Patent Publication Number: US-4547435-A

Title: Method for preparing fiber-reinforced metal composite material

Description:
This application is a divisional of application Ser. No. 285,975, filed on July 23, 1981, now U.S. Pat. No. 4,489,138. 
    
    
     The present invention relates to fiber-reinforced metal composite materials (hereinafter referred to as &#34;composite materials&#34;) having an excellent mechanical strength and comprised of an inorganic fiber as the reinforcing material and a metal or alloy as the matrix (hereinafter referred to as &#34;matrix metal&#34;). 
     Recently, novel composite materials comprising an inorganic fiber (e.g. an alumina fiber, a carbon fiber, a silica fiber, a silicon carbide fiber, a boron fiber) as the reinforcing material and a metal (e.g. aluminum, magnesium, copper, nickel, titanium) as the matrix have been developed and begun to be used in many industrial fields. 
     In combining an inorganic fiber with a metal, a reaction is caused at the interface between the matrix metal which is melted or maintained at a high temperature and the inorganic fiber to create a weakened layer so that the strength of the resultant composite material is, in many cases, lower than the theoretical value. For example, commercially available carbon fibers usually possess a strength of about 300 kg/mm 2 , and the theoretical strength of a carbon fiber-reinforced composite material is calculated to be about 150 kg/mm 2  according to the rule of mixture, the content of fiber being assumed to be 50% by volume, even when the strength of the matrix material is neglected. In fact, a carbon fiber-reinforced epoxy resin composite material shows a strength of 150 kg/mm 2  or larger, while the strength of a carbon fiber-reinforced metal composite material obtained by the liquid metal-infiltration method using aluminum as the matrix is only about 30-40 kg/mm 2  at a maximum. This is due to deterioration of the fiber caused by an interfacial reaction between the fiber and the melted metal as mentioned above. 
     For prevention of the above deterioration of fibers, various methods are adopted, including treatment of the fiber surface with a coating agent. In Japanese Patent Publication (unexamined) No. 30407/1978, for example, there is disclosed a procedure in which the surface of silicon carbide fiber is protected with metals or ceramics forming a compound being inactive or stable to carbon and then the fiber is combined with a matrix metal. Though this method is effective for a silicon carbide fiber, a sufficient result is not obtained for other inorganic fibers, and there is a problem of troublesome handling. Japanese Patent Publication (unexamined) No. 70116/1976 describes that the mechanical strength of a fiber-reinforced metal composite material is increased by addition of lithium in an amount of several percents to an aluminum matrix. However, this method is effective only in cases where the inorganic fiber is not compatible or does not react with the matrix metal. In the case where the inorganic fiber reacts with the matrix metal and its deterioration is caused, a substantial effect is not obtained, but the mechanical strength tends to be rather lowered. Thus, a practically useful method for overcoming the above mentioned drawbacks is not yet established. 
     For the purpose of increasing the mechanical strength of a fiber-reinforced metal composite material, an extensive study has been made. As the result, it has been found that, by incorporation of at least one element selected from the group consisting of metals belonging to the fourth or higher periods of the group (IA) in the periodic table (K, Cs, Rb, Fr) and to the fifth or higher periods of the group (IIA) in the periodic table (Sr, Ba, Ra) and Bi and In into a matrix metal of a fiber-reinforced metal composite material, the deterioration of the inorganic fiber due to its reaction with the matrix metal can be prevented, and the mechanical strength of composite material comprising such a matrix metal can be greatly increased. The present invention is based on this finding. 
     As the inorganic fiber to be used as the reinforcing material in the invention, there may be exemplified a carbon fiber, a silica fiber, a silicon carbide fiber containing free carbon, a boron fiber, an alumina fiber, etc. Among them, the alumina fiber described in Japanese Patent Publication (examined) No. 13768/1976 can afford the most notable metal-reinforcing effect. 
