Abstract:
A fiber-reinforced metal composite consisting essentially of continuous reinforcing fibers disposed in an aluminum alloy matrix containing about 0.005 wt % of cobalt.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fiber-reinforced metal composite (FRM) comprising reinforcing fibers and an aluminum alloy as a matrix. 
     2. Description of the Related Art 
     Recently, due to the superior strength and rigidity thereof, fiber-reinforced metal composites have been used for various machine parts and structural materials. Among these composites, a fiber-reinforced composite material of aluminum or an alloy thereof reinforced with inorganic fibers or metal fibers is light and has a high rigidity and high heat resistance. The inorganic fibers include, for example, continuous fibers such as Si--Ti--C--O fibers, SiC fibers, alumina fibers, boron fibers and carbon fibers, and short (staple) fibers, such as SiC whiskers, Si 3  N 4  whiskers and alumina whiskers. The aluminum or alloy thereof of the matrix is generally a standard product meeting the requirements of Japanese Industrial Standards (JIS), such as 1070 (pure aluminum), 6061 (Al--Mg--Si series), 2024 (Al--Cu--Mg series), and AC4C (corresponding to A356.0 of the Aluminum Association (AA)), or the like. Heretofore, such fiber-reinforced metal composites have been produced by methods such as infiltration, diffusion-bonding, and pressure casting. 
     In general, reinforcing fibers are used at a volume percentage of from 40 to 60% in the fiber-reinforced metal composite produced by a pressure casting method, and thus inevitably the fibers come into contact with each other, and this contact between the fibers prevents the obtaining of the expected strength of the fiber-reinforced metal composite. Further, sometimes the compatibility between the reinforcing fibers and the metal matrix is poor and a reaction occurs at the interface, which causes a deterioration of the reinforcing fibers. Furthermore, in the case of a matrix of aluminum or an alloy thereof, in particular, undesirable brittle crystals are generated. 
     It is considered that pure aluminum is most suitable as the matrix metal, since deterioration of the fibers and generation of brittle crystals do not occur when pure aluminum is used. Nevertheless, since pure aluminum has low strength, when continuous reinforcing fibers are used, the fiber-reinforced aluminum composite has poor strength in a transverse direction at a right angle to the continuous fiber orientation, and if a component part is formed only partially of fiber-reinforced aluminum, and the remainder thereof does not contain the reinforcing fibers but is formed of aluminum alone, such a remaining part has low strength. 
     To solve the above-mentioned problems, composite materials (fiber-reinforced metal composites) of an aluminum alloy matrix have been proposed. For example, an aluminum alloy containing 0.5 to 6.0 wt% of nickel (Ni) is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-124245, and another aluminum alloy containing at least one element selected from the group consisting of Bi, Sb, Sn, In, Cd, Sr, Ba and Ra is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-169034. Nevertheless, these proposed fiber-reinforced metal composites do not have the required strength or corrosion resistance. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a fiber-reinforced metal (aluminum) composite having an increased strength. 
     Another object of the present invention is to provide an aluminum-matrix composite reinforced with Si--Ti--C--O inorganic fibers. 
     These and other objects of the present invention are obtained by providing a fiber-reinforced metal composite consisting essentially of reinforcing fibers and an aluminum alloy containing 0.005 to 5 wt% of cobalt as a metal matrix. 
     Preferably, the reinforcing fibers are continuous inorganic fibers such as Si--Ti--C--O filers, SiC fibers, Si 3  N 4  fibers, alumina (Al 2  O 3  fibers, Al 2  O 3  --SiO 2  fibers, boron fibers, B 4  C fibers, and carbon fibers, or continuous metal fibers such as stainless steel, piano wire fibers, tungsten fibers, titanium fibers, molybdenum fibers and nickel fibers. The Si--Ti--C--O fibers are disclosed in Japanese Examined Patent Publication (Kokoku) Nos. 58-5286 and 60-1405 and U.S. Pat. Nos. 4,342,712 and 4,399,232, and are commercially produced by Ube Industries, Ltd. Instead of the continuous fibers, it is possible to use short (staple) fibers such as alumina short fibers, Al 2  O 2  --SiO 2  short fibers, zirconia short fibers as produced, and chopped fibers prepared by cutting the continuous fibers. It is also possible to use whiskers such as SiC whiskers, Si 3  N 4  whiskers, carbon whiskers and Al 2  O 3  whiskers, K 2  O;6TiO 2  whiskers, K 2  Ti 2  O 5  whiskers, B 4  C whiskers, Fe 3  C whiskers, chromium whiskers, copper whiskers, iron whiskers and nickel whiskers. 
