WO1990002824A1

WO1990002824A1 - Reinforced composite material

Info

Publication number: WO1990002824A1
Application number: PCT/DK1989/000204
Authority: WO
Inventors: Niels Hansen; Dorte Juul Jensen; Hans Lilholt; Yi Lin Liu; Palle Nielsen
Original assignee: Forskningscenter Risø
Priority date: 1988-09-02
Filing date: 1989-09-01
Publication date: 1990-03-22
Also published as: DK490388D0; AU4305689A

Abstract

A reinforced aluminium or aluminium alloy composite material having a matrix of aluminium or aluminium alloy grains and comprising 0.1-10 % by volume of small refractory particles which are substantially homogeneously distributed in the matrix and which have a mean diameter of at the most about 1 νm, and 1-50 % by volume of high temperature resistant reinforcing fibres and/or particles having at least one dimension of at least about 1-2 νm, and a method of preparing same. The small refractory particles especially comprise Al2O3, and the reinforcing fibres and/or particles especially comprise SiO or Al2O3. The composite material has excellent properties with respect to strength and elasticity, notably at high temperatures.

Description

REINFORCED COMPOSITE MATERIAL

FIELD OF THE INVENTION

The present invention relates to a high strength reinforced aluminium or aluminium alloy composite material comprising aluminium or an aluminium alloy, small particles, and reinforcing fibres and/or particles, the material being especially useful at high temperatures.

TECHNICAL BACKGROUND

Metal matrix composites are materials which comprise a metallic phase consisting of a matrix of a metal or an alloy, and up to about 50% by volume of strong reinforcing fibres and/or particles. The reinforcing fibres or particles comprise materials which are significantly stronger than the metal or alloy of the matrix, and the metal matrix is thereby strengthened according to the strength of the reinforcing fibres or particles and the amount of reinforcing fibres or particles included in the composite material. The fibres or particles normally have a diameter of about 0.1-5 μm and a ratio between length and diameter of, for example, about 1-100. These materials are primarily fabricated by powder metallurgical techniques, fusion metallurgy or diffusion bonding, followed by secondary fabrication involving conventional metal-working processes such as extrusion, cold-rolling or drawing.

The development of strong matrix metal composites has generally involved optimization of the properties and distribution of the reinforcing fibres or particles, as well as strengthening of the alloy matrix of the material, for example by altering the composition of the alloy. The microstructure of the metallic matrix is, however, also an important factor in the strength of the finished material, especially when the processing of the composite material involves thermomechanical treatment. Such microstructural features include the concentration and arrangement of dislocations, the grain size and the crystallographic texture. It is known that the grain size in a metal matrix composite, as well as the strength of the resulting material, is influenced by the size of the particles in the material.

Nucleation in metals, i.e. initiation of crystallization, the crystallization typically being obtained by subjecting the material to a heat treatment ("recrystallization") , is associated with sites where large misorientations exist, such as grain boundaries, deformation bands, shear bands and deformation zones at large particles. Incorporating relatively large particles, such as SiC (silicon carbide) fibres, into a composite material results in an increase in the number of nucleation sites, and therefore in a greater number of crystals in the material after recrystallization. The addition of fibres and/or particles with a diameter of more than about 1 μm, i.e. of a size normally employed in the manufacture of fiber reinforced composite materials, thus leads to the formation of small grains in the material upon recrystallization. This is a disadvantage for materials designed for high temperature use, since smaller grain size is associated with reduced strength under high temperature conditions. Therefore, it is desirable that the size of the grains in fibre reinforced composite materials designed for high temperature use can be increased, so that the full advantage of adding the reinforcing fibres and/or particles to the material can be attained in the finished product.

The strength of the metal matrix is also a determining factor for the strength of the reinforced composite material. In order to achieve high strength at high temperatures, it is necessary that the matrix itself have sufficient strength at such temperatures. Only thereby will it be possible to transfer the stress from the matrix to the fibres. As explained below, it has now been found that such strengthening of the matrix can be achieved by adding small stable particles to the metal or alloy of the matrix. BRIEF DISCLOSURE OF THE INVENTION

It has now been found that in alloys with both small and large particles, the two types of particles will cooperate to give a net result in the finished material which can be carefully controlled and which depends on parameters such as the size and distribution of the particles, the initial grain size and the degree and mode of deformation. More importantly, it has furthermore been found that these principles can be utilized such that a reinforced aluminium or aluminium alloy composite material having excellent properties with respect to strength and elasticity, notably at high temperatures, can be obtained by preparing a composite material comprising controlled amounts of both relatively small refractory particles and relatively large reinforcing fibres and/or particles.

Accordingly, in one aspect, the present invention relates to a reinforced aluminium or aluminium alloy composite material having a matrix of aluminium or aluminium alloy grains and comprising

0.1-10% by volume of small refractory particles which are substantially homogeneously distributed in the matrix and which have a mean diameter of at the most about 1 μm, and

1-50% by volume of high temperature resistant reinforcing fibres and/or particles having at least one dimension of at least about 1-2 μm.

