Note: Descriptions are shown in the official language in which they were submitted.
- 1 -
F~ OF I~E ~TION
The present invention also relates to novel
dispersion strengthened composite metal powders having
an average particle size of less than about 50 microns
and an average grain size within the particle of about
0.05 to 0.6 microns. The powders are produced by
mechanical compositing wherein cryogenic conditions are
used in the milling step.
BACK5RO~ND OE THE INVENTION
Th~re is a great need for metal alloys
having high strength and good ductility which can with-
stand adverse environments, such as corrosion and
carburization, at increasingly higher temperatures and
pressures. The upper operating temperature of conven
tional heat resistant alloys is limited to the tempera-
ture at which second phase particles are substantially
dissolved in the matrix or become severely coaesened.
Above this limiting temperature, the aLloys no longer
exhibit useful strength. One class oE alloys which is
exceptionally promising for such uses are dispe~sion
strengthened alloys obtained by mechanical alloying
techniques. These dispersion strengthened alloys,
especially the oxide dispeesion strengthened alloys,
are a class oE materials containing a substantially
homogen~ous dispersion of fine inert particles, which
alloys can exhibit useful strengt'n up to temperatures
approaching the melting point oE the alloy material.
, ~
-- 2 --
The primary re~uirement of any technique
used to produce dispersion stren~thened metallic
materials is to create a homogeneous dispersion oE a
second (or hard) phase which has the ~ollowing charac-
teristics.
(i) small pacticle size (<sO micron), prefer-
ably oxide particles;
(ii~ low i.nterparl:icle spacing (<200 micron);
(iii) chemically ~table sec~nd phase, [The
negative free energy OL formation
should be as large as possible. The
second phase should not exhibit any
phase transformation within the opera-
tion range oE the alloyl;
(iv) the second phase should be substan-
tially insoluble in the m.etallic
matrix.
Dispersion strengthened alloys are generally
produced by conventional mechanical alloying methods
wherain a mixture oE metal powder and second, oe hard
phase particles are inten~ively dry milled in a high
energy mill, such as the Szeguari attritor. Such a
process is taught in U.S. Patent No. 3,591,362 Evf
producing oxid~ dispersion strengthened alloy.s.
Th~ high
energy milling-causes repeated welding an~ fracturing
of the metallic phase, which.is accompanied by refine-
ment and dispersion o~ the hard phase particles. The
resulting composite powder particles are generally
comprised oE a ~ubstantially homogeneous mixture o~ the
-
.
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metallic components and an adequate dispersion of the
second, or hard phase. The bulk material is then ob-
tained by hot or cold compaction and extrusion to final
shape.
One reason for the lack of general adoption
~of commercial dispersion strengthened alloys, Eor
example oxide dispersion strengthened alloys, by
industry has been the lack oE technically and economi-
cally suitable techniques for obtaining a uniform dis-
persion of fine oxide particles in complex metal
matrices that are free oE microstructural defects and
that can be shaped into desirable orms, such as tubu-
lars. Although research and development on oxide
dispersion strengthened material have continued over
the last two decades, the material has failed to reach
i~s full commercial potential. T~his is because prior
to the present invention, development oE microstructure
during processing which would permit the control of
grain size and grain shape in the alloy product was not
understood. Furth2rmore, there was no explanation of
the for~nation of intrinsic microstructural defects
introduced du~ing processing, such as oxide stringers,
boundary cavities, and porosity.
Oxide stringers consist of elongated patches
of oxides of the constituent metallic elements. These
stringer~ act as planes of weakness across their length
as well as inhibiting the control of grain size and
grain shape during subsequent recrystallization.
Porosity, which includes grain boundary cavities, is
detrimental to dispersion strengthened alloys because
it adversely afEects yield strength, tensile strength,
ductibility, and cr-ep rupture strength.
. ' ~
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Consequently, there is a need in thé art for
methods of producing dispersion strengthened alloy~s
free of such defects as oxide stringers and porosity.
