CA1055733A - Reinforced aluminum alloy composite - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A composite reinforced with polycrystalline alumina fibers in a matrix of an aluminum alloy containing 0.5-5.5% by weight of the matrix of lithium is prepared by infiltrating alumina fibers with a molten alloy containing aluminum and 1-8% by weight of lithium for a time sufficient to form a reaction sheath on the fibers of a thickness less than about 15% of the total fiber diameter.
Composites of the invention are useful as structural members in applications that require strong, stiff and light-weight members, such as in aircrafts and missiles, as well as in high temperatures applications such as aircraft engines and turbines.

Description

~55733 BACKGROTJ~D OF THE INVENTION :` ~
t _ .
: This invention relates generally to the rein-forcement of metals with fibers and, more particularly, to the preparation of composites of alumina fibers and -aluminum. -Polycrystalline A1203 fiber has long been con-sidered ideal for reinforcing metals because of its high modulus and strength, chemical inertness and high tempera-ture stability. Aluminum is ideally used in the aerospace, transportation and other industries because of l; its light ~eight, excellent corrosion resistance and low~ -cost. It is the prime candidate for reinforcement with continuous polycrystalline ~-A1203 fibers prepared, for example, as described by Seufert- ln U.S. Patent~No.
3,808,015. The major problem limit1ng the utilization of ~ -A1203-Al composites has been~the lack of a practical method to fabricate them due to the fact that aluminum does not adequately wet alumina. ---Considexable effort has~been expended in attempts to obtaln wetting of the surface~of A1203 fibers with alumi~
num,~for example,by coating the surface of A1203 fibers with metals like Ni, Ti, Cu and the like. However, these metal coating methods are slow, expensive, difficult to scale up and sometimes yield composites with brittle interfaces.~ -Brittle interfaces tend to lower t~e mechanical properties~
such as flexural and shear streng~hs.

- 2 - --~35~733 SUr~MARY OF THE INVENTION
It has now been found that the difficulties noted above can be overcome by i~filtrating polycrystalline alumina fibers with a molten aluminum alloy containing about 1-8% by weight of lithium to form a reinforced composite having a reaction sheath on the fibers of a thickness of less than about 15% of the total fiber diameter and then cooling the composite. In this manner, composites which are substantially free of brittle interfaces and which have'good longitudinal and transverse mechanical propertie~
have been prepared. Such composites contain from about 10-80 volume percent of alumina'fibers, a matrix of an aluminum alloy containing from about 0.5 to about 5.5%
by weight of the matrix of lithium and a reaction sheath ' 15 on the fibers which has a thlckness of less than 15% of - ' '~
the total fiber diameter.
FIGURE
The figure is a photomicrograph of A1203 fibers which are about 22 microns in diameter.

DETAILED DESCRIPTIO~ OF THE INVENTION
The composites of this invention can contain fiber volume fractions within the-range of 10-80 volume percent, preferably about 15-7~/o volume percent. Below about 15 volume percent, there is little practical advantage in terms of strength or modulus. At greater than about 70 ' '' volume percent, the fibers tend to contact each other and each contact point is a stress area from which fractures can emanate. The composite can contain either continuous filaments or discontinuous fibers of polycrystalline alumina. ''"
.
~ 30 As used herein, "continuous filament" denotes ; - 3 _ .
- ....

a fiber having a length about as long as that of the composite as measured in the direction in which the fiber is aligned. Discontinuous fibers have a minimum length of about 0.10 mm, preferably at least 3 mm. When the composite contains substantially continuous filaments, fiber fractions of about 30-60 volume percent are pre-ferred for best fiber distribution and packing in the composite. when the composite contains substantially xandomly oriented discontinuous fibers, about 15-30 .. . . .
- 10 volume percent is the preferred fiber fraction.
The fibers in the composite can be aligned in any direction or directions in which maximum strength or modulus is desired. Such alignment may be parallel, perpendicular or at any other angle with respect to any axis in the composite. The fibers may also be randomly oriented in the composite structure.
Examples 1-4 show unidirectional ~iber-reinforced ;
composites; such composites have highest strength and modulus in the direction of fiber alignment. For some applications, more isotropic properties are desired and can be obtained by - `
using parallel layers (plies) of unidirectionally orlented fibers and crossing the plies at different directions (e.g., 45) to the adjacent ply. More isotropic properties can `
also be obtained by using a random orientation of the dis-continuous fibers throughout the composite but such a fiber distributlon limits the flber loadlng to a maximum of - about 35 volume percent of the composite.
, The fibers employed herein are high modulus, high strength, polycrystalline alumlna fibers. Preferred fibers conta~n at least 6~/o aluminum oxide (A1203) by weight. All ~ , ~ - 4 - ~

, ~ ~
.

~05~733 other things being equal, the mechanical properties of a composite, such as maximum modulus and high temperature resistance, generally increase as the amount of A12O3 in the fiber increases. Accordingly, fibers containing at least 8~/o of A1203, preferably at least 95% A1203 are most preferred. Generally, the most preferred fibers contain A12O3 in the form of alpha alumina. The fibers can be prepared as described by Seufert in U.S. Patent ~o. 3,808,015 and by D'Ambrosio in U.S. Patent ~o. 3~853~68~. The strength of some such alumina fibers is increased by a silica coating having a thickness of about 0.01 to 1 micron (~). Methods ~ -of providing fibers with such silica coatings are described by Tietz and Green in U.S. Patents No. 3,837,891 and

3~849~181~ respectively. Such silica coatings also promote wetting by aluminum-lithium alloys and permit the preparation of high quality composites using alloys with a lower lithium content than would be required with uncoated fibers using the same time and temperature of infiltration.
In addition to being as strong as possible, it 20 is also desirable that the fibers be as dense as possible since a higher degree of fiber strength is retained in the composite as the density of the fiber increases. When fibers are made by the general process described by Seufert, the density of the fibers is increased by conducting the ~5 final sintering (or firing) of uncoated fibers at a temper-.
ature slightly above the temperature at which the uncoated i fiber achieves its maximum tensile strength. Since there is an interrelationship between the speed of the ~lberthroùgh the flame, the type of equipment used, the denier ~, 30 - and number of-the fibers, and so on, the precise temperature .;, - - - : ~

. ~ . .