In a further aspect, the present invention relates to the preparation of a reinforced aluminium or aluminium alloy composite material.

Thus, the reinforced aluminium or aluminium alloy composite material may be prepared by mixing aluminium powder and/or aluminium alloy powder with:

1-50% by volume of high temperature resistant reinforcing fibres and/or particles having at least one dimension of at least about 1-2 μm, and 0.1-10% by volume of small refractory particles having a mean diameter of at the most about 1 μm, the small particles being present on the surface of the aluminium powder and/or aluminium alloy powder and/or at least part of the small particles being added separately as a powder,

so as to obtain a substantially homogeneous distribution of at least the small particles, and compacting the mixture to obtain a sub¬ stantially dense matrix of aluminium or aluminium alloy grains.

Furthermore, the composite material may be prepared by incorporating 1-50% by volume of higT temperature resistant reinforcing fibres and/or particles having at least one dimension of at least about 1-2 μm, and 0.1-10% by volume of small refractory particles having a mean diameter of at the most about 1 μm into an aluminium or aluminium alloy melt and allowing the melt to solidify.

The composite material may additionally be prepared by incorporating 1-50% by volume of high temperature resistant reinforcing fibres and/or particles having at least one dimension of at least about 1-2 μm, and 0.1-10% by volume of small refractory particles having a mean diameter of at the most about 1 μm into a gas phase containing aluminium or aluminium alloy particles in a solid or semi-solid state, so as to obtain a substantially homogeneous distribution of at least the small particles, and allowing the resulting mixture to solidify as a solid or precipitate as a powder, and compacting the mixture to obtain a substantially dense matrix of aluminium or aluminium alloy grains.

The reinforced composite material according to the invention is useful for all applications where a lightweight material with excellent strength and elasticity is desired. The material will typically have a large grain size attained through recrystallization, and it is therefore especially useful under high temperature conditions, e.g. at a temperature in the range of about 300"C to about 600°C, for example in the aerospace industry and in the manufacture of internal combustion engines, turbines, compressors, etc. The reinforced composite material may also be used advantageously even without being subjected to recrystallization, since the novel combination of both small refractory particles and larger reinforcing fibres and/or particles lend it excellent strength characteristics over a broad range of temperature conditions.

It was known from non-reinforced aluminium and aluminium alloys that the addition of particles having a size of not more than about 0.1-1 μm will tend to reduce the number of nucleation sites and thereby lead to the formation of recrystallized grains of a larger size, and will tend to have a beneficial effect on the strength of metal matrix materials at high temperatures. It was also known that the kinetics of recrystallization are strongly affected by the dispersion of particles, and that small and closely spaced particles inhibit recrystallization, while large and widely spaced particles promote recrystallization, and that small particles of, for example, I2O3, retard recrystallization in aluminium or aluminium alloy materials when the small particles are homogeneously distributed.

However, the combination of the known fibre reinforcement of aluminium or aluminium alloys and the incorporation of substantially homogeneously distributed small particles, with the surprising result that the combination of the two components has a cooperative effect with respect to beneficial metal matrix properties, has not been described. While it might on the onset seem that these two principles would counteract each other, it has surprisingly been found that they in fact complement each other.

As it appears from the following detailed description of the invention, this combination according to the invention has been found to provide hitherto unknown beneficial increases in the quality of aluminium and aluminium alloy composite materials.

DETAILED DESCRIPTION OF THE INVENTION

The term "reinforced composite material" or "composite material" in the context of the present invention is understood to mean a material having a matrix of aluminium or an aluminium alloy and additionally comprising 0.1-10% by volume of small refractory particles and 1-50% by volume of high temperature resistant reinforcing fibres and/or particles having at least one dimension of at least about 1-2 μm distributed in the matrix.

The matrix, which comprises from about 40% to about 99% of the volume of the composite material of the invention, may comprise aluminium or an aluminium alloy. The alloy may be of any type which is normally employed in the production of high strength aluminium alloy materials, or which is suitable for use in such materials (see for example "Metals H dbook, Ninth Edition, Volume 2 , Properties and Selection: Nonferro s Alloys .and Pure Metals" , American Society for Metals, Metals Park, Ohio, 1979). The alloy is especially of a type which is suitable for use in the production of high temperature resistant aluminium alloy materials. Such an alloy will typically comprise a variety of different elements, and will normally include about 0-20% by weight of Mg, Mn, Cu, Zn and/or Li. The alloy may additionally comprise Si, Fe, Co, Ni and/or Cr.