     This alumina fiber is obtained by admixing a polyaluminoxane having structural units of the formula: ##STR1## wherein Y is at least one of an organic residue, a halogen atom and a hydroxyl group with at least one compound containing silicon in such an amount that the silica content of the alumina fiber to be obtained becomes 28% or less, spinning the resultant mixture and subjecting the obtained precursor fiber to calcination. Particularly preferred is the alumina fiber which has a silica content of 2 to 25% by weight and which does not materially show the reflection of α-Al 2  O 3  in the X-ray structural analysis. The alumina fiber may contain one or more refractory such as oxides of lithium, beryllium, boron, sodium, magnesium, silicon, phosphorus, potassium, calcium, titanium, chromium, manganese, yttrium, zirconium, lanthanum, tungsten and barium in such an amount that the effect of the invention is not substantially reduced. 
     The content of the inorganic fiber in the composite material of the invention is not particularly limited. Preferably, it may be from 15 to 70% by volume. When it is less than 15% by volume, the reinforcing effect is insufficient. When the volume is more than 70%, the strength is rather decreased due to the contact between fiber elements. The shape of the fiber may be long or short, and depending on the purpose or the use, there may be employed either a long fiber a short fiber, or both in combination. For obtaining the desired mechanical strength or modulus of elasticity, a suitable orienting method such as unidirection ply, corss ply or random orientation ply may be selected. 
     As the matrix metal, aluminum, magnesium, copper, nickel, titanium, etc. may be employed. Their alloys are also usable. In the case that a light weight and a high mechanical strength are required, the system containing as the matrix aluminum, magnesium or their alloy is desirable. When a thermal resistance and a high strength are required, the system containing nickel or titanium as the matrix is favorable. These metals may contain a small amount of impurities insofar as they can be used in an ordinary way without trouble. 
     The characteristic feature of the present invention is that at least one element selected from the group consisting of metals belonging to the fourth and higher periods of the group (IA) in the periodic table (potassium, cesium, rubidium, francium) and to the fifth and higher periods of the group (IIA) in the periodic table (stronthium, barium, radium) and bismuth and indium is incorporated in the matrix metal or the inorganic fiber, whereby the mechanical strength of the resulting fiber-reinforced metal composite material is greatly increased. The mechanism for such increase of the strength is still unclear but may be assumed as follows. 
     When the said element is added to the matrix metal, the concentration of such element at the surface of the matrix metal becomes higher than the average concentration. In case of aluminum, for example, addition of bismuth, indium, stronthium or barium in an amount of 0.1  mol % decreases the surface tension of aluminum by 400, 20, 60 or 300 dyn/cm, respectively, in comparison with the surface tension of pure aluminum. This is attributable to the fact that the concentration of the element at the surface portion is higher than the average concentration in the matrix as shown by the Gibbs&#39; adsorption isotherm. It is thus suggested that, in a fiber-reinforced metal composite material which comprises a matrix metal containing the said element, the element is accumulated in a high concentration at the fiber-matrix interface. This has been actually confirmed by the aid of Auger&#39;s scanning microscope and EPMA (Electron Probe Micro Analyser). 
     Observations with a scanning electron microscope of the broken surface of an inorganic fiber-reinforced metal composite material, prepared from a matrix metal containing the said element according to the liquid metal infiltration method, reveals that, the bonding strength of the fiber-matrix interface in the fiber-reinforced metal composite material comprising bismuth- and/or indium-containing aluminum as the matrix is weaker than that in the fiber-reinforced metal composite material not containing such element. Furthermore the reaction phase with the matrix metal having been observed at the extraperipheral surface of the fiber disappears, whereby it is understood that the reaction at the fiber-matrix interface is diminished. Namely, the said element is present in a high concentration at the fiber-matrix interface and controls the reaction at the interface so that the mechanical strength of the composite material is greatly increased. 
     In case of the fiber-reinforced metal composite material comprising a matrix metal containing one or more chosen from elements belonging to the fourth and higher periods of the group (IA) in the periodic table (K, Rb, Cs, Fr), elements belonging to the fifth and higher periods of the group (IIA) in the periodic table (Sr, Ba, Ra) and Bi and In, the combination at the fiber-matrix interface is not weakened in comparison with the system containing no additional metal, and nevertheless the reaction phase with the matrix metal having been observed at the extraperipheral surface of the fiber disappears. When the composite material is treated with an aqueous hydrochloric acid solution to remove the matrix metal and the recovered fiber is subjected to determination of the tensile strength, a considerable decrease of the tensile strength is observed in the system not containing the said element, compared with the tensile strength of the fiber before used. In the system containing the element, no material decrease of the tensile strength of the fiber is observed. 
