Note: Descriptions are shown in the official language in which they were submitted.
117~ 6
Description
Dispersion Strengthened Aluminum Alloys
Technical Field
This invention relates to aluminum alloys processed
by powder metallurgy techniques which can be used to form
articles which have useful mechanical properties at ele-
vated temperatures, at least up to 350C.
Background Art
Attempts have been made in the prior art to provide
improved aluminum alloys by powder metallurgy techniques.
These techniques have employed increased solidification
rates over those rates generally obtained in conventional
casting. However, the solidification rates obtained
have not been sufficiently great to produce useful meta-
stable phases in the limited number of alloy systems which
have been studied.
The following journal articles deal with rapid solid-
ification processing of aluminum alloys;
"Exchange of Experience and Information, Structures
and Properties of Al-Cr and Al-Fe Alloys Prepared
by the Atomization Technique". A.A. Bryukhovets,
N.N. Barbashin, M.G. Stepanova, and I.N. Fridlyander.
Moscow Aviation Technology Institute. Translated
from Poroshkovaya Metallurgiya, No. 1 (85), pp. 108-
2~ 111, January, 1970.
"On Aluminum Alloys with Refractory Elements, Ob-
tained by Granulation" by V.I. Dobatkin and V.I.
Elagin. Sov. J. NonFerrous Metals Aug. 1966, pp 89-
93.
"Fast Freezing by Atomization for Aluminum Alloy
Development" by W. Rostoker, R.P. Dudek, C. Freda
and R.E. Russell. International Journal of Powder
Metallurgy. pp 39-143.
~_4~q~ ~y
28~i
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U.S. patents 4,002,502, 4,127,426, 4,139,400 and
4,1~3,822 all relate to aluminum alloys containing iron
as a ma~or alloy ingredient. U.S. patent 4,127,426 also
describes the rapid solidification of an alloy containing
up to 5% iron.
Disclosure of Invention
It is a major object of this invention to provide
aluminum alloy articles having useful mechanical properties
at temperatures up to at least 350C.
It is another object of this invention to describe
a class of aluminum alloys which may be processed by
powder metallurgy techniques to provide high strength
articles.
Yet another ob~ect of this invention is the descrip-
tion of powder metallurgy processes which may be employed
with a class of aluminum alloys to provide articles with
exceptional mechanical properties at elevated temperatures.
This invention concerns a new class of aluminum al-
loys which are strengthened by a novel precipitate. Pre-
cipitation strengthened aluminum alloys are known in the
prior art. Such alloys are typified by the alloys based
on the aluminum-copper system (such as 2024). In such
a classic precipitation hardening system advantage is
taken of decreasing solid solubility of one element in
another so that a controlled precipitate can be produced
by a thermal treatment. In the case of the aluminum-
copper system the decreasing solid solubility of copper
and aluminum makes possible the development and control
of precipitate particles based on CuA12. Since the solid
solubility of copper and aluminum increases with tempera-
ture, such materials have only limited capability to re- ~
sist stresses at elevated temperatures since the pre-
cipitate phase tends to dissolve at elevated temperatures.
117'~6
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Another class of alloys which is strengthened by particles
are those known as SAP alloys. SAP alloy articles are
produced by powder metallurgy techniques in which aluminum
alloy powder is oxidized and then compacted and severely
cold worked. The result of this treatment is the develop-
ment of a structure containing fine discrete particles
of aluminum oxide. Since aluminum oxide is essentially
insoluble in aluminum, this class of alloys is more
stable at elevated temperatures than are the precipitation
alloys formed by a true precipitation phenomenon.