The term "small particles" or "small refractory particles" as used in the context of the invention refers to high temperature resistant particles having a mean diameter of at the most about 1 μm, typically less than about 0.5 μm, preferably less than about 0.2 μm, especially less than about 0.1 μm, such as less than 0.01 μm. The small particles may comprise oxides, carbides, nitrides, suicides, borides and/or metals. Typical examples of materials which are suitable for use in the small particles of the invention are I2O3, SiC, SiN, TiC, ZrC, WC, NbC, TiN, BN, Si₃N₄, MgO, Si0₂, Zr0₂, Fe₂0₃, CuO, Y2O3, A1N, steel and graphite. A preferred material for the small particles is I2O3. The small particles can be substantially spherical, plate or disc shaped or irregular.

The term "reinforcing fibres and/or particles" refers to high temperature resistant fibres or particles having at least one dimension of at least about 1-2 μm, and having a diameter of about 0.1-100 μm, for example about 0.2-10 μm, preferably about 0.5-5 μm, typically about 0.5-2 μm, in particular about 0.5-1 μm, and a ratio between length and diameter of about 1-100, in particular about 1-20, typically about 1-10, for example about 1-5. Thus, the shape of these particles can vary from being more or less substantially spherical to being quite long in comparison to the diameter, i.e. having a fibrous shape in the latter case. They can additionally be in the shape of platelets or discs, or they can have an irregular shape.

The reinforcing fibres and/or particles are present in the composite material in an amount of about 1-50% by volume, e.g. about 2-40%, typically about 5-35%, preferably about 10-30%, more preferably about 15-25%, for example about 18-22%. A minimum content of about 1% by volume is thought to be necessary to obtain the desired reinforcing effect of the fibres and/or particles, and a content of the fibres and/or particles of more than about 50% by volume would tend to have an adverse effect on the properties of the composite material, for example its ability to be formed into the desired shape.

The reinforcing fibres and/or particles may comprise any suitable material which increases the strength of the matrix, and which has a melting point which is sufficiently high to ensure that the fibres and/or particles will not be adversely affected by the high temperatures to which the composite material is to be subjected. They preferably comprise carbides, oxides, nitrides, suicides, borides and/or metal fibres. Typical examples of materials which are suitable for use in the reinforcing fibres and/or particles of the invention are SiC, Al₂0₃, SiN, TiC, ZrC, WC, NbC, TiN, BN, Si₃N₄, MgO, Si0₂, Zr0₂, F^β2θ3, CuO, Y2°3- ^A-***^- ^stee**- ^a~rι- graphite. Preferred materials for the reinforcing fibres and/or particles are SiC and AI2O3, as composite materials reinforced with either SiC or AI2O3 can be readily shaped and machined using conventional metal fabrication techniques, and also due to the fact that SiC and AI2O3 are readily commercially available.

The small particles are present in the composite material in an amount of about 0.1-10% by volume. The exact amount of small particles to be used in the material will of course vary, depending on the composition and intended use of the material, and it will also depend on the amount of reinforcing fibres and/or particles in the material. As mentioned above, in alloys with both small and large particles, the two types of particles will have complementary effects, and the net result with respect to recrystallization will depend on, among other things, the size and distribution of the particles. Therefore, an increased proportion of reinforcing fibres and/or particles will generally require that the material includes an increased proportion of small particles.

Thus, a content of reinforcing fibres and/or particles of, for example, 1% by volume will require a small amount of small particles, e.g. about 0.1-0.5% by volume. On the other hand, a content of reinforcing fibres and/or particles of, for example, 50% by volume will require a large amount of small particles, e.g. about 5-10% by volume. Similarly, a content of reinforcing fibres and/or particles of 30% by volume will require a small particle content of e.g. about 4-8% by volume, such as about 6%. In an experiment with a composite material having a content of reinforcing SiC fibres of 2% by volume, good results were obtained with a content of AI2O3 small particles of 0.8% by volume (see Example 1). It is understood that the above ranges are non-limiting examples, and that the small particle content in each case will be determined on the basis of the content of reinforcing fibres and/or particles and the intended use of the material.

A preferred embodiment of the invention is a reinforced composite material which comprises, as the reinforcing fibres and/or particles, SiC fibres and/or particles having a diameter of about 0.1-1 μm and a ratio between length and diameter of about 1-20, for example about 1-10, in an amount of about 10-25% by volume, especially about 15-20%, and as the small refractory particles, AI2O3 particles having a mean diameter of less than about 0.2 μm, preferably less than about 0.1 μm, such as less than about 0.01 μm, in an amount of about 2-5% by volume, especially about 3-4%.

In order to obtain the desired effect, it is important that the small particles be substantially homogeneously distributed in the matrix, as the effect of the small particles is dependent not only upon the amount of small particles in the material, but also upon their spacing, a more homogeneous distribution giving a better effect with respect to improving the high temperature strength of the material.