~55733 to be used in any given case is necessarily determined by the overall balance among the factors which enter into the interrelationship. For example, the sintering o~ a 1700-1800 denier yarn of about 200 fibers at a speed of 60 feet per minute through a fla~e in a chimney at a temperature of about 20-100C. higher than that at which the fiber achieves its maximum tensile strength affords preferred fibers for use in the composites of this inven-tion. while the fiber composition is the same, the higher fired material is more compact and dense and has a differ-ent microstructure as judged by scanning electron micro-graphs. As a result, the higher fired fiber can withstand longer exposure to the aluminum-lithium alloy with substan- ' tial retention of the original fiber strength than the same fiber fired at the lower temperature. Accordingly, while the strength of the fiber going into the composite may be lower than the maximum tensile strength of the fiber as a --result of the higher firing temperature, the strength of the fiber in the composite could be considerably higher than the strength of the maximum tensile fiber since the more dense fiber is not subject to the same degree of attack by the alloy. The preferred fibers may also be coated with silica.
In ascertaining increases in density which~result from the above higher firing treatment, the conventionally employed density measurements are insufficiently sensitive to distinguish the higher from the lower fired structure.
A more sensitive test is required. It has been found that as-the density of the flber increases as a result of the , 30 higher firing temperature, the microstructure changes and : , :
;A~ ' .i; , _, ' - 6 - ~
~- . .
.' ' : - ' ., ' `,...... . ,. , .. , .. ... .. . ... ~'~:

the fiber tends to transmit more light. Utilizing this phenomenon, a dense fiber which results from the higher firing treatment can be distinguished from a less dense ;~
fiber of similar composition and diameter by its trans-S lucency number. The translucency number is an average of determinations on thirty random samples obtained by viewing a length of fiber in air at 600-1200X, preferably 12QOX, using transmitted light. The amount of light transmitted by a fiber is rated in terms of translucency number on a scale of from zero to six, and the accompanying Figure can be used as a standard in rating the fibers. A translucency number of six is given to the most translucent fibèr of a given composition and diameter. Such a fiber displays a - bright band in the center and along the entire length of the fiber. The bright band has a width of about 1/3 the fiber diameter and is contained between two opaque (black) bands, each extending from a fiber edge to the outer edge of the bright center band. As the Figure shows, when the intensity of the light transmitted by the center band decreases, the translucency number lowers. At a trans-lucency number of zero the fiber appears opaque and no center band can be distinguished. The Figure illustrates that the relationship between the opacity of the fiber and -its translucency number is approximately linear. Fibers having a translucency number of 4-5.5 are preferred for -~
this invention. The translucency number can also be ~ -determined on fibers contalning a coating of silica in ~ -the same manner.
Preferred fibers-also have a diameter of between about 15 and 30 ~, a tensile strength of at least 100,000 psi, _ ~,:

..... ::: :~

~055733 preferably greater than 200,000 psi, and a Young's modulus of at least 20 million psi. In addition to Al2O3, the fib~rs can contain other refractoxy oxides and~or refractory oxide systems such as SiO2, MgO, ThO2, zro~, ZrO2-CaO, ZrO2-~lgO~ zro2-sio2~ Ce2O3, Fe2O3, Nio, CoO, Cr203, ~2 Tio2 and the like. Th~se fibers should have a melting point of at least 1000 C. Preferably, the fibers will be ~ I
employed in the form of a tow of continuous alumina fila-ments.

10 ' The composite matrix will contain at least 6~/o 1: ' by weight of aluminum, preferably at least 9~/O~ and 0.5-5.5% by weight of lithium. The concentration of lithium in the matrix is generally lower than its con-centration in the starting alloy since some of the lithium is consumed in orming the reaction sheath around the fibers. Additional losses may occur during fabrication by reaction with the crucible, sublimation and/or oxidation.
A~third component comprising one or more metals capable of being alloyed with'a1umi'num may also be present - in the matrix at a concentration of~up to about 1~/~ by weight of the matrix. Since only a limited number of metals can be alloyed wlth aluminum to produce alloys of ' practical signlficance, the constituents of the aluminum alloys useful as a matrix in the~composites of this inven- ~ ' tion are similarly limited.~ sultable primary~metals~for ; ~- ;
alloying with~aluminum includelcopp2r, iron, magnesium, ~ -"
manganese, nickel, sll~icon, tln, zlnc, titanium snd~the~

~ ~ : . ~ - - . .

~055~733 1 ike and ~ixtu~es thereof and trace amounts of 1% by weight of the matrix or less of secondary metals such as beryllium, bismuth, boron, cadmium, calcium, chromium, cobalt, galliùm, lead, sodium, strontium, vanadium, zir-conium and the like and mixtures thereof.
It will be appreciated that for most applicationsthe matrix in the composite should be ductile. The ductility of the matrix is evidenced by a strain-to-failure of greater than 0.~O for the composite. When continuous alumina filaments are used, the upper limit of strain measured in the direction of filament alignment can be as high as the strain-to-failure of the alumina filaments. When discontinuous alumina fibers (staple fibers) ;
are used, the strain-to-failure of the composite is limited - 15 by the ductility of the matrix, amount of loading, orienta- ;
tion of the fibers and other such considèrations. AIl of the matrices of the examples of the invention are ductile by this definition.
The lithium-aluminum alloys used in this inven-..- . ...
tion chemically wet the fibers, thus providing excellent fiber-matrix bonding and composites which have good high `
temperature performance properties. The preferred com-posites of this invention have longitudlnal short beam shear strengths of at least 7000 psi ~measured at room 25 temperature 25C.). The short beam shear value is a ;~
measure of overall composite quality including the degree of~bonding between the fibers and the matrix, the strength of the matrix and the in situ f1~er strength. Composites having a room temperature short beam shear of less than 7000 psi do not possess best overall composite qualities~
:
_ g ~
. .