The present invention concerns a class of alloys
which in some respects combines the advantages of both
types of materials previously described. The invention
aluminum alloys are strengthened by a precipitate based
on iron and one or more refractory elements. Both iron
and the refractory elements have an extremely small solid
solubility i~ aluminum and for most practical purposes
may be said to be insoluble in aluminum. As a consequence
precipitate particles based on iron and the refractory
elements are quite stable in aluminum even at elevated
temperatures. The alloys are prepared by a process which
includes rapid solidification from the melt at rates which
preferably exceed 10 C per second. This rapid solidifi-
cation rate ensures that the precipitate particles, which
form upon solidification from the melt, are fine and uni-
formly dispersed. The short time involved in the solidi-
fication does not permit significant particle growth. If
the solidification-rate is sufficiently high, formation
of amorphous or non crystalline regions rich in iron and
the refractory elements will result. This is a preferred
result since these amorphous regions can be controllably
decomposed through thermal treatment to provide an ex-
ceptionally fine dispersion of precipitate particles.
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Any cooling rate which exceeds about 105 C per
second will provide iron-refxactory metal compounds which
have a non equilibrium metastable structure. In the ex-
treme case the structure will be amorphous while at lower
cooling rates a series of different non equilibrium crys-
talline precipitate structures will occur. It is believed
that the precipitates transform through these different
structures towards the equilibrium structure during ex-
posure at elevated temperatures.
The aluminum alloy powder so produced is compacted
to form a bulk article. A variety of compacting techni-
ques can be used so long as the alloy temperature does
not rise significantly above about 350C for any signifi-
cant length of time.
Other features and advantages will be apparent from
the specification and claims and from the accompanying
drawings which illustrate an em~odiment of the invention.
Brief Description of Drawings
- Figure 1 shows the ultimate tensile strength as afunction of temperature of several conventional aluminum
and titanium alloys and an alloy of the present invention.
Figure 2 shows the yield strength as a function of
temperature for several conventional aluminum and titanium
alloys and an alloy of the present invention.
Figure 3 shows stress rupture properties as a function
of temperature for several conventional aluminum and
titanium alloys and an alloy of the present invention.
Figure 4 shows a photomicrograph of an alloy of the
present invention after exposure at an elevated tempera-
ture.
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Best Mod~ For Carrying Out The Invention
Turning now to the specifics of the invention, the
alloys are based on aluminum and contain from 5 to 15%
iron by weight and from 1 to 5% by weight of at least
one refractory metal selected from the group consisting
of niobium, zirconium, hafnium, titanium, molybdenum,
chromium, tungsten and vanadium and mixtures thereof.
Preferably the refractory metal is present in an amount
of from 15 to 35~ of the iron content. These refractory
elements combine with iron to form a strengthening pre-
cipitate phase based on A13Fe with the refractory metal
partially substituting for some of the iron.
I believe that my invention is in large measure a
discovery of this novel useful strengthening phase and I
am aware that many other elements could be added to this
alloy for a variety of purposes including improved solid
solution strengthening and improved corrosion resistance
without materially affecting the strengthening affect
which is obtained from the novel precipitate of the in-
vention. I therefore broadly describe my invention as
being an aluminum solid solution matrix which may contain
up to 5% by weight of a solid solution strengthening ele-
ment, which also contains from about 5 to about 30 volume
percent of a strengthening precipitate based on iron and
at least one of the aforementioned refractory metals.
These strengthening particles have an average diameter of
less than 500 angstroms and preferably less than 300 ang-
stroms and are typically spaced less than ~000 angstroms
apart.
Such a structure can to my knowledge only be obtain-
ed through a high rate solidification. To obtain such a
structure it is necessary to provide the alloy in a melted
form with a significant amount of super heat and to
solidify this alloy in particulate form at a rate in
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excess of 105 C per second. If the iron and refractory
metal contents are increased, a higher cooling rate will
be necessary to achieve the same non equilibrium structure.
While there are several techniques known which can pro-
duce such rapid solidification rates, these techniquesare mainly suited for laboratory production of small
quantities of material. The technique which I prefer
to use to produce commercial quantities of this material
is known as the RSR technique. This technique employs a
horizontally disposed disk which is spun at a rate of
about 20,000-30,000 rpm while the material to be atomized
is poured on a disk The spinning disk throws the liquid
material off where upon it is cooled by jets of helium gas.