The composite material according to the invention is typically recrystallized, such that the aluminium or aluminium alloy grains in the matrix obtain a diameter of at least about 1 μm, typically at least about 5 μm, and preferably as large as possible. As explained above, the grain size is a factor of critical importance with respect to the strength of the composite material under high temperature conditions. By the process of recrystallization, which typically comprises deformation and heat treatment, the relatively small subgrains, i.e. typically about 0.5-2 μm, in the matrix coalesce and grow to form grains which are significantly larger, thus giving the recrystallized composite material improved heat resistant properties. The recrystallized grains typically have a diameter of at least about 10 μm, in particular at least about 50 μm, such as at least about 100 μm, preferably at least about 200 μm, more preferably at least about 500 μm, most preferably at least about 1 mm or even at least about 5 mm. The grains can after recrystallization be more or less cubic or spherical in shape, or they can be oblong, and they can have a cross section which may be more or less rectangular or circular or possibly irregular. They can have a length/diameter ratio of between about 1 and 50, typically between about 1 and 20, for example between about 1 and 10.

While the composite material according to the invention is typically recrystallized, so as to obtain the above-mentioned advantage with respect to high temperature strength, it is also contemplated that the composite material may also advantageously be prepared and utilized without recrystallization. It has been shown that the novel combination of a reinforced composite material comprising both large and small particles can have an additive effect which is independent of the recrystallization process. In such a material, the relatively large reinforcing fibres and/or particles provide high strength under conditions of low temperature, e.g. at room temperature, while the substantially homogeneously distributed small refractory particles contribute to high strength under high temperature conditions. The contribution of the small particles to the strength of the material involves a direct effect due to an increase in the matrix strength as well as an indirect effect, in that a strong matrix is necessary to ensure transfer of stress from the matrix to the reinforcing fibres. Thus, to obtain a beneficial effect of the reinforcing fibres at high temperatures, a strong matrix is required. This is obtained by the introduction of the small refractory particles into the matrix. This additive effect, which is a result of the interaction and complemen¬ tary effects of the large and small particles, thus results in reinforced composite materials with improved strength characteristics over a broad range of temperatures in non-recrystallized reinforced composite materials according to the invention.

As mentioned above, the reinforced composite material of the_. invention may be prepared using powder metallurgical techniques, fusion metallurgy or diffusion bonding, i.e. conventional processes for metal fabrication. »

More specifically, the reinforced composite material may, as explained above, be prepared by mixing aluminium or aluminium alloy powder with 1-50% by volume of reinforcing fibres and/or particles having at least one dimension of at least about 1-2 μm and 1-10% by volume of small refractory particles having a mean diameter of at the most about 1 μm, followed by compaction.

The small particles may be incorporated into the material by adding them directly, i.e. as a powder, or they may be added as particles on the surface of the aluminium or aluminium alloy powder particles, e.g. as surface particles of AI2O3. In the first case, i.e. when the powder comprising the small particles is added directly to the aluminium or aluminium alloy powder, this can for example be accomplished according to the method disclosed in U.K. Patent Specification No. 977,245. Of course, a combination of these methods of adding the small particles may be employed. The aluminium or aluminium alloy matrix may, for example, be provided by mixing aluminium powder and/or aluminium alloy powder with 1-50% by volume of high temperature resistant reinforcing fibres and/or particles, the small particles being present on the surface of the aluminium powder and/or aluminium alloy powder and/or at least part of the small particles being added separately as a powder, so as to obtain a substantially homogeneous distribution of at least the small particles.

In the case of the small particles being present on the surface of the aluminium or aluminium alloy powder particles, i.e. as a surface oxide, the concentration of the small particles can be varied by varying the size of the aluminium or aluminium alloy particles. A reduction in the size of the aluminium or aluminium alloy particles will thus result in an increase in the composite materials content of small particles, since the surface area of the aluminium or aluminium alloy particles will be increased. •Similarly, the composite materials content of small particles can be reduced by increasing the size of the aluminium or aluminium alloy particles. Thus, an aluminium powder with a mean particle size of about 6 μm will comprise about 0.8% by volume of small particles, which are present on the surface of the aluminium or aluminium alloy particles as AI2O3, and an aluminium powder with a mean particle size of about 1 μm will comprise about 5% by volume of small particles present as surface aluminium oxide particles. The content of small particles is substantially inversely proportional to the size of the aluminium or aluminium alloy particles, so that aluminium or aluminium alloy particles with a mean size of about 0.5 μm will provide about 10% by volume of small AI2O3 particles, while aluminium or aluminium alloy particles with a mean size of about 50 μm will provide about 0.1% by volume of small AI2O3 particles.

If the small particles are added as aluminium oxide particles which are present on the surface of the aluminium or aluminium alloy powder particles, the aluminium or aluminium alloy matrix is produced from aluminium or aluminium alloy powder with a mean particle size before processing of about 0.5 μm to about 1 mm, typically from about 1 to about 100 μm, for example from about 1 to about 50 μm, especially from about 1 to about 20 μm, in particular from about 1 to about 10 μm.