The composites of this invention also have a porosity of less than about 10%, preferably 5% or less, and most preferably less than 2%. At a porosity of 1~/~
or more, the composite has lower overall mechanical prop- , erties~ Such composites are not satisfactory for structural applications, for example, since the pores in a composite act as points of stress concentration. At porosities of 10% or more, the stress concentration phenomenon can result in poor fatigue behavior. A porosity of 25% or more is considered evidence that the alloy has not wet the fibers.

The preferred composites of this invention also have a longitudinal modulus at room temperature of at least 15 x 106 psi and a modulus of about 25 x 10 and up to about 45 x 10 psi is most preferred. The more preferred composites have a flexural strength at room temperature equal to or greater than the product of 1900 psi and the volume of fibers in % between the range of 30-60 volume percent. Thus, a composite containing 50 volume percent fibers would have a flexural strength of at least 95,000 ¦
psi.

Method of Preparation ., The composite matrix is prepared from an aluminum alloy containing at least about 600/o~ preferably 90~/0, by weight of the alloy of aluminum and about 1-8~o~
preferably 2-5%, by weight o lithium. Composites having the best mechanical properties are prepared at the preferred concentrations of lithium and aluminum in the alloy melt and the matrix.

- 10 - ' ~ ' ~" ' ': : ' ~ ~ ' ~055733 The composite structure is prepared by infiltra-ting the molten aluminum alloy into A1203 fibers contained in molds. Details of a general procedure for infiltration have been described by Dhingra in U.S. Patent No. 3,828,839.
In this case, excellent composites can be made by infiltrating alumina fibers with an aluminum alloy melt - containing a small amount of lithium under the proper con-ditions of temperature and time of heating. The fiber undergoes a reaction with the lithium in the alloy melt, and it is believed that this reactlon is responsible for the wetting of the fibers by the molten metal and for good fiber-matrix bonding. The reaction forms a sheath around the fiber. At the minim~m useful extent of . I .
reaction, the sheath may not be visible in cross section~.
However, whenever reaction takes place, no matter how slight, the fiber surface becomes black or gray in contrast to its original white-color. The presence of LiA102 has been detected by X-ray analysis on fibers recovered from composites. Therefore, although the sheath may not be visible in cross sections, the fiber can be leached out o~
the composite by dissolving away the matrix, for example, in 2~o aqueous hydrochloric acid and the fact of reaction~
determined from the color change.~ As the amount of reac-tion which has taken place increase~s, the sheath on the 2~ fiber becomes progressively larger and visible while the (apparently) unreacted core becomes smaller. As ~he~ :
sheath grows to about 2~/~ of the fiber diameter, cracks or wedges frequently form. In extreme cases of reaction,~
the fiber core may be broken into several portions.
~ In order to rètain a useful strength in the 1~55733 fi~ers, the reaction sheath in the composite sh~uld have a thickness of less than about 15% of the total dlameter of the reacted fiber (including sheath). If the total diameter of the fiber including the reaction sheath is designated as (dl), then the thickness of the reaction sheath (t) is one half of the difference between dl and the diameter of the unreacted fiber core (d2), and the percent reaction sheath ; is t/dl (100). Accordingly, reaction conditions should be controlled so that a reaction sheath of 15% or more is not obtained. Since the amount of reaction that takes place increases with increasing temperature, increasing reaction time and increasing concentration of lithium in the melt, the interrelationship among these factors must be care-fully controlled. For example, since lithium is a very reactive metal, as the concentration of lithium in the alloy increases, the alloy melt becomes more highly reactive ;~
~ .
with the A1203 fibers. It then becomes necessary to carry out the infiltration of the fibers with the alloy ~ -melts containing higher lithium concentrations at a lower temperature or within a shorter time, or both, than would be used with melts containing a lower concentration of lithium.
Generally, a composit having a reaction sheath of a thickness less than 15% of the total diameter of the fiber can be prepared at temperatures in the range of 25 -100 C. above the melting point of an aluminum alloy containing about l-~/o~ preferably 2-5%, by weight of llthium with a reaction time of less than about fifteen minutes. Satisfactory compositeg can be prepared at 30 shorter reaction times and at a lower temperature ~ -- 12 ~

:: . ~ . . -,. .. . .

~055733 when the alloy contains more than about 5% lithium.
On the other hand, similar composites can be obtained at temperatures as high as 200 C. above the melting point of an alloy containing 2~ by weight of lithium with a short reaction time. Thus, the reaction time, temp~rature and the lithium concentration ln the aluminum alloy melt can be adjusted with respect to onle another as required to achieve a reaction sheath on the fibers in the composite having a thickness of less than 15% of the diameter of the entire cross section of the fiber.
The mechanics of composite preparation in which the ~bove conditions are to be observed may differ depend-ing on the size of the composite to be produced. Small composites (Example 1 herein) have been prepared by 15 inserting the fibers by hand into small unitary molds ¦;
while larger composites (Example 3 herein) have been prepared by in,serting a preform of the fibers in an organic, polymeric matrix into the mold which is then heated to remove the organic polymer, cooled and vibrated. For infiltration, the mold containing the fibers or a tube leading to the mold is inserted into a crucible containing the molten alloy. Stainless steel molds and silicon car-bide crucibles have been found satisfactory. The molten -metal may be from about 25-200 C. above its melting pQint.
Small molds, as described in Example 1 herein, may be inserted directly into the melt and allowed to reach `~ thermal equilibEium while the pre~erred procedure for larger molds is ts~ preheat the mold containing the fibers -before infiltrating. ~ -The longitudinal axis of a mold can vary from - 13 - ~ -:j : :

near horizontal to vertical during infilt~ation depending upon the len~th of the mold. The use of horizontal or near horizontal attitude affords better control of the tempera-ture of the mold and reduces any tendency to distortion and buckling in addition to providing a lower pressure head of the molten metal.
Infiltration of the molten metal into the mold containing the fibers is accomplished by creating a pres-sure differential either by applying a vacuum to the moldor a positive pressure to the metal or a oombination of both. The pressure differential should be sufficient to overcome the resistance to flow caused by the mold and ':~
fibers and the pressure due to the head of the molten metal. Excessive pressures can cause channeling in the mold. For fabricating the alumlna-aluminum composites, a pressure differential of about 2 to 14 pounds/square inch (psi) has been satisfactory.
After the mold containing the fibers is thoroughly infiltrated, it is removed from the molten metal and àllowed to cool to room temperature. The mold can be left on ~s a cladding or it can be removed. Cladded composites are preferred for subsequent rolling, swaging, drawing, hydro- ¦
static extruding or hot isostatic pressing operations.