The process is described in U.S. Patents 4,025,249,
4,053,264 and 4,078,873. While this is the preferred pro-
cess, what is important is the cooling rate rather than
the specifics of the process used to obtain the cooling
rate. Another advantage of the preferred process is the
cleanness of the powder which is produced. ALuminum is a
reactive element and it i5 desirable that oxidation of
the powder be minimized or avoided. This requires a clean
processing apparatus and the previously described process
satisfied these needs.
Having produced the material in a particulate form
the material is then compacted to form an article of
useful dimensions. Such compaction may be performed
using a variety of processes known to those skilled in
the metallurgical arts. A necessary condition however,
is that the material not be exposed to temperatures
significantly in excess of 350C for any significant
period of time. Exposures to temperatures in excess of
about 350C will result in an undesirable amount of
coarsening of the strengthening precipitates and a re-
,~
~177'~8F~
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duction in mechanical properties. Compaction techni-
ques which have been successfully employed include ex-
trusion at temperatures of about 300C. Another com-
paction technique which appears practical is dynamic
compaction using a shock wave to bond the powder particles
together without inducing significant temperature rise.
As previously indicated this class of alloys can
display a range of precipitate structures varying from
amorphous to the equilibrium crystal structure. If
extremely high solidification rates have been employed
so that a substantial amount of the amorphous phase
is present, it may be desirable to controllably trans-
form this phase into another more stable crystalline
phase prior to placing the article in service. This may
readily be obtained by heat treating the compacted article
at a tempera_ure between about 50 and 300C for a period
of time sufficient to cause a desired transformation.
- The previously described features of the present in-
vention may be better understood through reference to
the figures. Figures 1, 2 and 3 illustrate the mechan-
ical properties of one specific composition processed
according to the present invention compared with several
exist~ng high strength aluminum alloys and two common
titanium alloys. The compositions of the aluminum alloys
are shown in Table 1 below.
TABLE 1
2014 4.4% Cu, .8~ Si, .8% Mn, .4% Mg
2219 6.3% Cu, .3% Mn, .1% V, .15% Zr
2618 2.3% Cu, 1.6% Mg, 1.0% Ni, 1.1% Fe
7075 5.6% ~n, 1.6% Cu, 2.5% Mg, .3% Cr
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Such titanium and aluminum alloys are commonly used
in applications where high strength and low density are
required. Titanium alloys are in general stronger and
maintain their strength at higher temperatures than do
aluminum alloys. However titanium is much more expensive
than aluminum and there is consequently a great need for
higher strength aluminum alloys, especially those which
can maintain their strength at elevated temperatures. Al-
loys of the present invention bridge the gap in properties
between conventional aluminum alloys and titanium alloys.
For application in rotating machinery where the
stresses imposed on a component are largely the result
of centrifugal force acting on the component, it is not
the absolute strength which is of importance so much as
the ratio of strength to density. Obviously a high
density article will generate greater internal stresses
than an identical article of lesser density. Titani~
alloys are somewhat more dense than alumin~m alloys.
Figures 1, 2 and 3 each contain a dotted line which repre-
sents a theoretical allo~ with the strength/density ratioof a common titanium (Ti-6Al-4V) alloy combined with the
density of a typical aluminum alloy. If an aluminum
alloy could be developed which equaled or exceeded the
properties designated by the dotted line, such an alloy
would be equ;valent to titanium in many respects for high
performance applications, especially in rotating machinery.
One invention alloy composition was prepared and
from this speciic alloy certain mechanical properties
determined. The alloy was a simple one containing 8
weight percent iron, 2 weight percent molybdenum balance
aluminum and was prepared using the previously described
rapid solidification rate process with a cooling rate
in excess of about 106 C per second. The result of this
cooling process was a powder material which was com-
1~77286
pacted and hot extruded to produce a material from which
test samples were machined.