     To the contrary, in case of the fiber-reinforced metal composite material comprising as the matrix an aluminum alloy containing 0.5% by weight of sodium or lithium of the group (IA) in the periodic table or 5% by weight of magnesium of the group (IIA) in the periodic table, the strength is greatly decreased, and the presence of the reaction phase at the extraperipheral surface of the fiber is confirmed in observations of the broken surface by the aid of a scanning electron microscope. The tensile strength of the fiber recovered after elimination of the matrix metal is greatly lowered in comparison with the tensile strength of the fiber previously used. Apparently, the element chosen from the fourth and higher periods of the group (IA), the fifth and higher periods of the group (IIA) and Bi and In react with the fiber at the interface, but due to their large atomic diameters, their diffusion into the fiber is difficult so that deterioration of the fiber is not caused and the bonding strength of the fiber-matrix at the interface is increased. 
     It is thus supposed that the said elements accumulate in high concentrations at the fiber-matrix interface and react with the fiber in a single layer to control the reaction between the fiber and the matrix metal, which results in great increase of the mechanical strength of the composite material. 
     The said element may be employed in the form of either simple substance or an inorganic or organic compound. It is surprising that the element incorporated in the form of a compound can afford similar effects as the one incorporated in the form of a simple substance. Supposedly, a part of or the whole portion of the inorganic or organic metal compound is decomposed or reduced before or after the combination of the fiber with the matrix metal and exerts a similar activity to that of the simple substance itself. The use of the element in the form of a compound is particularly advantageous when its simple substance is chemically unstable and can be handled only with great difficulty. As the inorganic and organic compounds of the element, there may be exemplified halides, hydrides, oxides, hydroxides, sulfonates, nitrates, carbonates, chlorates, carbides, nitrides, phosphates, sulfides, phosphides, alkyl compounds, organic acid compounds, alcoholates, etc. 
     The amount of the element in the form of a simple substance or of a compound to be incorporated may be usually from 0.0005 to 10% by weight (in terms of element) to the weight of the matrix metal. When the amount is less than 0.0005% by weight, the technical effect is insufficient. When the amount is larger than 10% by weight, the characteristic properties of the matrix metal are deteriorated to cause decrease of corrosion-resistance, reduction of elongation, etc. 
     The incorporation of the element into the matrix metal of the fiber-reinforced metal composite material may be effected by various procedures. For example, the simple substance or the organic or inorganic compound may be applied to the surface of the inorganic fiber to form a coating layer thereon, and the fiber is then combined with the matrix metal. The use of the organic or inorganic compound of the metal element is particularly advantageous when handling of the simple substance is troublesome. The formation of the coating layer on the surface of the inorganic fiber may be effected by various procedures such as electroplating, non-electrolytic plating, vacuum evaporation, spattering evaporation, chemical evaporation, plasma spraying, solution immersion and dispersion immersion. Among these procedures, the solution immersion method and the dispersion immersion method are particularly preferable for formation of a coating layer of the inorganic or organic compound of the element on the surface of the fiber. In these methods, the compound of the element is dissolved or dispersed in a suitable solvent, and the inorganic fiber is immersed therein and then dried. The thus treated fiber is then combined with the matrix metal to obtain a fiber-reinforced metal composite material having a high strength. This is an extremely simple and economical procedure in comparison with other procedures for coating layer-formation. 
     The coating layer is desired to have a thickness of 20 Å or more. When the thickness is less than 20 Å, a sufficient effect is not obtained. 
     It is characterized in this invention that a good result can be obtained in the combination with the matrix metal even when the coating layer of the element in the form of a simple substance or a compound form made on the surface of the inorganic fiber has not a uniform thickness. This is probably explained by the reason that a part of the element applied on the fiber surface is dissolved in the matrix metal and is present in a high concentration at the fiber-matrix metal interface by the above mentioned mechanism. 