The p~oducts of this invention are useful as structural members in applications that require light-weight and high stiffness and strength, especially in aircraft and missiles. These products are also useful for structural applications at elevated temperatures such as in aircraft engines and turbine~.
' ~ ,, ~ ', , . ;

, `
' TEST PP~OCEDURES AND STANDARDS
Metalloqraphic Examination To assess the thoroughness of infiltration of metal matrix between the alumina fibers and the extent of reaction between the fibers and the alloy matrix, transverse sections of the composite specimens are exam-ined metallographically. Specimens are mounted in a suitable resin such as phenolform~aldehyde, epoxy or poly-ester and are polished with a series of polishing grits beginning with approximately 100 grit and going down to 0.3 micron (,u) diamond paste.
Fiber/Matrix Reaction _ Metallographic examination of the polished cross sections at a magnification of 600X can be used to determine a reaction sheath having a different appearance from the apparently unreacted core, and the thickness of t~is reaction sheath is reported in the -Examples in microns (u). Those samples in which the :.
~ibers appear to be unreacted are considered to have undergone a minimal amount of reaction with the matrix and the fiber reaction sheath is reported in the Examples as : ~ ~
" < 0.5 ~". That reaction has taken place can be ~ .. --ascertained by extracting the fibers from the composites -:
and noting their black or gray color.
ComPosite Quality .-~ --.
The polished cross-sections described above are examined at about 60X magnification and the porosity of . thè cross-section is obtained by estimating the areas of - voids compared to the total area in the cross-sectlon.
Porosity is caused by faulty techniques of composite ,,- ~ ~ , . ~

~055733 preparation and/or an insufficient wetting of the fibers by the metal. Hence, the degree (%) Gf porosity i~ a useful quality control index.
The porosity can be conveniently and precisely S determined by vacuum depositing aluminum on the above polished cross-sections and then analyzing each cross- ;
section with a reflected light microscope and a Quantimet 720 instrument (an image analyzing computer made by Imanco, ~`
~ew York, N.~. as reported in an article by M. Cole in lD American LaboratorY, June 1971). The cross-section is analyzed by viewing the number of separate and distinct fields of view that approximate the entire cross-section.
The system is operated under conditions that detect voids as small as 2 ~ in diameter. The voids are "seen" as black against the relatively high reflectance of the aluminum-coated fibers and solid matrix.
Fiber Properties ; Single fibers are broken on an Instron tensile testing machine Model TM at a crosshead sp~ed of 0.02 inch per minute using gauga lengths of 0.25 and 10 inches~
Tensile strength is obtained from the 0.25 inch gauge results. The modulus (Young's) is obtained by plotting the reciprocal of the modulus (1/M) obtained at the 2 gauge lengths against the reciprocal of the gauge length ;
(l/G) and using the value of l/M~at l/G (extrapolated~
- of zero to calculate the~reported modulus. This method~
is followed to correct for any fiber slippage in the test. Similax result~ can be obtained with shortcr fibers by using gauge lengths of,~for example, 0.25 and .
1 inch.

~055733 Composite Mechanical Properties In addition to metallographic examination of the composites as described above, mechanical properties are another measure of the quality of the composites. The mechanical properties such as flexural strength and modulus are indicative of mechanical performance of the composites, particularly as structural materials. A
measure of the s~rength, stiffness, and strain-to-failure of the composites is obtained from flexural tests.
Flexural strength, modulus and strain-to-failure are determined using the method of ASTM D-790-71 except that round rods are used instead of rectangular bars.
Short beam shear (S.B.S.) strength values are determined on composites containing randomly orient~d as well as aligned fibers using the method of ASTM D-2344-67.
~ormally, the portions of the specimen remaining after a .: . .
flexural strength determination are used for this test.
Unless stated otherwise, all vaiues in the - -examples are longitudinal, i.e., measured on a bar or rod having the fibers aligned along its length, and are measured at room temperature.
Metal Analysis The alloys and the composites themselves are analyzed for metals by dissolving the~matrix from about a 0.25 gram sample of the composite in-20 milliliters (ml) . . .
-of a mixture of equal volumes of concentrated hydrochloric acld ~35%) and water. The resulting solution is diluted , to 100 ml with water and analyzed in an Atomic Absorption Spectrophotometer ~Perkin-Elmer Model 503). The fibers in the composite do not appear to be affected by the acid.

-- 17 ~--`: :, 1 . ~ ~ . ,.: .:

:~ ' '. . ' .

-` lOS5733 Characterizations obtained by the tests described above and other detailed information concerning the com-posites are set ~orth in the following illustrative examples in which all parts and percentages are by weîght unless otherwise specified. All fibers in the ~xamples are made generally by the procedure outlined in U.S.
Patent 3,808,015.

A lithium-aluminum alloy was made by (1) heating a silicon carbide crucible to 700 C. in a pot furnace, (2) adding sufficient flux (LiCl:LiF, 3:1 by weight) to form an 0.5 inch layer of molten salt, (3) adding 500 grams of commercially pure aluminum shot (99.5 Alcoa), melting and then adding additional flux to totally cover the aluminum, (4) adding small pieces ~0.5 x 0.5 x I.O inch) of a commercial Al-Li alloy with a nominal Li content of 10%~ submerging the alloy pieces and (5) stirring with a stainless steel rod. Additional flux was added to minimize the loss of lithium. The addition of Al-Li alloy was repeated until SOO grams of the alloy~had been added. Analysis of the melt showed 3.~/0 lithlum; the resulting alloy had a melting point of about 637 C.
To make a four inch long composite, four inch lengths of yarn, each containing 95 continuous filaments -~
(average diameter of 23.3 + 4.3 ~) of polycrystalline alumina (nominal tensile strength of 200,000 to 239~ 000 pSiJ
tensile modulus of 50-x 10 ) were used. The filaments contain about 0.2% MgO with the~remainder A1203, predomi-nantly (greater than 9~/0) in the alpha form. The filament~ -were coated with about 0.02-0.2~ thick layer of silica.~ ~ -- 18 - ~
.