The material obtained by the above-described powder blending is typically compacted after mixing to consolidate it to full density, the compacting typically being accomplished by cold and/or hot compaction, e.g. by rolling and/or compressing. Cold compacting is typically carried out at a temperature of around room temperature, while hot compacting is typically performed at a temperature of about 300-600°C, for example from about 400°C to about 600°C. The compac¬ tion process results in a material having substantially full density, e.g. a density of about 2-4 g/cm , typically about 3 g/cmr .

The reinforced composite material may also be prepared by incorpora¬ ting 1-50% by volume of reinforcing fibres and/or particles and 1-10% by volume of small refractory particles into an aluminium or aluminium alloy melt arid allowing the melt to solidify. In this method, the small particles may also be formed in situ by precipita¬ tion hardening upon solidification or by precipitation after solidification by heat treatment of the solid material.

Finally, the reinforced composite material may be prepared by incorporating the reinforcing fibres and/or particles and the small refractory particles into a gas phase containing aluminium or aluminium particles in a solid and/or semi-solid state, and allowing the resulting mixture to solidify or precipitate. The metallic particles can thus be combined with other components, i.e. in this case the reinforcing fibres and/or particles and the small particles, by incorporating the latter components into the gas phase. In this method, the mixture will thus attain the form of a solid or a powder, and the material must subsequently be consolidated to full density by compaction. Compaction can be accomplished using the same processes as described above for materials produced by powder blending.

The reinforced composite material of the invention is typically recrystallized, the recrystallization typically comprising a deformation and a heat treatment. This may optionally be performed as a series of deformations and heat treatments, the two sub-processes being performed alternately. Deformation and heat treatment may also be performed simultaneously in a single operation known as hot deformation. Deformation of the material, in other words the formation of subgrains, is typically accomplished by cold and/or hot working, e.g. by extrusion, compression and/or drawing, so as to obtain subgrains of a size of, for example, about 0.5-4 μm, typically about 0.5-2 μm. The deformation temperature can vary from about room temperature to about 550°C or even higher. The degree of deformation, which is defined as the reduction in the cross-sectional area of the material, is typically in the range of about 10-99%. A lower degree of deformation results in a larger grain size after recrystallization.

Recrystallization of the material includes, as mentioned above, a heat treatment, which, however, may also be performed simultaneously with the deformation. The heat treatment typically comprises a treatment of from about 1 minute to several days, more typically for about 1 to 24 hours, at a temperature that typically lies in the range of about 250^βC to about 650^βC.

The temperature used for the recrystallization process is chosen according to such factors as the proportion of large and small particles in the material, the composition of the alloy and the degree of deformation. It will therefore be understood that a precise disclosure of recrystallization temperatures cannot be given, since the optimum temperature will vary depending on the above-mentioned factors as well as the desired use for the material in question. In general, however, a higher annealing temperature will result in a faster recrystallization and a smaller grain size, and this must therefore be taken into consideration. For example, if a large grain size is desired, this may be accomplished by a small to medium degree of deformation and a relatively high annealing temperature. If, however, the material is subjected to a high degree of deformation, and a large grain size is desired, it will normally be necessary to recrystallize the material using a relatively low annealing tempera¬ ture. These principles are illustrated in Example 3.

The invention will be further illustrated in the following non- limiting examples. EXAMPLE 1

Preparation of a reinforced composite material comprising SiC fibres and I2O3 particles

An l-Al2θ3"SiC composite was prepared by blending aluminium powder with aluminium oxide particles on the surface thereof and SiC fibres, using a high-speed standard commercial blender (Hansen, N. , "Dis¬ persion strengthened aluminium products manufactured by powder blending", Powder Met. , 12, 23-44, 1969). The aluminium powder was 99.5% pure, the remainder comprising mainly Fe and Si, and the powders content of aluminium oxide, which was present on the surface of the aluminium powder particles, was 0.8% by volume. The mean particle size of the powder was 6.4 μm. The SiC fibres (supplied by Tokai Carbon Co.) consisted of -type (cubic) SiC single crystals 0.1-1 μm in diameter and 30-100 μm in length. The composition of the composite material is given in Table 1:

Table 1

Composition of the Al-Al2θ3-SiC composite

SIC A1₂0₃ Fe Si vol.% vol.% wt.% wt.%

0.8 0.26 0.18

The thermomechanical processing consisted of cold compaction at a pressure of 30-35 kg/mm² to a specific gravity of about 2.3 g/cπ , hot compaction at about 50 kg/mm^*** at 550-600°C to a specific gravity of about 99% of the theoretical specific gravity and extrusion at 50-75 kg/mm² at about 550^CC, with an extrusion ratio of 15 to 1. The extruded rods were then treated either by heat treatment for 6 h at 500°C ("extruded"), or by cold drawing to a reduction in cross- sectional area of 30% and recrystallization for 24 h at 600°C ("recrystallized") .