SUMMARY OF THE INVENTION
In accordance with the present invention,
there is provided dispersion strengthened composite
metal ~owders comprised of one or more metals and one
or more refractory compounds which powder is charac-
terized as (a) having the refractory sub-tantially
homogeneously dispersed throughout the metal matrix,
and (b~ being substantially free of oxide scale. Pr2-
ferably the composite powders will have a mean particle
size less thant about 50 micxons and a mean grain size
less than about 0.6 microns.
The metallic constituent may be comprised of
one or more metals which melt at high temperatures
selected from the group consisting of yttrium, silicon
and metals from Groups ~b, 5b, 6b and 8 or one or moce
metais which melts at a lower temperature such as those
selected from Groups lb, 2b except ~g, 3b, 5a, 2a, 3a
except s, and 4a except Si.
The refractory constituent is selected from
the group consisting of refractory oxides, carbides,
nitrides, borides, oxy-nitrides and carbo-nitrides. In
preferred embodiments of the present invention the
refractory constituent is a metal oxide such as thoria,
yttria and 5~1203.3y2o3.
.
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Also provided is a method ~or producing such
composite metal powders which method comprises:
(a) mixing one or more metallic powders with another
powder comprised of one or more refractory compounds
selected Erom the group consisting of refractory
oxides, carbides, nitrides, and boridesj and
(b) milling t'ne powder mixture with a cryogenic
material at a temperature which i5 low enough to
substantially suppress the annihilation oE dislocations
of the powder particles but not so low as to cause all
the StLain energy incorporated into the particles by
milling to be dissipated by Eracture.
In preferred embodiments of the present
invention the temperature is provided by a cryogenic
material such as liquid nitrogen and the metal is
aluminum, nickel or iron base.
BRIEF QESCRIPTION OF THE FIGURES
Figure 1 i3 a theoretical plot oE milling
time versus resulting grain size for an iron base
yttria dispersion strengthened material at various
temperatures.
Figures 2A and 2B are photomicrographs of
iron base yttria dispersion strengthened composite
particles which were removed Erom milling prior to
complete homogenization Figure 2A shows a composite
particle after being milled in research grade argon for
15 hours in accordance with Comparative Example B here-
of and Figure 2B shows a composite particle a~ter being
milled in liquid nitrogen for 5 hours.
....
~ ' , .
-- 6
Figures 3A and 3B are photomicrographs of
iron base yttria dispersion strengthened composite
particles after completion of milling. Figure 3A shows
such a particle after being milled in air for 24 hours
wherein an oxide scale about: 10 microns thick can be
~seen on the outer surface of the particle. Figure 3B
i3 a particle of the iron base alloy after being milled
in liquid nitrogen for 15 hours which evidences the
absence o~ such an oxide scale.
Figures ~A and 4a are photomicrog~aphs of
iron base yttria dispersion strengthened composite
particles after milling and after a 1 hour heat treat-
ment at 1350C, showing the recrystallized grain
structure. Figure 4A shows such a particle after mill-
ing in argon for 24 hours and heat treating and Figure
4B shows such a particle a~ter milling in liquid nitro-
gen foe 15 hours and heat treating. The mean grain
size of the particle milled in liquid nitrogen is finer
than that of a particle milled in argon.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is based on the view
that all defects observed in a mechanically composited
oxide dispersion strengthened product can be traced to
events that take place during the powder milling opera-
tion, that is, the first step in a mechanical alloying
process.
~ s previously discussed, oxide stringers are
elongated patches of oxides of constituent metallic
elements, such as aluminum, chromium, and iron. We
have surprisingly discovered that these oxide stringers
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initiate from oxide scale formed on the particles
during ball milling in air, and even more surprisingly
in industrial grade argon, when such metals as alumi-
num, chromium and iron react with available oxygen to
form external oxide scales on the metal powders during
milling. These scales break during subsequent consoli-
dation and elongate during extrusion to form oxide
stringers. The stringers act: as centers of weakness in
the bulk material as well as serving to inhibit grain
boundary migration during annealing. By doing so, they
interfere with control of grain size and grain shape
during the final thermomechanical treatment steps.