~ . .

~L055~33 - The filaments were packed tightly into one end of a 12-inch length of a stainless stèel tube (0.25 inch 0.D. x 0.035 inch wall) to obtain a loading o about 60 volume percent.
The filaments were separated and d:tstributed uniformly across the insid~ diameter of the tube by holding the tube in a vertical position against a vertical rod-type vibrator ;
(Type EI made by A. Go FurChemie - Apparatebau, zuruck).
The upper end of the tube was connected by a Y-connectiOn - and flexible vacuum hose to à u-tube mercury manometer and ~- -to needle valve (closed) in series with a vacuum source. ..

The lower portion of the tube containing the aligned fila- ~.
ments was placed below the surface of the flux and the melt ;
of the alloy at 680-700 C. and held for about one minute for the tube and fibers to reach the melt temperature. T~en, :~... ...
the valve was slowly opened so that the pressure in th~

mold changed from atmospheric to 60-70 cm of mercury`over a period of 2-4 minutes. During this.procedure the melt : ent~æd the mold, infiltrated the fibers and immediately solidified in the mold above the level o~ the melt. The ~ tube was immediately removed from the mslt and.allowed to cool. After removing t~e flux and alloy on the outer surface, the steel tube was machined off and the remaininy composite was centerless ground to a 0.139 + .001 inch . diameter rod four inches in length.
~ 25 Mechanical properties of the ground rods at room . .
temperature are given under item a in Table I. Values : ;
- obtained at 600 F. and 900 F. follow: flexural strengths ..
of 134 x 103 and 105 x 103 psi; moduli (Mi) of 38 x 10 ~ .

and 33 x 10 psi and short beam;shears (S.B.5.) of 14 x 103 and 6.9 x 103 psi, respectively.

19 ~
- ~ : -. .

' - ~ - ,'. :

1~55733 Metallographic examination of polished cross-sections of the composite revealed none or very little of a second phase in the matrix. It may be that a "super saturated" solid solution of lithium in the aluminum matrix was produced.
Samples of the composites were treated with aqueous HCl, the fibers recovered from the acidic solu tion of the matrix and the solution analyzed for lithium.
The analysis'showed l~/o lithium in the matrix against' 3.~/0 in the melt used. It is believed that a considerable amount of the original lithium in the melt is in an acid-insoluble form in the reaction sheath. The recovered fibers were biack instead of the original white color, indicating a reaction with the lithium, and retained ~3~/0 or more of their original tensile strength.

The items of this example show the effect'of lithium content and infiltration temperature and time on composites of Al-Li/polycrystalline alumina fibers.
Following the general procëdure of Example 1, compocites were made with various Al-Li alloys using . :
yarns containing silica coated filaments (B) similar to ~;
those of Example 1 and yarns of uncoated filaments (A)- Yarns of ' items a, b, f, g, k and q in TABLE I contained 95 contin~- '-' ous filaments and the remainder contained 210 cont1nuous filaments of polycrystalline alumina cont`aining about ' ' ' , . :
99.~0 A1~03, predominantly in the alpha form, each having - a diameter of about 23+ 4 ~u~ Fiber codes in Table I '¦ ' indicate nominal tensile strengths of the starting fllaments as follows: '~' '~' "

~ ~ , .

10~;5733 A 180,000 to ~00,000 psi B 200,000 to 239,000 psi B-2 ~40,000 to 260,000 psi At equal fiber loadings, the maximllm possible flexural strength of the composites is directly related to thè
strength and density of the original fibers. All of the fibers had a modulus of about 50 x 10 psi. Three of the composites, items n, o and ~ were made with a matrix of a ternary alloy. Item nwas a ternary alloy of al~uminum (95.~/O)~ lithium (~%) and magnesium (2~3%~; item o was a ternary alloy of aluminum (91.5%), lithium ( 2 . 2%) and zinc-(6.3%); item pwas a-ternary alloy of aluminum (91o/o)~
lithium (4.6%) and silicon (4.4%). All composites con-tained about 50 volume percent of fibers except items a (Example 1) and b which contained about 60 volume percent and item f which contained about 55 volume percent. After infiltration, all composites were removed from the metal bath within three minutes except item c which was held ~
in the bath~for 15 minutes and item h which was held in ~ ~ -20 the bath for 5 mlnutes. ~; ;
- - Items a, b d, e, f, ~, h, k, n, o and represent preferred products of the invention. The extent of fiber reactionW2S minimal and the thickness of the .
,~ fiber reaction sheathwas less than about 2% of the total fiber diameter.
A~usefuI but less~preferred group of composites included items~i, i and~l with a maximum fiber reaction ~ ~sheath thickness~of 3~u (about 13%~of tQtal fiber diameter).
1 ~ ~ Items c and m were comparative examples of t~ ~ 30 composites beyond this inventlon~with fiber reaction~sheath ~::

lOSS733 thicknesses of from ~-8 ~u (17-33% of the reacted fiber diameter of about 24 ~). A comparison of the flexural properties of items c and d shows the adverse effect of a thick reaction sheath caused by too long an exposure time at 700C. Item m had a porosity of greater than lG% on the average and showed the adverse effect of a higher than normal infiltration t:emperature (900C.~.
Two other composites g and r were made similarly to item k from alumina fibers C and D, respectively, (about 50 volume per~ent) and Al-Li alloys containing about 4.8-5.5% lithium. Fiber C was a polycrystalline alumina fiber (diameter about ~3 ,u) made in a manner similar to Example 8 of U.S. Patent No. 3,808,~15 to seufert except that the solid particles in the spin mix (which provide 6C% of the final A1203 in the fiber) consisted of 7P/O of alpha-alumina ¦~

particles (5~/O with an equivalent diameter between 0.2 and 5 ~) and 23% of gamma-alumina particles having a diameter of 0.005 ~ to 0.07 ~. Fiber D (diametèr about 23 ~) wa~
made as was fi~er C except-that the gamma-alumina particles ~ -constituted about 4~O of the solid particles of the spin mix. Fibers C and D were used in the form of yarns con- ¦
taining 95 continuous filaments. ~
The silica coated filaments of this example con- ~ -tain~about 0.19 to 1.9% Sio2 which is equivalent to a silica coating thickness of from about 0.02 to 0.2 ~ for a 23 ~ diameter starting filament.
: ~.