The microstructure of the Al-Al2θ -SiC composite consisted of an aluminium matrix with a dispersion of three different types of fibres or particles. These were the SiC fibres, the AI2O3 platelets originating from the surface oxide on the aluminium powder particles, and a relatively small amount of 5.ntermetallic iron-aluminium particles present in the aluminium powder.

In the extruded and heat treated rods, the aluminium matrix consisted of subgrains with a mean diameter of about 1.2 μm. In the cold drawn and recrystallized rods, the grains were elongated in the extrusion (drawing) direction. The mean grain diameter in the recrystallized state was approximately 0.4 mm, and the grain aspect ratio, i.e. the ratio of length to diameter, was about 5.

The SiC fibres were fragmented during processing and the length was reduced to about 2 μm. The mean diameter was approximately 0.5 μm. Many of the SiC fibres were uniformly distributed, but a relatively large fraction (about 60%) were present in agglomerates or groups. The size of such groups of fibres ranged from 0.5 to 5 μm. The spacing of the fibres in the groups was of the order of about 0.5 μm.

The AI2O3 particles, which were plate or disc shaped, had a diameter of about 50 nm and a thickness of about 8 nm. The AI2O3 particles were substantially homogeneously distributed in the matrix with an average planar spacing of the order of about 0.2 μm.

The tensile creep properties, i.e. the high temperature strength properties, were measured in air at 673 K. The creep specimens had a gauge length of 50 mm and a diameter of 4.5 mm with threaded ends. The specimens were stressed under constant load conditions, but since the elongation was very low, essentially constant stress prevailed throughout the test. The creep strain was measured over the gauge length of the specimens using an extensometer having a resolution of 2 μm. All specimens were tested parallel to the extrusion (drawing) direction. The results of the measurements of creep strength showed that the cold drawing and recrystallization treatment significantly improved the creep strength of the material. In Table 2 below, the results of measurements of creep strength on the extruded and the recrystallized materials are shown. For the purpose of comparison, results are also shown for a material prepared as described above, but without the addition of the SiC fibres.

Table 2

Values of applied stress giving a creep rate of 10"° s^~- at 673 K

Material State Applied stress, MPa

A1-A1₂0₃ Extruded 22 Recrystallized 29

Al-Al₂0₃-SiC Extruded 29

Recrystallized 36

It can be seen from Table 2 that for the specimens containing both the SiC fibres and the small particles, the creep strength was considerably increased in both the extruded and the recrystallized materials.

EXAMPLE 2

Effects of small and large particles on nucleation

A composite material was prepared as in Example 1. The fabrication procedure of the Al-Al2θ3-SiC composite was essentially the same as in Example 1, in that it consisted of blending atomized Al powder and a 2% by volume fraction of SiC whiskers, cold compacting, hot compacting at 550°C, extruding at 350^CC with a reduction ratio of 15 to 1, followed by a 50% reduction by cold rolling in the extrusion direction. To reduce the effect of impurities in solid solution the two materials were heat treated in a vacuum (10^"*-^* mm Hg) for 24 h at 600°C followed by slow cooling. The final cold deformation prior to the recrystallization treatment was either a 50 or 90% cold rolling reduction in thickness in the original extrusion direction. Differen¬ tial thermal analysis (DTA) showed that the Al-Al2θ₃-SiC composite was stable up to 640°C. Bulk cold-rolled specimens were heat treated for one hour at a temperature sufficient to stimulate formation of recrystallization nuclei. As in Example 1, a material was also prepared without SiC fibres for the purpose of comparison.

The subgrain size decreased after adding SiC whiskers and with increasing the degree of deformation. It was observed that subgrains frequently grew at the interface of SiC fibres and the aluminium matrix. However, only a few of these subgrains developed into nuclei, and fine particle pinning of the subgrain boundaries was seen clearly. Thus, the small particles reduced the growth of the subgrains in the material containing SiC fibres. During recrystalli¬ zation, the nucleation and grain growth in the material without SiC fibres was very inhomogeneous, while nucleation in the Al-Al2θ3-SiC material was more frequent and quite random, with growth being more uniform.

It was calculated that in the 90% cold-rolled Al-Al₂0₃-SiC alloy, the concentration of nuclei groups, which are potential nucleation sites, was about two orders of magnitude larger than the concentration of nuclei which grew into recrystallized grains. This effect may be primarily attributed to the presence of the fine AI2O3 particles, which exert a pinning effect on the sub-boundaries, thereby reducing the nucleation efficiency. In the presence of the fine particles, only those nuclei which have favourable conditions for growth can develop into a larger grain containing the fibre or fibre group.