     The incorporation of the element into the matrix metal may be also effected by adding it in the form of either the simple substance or compound to the matrix metal. This method is advantageous in that the operation of coating of the fiber surface is unnecessary. The addition of the element into the matrix metal may be effected by a conventional procedure usually adopted for preparation of alloys. For example, the matrix metal is melted in a crucible in the air or in an inactive atmosphere, and after the element in the form of a simple substance or a compound form is added thereto, the mixture is stirred well and cooled. In some cases, powdery matrix metal may be admixed with powdery inorganic or organic compound of the element. 
     The preparation of the composite material of the invention may be effected by various procedures such as liquid phase methods (e.g. liquid-metal infiltration method), solid phase methods (e.g. diffusion bonding), powdery metallurgy (sintering, welding), precipitation methods (e.g. melt spraying, electrodeposition, evaporation), plastic processing methods (e.g. extrusion, compression rolling) and squeeze casting method. Among these procedures, particularly preferred are the liquid-metal immersion method and the high pressure coagulation casting method in which the melted metal is directly contacted with the fiber. A sufficient effect can be also obtained in other procedures mentioned above. 
     The thus prepared composite materials show a great increase in mechanical strength as compared with the system not containing the element of the invention. It is an extremely valuable merit of the invention that the preparation of this composite material can be realized in a conventional manner by the aid of usual equipments without any alteration. 
     The present invention will be hereinafter explained further in detail by the following Examples which are not intended to limit the scope of the invention. 
    
    
     EXAMPLE 1 
     In a crucible made of graphite, aluminum having a purity of 99.99% by weight was melted under heating up to 700° C. in an argon atmosphere. A designed amount of the element in the form of simple substance as shown in Table 1  was added thereto, and the contents were stirred well and cooled to obtain a matrix alloy. 
     As the inorganic fiber, the following substances were employed: (1) alumina fiber having an average fiber diameter of 14 μm, a tensile strength of 150 kg/mm 2  and a Young&#39;s modulus of elasticity of 23,500 kg/mm 2  (Al 2  O 3   content, 85% by weight; SiO 2  content, 15% by weight); (2) carbon fiber having an average fiber diameter of 7.5 μm, a tensile strength of 300 kg/mm 2  and a Young&#39;s modulus of elasticity of 23,000 kg/mm 2  ; (3) free carbon-containing silicon carbide fiber having an average fiber diameter of 15  μm, a tensile strength of 220 kg/mm 2  and a Young&#39;s modulus of elasticity of 20,000 kg/mm 2  ; (4) silica fiber having an average fiber diameter of 9 μm, a tensile strength of 600  kg/mm 2  and a Young&#39;s modulus of elasticity of  7,400 kg/mm 2  ; and (5) boron fiber having an average fiber diameter of 140 μm, a tensile strength of 310 kg/mm 2  and a Young&#39;s modulus of elasticity of 38,000 kg/mm 2 . The inorganic fiber was introduced in parallel into a casting tube having an inner diameter of 4 mmφ. Then, the above obtained alloy was melted at 700° C. in an argon atmosphere, and one end of the casting tube was immersed therein. While the other end of the tube was degassed in vacuum, a pressure of 50 kg/cm 2   was applied onto the surface of the melted alloy, whereby the melted alloy was infiltrated into the fiber. This composite material was cooled to complete the combination. The fiber content of the composite material was regulated to become 50±1% by volume. 
     For comparison, a fiber-reinforced metal complex material comprising pure aluminum (purity, 99.99% by weight) as the matrix was prepared by the same procedure as above. The thus obtained fiber-reinforced metal composite materials were subjected to determination of flexural strength and flexural modulus. The results are shown in Table 1. In all of the composite materials comprising the alloy matrix, the mechanical strength was greatly increased in comparison with the composite materials comprising the pure aluminum matrix. 