I' :- -~ - 22 1055'733 U~ ~
~, ~ ~ ~) r` ~ ~D 10 ~ !rl N ~ ~ N ~ 11'1 O ~ d' ~ (r~ ~) ~ ~) ~) ~) (~) N ~) ~) ~ N
~ ~ X . ..
U~ ~
d'--I ~ N ~ ~r) ~ ~ N ~ _I _I N N CO
m ,l o ~ ~ ~ ~ ~ ~ ~
. . m ~
U~ ~
~Z 1~ ~ ~ d' ~ ~ X N

1 O O O O' O O O O O O O O O O O
U~ ~4 ' ' ' O
r` ~D O N ~ ~ ~ ~ ~ t~ ~ ~ ~ O
~Z X ~ ~ lS') O

0 0 U') - t'~t I ~ ~1 ~1 I tl~ _I .
I H 0 t~ N N In 10 N N N N N N N CQ ~ N N N N
Oo~ta ~ JV~ V~ a~
~ ~ ~ 0 . .
t Z ~ ` ' ,, , ', .
C , . .
H ~ E- t~ Ll~ u-) ~ Il) 111 ~0 ~) 111 0 U') O CO ~ 0 m~ .. ....... .. ~ . , . . . -~;t H 1:~ ~ 0 0 d' 0 0 0 0 0 _I N O ~) ~ O O O O ~ . . .
~ 14 ~ VV VV\,(VV ~ V V
1~5 o l oo O ' ' O , ...
;t ~ ~ r`
~ 1¢ ~ O 00,0000000 000 0 0 0 0 O' Z C~ ~ tlD O O O O O O O U) O O O O O ' o O o 1 H E-' E~ 1~ I` l` 1~ 1~ Ci~ 1~ 1~ I~ 1~ 1~- -P~N N N ~ t~ N
t'~tI l mH m m ~ m m ~ m m m m m m m m c~ ~
tl~l , : ::
. ' : .:, ;O

H E-l ~ 1~ I` U) n 0 0 U~ ~D 11') ~ ~ ~` ~.D ~D :

~ Nd' ~ ~D : ":
~ H ~ ~ ~) N ID Ir) O ao ~ ~ r~ ~; ~ ~ N . ::
O ~1 ~ N _I N ~1 --I N ~) ~rl O ~ ~J --~ ~ N ~ ~, ~ ~t V~

~ ~ ~ Ql ~ O co ~ In O In 5~ . 0 -~ - - g U~ Irl U~
.~ ~) ~ 10 IQIf~ ~I N5.~ N Ir~ ~ ~J tr~ N ~N ~ p~

H~ R U ~ q~ S ~ X -~ t- ~: O

23 ~~

.~ . . : .
~: ~

1055~33 EX~MPLE 3 Using the preform technique of Example 1 of U.S~
Patent No. 3,828,839 to Dhingra, a yarn of 95 continuQus filaments of polycrystalline alumina containing about 99.~/O
A1203, predominantly (greater than 9C/O) in the alpha form, (nominal tensiIe strength of 150,000 to 179,000 psi; tensile modulus 50 x 106) with a diameter of ahout 23 ~ was made into a tape by winding the yarn on a mandrel, coating the yarn layer with a 5% solution of poly(ethylacrylate) in methyl ethyl ketone, drying in air for about five minutes and repeating the winding and coating. The tape was removed from the mandrel, cut to size and compressed to fit a sectional rectangular mold. The mold of stainless steel had -inside dimensions of 5 x 3.5 x 0.5 inches with a tube attached to one 3.5 inch edge and the other 3.5 inch edge open. The poly(ethylacrylate) was removed ky heating the loaded mold at 600 C. for four hours while drawing air through it. The fibers in the mold were rinsed with acetone, dried and then vibrated.
The fibers in the mold were infiltrated at 710 C. for about 5 minutes with an Al-Li alloy containing

4.~/O Li and having a melting point of about 630 C. ~The mold was removed by machining and~the composite containing 45 volume percent of fibers was dlvided into two 3/16 25 ~ inch thick pieces~which were finish ground to 1~8 inc~
thicknesses. The composite contained 2. ~/o lithium in th~
matrix, a fiber reaction sheath;having a thickness~of~
2.5 ~ (about 11% of the total~fiber~diameter) and had a porosity of < ~/~. Average,~transverse strength and~modulu properties of specimens cut from~the~ above pieces~foll~w~

Test Strength Modulus -6 TemPeratu-re psi-x 10 3 psi x 10 ca. 70 F. 25 14 600 F. 27 13 It was unexpected that the composites would

5 retain these transverse properties at 600 F~ since other ~ -metal/fiber composite systems such as Al/B, Mg/A1203 and . t' Al/C show a significant loss in properties at 600 F.
other test specimens taken from the same com-posite had the following average room temperature - 10 longitudinal flexural properties. strength 79 x 10 psi, modulus 26 x 10 psi and S.B.S. 11 x 10 psi. The lower strength compared to Example 1 is due to the fiber loading (about 45 volume percent), the use of a weaker starting fiber and the greater extent of fiber reaction.
EXAI~PLE 4 "
A mixture of 30~/0 alpha-alumina particles, 41.3%
solid aluminum chlorohydroxide [A12 (OH)5Cl 2. 2H20] ~ O~6% ¦
MgC12- 6H20~ 2. 2% concentrated hydrochloric acid and 25~/o f ¦
waterwas made and concentrated by removal of water to give ~0 a spin dope with a viscosity of about 800 poises at 30C. i` !:
The spin dopewas extruded from a spinneret into a heated spinning column and the precursor fibers are forwarded by 2 feed rolls at about 900 feet/minute and wound up on a collapsible, refractory bobbin. The bobbins ~ e stored in a room at 10~/o relative humidity until fired. The bobbins ~re placed in a cold oven which is then heated to 550C.