Because mechanical milling of one or more
metals is a process in which initial constituent pow-
ders are repeatedly fractured and cold welded by the
c`ontinuous impacting action of milling elements, consi-
derable strain ene~gy is stored during this operation.
During subsequent reheating prior to extrusion, recry-
stallization of the resulting composite powder occurs.
It is well-known that the grain size produced by recry-
stallization after cold working critically depends on
the degree of cold working. However, there is a lower
limit of work below which recrystallization does not
occur. Inasmuch as the degree of cold work is a mea-
sure of the strain energy stored in the material, we
have found that a decrease in the milling temperature
leads to an increase in the amount of work that can be
stored in the material over a given period of time and
the amount of work that can be stored prior to satura-
tion. Accordingly, a decrease in milling temperature
leads to an increase in the rate of reduction of recry-
stallized grain size as well as a decrease in the grain
size achieved at long milling times, as shown in Figure
1 hereof.
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The production of ultra--fine grains during
recrystallization prior to extrusion serves to alle-
viate the tendency of the material to form grain boun-
dary cavities during extrusion and subsequent working.
We believe the reason for this is that as the grain
size is refined, more and more of the sliding deforma-
tion can be accommodated by diffusional processes in
the vicinity of the grain boundaries. ~s a result, the
concentration of slip within the grains is reduced and
grain boundary concentration of slip bands is propor-
tionally reduced.
The properties of the materials produced by
the practice of the present invention herein include:
substantially homogeneous dispersion of the refractory
(which in the case of the lower melting metals has
never before been produced); freedom from oxide scales
and, therefore, superior strength of products Eormed in
any manner from these materials (e.g. extrusion, com-
paction), and a far greater ability to form extruded
products substantially free of texture under commer-
cially feasible conditions. Oxide scales formed in-
situ which are deleterious are distinguished from
desirable oxide dispersoids which are purposely added
to the material.
~- Types of materials, that is, a single metal
or metal alloys which are of particular interest in the
practice of the present invention are the dispersion
strengthened materials. The term dispersion streng-
thened material as used herein are those materials in
which metallic powders are strengthened with a hard
phase.
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The hard phase, also sometimes referred to
herein as the dispersoid phase, may be refractory
oxides, carbides, nitrides, borides, oxy-nitrides and
carbo-nitrides and the like, of such metals as thorium,
zirconium, hafnium, and titanium. Refractory oxides
suitable for use herein are generally oxides whose
negative free energy of formation of the oxide per gram
atom of oxygen at about 25C is at least about 90,000
calories and whose melting point i5 at least about
1300C. Such oxides, as well as those listed above,
include oxides of silicon, a:Luminum, yttrium, cerium,
uranium, magnesium, calcium, beryllium, and the like.
~lso included are the following mixed oxides of alumi-
num and yttrium: A12O3.2Y2O3 (YAP), A12O3.Y2O3 (YAM),
and 5Al203.3Y2O3 (YAG). Preferred oxides include
thoria, yttria, and YAG, more preferred are yttria and
~AG, and most preferred is YAG.
The amount of dispersoid employed herein
need only be such that it furnishes the desired charac-
teristics in the alloy product. Increasing amounts of
dispersoid generally provides necessary strength but
further increasing amounts may lead to a decrease in
strength. Generally, the amount of dispersoid employed
herein will range from about 0.5 to 25 vol.%, prefer-
ably about 0.5 to lO vol.%, more preferably about 0.5
to 5 vol.%.
