Fiber-Reinforced Metal Matrix Composites

A circular preform of size 100 mm diameter and 15 mm thickness was prepared from polycrystalline alumina fibers by a hand lay-up technique.

Circular samples (100 mm diameter) were cut from a sheet or mat of essentially-aligned, nominally-continuous, polycrystalline alumina fibers fired at 1300.degree. C. The density, tensile strength and modulus of the fibers were 3.3 g/ml, 2,000 MPa and 300 GPa. The mat had a lateral strength cf 42,500N,/m.sup.2.

The samples of fiber mat were sprayed with an aqueous silica sol in an amount providing a pick-up of silica (dry weight) of about 5% by weight of the fibers. Immediately following the silica application, the sample were sprayed with an aqueous solution of starch and a retention aid available under the trade name "Percol" in an amount to provide a pick-up (dry weight) of 5% starch and 2% "Percol" by weight of the fibers. The starch/"Percol" solution serves to flocculate the silica sol onto the fibers and retain the silica on the fibers.

Impregnated circular samples of the fibers were laid-up by hand in a cylindrical mould such that the fibers in the several layers were aligned in the same direction and the assembly was compressed to a predetermined density corresponding to a predetermined volume fraction of fiber. The assembly was dried in air at approximately 110.degree. C. for about 4 hours and then was fired at 1200.degree. C. for 20 minutes to consolidate the assembly and burn out any organic materials. Using this technique, preforms were produced of fiber volume fractions 0.2 and 0.5 which were designated "Preform A" and "Preform B" respectively.

Two further preforms, designated "Preform C" and "Preform D" of fiber volume fraction 0.2 and 0.5 respectively were produced by the above technique from a mat of essentially-aligned, nominally-continuous polycrystalline alumina fibers fired at 900.degree. C. The density, strength and modulus of the fibers were 2.1 g/ml, 2100 MPa and 210 GPa. The mat had a lateral strength of 35,000N/m.sup.2. In making Preforms C and D the temperature at which the assembly of fibers was fired was 900.degree. C. instead of 1200.degree. C.

MMCs were made from the preforms as follows. Each of the preforms A and B was placed in a die preheated to 500.degree. C. and molten metal at a temperature of 840.degree. C. was poured onto the preform. Each of preforms C and D was preheated at 840.degree. C. in a die and molten metal at 840.degree. C. was poured onto the preform.. The metal was an aluminium alloy available as Al 6061 and of approximate percentage composition 97.95 Al, 1.0 Mg, 0.6 Si, 0.25 Cu, 0.25 Cr.

The molten metal was forced into the preforms under a pressure of 30 MPa applied by a hydraulic ram for a period of 1 minute. The resulting billet (MMC) was demoulded and given a T6 treatment (520.degree. C. for 8 hours solution treatment and 220.degree. C. for 24 hours precipitation treatment). The resulting tempered billet was cooled to room temperature and its properties were measured. The results are shown in Table 1 below.

Four preforms, designated "Preforms A-D", were prepared as described in Example 1.

MMCs were made from the preforms by the squeeze infiltration technique described in Example 1 but using a magnesium alloy, Mg-ZE63 of approximate %age composition 90 Mg, 5.8 Zn, 2.5 rare earth metals and 0.7 Zr, instead of an aluminium alloy. The molten magnesium alloy under a blanket of 2% SF.sub.6 in carbon dioxide and at a temperature of 800.degree. C. was poured onto the preform (preheated at 500.degree. C. in the case of preforms A and B and 800.degree. C. in the case of preforms C and D) and squeezed into the preform under a pressure of 30 MPa applied for 1 minute.

The resulting MMC was demoulded and cooled and its properties were determined and are shown in Table 2.

Fiber tows of length approximately 5-7 cm produced from a blanket of essentially-aligned alumina fibers of mean diameter 3 microns which had been heat-treated in steam and then heated at 950.degree. C. were weighed and laid in layers in the lower half of a mould comprising two half-round members which form a cylinder of diameter 1-1.5 cm when the mould is closed. The mould was closed to compress the fibers, both halves of the mould moving to reduce uneven pressures and dead zones. The mould is open-ended, thereby providing access to the ends of the compressed bundle of fibers. The volume fraction of fibers in the compressed bundle was 0.57 (Example 3).

The mould was turned through 90.degree. so that the fiber bundle was vertical and its lower end was closed and connected to an Edwards 5 single stage vacuum pump. Using a funnel, a liquid methyl methacrylate resin (Modar 835) was poured into the top of the mould whilst vacuum was applied to the bottom of the mould to suck the resin into the mould to impregnate the bundle of fibers. The connection to vacuum was removed and the resin was left to cure for 2 hours at room temperature. The mould was then opened and the resin-bonded fiber preform was removed and finished on a lathe.

The finished preform was fitted into a mild steel tube which was then heated to about 700.degree. C. to burn out the resin and allow the aligned fibers to relax within the tube. The tube was then placed in a squeeze-infiltration machine and infiltrated at 600.degree. C. with a molten aluminium alloy (6061) of approximate composition Al 97.95% Mg 1%:Si 0.6%:Cr 0.25%:Cu 0.25%. The tube was then allowed to cool; the composite was not aged.

In a further experiment (Example 4), a rod-like metal matrix composite was prepared as described above except that the volume fraction of alumina fibers was 0.56 instead of 0.57.

The modulus of the metal matrix composites were:

Ex.3 Modulus--160 GPa

Ex.4 Modulus--154 GPa.

A rod-like metal matrix composite was prepared as described in Example 3 except that the volume fraction of alumina fibers was 0.45 and the fibers were taken from a blanket which had been heated in air at 1300 .degree. C. instead of 950.degree. C.