Thus, the silicon carbide whiskers in the material stimulated nucleation, while the aluminium oxide particles had the opposite effect. EXAMPLE 3

Effect of degree of deformation and annealing temperature

The same materials as in Example 2 were used to test hardness as a function of degree of deformation and annealing temperature. Specimens of materials which had been subjected to either a 50% or a 90% deformation by cold-rolling were annealed for one hour at different temperatures.

Fig. 1 shows curved of hardness (load 1000 g) plotted against temperatures at which the specimens were annealed for one hour, with (a) showing hardness after 50% cold rolling, and (b) showing hardness after 90% cold rolling.

The results, as seen in Fig. 1, show that the addition of SiC lowered the recrystallization temperature for both the 50% and the 90% cold- rolled materials, with this difference in recrystallization tempera- ture between the materials with and without SiC decreasing as the degree of deformation increased from 50% to 90%. It is further observed from Fig. 1 that an increase in the degree of deformation lowered the recrystallization temperature.

Fig. 2 shows the effect of annealing temperature for 90% cold rolled alloys containing SiC fibres on recrystallized grain size in the rolling plane. It can be seen that the recrystallized grain size decreases with increasing annealing temperature.

It was furthermore observed in the same study that the aspect ratio of the grains, i.e. the ratio between length and diameter, decreased with increasing annealing temperature and increasing degree of deformation prior to annealing.

Claims

1. A reinforced aluminium or aluminium alloy composite material ^• having a matrix of aluminium or aluminium alloy grains and comprising

2. A composite material according to claim 1, in which the small refractory particles have a mean diameter of less than about 0.5 μm, preferably less than about 0.2 μm, especially less than about 0.1 μm, such as less than about 0.01 μm.

3. A composite material according to claim 1 or 2, in which the small refractory particles are present in an amount of about 0.5-10% by volume, typically about 1-5%.

4. A composite material according to any of claims 1 to 3, in which the small refractory particles comprise oxides, carbides, nitrides, suicides, borides and/or metals, e.g. AI2O3, SiC, SiN, TIC, ZrC, WC, NbC, TiN, BN, 813^, MgO, Si0₂, Zr0 , Fe₂0₃, CuO, Y2O3, AlN, steel or graphite.

5. A composite material according to claim 4, in which the small refractory particles comprise AI2O3.

6. A composite material according to any of claims 1 to 5, in which the small refractory particles are substantially spherical, plate or disc shaped or irregular.

7. A composite material according to any of claims 1 to 6, in which the reinforcing fibres and/or particles are present in an amount of about 2-40% by volume, typically about 5-35%, preferably about 10-30%, more preferably about 15-25%, for example about 18-22%.

8. A composite material according to any of claims 1 to 7, in which the reinforcing fibres and/or particles have a diameter of about 0.1- 100 μm, for example about 0.2-10 μm, preferably about 0.5-5 μ , typically about 0.5-2 μm, in particular about 0.5-1 μm, and a ratio between length and diameter of about 1-100, in particular about 1-20, typically about 1-10, for example about 1-5.

9. A composite material according to any of claims 1 to 8, in which the reinforcing fibres' and/or particles comprise carbides, oxides, nitrides, suicides, borides and/or metal fibres, e.g. SiC, AI2O3, SiN, TiC, ZrC, WC, NbC, TiN, BN, Si₃N , MgO, Si0₂, Zr0₂, Fe₂0₃, CuO, Y2O3, AlN, steel or graphite.

10. A composite material according to claim 9, in which the reinfor- cing fibres and/or particles comprise SIC or AI2O3.

11. A composite material according to any of claims 1 to 10, in which the mean aluminium or aluminium alloy grain diameter is at least about 1 μm, typically at least about 5 μm, for example at least about 10 μm, in particular at least about 50 μm, such as at least about 100 μm, preferably at least about 200 μm, more preferably at least about 500 μm, most preferably at least about 1 mm or even at least about 5 mm.

12. A composite material according to any of claims 1 to 11, in which the grains have a length/diameter ratio of between about 1 and 50, typically between about 1 and 20, for example between about 1 and 10.

13. A composite material according to any of claims 1 to 12, in which the aluminium alloy matrix comprises about 0-20% by weight of Mg, Mn, Cu, Zn, Li, Si, Fe, Co, Ni and/or Cr.

14. A method of preparing a reinforced aluminium or aluminium alloy composite material, which method comprises mixing aluminium powder and/or aluminium alloy powder with: 1-50% by volume of high temperature resistant reinforcing fibres and/or particles having at least one dimension of at least about 1-2 μm, and

0.1-10% by volume of small refractory particles having a mean diameter of at the most about 1 μm, the small particles being present on the surface of the aluminium powder and/or aluminium alloy powder and/or at least part of the small particles being added separately as a powder,

15. A method according to claim 14, wherein at least part of the small particles are present as aluminium oxide particles on the surface of the aluminium or aluminium alloy powder, and in which the powder has a mean particle size of from about 0.5 μm to about 1 mm, typically from about 1 to about 100 μm, for example from about 1 to about 50 μm, especially from about 1 to about 20 μm, in particular from about 1 to about 10 μm.