~ Prior to the present invention it was not
practical to mechanically alloy the relatively low
melting more malleable metals such as aluminum. This
was so because such metals have a tendency to stick to
the attritor elements and the walls of the mill. By
the practice of thè present invention such metals and
alloys based on such metals may now be successfully
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mechanically alloyed by cryogenic milling to produce
dispersion strengthened composite particles having a
substantially homogeneous dispersion of dispersoid
particles throughout the matrix. For purposes of the
prPsent invention these more malleable metals will be
identified as those metals for which room temperature
~(25C) is the homologous temperature and is between 0.2
and 0~5. Homologous temperature, as used herein is the
absolute temperature expres;sed as a fraction of the
melting te~perature oE the metal. rhat i5, the
homologous temperature (HT~, can be expressed as
HT = RTk
MTk
~here RT is room temperatu~e.and Ml~ is the melting
temperature of any given metal. Non-limiting exaM ples
of such metals include those selected fro.~ ~roups lb,
2b except Hg, 3b, 5at 2a~ 3a except B, and 4a except Si of
the Periodic Table of the Elements. Preferred is alu~inum. The
metals which have a high melting temperature, which are
pre~erred in the practice of the present inven~ion,
have a homologous temperature less than about 0.2 and
include those metals selecte~ from Groups 4a, Sb, 6b,
and 8 of the Periodic Table of the Elements, as well as
alloys based on such metalsu Preferred are Group VIII
metals, more preferred is nickel and iron, ~nd most
preferred is iron. The Periodic TabLe of the Elements
refeered to herein is the table shown on the inside
cover of The Handbook oE Chemistry and Physics, 65th
Edition (1984-1985~ r ~RC Press. High temperature
alloys of particular interest in the practice of the
present invention are the oxide dispersion strengthened
alloys which may contain, by wei~ht; up to 65~, pre-
ferably about 5~ to 30% chromium; up to 8~, preferably
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about 0.5% to 6.5~ titanium; up to about 40% molyb-
denum; up to about 20% niobium; up to about 30~
tantalum; up to about ~0% copper; up to about 2%
vanadium, up to about 15% manganese; up to about 15%
tungsten, up to about 2% carbon, up to about 1%
silicon, up to about 1% boron; up to about 2% zirco-
~nium; up to about 0.5% magnesium; and the balance beingone or more of the metals selected from the group con-
sisting of iron, nickel~and cobalt in an amount being
at least about 25%. The term, based on, when referred
to alloys suitable for u~e in the practice of the
instant invention, means that the metal o~ highest
concentration in the alloy i6 the metal on which the
alloy is based.
In general, the present invention is prac-
ticed by charging a cryogenic material, such as liquid
nitrogen, into a high energy mill containing the mix-
ture oE metal powder and dispersoid particles, thereby
forminy a slurry. The high energy mill also contains
attritive elements, such as metallic or ceramic balls,
which are maintained kinetically in a highly activated
state of relative motion. The milling operation, which
is conducted in the substantial absence of oxygen, is
continued for a time sufficient to: (a) cause the con-
stituents o~ the mixture to comminute and bond, or
weld, together and to co-disseminate throughout the
resulting metal matrix of the product powder, and (b)
to obtain the desired particle size and fine grain
structure upon subsequent recrystallization by heating.
By substantial absence of oxygen, we mean preferably no
oxygen or less than an amount which would cause the
formation of oxide scale on the metallic powders. The
material resulting from this milling operation can be
characterized metallographically by a cohesive internal
structure in which ~the constituents are intimately
: ~ :
united to provide an interdispersion of comminuted
fragments of the starting constituents. The material
produced in accordance with the present invention dif-
fers from material produced from identical constituents
by conventional milling in that the present material is
substantially free of oxide scale and generally has a
smaller average particle and grain size upon subsequent
thermal treatment. For example, the composite powders
based on metals having a homologous temperature of less
than 0.2 produced in accordance with the present inven-
tion have an average size of up to about 50 microns,
and an average grain size of 0.05 to 0.6 microns, pre-
ferably 0.1 to 0.6 microns.