With reference now to Figure 1, the ultimate ten-
sile strength as a ~unction of temperature of several
conventional aluminum and titanium alloys are shown.
Also shown is a curve illustrating the properties of the
previously described Al-8~ Fe-2% Mo alloy as well as a
dotted line showing the ultimate tensile strength of a
theoretical alloy having the same strength/density ratio
as Ti-6%Al-4%~ and the density of aluminum. An aluminum
alloy with this com~ination of strength and density
could be directly substituted for Ti-6Al-4V in rotating
mach nery applications. It can be seen that in terms
of ultimate strength at elevated temperatures the in-
vention alloy is substantially superior to the conven-
tional high strength aluminum alloys. From temperatures
of 100C upwards the invention alloy is stronger than the
prio- art aluminum alloys. At elevated temperatures such
as 290~C the superiority of the invention alloy is notable,
since at 290~ the strongest conventional aluminum alloy
had an ultimate tensile strength of about 20 ksi whereas
the invention alloy has double that strength, 40 ksi.
By way of comparison the theoretical aluminum alloy with
the strength to density ratio of titanium would have an
ultimate tensile strength of 60 ksi. Thus in terms of
ultimate tensile strength as a function of temperature,
the invention alloy bridges the gap between conventional
alloys and titanium alloys.
Figure 2 shows a similar comparison of strength
versus temperature except that the strength parameter
shown i- yield strength (measured at .2% offset). Again
curves are shown for conventional high strength aluminum
and titanium a~loys and a dotted line shows the yield
strength of an alloy having the yield strength to
density ratio of Ti-6Al-4V. In terms of yield
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strength the invention alloy (Al-8Fe-2Mo) is very near
the theoretical alloy and is markedly superior to the
conventional high strength aluminum alloys. A signi-
ficant feature which is evident in Figure 2 is that the
conventional high strength aluminum alloys all have a
significant drop in yield strength in the temperature
range of about 125C and about 250C. The invention
alloy does not show a sharp decrease in yield strength
until a temperature approaching 350C. This is an in-
crease of about 100C in useful operating temperatures
and this is a significant advantage of the material o
- the present invention. The increased softening tem-
perature of the present alloy is indicative of greater
alloy stability.
Figure 3 shows stress rupture properties of various
high strength aluminum and titanium alloys as a function
of temperature. Again, the properties of a theoretical
aluminum alloy with the strength to densitv ratio of
Ti-6%Al-4%V are also shown. The curves shown indicate
the stress re~uired at a given temperature to produce
failure in a sample after 1000 hours of exposure. Again
the invention alloy is shown to be superior to the con-
ventional high strength aluminum alloys.
Figure 4 is a transmission electron micrograph of
the prevîously described aluminum - ~Fe--2Mo material
after exposure at 290C for 4 hours. The significant
feature seen in the photomicrograph is that the pre-
cipitate phase is extremely fine even after exposure for
temperatures and times which would produce substantial
softening in all conventional aluminum alloys. The
precipitate structure is generally seen to be on the
order of lOQ angstroms in size after this treatment.
~L177Z86
The invention alloys also have higher moduli of elas-
ticity than do conventional aluminum alloys. The modulus
of elas~icity relates to the stiffness of the alloy and
high modulus values are desired for structural applications.
Conventional aluminum alloys have modulus values of about
10 x 106 psi and conventional titanium alloys have modulus
values of 14-16 x 106 psi. The measured value for modulus
for the previous described Al-8%Fe-2% Mo alloy is 12.4 x
106 psi. The range of modulus values for the invention
alloys will be from 12-16 x 106 psi.
It should be understood that the invention is not
limited to the particular embodiments shown and described
herein, but that various changes and modifications may be
made without departing from the spirit and scope of this
novel concept as defined by the following claims.