.
in about 2 hours, held at 550 C. for 45 minutes and cooled.

- The yarnwas then passed at 60 feet/minute vertically down-ward through a chimney with a ring burner with propane-oxygen flames and fired at an apparent yarn temperat~are ,~ ~ `: ' - 25 - ~

~ , , ~ff~fff~f7 3 3 of about 1555 c. measured with an optical pyrometer with no correction for emissivity. This temperature was above the temperature at which the fiber develops maximum strength. The yarn is designated as (F) below.
A second yarn (E below) was prepared in the same , . :
manner from an equivalent spfin mix lexcept that it was spun on a differant day and in a slightly different chim~ey with the propane-oxygen rat`io adjusted to develop about the maximum tensile strength of the fibers. It is estimated that these firing conditions in the firing unit used for yarn F would have caused an apparent yarn temperature of about 1500-1530 C. A comparison of the two yarns foll fWS: j Yarn E Yarn F ',-- Denier 2040 1790 f 15 Number of continuous filaments 196 198 Average Fiber diameter, p20.0 18.2 Average Tensile stren~th, psi 249,000 196,000 Translucency num~fer 4.3 5.1 Density from scanning electron Denser than microscope view of fractured E ,~
- 20 cross sections Both yarnswere then treated to give them a 0.02 to 0.05~ thlck coating of silica on the fibers.
A lithium-aluminum alloywas made using the If-:
procedure of Example 1 with pure lithium ( 99 . 98%) . i~
Composites (s and t) were made from yarns E and F, raspec~
tively, using the procedure described in Example 1.
Details and results of the product characterizations are - given in the Table II. Both composites contained 60 volume ,, ' percent of fibers.
::
The fibers ~black-) recovered from the composites~ ~
, ~ - 26 ~
, : , ~: .-~ .
: ~ - , . : .

~055~33 after dissolution of the matrix in about 18% hydrochloric acid had tensile strengths of 70 and 1O0D~O of the original fibers for items s and t, respectively. Thus the more :
dense starting fiber of item t gave a stronger composite than item s even though the starting ~iber for t had a lower initial tensile strength.

,',.'' ~:
- 10 , ,.
`"',' .

1 .' ' ,~ , . ~'' .
, ~
~ ~ !

1~5733 ."
u~ In Q~D
I r~
~_~o ~-~
~X ~ ~ .
U~
., m ,lo . . .
. ~q ~ .
U~ Ql I
ZI -~ d' .
HI ~
1 IO o U:~
. . `~.
~O U~ ' , ~, .
~ ~D ~ . .
X ~ X

~ Q .

I H O E~
æO O ~ U~ V V ~" ' ~ ~1 ~ O , . ' ~ ', .~ I ... .
H O ' 1~
¦ H ~ l~i V V
~ . . ' . '' ' .
~0 -.,.
~:~ H ¦ Oo Oo . :

H E~ l O O : .~

Hj li3 ~ ~ ~ ;

oUou ~ ~ :

- ~:1 H ~1 ~ d' :,,~ -, :
~:
X~ o ~055733 For comparative purposes, the translucency numbers for fibers used to make items a, b, c, d, g and j in Table I were 5~1, 5.1, 2.2, 2.2, 5.0 and 2.7, respectively.

A yarn containing 210 continuous filaments (average diameter of 23ju) of silica-coated, polycrystalline alumina (tensile strength 219,000 + 19,000 psi), 9~/O A12O3, predominantly (greater than 90~/~) in the alpha form, ~s cut into about 0.125 inch lengths and dropped into a stainless steel mold (5.75" x 2~75" x 0.50") with one narrow side (.50" x 2.75") sealed except where itwas welded in the middle to a 0.25" OD tube while the opposite narrow side was open. After the addition of each 0.25" layer of fi~er, the fiber ~s gently packed using a 7/16" solid rod. The gentle packing resulted in a randomly oriented loading of the fibers substantially in one plane (the plane parallel to the 0.5" x 2.75"
I
, sides of the mold)~ The final loading of the fiber~as 20 .
volume percent and sufficiently tight so that the fibers did not fall when the moldwas inverted for infiltration.
- The fibers in the moId wereinfiltrated at 680-700 C. with an aluminum alloy containing about 4.~h ~, ..
lithium (melting point- about 640 C.). The mold was ~ , machined off and test,coupons were~sIiced,from the composite~
,~ 25 plate in planes parallel to the 0.5" x 2.75" sides. The~
final composite test coupon dimensions after finish grind~
ing were.l25" x O450 x 2.70" with the plane containing the~
randomly orientecl fibers parallel to the .450" x 2.70" sid~s. , The composite contained 3.7% lithium in the matrix, a ~iber reaction sheath having an average thickness of 1 u and had , - 29 _ - .

- : --:

~55733 a porosity of about 5%.
Average room temperature tensile strength and modu-lus flexural properties of the test coupons were 17,400 psi and 12.5 x 10 psi, respectively. For random fiber orientation in a plane, the strength of the composite is. :
given by - .
= ~M (1 - VF - Vp) + 1/3 ~F VF
where: :

. C = composite strength ~ = flow stress of aluminum matrix at the M fiber fracture strain VF = fiber volume fraction Vp = volume fraction of porosity - ~F = fiber strength The flow stress (~M) of a comparable as-cast Al- 4 wt %
Liwas 6,200 psi. The calculated composlte strength was:
- 19,250 = 6,200 (1 - .20 - 0.05) ~ 1/3 (219,000~ 0.20 ~ . :
Hence, over 907~o of the rule of mixtures was observed. The observed modulus performance represents approximately 23% ~ ~ :
20 stiffness improvement over the unreinforced alloy ~ -` ~10.2 x 106 psi). -Other test specimens taken from the same composite ~ : ::
` had the following average room temperature flexural : strength properties: strength 41 x 10 psi and S.B.S.:5:.4 -:
x 10 psi. The lower strength compared to Example l was due to the lower volume loading (20 volume percent) and~
random orientat1on of the fiber.