     According to the present invention, the aluminum alloy matrix contains 0.005 to 5 wt%, preferably 0.5 to 3 wt%, of cobalt, whereby fine crystals having diameters of 0.1 tm or less are generated in quantity at the interface between the reinforcing fibers and the matrix, and as a result, contact between the fibers is reduced to a minimum and the compatibility between the fibers and the matrix is remarkably improved. The above phenomena can be recognized by using an optical microscope, an Auger electron spectroscope (AES), a scanning electron microscope (SEM), an electron probe microanalyzer (EPMA), and a transmission electron microscope (TEM) or the like. Therefore, the strength of the fiber-reinforced metal composite according to the present invention is superior to that of conventional fiber-reinforced aluminum composites. 
     Furthermore, the pressure casting method contributes to the formation of fine crystals, as compared with a gravity casting method. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more apparent from the description of the preferred embodiments set forth below, with reference to the accompanying drawings, in which: 
     FIG. 1 is a sectional view of a fiber-reinforced metal composite test piece which is bent by a load applied in parallel to the fiber orientation; 
     FIG. 2 is a sectional view of a fiber-reinforced metal composite test piece which is bent by a load applied at a right angle to the fiber orientation; 
     FIG. 3 is a graph showing relationships between the cobalt content and flexural strengths of fiber-reinforced metal composites; 
     FIG. 4 is a photomicrograph (×1000) of a fiber-reinforced metal composite having a metal matrix of Al--0.5%Co., in a transverse direction to the fiber orientation; and 
     FIG. 5 is a photomicrograph (×1000) of a fiber-reinforced metal composite having a metal matrix of Al--1.6%Co. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     EXAMPLE 1 
     Fiber-reinforced metal (aluminum) composites were produced in the following manner. 
     Many Si--Ti--C--O continuous fibers were unidirectionally arranged to form a fiber preform held by a frame. The fiber preform was preheated at 700° C. for 30 minutes in a furnace under an ambient atmosphere, and a metal mold and a plunger of a pressure casting apparatus were heated at 300° C. by a heating means. A pure aluminum melt and binary aluminum alloy melts containing cobalt (Co) in various amounts of 0.005 to 6 wt% were prepared, respectively, and heated at 720° C. 
     The fiber preform was placed in a cavity of the metal mold and the prepared melt was poured into the cavity to cover the fiber preform. Subsequently, the plunger was inserted into the cavity of the metal mold and a pressure of 1000 kg/cm 2  was applied to the melt, and then the mold and plunger were cooled to allow the melt to solidify under the pressure. The thus obtained fiber-reinforced metal composite was taken out the cavity and machined to form test pieces 1A and 1B, as shown in FIGS. 1 and 2, for the bending tests. The test pieces of the fiber-reinforced metal composite had a fiber content of 50 vol%. 
     In one of the test pieces 1A, the fibers 2 were oriented at a right angle to the longitudinal axis of the test piece, as shown in FIG. 1, and in the other test piece 1B, the fibers 2 were oriented in parallel to the longitudinal axis of the test piece, as shown in FIG. 2. The test pieces 1A and 1B contained a metal matrix of pure aluminum and binary aluminum alloys containing different cobalt contents, respectively. 
     The test pieces 1A and 1B were tested by applying a bending load P thereto, as shown in FIG. 1 or 2, to measure the flexural strength of each test piece 1A and 1B. In FIG. 1, the load P was applied in parallel to the fiber orientation, and in FIG. 2, the load P was applied at a right angle to the fiber orientation. 
     The results of the bending test (the obtained flexural strength values) are shown in FIG. 3, wherein the abscissa represents the cobalt content and the ordinate represents the flexural strength. 
     As can be seen from FIG. 3, the flexural strength of the test piece 1B to which the load P was applied at a right angle to the fiber orientation varies upward and then downward, as the cobalt content is increased. The maximum flexural strength value was obtained at the cobalt content of the metal matrix of 1 to 2 wt%. Where the cobalt content is from 0.005 to 5 wt%, the flexural strength of the fiber-reinforced aluminum alloy composite is greater than the flexural strength of the fiber-reinforced pure aluminum composite. 
     FIGS. 4 and 5 are photomicrographs (×1000) of the test pieces having a metal matrix containing 0.5 wt% and 1.6 wt% of cobalt, respectively, in a transverse direction to the fiber orientation. As shown in FIGS. 4 and 5, fine acicular crystals of eutectic Co Co 2  Al 9  are nonuniformly generated at the interface between the reinforcing (Si--Ti--C--O ) fibers and the alloy matrix, and such crystals increase as the cobalt content increases. Although the eutectic crystals are generated nonuniformly, the strength of the composites is improved, because the crystals have a very fine acicular shape which produces a strengthening effect due to the particle dispersion, and an addition of cobalt improves the wettability of the aluminum alloy melt on the reinforcing fibers, and the state, composition and mechanical properties of the generated crystals are different from those of conventionally generated crystals which impair the mechanical strengths of fiber-reinforced composites. Nevertheless, a matrix containing more than 5 wt% of cobalt has a lower flexural strength, since coarse primary crystals Co 2  Al 9  are crystallized and cause stress concentration under a load. 