The modulus of the composite was 151 GPa.

Rod-like metal matrix composites were prepared as described in Example 3 containing the fiber volume fractions shown below together with the properties of the composite.

The density of the composites in Examples 14 and 15 (Mg matrix) was less than 2.0 g/ml. In all Examples the strength and modulus of the composites were as predicted from the corresponding properties of the fibers and the matrix metal.

These Examples illustrate the preparation of metal matrix composites from chopped alumina fibers of mean diameter 3 microns and an alloy (Lital) of approximate percentage composition Al 95.55:Li 2.5: Mg 0.6:Zr 0.12.

Chopped alumina fibers of nominal length 64 microns were blended at room temperature with powdered alloy in a Kenwood food mixer. Isopropanol was added to the mixture in an amount just sufficient to prevent the mixture from "balling" and thus ensure that a shearing action rather than rolling was imparted to the mixture. The isoproPanol was allowed to evaporate from the mixture which was then packed into an aluminium alloy "can" of diameter 7 cm and length 22.5 cm and wall thickness 10 mm. A lid was fitted to the "can" which then was heated at 300.degree. C. for 1.25 hours. The "can" was then extruded at 350.degree. C. through a preheated round die fitted with a 120.degree. tapered ring to provide an extrusion ratio of 10:1.

Three extruded metal matrix composites (Examples 16, 17 and 18) were produced in this way, containing volume fractions of alumina fibers of 0.12, 0.2 and 0.2 respectively. In the third experiment (Example 18) the extrusion ratio was 7:1 rather than 10:1.

In each Example, the modulus of the metal matrix composite, which was not subjected to a subsequent solution treatment, was slightly greater than 100 GPa indicating the drawing of about 200 GPa from the alumina fibers. In each composite at least 95% of the alumina fibers were aligned within 5.degree. of the direction of extrusion of the composite.

Using the procedure described in Example 16, a metal matrix composite was prepared containing a volume fraction of aligned, chopped yttria-stabilized zirconia fibers and titanium metal fines. The metal showed no signs of oxide attack and had not become embrittled.

These Examples illustrate the preparation of bound alumina fiber preforms comprising essentially aligned fibers and suitable for use in the manufacture of metal matrix composites using, for example, the procedure described in ExamPle 14.

A blend of fibers and binders was prepared as follows in the chamber of an extrusion machine and under vacuum. Approximately one half of the total of chopped alumina fibers ("Saffil" RF grade - mean diameter 3 microns, nominal chopped length 160 microns) was mixed with powdered polyvinylalcohol and then silica sol and about one half of the chosen volume of water were added and mixed in. The silica sol was 1030 from Nalfloc Ltd containing 30% by weight silica. Cellulose pulp was then added (Examples 21 and 22), followed by the remainder of the water and finally by the remainder of the chopped alumina fibers. The total mixing time was about 60 minutes to produce a blend of uniform consistency.

The vacuum in the mixing chamber was reduced to 720 mm Hg and the blend of fibers and binders was extruded through a round die. The resulting extrudate was fired at 600.degree. C. to burn off the polyvinylalcohol.

Preforms were prepared to the following formulations:

After firing, the preform had a density of 1.6 gm/ml, and the volume fraction of fibers was 0.48.

After firing, the preform had a density of 0.55 g/ml and the volume fraction of alumina fibers was 0.17.

After firing, the preform had a density of 1.0 g/ml and the volume fraction of alumina fibers was 0.3.

Circular samples of diameter 100 mm were cut from a mat of aligned alumina fibers and assembled in a circular vacuum-infiltratior mould (diameter 100 mm) with the fibers in all the layers being aligned in the same general direction. The thickness of the fiber assembly was built up to a level at which compression to 15 mm thickness would yield a preform cf density 1.2 g/ml. The assembly was then infiltrated with a dilute solution of silica sol (1030W silica sol) containing 30% by weight silica to achieve a pick-up of 5% by weight of silica based on the weight of the fibers. The silica was precipitated onto the fibers by passing through the assembly firstly a 2.5% starch solution and secondly a 0.5% solution of a floculating agent (Percol 292). The assembly was then compressed to a thickness of 15 mm in a press and allowed to dry overnight at about 110.degree. C. to yield a silica-bound preform.

A rectangular sample cut from the preform was boxed in an open-ended rectangular box and heated to 750.degree. C. to burn out any organic material. The boxed preform (at 750.degree. C.) was placed in a casting die preheated to 300.degree. C. and squeeze-infiltrated with an aluminium alloy (LMlO containing 10% magnesium) at 820.degree. C. using a pressure of 30 MPa applied by a ram assembly preheated to 350.degree. C. The resulting MMC was demoulded and surplus aluminium was removed by machining. The (boxed) MMC was cut into rectangular bars and its tensile strength and modulus were determined.

For purposes of comparison an MMC was made by the above procedure from a mat of randomly-orientated, short (up to 5 cm) alumina fibers of mean diameter 3 microns. In order to avoid damaging the fibers on compression, the volume fraction of fibers was limited to 20%.

Using the extrusion technique described in Example 16, an MMC was made from chopped alumina fibers and a powdered aluminum alloy (Atomised 6061). The volume fraction of the fibers was 20% and the MMC was subjected to a T6 treatment.

For purpose of comparison, an MMC containing 20% volume fraction fibers was made by hot-pressing a mixture of chopped alumina fibers and powdered alloy (Atomised 6061). The MMC in which the fibers were randomly orientated, was subjected to a T6 treatment.