     
                                           TABLE 1                                 
__________________________________________________________________________
               Element added                                              
                           Flexural                                       
                                 Flexural                                 
                     Amount                                               
                           strength                                       
                                 modulus                                  
Run No.                                                                   
     Inorganic fiber                                                      
               Kind  (% by wt.)                                           
                           (kg/mm.sup.2)                                  
                                 (kg/mm.sup.2)                            
__________________________________________________________________________
Example                                                                   
1    Alumina fiber                                                        
               Potassium                                                  
                     0.05  78.6  12800                                    
2    Alumina fiber                                                        
               Rubidium                                                   
                     0.05  108   12900                                    
3    Alumina fiber                                                        
               Cesium                                                     
                     0.005 89.2  12800                                    
4    Alumina fiber                                                        
               Cesium                                                     
                     0.05  110   12900                                    
5    Alumina fiber                                                        
               Cesium                                                     
                     0.10  115   12400                                    
6    Alumina fiber                                                        
               Strontium                                                  
                     0.008 78.1  12700                                    
7    Alumina fiber                                                        
               Strontium                                                  
                     1.0   122   13200                                    
8    Alumina fiber                                                        
               strontium                                                  
                     4.0   77.8  13800                                    
9    Alumina fiber                                                        
               Barium                                                     
                     0.004 98.8  13400                                    
10   Alumina fiber                                                        
               Barium                                                     
                     1.0   149   13400                                    
11   Alumina fiber                                                        
               Barium                                                     
                     4.0   118   12800                                    
12   Alumina fiber                                                        
               Bismuth                                                    
                     0.005 92.2  12100                                    
13   Alumina fiber                                                        
               Bismuth                                                    
                     0.5   130   12200                                    
14   Alumina fiber                                                        
               Indium                                                     
                     0.01  80.6  13100                                    
15   Alumina fiber                                                        
               Indium                                                     
                     1.0   88.0  12900                                    
16   Carbon fiber                                                         
               Cesium                                                     
                     0.05  64.4  12900                                    
17   Carbon fiber                                                         
               Barium                                                     
                     0.004 56.4  13800                                    
18   Carbon fiber                                                         
               Barium                                                     
                     1.5   65.8  12900                                    
19   Carbon fiber                                                         
               Bismuth                                                    
                     0.5   62.3  12800                                    
20   Silicon carbide fiber                                                
               Cesium                                                     
                     0.05  64.4  12900                                    
21   Silicon carbide fiber                                                
               Barium                                                     
                     0.004 63.2  11900                                    
22   Silicon carbide fiber                                                
               Barium                                                     
                     0.3   88.4  12000                                    
23   Silica fiber                                                         
               Bismuth                                                    
                     0.5   42.5   750                                     
24   Boron fiber                                                          
               Bismuth                                                    
                     1.0   76.1  20300                                    
Compar-                                                                   
ative                                                                     
Example                                                                   
25   Alumina fiber                                                        
               --    --    70.0  12600                                    
26   Carbon fiber                                                         
               --    --    43.0  13000                                    
27   Silicon carbide fiber                                                
               --    --    32.5  12100                                    
28   Silica fiber                                                         
               --    --    31.1   7300                                    
29   Boron fiber                                                          
               --    --    35.1  18200                                    
__________________________________________________________________________
 
    
     EXAMPLE 2 
     In a crucible made of graphite, aluminum having a purity of 99.99% by weight was melted under heating up to 700° C. in an argon atmosphere. A designed amount of the element in the form of compound as shown in Table 2 was added thereto, and the mixture was stirred well and then cooled to obtain a matrix alloy. 
     As the inorganic fibers, the same alumina fiber, carbon fiber and silicon carbide fiber as used in Example 1 were employed, and the same procedure as in Example 1 was used to obtain fiber-reinforced metal composite materials. The fiber content of the composite material was regulated to become 50±1% by volume. 
     The thus prepared fiber-reinforced metal composite materials were subjected to determination of flexural strength at room temperature. The results are shown in Table 2. All of the composite materials produced the marked increase of the mechanical strength in comparison with Comparative Example as shown in Table 1. 