Furthermore, by practice of the present
invention, the time required for complete homogeniza-
tion by milling is substantially reducedO For example,
dispersion strengthened alloy powders prepared in
accordance with the present invention in about 8 hours
show a similar degree of homogeneit~ of chemical compo-
sition to identical alloy powders obtained a~ter
milling for 24 hours at room temperature, although only
under the cryogenic temperatures employed herein can
average grain sizes of less than about 0.6 microns be
achieved.
The term cryogenic temperature as used here-
in means a temperature low enough to substantially
suppress the annihilation of dislocations of the par-
ticles but not so low as to cause all the strain energy
to be dissipated by fracture. Temperatures suitable
for use in the practice of the present invention will
generally range from about -240C to 150C, preferably
from about -1~5C to -195C, more preferably about
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-1~5C. It is to be understood that materials which
are liquid at these cryogenic temperatures are suitable
for use herein.
Non-limiting examples of cryogenic materials
which may be used in the practice of the present inven-
tion include the liquified gases nitrogen (b.p.
-195C), methane (b.p. - 164C), argon (b.p. -185C)~
and krypton (b.p. 152C).
The following examples serve to more fully
describe the present invention. It is understood that
these examples in no way serve to limit the true scope
of this invention, but rather, are presented for
illustrative purposes.
The component metal powders used in the
following examples were purchased from Cerac Inc. who
revealed that: the Cr and Ti powders had been produced
by crushing metal ingots; the ~1 powder had been pro-
duced by gas atomization, the Fe powder had been pro-
duced by an aqueous solution electrolytic technique;
and the Y2O3 particles were produced by precipitation
techniques.
- Comparative Example A
;': :
1500g of a metal powder mixture comprised of
300g Cr, 67.5g ~1~ 15g Ti, 7.5g Y2O3, and lllOg Fe was
charged into a high speed attritor (ball mill) manu-
factured by Union Process Inc., Laboratory Model I-S.
The attritor contained 1/4" diameter steel balls at an
lnitial ratio, by volume, of balls to powder of 20~
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Mill;ng was carried out in air at room
temperature (about 25C) and 50g samples of milled
powder were taken for analysis after 1, 2, 3, 6, 9, 12,
15, 18, 21, 2~, 27, and 30 hours. Of course, the ball
to powder volume ratio increases as samples are with-
drawn. For example, after 30 hours the ball to powder
ratio had increased to about 32:1. Throughout the
milling operation the average ball to powder ratio was
about 25:1.
Each of the samples was mounted in a trans-
parent mounting medium, polished, and examined opti-
cally in a ~,etallograph for particle size and particle
shape. The samples were also examined by scanning
electron microscopy, and X-ray emission spectrometry
for X-ray mapping of Fe, Cr, and Al. Micrographs were
taken of one or more of the resulting composite par-
ticles chosen at random and other micrographs were
taken of particles above average size to show as much
detail as possible. In addition, samples taken after
6, 9, 15, 21 and 30 hours of milling and were encapsu-
lated in quartz-tubes and heat treated under vacuum at
1350C for one hour. Optical and scanning microscopy
as well as x-ray mapping were performed on each sample.
The samples were analyzed as indicated above
for the following: (i) the change in particle size and
shape with milling time, (ii) the change in homogeneity
of the powder particles as a function of milling time,
and (iii) the influence of the degree of milling on the
recrystallization of the alloy powder particles after
heat treatment.
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Results
The morphology of the composite powder
particles after final milling showed relatively large
agglomerates having a mean diameter of about 62 microns
( m). The particle size as a function of milling time
, is shown in Table I below. Metallographic analysis
showed that chemical homogenization was completed after
18 hrs and that further mil'Ling did not produce signi-
ficant further refinement of the particle size, nox an
increase in the degree of homogenization. The grain
size within the particles produced upon heating at
1350C is also shown in Table I below.