16. A method according to claim 14 or 15, wherein the mixture is compacted by cold and/or hot compaction, e.g. by rolling and/or compressing, and which optionally includes a sintering step.

17. A method of preparing a reinforced aluminium or aluminium alloy composite material, which method comprises incorporating 1-50% by volume of high temperature resistant reinforcing fibres and/or particles having at least one dimension of at least about 1-2 μm, and 0.1-10% by volume of small refractory particles having a mean diameter of at the most about 1 μm into an aluminium or aluminium alloy melt and allowing the melt to solidify, so as to obtain a substantially homogeneous distribution of at least the small particles and a substantially dense matrix of aluminium or aluminium alloy grains.

18. A method according to claim 17, wherein the small particles are formed in situ by precipitation hardening upon solidification or by precipitation after solidification by heat treatment of the solid material.

19. A method of preparing a reinforced aluminium or aluminium alloy composite material, which method comprises incorporating 1-50% by volume of high temperature resistant reinforcing fibres and/or particles having at least one dimension of at least about 1-2 μm, and 0.1-10% by volume of small refractory particles having a mean diameter of at the most about 1 μm into a gas phase containing aluminium or aluminium alloy particles in a solid or semi-solid state, so as to obtain a substantially homogeneous distribution of at least the small particles, and allowing the resulting mixture to solidify as a solid or precipitate as a powder, and consolidating the mixture by compaction to obtain a substantially dense matrix of aluminium or aluminium alloy grains.

20. A method according to any of claims 14 to 19, wherein the materi¬ al is deformed by cold and/or hot working, e.g. by extrusion, compression and/or drawing, so as to obtain aluminium or aluminium alloy subgrains of a size of about 0.5-4 μm, typically about 0.5-2 μm.

21.. A method according to any of claims 14 to 20, wherein the material is recrystallized, e.g. by heat treatment or hot deforma¬ tion, the temperature of the heat treatment or hot deformation, the degree of deformation and the amount of the small particles being adapted to secure the attainment of aluminium or aluminium alloy grains having a diameter of at least about 1 μm, typically at least about 5 μm, more typically at least about 10 μm, in particular at least about 50 μm, such as at least about 100 μm, preferably at least about 200 μm, more preferably at least about 500 μm, most preferably at least about 1 mm or even at least about 5 mm.

22. A method according to claim 21, wherein the grains have a length/diameter ratio of between about 1 and 50, typically between about 1 and 20, for example between about 1 and 10.

23. A method according to any of claims 14 to 22, wherein the small refractory particles have a mean diameter of less than about 0.5 μm, preferably less than about 0.2 μm, especially less than about 0.1 μm.

24. A method according to any of claims 14 to 23, wherein the small refractory particles are incorporated into the material in an amount of about 0.5-10% by volume, typically about 1-5%.

25. A method according to any of claims 14 to 24, wherein the small refractory particles comprise oxides, carbides, nitrides, suicides, borides and/or metals, e.g. Al₂0₃, SiC, SiN, TiC, ZrC, WC, NbC, TiN, BN, Si₃N₄, MgO, Si0₂, Zr0₂, Fe 0₃, CuO, Y₂0₃, AlN, steel or graphite.

26. A method according to claim 25, wherein the small refractory particles comprise AI2O3.

27. A method according to any of claims 14 to 26, wherein the small refractory particles are substantially spherical, plate or disc shaped or irregular.

28. A method according to any of claims 14 to 27, wherein the reinforcing fibres and/or particles are incorporated into the material in an amount of about 2-40% by volume, typically about 5- 35%, preferably about 10-30%, more preferably about 15-25%, for example about 18-22%.

29. A method according to any of claims 14 to 28, wherein the reinforcing fibres and/or particles have a diameter of about 0.1-100 μm, for example about 0.2-10 μm, preferably about 0.5-5 μm, typically about 0.5-2 μm, in particular about 0.5-1 μm, and a ratio between length and diameter of about 1-100, in particular about 1-20, typically about 1-10, for example about 1-5.

30. A method according to any of claims 14 to 29, wherein the reinforcing fibres and/or particles comprise carbides, oxides, nitrides, suicides, borides and/or metal fibres, e.g. SiC, AI2O3, SiN, TiC, ZrC, WC, NbC, TiN, BN, Si₃N₄, MgO, Si0 , Zr0₂, Fe₂0₃, CuO, Y2O3, AlN, steel or graphite.

31. A method according to claim 30, wherein the reinforcing fibres and/or particles comprise SiC or AI2O3.

32. A method according to any of claims 14 to 31, wherein the aluminium alloy comprises about 0-20% by weight of Mg, Mn, Cu, Zn, Li, Si, Fe, Co, Ni and/or Cr.