` ~ - Other test specimens taken from the same composite i` had the following average flexural properties measured at 600 F.: strength 46 x 10 pSl, modulus~8.4 x 106 psi and ~ 30 ~

S.~.S. 6 x 103 psi.
~ t should be noted that high quality Al-Li alloys cannot ~e practically fabricated in the same manner ~n which most conventional aluminum alloys are prepared since lithium metal has a much lower density, a much lower melting point, a higher vapor pressure than aluminum and oxidizes even at room temperature. Accordingly, it is recommended that the aluminum is melted initially and that the lithium is submerged in the molten aluminum pool. Such a procedure reduces lithium losses due to sublimation.
~uality alloys (i.e., free of oxide inclusions) can be prepared under an inert atmosphere such as argon (nitrogen atmospheres are not preferred since lithium nitride is formed at room temperature) or by the use of a suitable protective layer of flux on the molten metal surface. The use of a flux such as, for example, LiCl:LiF in a 3 to l ratio by weight, is preferred since it is an economical method to prevent oxidation and sublimation of the lithium.
An additional pràctical advantage of the fluxing technique over the use of an inert atmosphere is that the alloys can be prepared using more practical foundry techniques such as skimming away dross formation on the surface of the m~lt and replenishing the flux layer as required. After a high quality Al-Li alloy is prepared and allowed to cool to room .
25 temperature, a high quality melt can be obtained by reheat- -ing the alloy with a protective layer of the flux.
It is to be understood that the foregoing is solely for the purposes of illustration and that, although , the invention has been described in considerable detail herein, variations may be made by those skilled in the :.

., ~ . .
, .
, ~.

1~5733 art without departing from the spi.rit and scope of the invention.

..:
: . ~ :.: ....
. ~
:'-~.
.. ..

: . . : ~ : .... :
- : : .
. . ...

.: .. ..

~,.' - : ' '" .

Claims (33)

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

1. A composite reinforced with 10-80 volume percent of polycrystalline alumina fibers and having a matrix of an aluminum alloy containing about 0.5-5.5%
by weight of lithium in which the fibers have a reaction sheath of a thickness less than 15% of the total fiber diameter and the composite has a porosity of less than about 10%.

2. The composite of Claim 1 wherein the matrix contains at least about 60% by weight of aluminum.

3. The composite of Claim 2 in which the matrix contains at least 90% by weight of aluminum.

4. The composite of Claim 2 wherein the matrix contains up to about 10% by weight of one or more metals capable of being alloyed with aluminum in addition to lithium.

5. The composite of Claim 4 wherein the additional metal is magnesium, zinc or silicon.

6. The composite of Claim 1 wherein the reaction sheath is less than 0.5 micron thick.

7. The composite of Claim 1 having a porosity of 5% or less.

8. The composite of Claim 1 having a porosity of less than 2%.

9. The composite of Claim 1 containing about 15-70 volume percent of polycrystalline alumina fibers.

10. The composite of Claim 1 containing about 30-60 volume percent of continuous filaments of poly-crystalline alumina.

11. The composite of Claim 1 containing about 15-30 volume percent of discontinuous polycrystalline alumina fibers.

12. The composites of Claim 11 wherein the fibers have a minimum length of 0.1 mm.

13. The composite of Claim 1 wherein the poly-crystalline alumina fibers contain at least 60% by weight of Al2O3.

14. The composite of Claim 13 wherein the alumina fibers contain at least 80% by weight of aluminum oxide.

15. The composite of Claim 13 wherein the poly-crystalline alumina fibers contain at least 95% Al2O3.

16. The composite of Claim 13 wherein the Al2O3 is in the form of alpha alumina.

17. The composite of Claim 14 wherein the Al2O3 is in the form of alpha alumina.

18. The composite of Claim 13 wherein the fibers contain refractory oxides, refractory oxide systems or mixtures thereof and have a melting point of at least 1000 °C .

19. The composite of Claim 13 wherein the fibers have a diameter of 15-30 µ, a tensile strength of at least 100,000 psi and a Young's modulus of at least 20 million psi.

20. The composite of Claim 9 having a longitudinal modulus of at least 15 x 106 psi and a flexural strength at least equal to the product of 1900 psi and the volume of the fibers in %.

21. The composite of Claim 9 having a strain-to-failure greater than 0.2%.

22. The composite of Claim 9 having a short beam shear strength of at least 7000 psi.

23. A process comprising the steps of placing polycrystalline alumina fibers in a mold, forming a composite reinforced with 10-80 volume percent of said fibers and having a porosity of less than about 10% by infiltrating the fibers in the mold with a molten aluminum alloy containing about 1-8% by weight of lithium for a time sufficient to form a reaction sheath on the fibers which has a thickness of less than 15% of the total fiber diameter and cooling the composite.

24. The process of Claim 23 wherein the infiltration temperature is 25-100°C. above the melting point of the aluminum alloy and the time is less than about fifteen minutes.

25. The process of Claim 23 wherein the aliminum-lithium alloy contains at least about 60% by weight of aluminum.

26. The process of Claim 25 wherein the aluminum alloy contains at least about 90% by weight of aluminum.

27. The process of Claim 25 wherein the aluminum-lithium alloy contains 2-5% by weight of lithium.

28. The process of Claim 25 wherein the aluminum-lithium alloy contains up to about 10% by weight of one or more metals capable of being alloyed with aluminum.

29. The process of Claim 23 wherein the alumina fibers are coated with a 0.01-1 micron thick coating of silica.

30. The process of Claim 23 wherein the poly-crystalline alumina fibers have a translucency number of 4-5.5

31. The process of Claim 23 wherein the poly-crystalline alumina fibers have a diameter of 15-30 µ, a tensile strength of at least 100,000 psi and a Young's modulus of at least 20 million psi.

32. The process of Claim 23 in which a layer flux is positioned on the surface of the alloy.

33. The process of Claim 32 in which the flux is a 3:1 weight ratio of LiCl:LiF.