     
                       TABLE 2                                                     
______________________________________                                    
          Element added                                                   
                              Amount Flexural                             
Run  Inorganic                (% by  strength                             
No.  fiber        Kind        wt.)   (kg/mm.sup.2)                        
______________________________________                                    
Ex-                                                                       
am-                                                                       
ple                                                                       
30   Alumina fiber                                                        
                Cesium chloride                                           
                               0.05  108                                  
31   Alumina fiber                                                        
                Barium chloride                                           
                              0.5    97.1                                 
32   Alumina fiber                                                        
                Barium hydroxide                                          
                              0.5    90.3                                 
33   Alumina fiber                                                        
                Bismuth chloride                                          
                              1.0    85.5                                 
34   Alumina fiber                                                        
                Cesium sulfate                                            
                              0.1    98.6                                 
35   Alumina fiber                                                        
                Cesium nitrate                                            
                              0.1    96.9                                 
36   Alumina fiber                                                        
                Rubidium carbonate                                        
                              0.1    87.1                                 
37   Alumina fiber                                                        
                Strontium acetate                                         
                              0.5    85.7                                 
38   Alumina fiber                                                        
                Cesium ethyl oxide                                        
                              0.1    80.3                                 
39   Alumina fiber                                                        
                Barium methyl-                                            
                              0.5    81.2                                 
                sulfate                                                   
40   Carbon fiber                                                         
                Barium chloride                                           
                              0.5    64.2                                 
41   Silicon    Barium chloride                                           
                              0.5    73.9                                 
     carbide fiber                                                        
______________________________________                                    
 
    
     EXAMPLE 3 
     In this example, magnesium, copper or nickel is employed as the matrix metal. 
     In case of magnesium, commercially available pure magnesium (purity, 99.9% by weight) was melted under heating up to 700° C. in an argon atmosphere in a crucible made of graphite. A designed amount of the element in the form of simple substance as shown in Table 3 was added thereto, and the mixture was stirred well and cooled to obtain a matrix alloy, which was then combined with the same alumina fiber as used in Example 1 by the same procedure as in Example 1 to obtain a fiber-reinforced metal composite material. For comparison, a composite material comprising pure magnesium as the matrix was prepared by the same procedure as above. The fiber content of the composite material was regulated to become 50±1% by volume. 
     In case of copper, the same alumina fiber as in Example 1 was immersed into a dispersion obtained by dispersing copper powder (300 mesh pass) (98.0 g) and bismuth power (300 mesh pass) (2.0 g) in a solution of polymethyl methacrylate in chloroform to prepare an alumina fiber sheet whose surface was coated with powdery copper and bismuth. The sheet had a thickness of about 250μ and a fiber content of 56.7% by volume. Ten of the sheets were piled and charged into a carbon-made casting tool, which was placed into a vacuum hot press and heated at 450° C. with a vacuum degree of 10 -2  Torr to decompose polymethyl methacrylate as the sizing agent. The pressure and the temperature were gradually elevated, and the final condition of 10 -3  Torr, 650° C. and 400 kg/mm 2  was kept for 20 minutes to obtain a fiber-reinforced metal composite material. For comparison, a fiber-reinforced metal composite material comprising copper alone as the matrix was prepared by the same procedure as above. 
     In case of nickel, the same alumina fiber as used in Example 1 was immersed into a dispersion obtained by dispersing Ni-2.0% by weight Ba alloy powder in a solution of polymethyl methacrylate in chloroform to prepare an alumina fiber sheet whose surface was coated with Ni-2.0% by weight Ba alloy powder. This sheet had a thickness of about 250μ and a fiber content of 55.4% by volume. Ten of the sheets were piled and charged into a carbon-made casting tool, which was placed into a vacuum hot press and heated at 450° C. for 2 hours with a vacuum degree of 10 -2  Torr to decompose polymethyl methacrylate as the sizing agent. The pressure and the temperature were then gradually elevated, and the final condition of 10 -3  Torr, 900° C. and 400 kg/mm 2  was kept for 30 minutes to obtain a fiber-reinforced metal composite material. For comparison, a fiber-reinforced metal composite material comprising Ni alone as the matrix was prepared by the same procedure as above. 
     These complex materials were subjected to determination of flexural strength at room temperature. The results are shown in Table 3. All of the complex materials produced the great increase of the strength in comparison with Comparative Example as shown therein. 
