CA1177286A - Dispersion strengthened aluminum alloys - Google Patents

Abstract

Abstract Aluminum alloy compositions and related fabrication techniques are described. Articles made of the composition by the process contain a novel dispersed strengthening phase based on iron and refractory metals. Rapid solidifi-cation techniques are used to assure a fine distribution of this phase. Articles made according to the invention have mechanical properties significantly in excess of those of conventional aluminum alloys.

Description

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 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 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.

1~77~8~

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 1177~6 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~

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.

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 ~ i77~

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 ~77~6 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.

Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A high strength aluminum alloy article consisting essentially of an aluminum solid solution matrix containing a dispersion (present in an amount of about 5 to about 30 volume percent) of strengthening particles, said particles being based on the compound Al3Fe with from about 15% to about 35% of the Fe content being replaced with an element selected from the group consisting of molybdenum and vanadium and mixtures thereof, said particles having an average size of less than 500 angstroms and an average spacing of less than 2,000 angstroms.

2. An aluminum alloy article as in claim 1 in which the average particle size is less than 300 angstroms.

3. An aluminum alloy as in claim 1 in which the refractory element is molybdenum.

4. A method for producing a high strength aluminum alloy article including the steps of:
a. solidifying a molten aluminum alloy which contains 5 to 15 weight percent iron and 1 to 5 weight percent of an element selected from the group consisting of molybdenum and vanadium, and mixtures thereof, at a rate in excess of about 105° C/sec to form a solid particulate:
b. consolidating the particulate into a unitary mass at a temperature below about 350°C.

5. A method as in claim 4 in which the refractory element is present in an amount of from 15 to 35% of the iron content.

6. A method as in claim 4 in which the refractory element is molybdenum.