 weight percent
zinc-containing Nd-Fe-B magnet.
Figure 3 illustrates the demagnetization
curves for three different die-upset Nd-Fe-B magnets
each containing 0.5 weight percent of a different
additive: copper (solid line), nickel (dashed line) and
~392~
manganese (dotted line). Measurements were made
parallel to the press direction. As with zinc, the
addition of copper and nickel powders at approximately
0.5 weight percent, also increased the coercivity of
the die-upset Nd-Fe-B magnet, to 14.0 and 12.1 kOe,
respectively. In contrast manganese powder was also
used as an additive, but had no measurable affect on
the coercivity, Hci = 7.6 kOe. The copper-containing
magnet had a larger remanence, sr = 12.7 kG, than
magnets containing zinc, nickel or manganese wherein
the remanence equaled approximately 12 kG. However
this was most likely due to variations in press
conditions and not the additive.
To locate the position of the added elements
within the Nd-Fe-B magnetic alloy, electron microprobe
analysis was used to examine the polished surface of
the hot-worked samples containing approximately 0.5
weight percent zinc, copper, nickel and manganese. It
was determined that nearly all of the zinc powder had
reacted with the ribbon matrix. However, some of the
zinc was present within an inter-ribbon, or grain
boundary phase, with an approximate composition of
Zn4Nd31Fe65. ~he zinc may also have been present in
other less obvious intermetallic phases within the
boundary regions. However most of the zinc diffused
into the ribbons, or grains, themselves. Yet, due to
the small quantity of additive, the ribbons, or grains,
are believed to be primarily made up of the tetragonal
Nd2Fe14B phase.
Copper and nickel diffused throughout the
magnet in a manner similar to zinc. However, the
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diffusion of manganese, approximately 0.5 weight
percent, was limited to a region within 100-200
micrometers of the original grains of powdered
additive. Without the ability to diffuse, manganese
was less able to influence the coercivity of the
magnet.
Zinc levels varied from ribbon to ribbon and
showed a strong correlation with neodymium levels.
Zinc was more concentrated in ribbons which were also
richer in neodymium. The variation in neodymium
concentrations was probably due to production processes
since this pattern was also observed in the zinc-free
magnet. It is presumed that the zinc diffused into the
intergranular boundaries within the ribbons which are
neodymium-rich, and since neodymium-rich ribbons should
have a greater volume percent of this boundary phase, a
greater percentage of the zinc would collect in these
ribbons.
It should be noted that gallium, which has
resulted in the largest coercivity enhancement when
added to an ingot, was difficult to obtain and handle
as a powder because of its low melting temperature.
However, initial tests with a coarse gallium powder
revealed that although it diffused into nearby ribbons,
the bulk of the gallium was tied up as intermetallic
phases, and just as with the manganese, adding the
gallium did not alter significantly the coercivity.
Diffusion alloying has been shown to be an
effective process of introducing low-level additives
into hot-worked Nd-Fe-B magnets. Although similar
coercivities have been previously obtained adding
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elements to the initial ingot, diffusion alloying
during hot-working permits the final chemistry of the
magnet and, more specifically, the grain boundaries to
be determined during the final processing steps.
Elements which diffuse into the matrix, such as zinc,
copper and nickel, enhance the coercivity by as much as
100 percent in die-upset Nd-Fe-B magnetic alloys. The
coercivity was less affected by elements which did not
diffuse readily such as manganese. At optimum levels,
approximately 0.5-0.8 weight percent, the additives did
not diminish the remanence or energy product of the
alloy.
While our invention has been described in
terms of preferred embodiments, it is apparent that
other forms could be adopted by one skilled in the art,
such as by substituting compound powder additives for
elemental powder additives, or by substituting any of
the eleven elements believed to diffuse thoroughly
through the Nd-Fe-B magnetic alloys, i.e., cadmium,
copper, gold, iridium, magnesium, nickel, palladium,
platinum, ruthenium, silver and zinc, or by modifying
the heating and processing temperatures to promote
diffusion within the grain boundaries of the alloy. In
addition, it is foreseeable that other methods may be
used to introduce the additive into the rapidly
solidified Nd-Fe-B alloy, such as by using wet chemical
plating techniques which would result in homogeneous
ionic deposition of the additive on the surface of the
individual ribbons, or by plasma or metal spraying
techniques. Accordingly the scope of our invention is
to be limited only by the following claims.
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