     
                       TABLE 3                                                     
______________________________________                                    
                          Flexural strength                               
Run No.        Matrix metal                                               
                          (kg/mm.sup.2)                                   
______________________________________                                    
Example                                                                   
       42          Mg-0.08% Cs                                            
                              63.5                                        
       43          Mg-2.4% Ba 72.4                                        
       44          Mg-2.4% Bi 68.5                                        
       45          Cu-2.0% Bi 70.3                                        
       46          Ni-2.0% Ba 76.4                                        
Compar-                                                                   
       47          Mg         40.3                                        
ative  48          Cu         47.8                                        
Example                                                                   
       49          Ni         53.8                                        
______________________________________                                    
 
    
     EXAMPLE 4 
     As the inorganic fiber, alumina fiber, carbon fiber, silica fiber, silicon carbide fiber and boron fiber were employed. On the surface of each of these fibers, a coating layer of bismuth, indium, barium, strontium, radium, potassium, cesium or rubidium having a thickness of about 50 Å was formed by the vacuum evaporation method according to the fiber-metal combination shown in Table 4. The thus obtained metal-coated inorganic fiber was cut into 110 mm length in an argon atmosphere, and these pieces were bundled and introduced in parallel into a casting tube having an inner diameter of 4 mm. Into melted aluminum (purity, 99.99% by weight) kept at 700° C. in an argon atmosphere, one end of the casting tube was immersed, and while the other end was degassed in vacuum, a pressure of 50 kg/cm 2  was applied onto the surface of the melted aluminum, whereby the melted aluminum was infiltrated into the fiber. Then, the product was cooled to obtain a fiber-reinforced metal composite material. The fiber content was regulated to become 50±1% by volume. 
     The thus obtained fiber-reinforced metal composite material was subjected to determination of flexural strength and flexural modulus. The results are shown in Table 4. All of the cases using carbon fiber, aluminum fiber, silica fiber, silicon carbide fiber or boron fiber as the reinforcing material produced the great increase of the strength in comparison with Comparative Example as shown in Table 1. 
     
                       TABLE 4                                                     
______________________________________                                    
                            Flexural                                      
                                    Flexural                              
                  Coating   strength                                      
                                    modulus                               
Run No.                                                                   
       Fiber      element   (kg/mm.sup.2)                                 
                                    (kg/mm.sup.2)                         
______________________________________                                    
Example                                                                   
50     Alumina fiber                                                      
                  Indium    87.0    12900                                 
51     Alumina fiber                                                      
                  Barium    130     13000                                 
52     Alumina fiber                                                      
                  Strontium 95.4    12800                                 
53     Alumina fiber                                                      
                  Potassium 80.2    13200                                 
54     Alumina fiber                                                      
                  Cesium    98.1    13000                                 
55     Alumina fiber                                                      
                  Rubidium  96.9    13000                                 
56     Carbon fiber                                                       
                  Bismuth   60.5    12900                                 
57     Carbon fiber                                                       
                  Barium    62.3    13300                                 
58     Carbon fiber                                                       
                  Cesium    58.6    13200                                 
59     Silica fiber                                                       
                  Bismuth   41.4     9400                                 
60     Silica fiber                                                       
                  Strontium 42.8     9100                                 
61     Silica fiber                                                       
                  Rubidium  43.6     8800                                 
62     Silicon carbide                                                    
                  Bismuth   63.8    11900                                 
       fiber                                                              
63     Silicon carbide                                                    
                  Barium    66.2    12300                                 
       fiber                                                              
64     Silicon carbide                                                    
                  strontium 59.7    12200                                 
       fiber                                                              
65     Silicon carbide                                                    
                  Cesium    64.3    12300                                 
       fiber                                                              
66     Boron fiber                                                        
                  Bismuth   75.9    19800                                 
67     Boron fiber                                                        
                  Strontium 68.2    19600                                 
68     Boron fiber                                                        
                  Rubidium  70.1    20100                                 
______________________________________                                    
 
    
     EXAMPLE 5 
     As the inorganic fiber, the same alumina fiber, carbon fiber, silica fiber, silicon carbide fiber and boron fiber as in Example 1 were employed. Into a 2% by weight aqueous solution of barium chloride, cesium chloride or bismuth nitrate, the inorganic fiber was immersed according to the combination of inorganic fiber and metal as shown in Table 1 and then dried in a hot air drier at 130° C. for 3 hours. By observation of the fiber surface with a scanning electron microscope, it was confirmed that a coating layer having a thickness of 0.05-1.0 μm, though not uniform, was formed thereon. The thus treated inorganic fiber was cut into 110 mm long, and these pieces were bundled and introduced in parallel into a casting tube having an inner diameter of 4 mm. Into melted aluminum (purity, 99.99% by weight) kept at 700° C. in an argon atmosphere, one end of the casting tube was immersed, and while the other end was degassed in vacuum, a pressure of 50 kg/cm 2  was applied onto the surface of the melted aluminum, whereby the melted aluminum was infiltrated into the fiber. Then, the product was cooled to obtain a fiber-reinforced metal composite material. The fiber content was regulated to become 50±1% by volume. 