It can be seen in Table I that the grain
size decreased with time to 0.8 m after 30 hrs. Again,
no further refinement in grain size was observed with
additional milling. It was ob`served'that the powder
particles after milling had a thin external oxide scale
which was found to be A1203. ~
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- 16 -
TABLE I
POWDER ATTRITION IN AIR
-
ROOM TEMPERATURE (25C)
.
Milling Mean Particle Recrystallized
Time, hr.Diameter,~m Grain Size "~m
190
2 200
3 215
6 173 26
9 144 10
12 112
100 2.5
18 105
21 85 1.0 '
24 79
62 0.8
Comparative Example B
The procedure of Comparative Example A was
followed except the environment during milling was
argon instead of air. The argon employed was research
grade having no more than 2 ppm impurities and contain-
ing about 0. 5 ppm 2
Results
:
Particle sizes observed as a function of
milling time are shown in Table II below. The grain
size obtained after heat treatment at 1350C are shown
in column 2. It can be~seen that the argon environment
had little effect on either the~ particle size~or grain
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size developed on recrysta]lization. The argon atmos-
phere~ however, inhibited oxidation so that the milled
powder particles were relatively free of external oxide
scale. Micrographs and X-ray maps of the par-ticles
after milling were taken and showed no evidence of
higher than average concentration of any of the ele-
ments at the surface of the particles. This, of
course, further evidences the absence of oxide scales
on the surface of the particLes during milling.
TABLE II
POWDER ATTRITION IN ARGON
ROOM TEMPERATURE (25C)
Milling Mean Particle Recrystallized
Time, hr. Diameter,~(M Grain Size,;~m
.. .
3 161
8 105 12
81 3.2
21 71 0.9
56 0.9
Example l
The procedure of the above examples was
followed except the milling was carried out in a liquid
nitrogen slurry and the attritor was modified to permit
a continuous flow of liquid nitrogen so as to maintain
a liquid nitrogen phase in the attritor. Samples were
taken after l, 4, 8, and l5 hours of milling. The
powder particle size and recrystallized grain size are
shown in Table III below.
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- 18 -
TABLE III
POWDER ATTRITION IN LIQUID NITROGEN
Milling Mean Particle Recrystallized
Time, hr. Diameter,l m Grain Size,j~(m
1.0 136
4-0 90 1.1
8.0 25 0.6
0.16
This example illustrates that by milling
under cryogenic condi-tions, powder agglomerates can be
produced of very small particle size and ultra-Eine
grain size.
Example 2
Three additional runs were made by milling a
powder mixture as in the above examples for 5 hours at
various cryogenic temperatures. The first run was
performed in an environment created by continuously
supplying liquid helium which maintained the powder at
a temperature of about -207C. The liquid helium
established a gaseous environment during milling. Run
2 was performed in an environment created by contin-
uously supplying a flow of liquid nitrogen and gaseous
argon to the attritor at such a ratio that the powder
temperature was maintained at about -170C. Run 3 was
performed in an environment created by continuously
supplying a flow of liquid nitrogen and gaseous argon
to the attritor such that the powder temperature was
about -130Co
The powder particle size and the recrystaI-
lized grain size are shown in Table IV below.
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This data shows that neither the temperature
nor the nature of the gas appear to have a significant
influence on the recrystallized grain size as long as
the temperature is low enough to substantially suppress
the annihilation of dislocations of the particles but
not so low as to cause all of the strain energy to be
dissipated by fracture. The particle size, however,
appears to be less refined at: the lowest temperature,
-2007C.
TABLE IV
POWDER ATTRITION AT VARIOUS
CRYOGENIC TEMPERATURES FOR 5 HOURS
Temperature Particle Grain
. C Environment Size~Sm Size~;m
-207 He 100 1.1
-170 N2 + Ar 65 1.2
-130 N2 + ~r 45 .95
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