     On the other hand, as shown in FIG. 3, the flexural strength of the test pieces 1A to which the load P was applied in parallel to the fiber orientation is slightly increased with an addition of cobalt. In this case, the strengthening effect of the reinforcing fibers for the test pieces 1A is very low, compared with that of the test pieces 1B. Namely, the strength of the metal matrix has an influence on the flexural strength of the test piece (i.e., fiber-reinforced metal composite). That is, the tensile strength of the matrix varies, as shown in Table 1, with an increase of the cobalt content, whereby the flexural strength also varies. 
     
                       TABLE 1______________________________________Matrix          Tensile StrengthComposition     of Matrix only______________________________________pure Al         6          kg/mm.sup.2Al-0.5     wt % Co  9          kg/mm.sup.2Al-1       wt % Co  11         kg/mm.sup.2Al-1.6     wt % Co  10.5       kg/mm.sup.2Al-2.3     wt % Co  10         kg/mm.sup.2______________________________________ 
    
     EXAMPLE 2 
     Many carbon continuous fibers were uni-directionally arranged to form a fiber preform held by a frame. The fiber preform was preheated at 700° C. for 30 minutes in a furnace under an argon atmosphere, and a metal mold and a plunger of a pressure casting apparatus used in Example 1 were also preheated at 300° C. by a heating means. A pure aluminum melt and an Al-1 wt%Co melt were prepared, respectively, and heated at 720° C. 
     The carbon fiber preform was placed in a cavity of the mold and the melt of pure aluminum (or Al1 wt%Co) was poured into the cavity. Subsequently the plunger was inserted into the cavity and a pressure of 1000 kg/cm 2  was applied to the melt, and then the mold and the plunger were cooled to allow the melt to solidify under pressure. Each of the thus obtained fiber-reinforced metal composites was taken out the cavity and then machined to form test pieces 1A and 1B, as shown in FIGS. 1 and 2, for a bending test. The test pieces of the fiber-reinforced metal composites had a fiber content of 60 vol%. In one 1A of the test pieces, the (carbon) fibers 2 were oriented at a right angle to the longitudinal axis thereof, as shown in FIG. 1, and a bending load P was applied to the test piece 1A in parallel to the fiber orientation. In the other test piece 1B, the (carbon) fibers 2 were oriented in parallel to the longitudinal axis thereof, as shown in FIG. 2, and the bending load P was applied to the test piece 1B at a right angle to the fiber orientation. The results (the obtained flexural strengths) of the bending test are shown in Table 2. 
     
                       TABLE 2______________________________________       Flexural Strength (kg/mm.sup.2)         Test Piece 1B                     Test Piece 1A         Load at Right                     Load ParallelMatrix        Angle to Fiber                     to FiberComposition   Orientation Orientation______________________________________Pure Al       105          5Al-1 wt % Co  140         12______________________________________ 
    
     As can be seen from Table 2, the fiber-reinforced metal composite having an Al-1 wt%Co matrix according to the present invention has a greater flexural strength than that of the fiber-reinforced metal composite having a pure aluminum matrix. 
     Suitable elements such as Si, Mn, Mg, Cn, Zn and the like can be added, to improve the strength of the binary (Al-Co) alloy of the metal matrix of the fiber-reinforced metal composite according to the present invention. Furthermore, instead of the Si--Ti--C--O fibers and carbon fibers used in Examples 1 and 2, other continuous inorganic fibers, such as SiC. fibers, fibers, Si 3  N 4  fibers, Al 2  O 3  -SiO 2  fibers, B 4  C. fibers, and B fibers, or continuous metal fibers, such as stainless fibers, piano wire fibers, W fibers, Mo fibers, Be fibers, Ti fibers, and Ni fibers can be used. It is also possible to use short fibers such as short fibers, Al 2  O 3  --SiO 2  short fibers, ZrO 2  short fibers as produced, and chopped fibers prepared by cutting the continuous fibers. Further, in addition to the above-mentioned fibers, whiskers, such as SiC. whiskers, Si 3  N 4  whiskers, carbon whiskers, Al 2  O 3  whiskers, K 2  O6Ti 2  whiskers, K whiskers, K 2  Ti 2  O 5  whiskers, B 4  C whiskers, Fe 3  C whiskers, Cr whiskers, Cu whiskers, Fe whiskers and Ni whiskers can be used as the reinforcing fibers. The aluminum alloy containing 0.005 to 5 wt% of cobalt is used as the metal matrix to improve the compatibility between the reinforcing fibers and the matrix. 
     It will be obvious that the present invention is not restricted to the above-mentioned embodiments and that many variations are possible for persons skilled in the art without departing from the scope of the invention.