     The thus obtained fiber-reinforced metal composite material was subjected to determination of flexural strength and flexural modulus. The results are shown in Table 5. All of the cases using carbon fiber, aluminum fiber, silica fiber, silicon carbide fiber or boron fiber as the reinforcing material produced the great increase of the mechanical strength in comparison with Comparative Example as shown in Table 1. 
     
                       TABLE 5                                                     
______________________________________                                    
                Metal compound                                            
                             Flexural                                     
                                     Flexural                             
Run             used in surface                                           
                             strength                                     
                                     modulus                              
No.  Fiber      treatment    (kg/mm.sup.2)                                
                                     (kg/mm.sup.2)                        
______________________________________                                    
Ex-                                                                       
am-                                                                       
ple                                                                       
69   Carbon fiber                                                         
                Barium chloride                                           
                             57.2    13000                                
70   Carbon fiber                                                         
                Bismuth nitrate                                           
                             59.4    12800                                
71   Alumina fiber                                                        
                Barium chloride                                           
                             105     12800                                
72   Alumina fiber                                                        
                Cesium chloride                                           
                             110     12900                                
73   Alumina fiber                                                        
                Bismuth nitrate                                           
                             107     12500                                
74   Silica fiber                                                         
                Bismuth nitrate                                           
                             46.5     9200                                
75   Silicon carbide                                                      
                Barium chloride                                           
                             67.1    12500                                
     fiber                                                                
76   Silicon carbide                                                      
                Cesium chloride                                           
                             73.4    12600                                
     fiber                                                                
77   Boron fiber                                                          
                Bismuth nitrate                                           
                             70.8    18500                                
78   Boron fiber                                                          
                Barium chloride                                           
                             75.4    18200                                
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     EXAMPLE 6 
     On the surface of the same alumina fiber as used in Example 1, a coating layer of bismuth having a thickness of about 1000 Å was formed by the plasma spray method. Using the thus treated alumina fiber and magnesium (purity, 99.99% by weight) melted at about 700° C. in an argon atmosphere, a fiber-reinforced metal composite material was prepared in the same manner as in Example 1. Then, another fiber-reinforced metal composite material was prepared from the same alumina fiber as above and copper (purity, 99.99% by weight) melted at 1100° C. in an argon atmosphere in the same manner as in Example 1. These composite materials were subjected to determination of flexural strength. The results are shown in Table 6. In both cases, a higher flexural strength was obtained in comparison with Comparative Example as shown in Table 3. 
     
                       TABLE 6                                                     
______________________________________                                    
                       Coating  Flexural strength                         
Run No.     Matrix metal                                                  
                       metal    (kg/mm.sup.2)                             
______________________________________                                    
Example                                                                   
       79       Magnesium  Bismuth                                        
                                  62.8                                    
       80       Copper     Barium 63.5                                    
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     EXAMPLE 7 
     The same alumina fiber as in Example 1 was immersed into a 2% aqueous solution of barium chloride and then dried. The alumina fiber was subjected to reduction at 700° C. in the stream of hydrogen to precipitate out barium metal on the surface of the alumina fiber. Then, combination of the thus treated alumina fiber with aluminum was effected in the same manner as in Example 1 to obtain a fiber-reinforced metal composite material. The flexural strength of this composite material at room temperature was 124 kg/mm 2 . Thus, the great increase of the flexural strength was attained in comparison with Comparative Example in Table 1.