Abstract:
A metal matrix composite was fabricated by adding particles of calcium hexaboride to a metal of aluminum, magnesium or titanium and their alloys. The resulting metal matrix composite is light weight has improved strength, increased elastic modulus and reduced thermal coefficient of expansion, thus making the metal matrix composite more useful in industry.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to the improved mechanical and physical properties including strength, elastic modulus and reduced thermal expansion of metal bodies by using metal matrix composites, and more particularly to the reinforcement of aluminum, magnesium and titanium by forming metal matrix composites of these metal using calcium hexaboride particles.  
         BACKGROUND OF THE INVENTION  
         [0002]    The light weight metals of aluminum and magnesium have very large markets for they are utilized in a wide variety of industries. In a lesser way, titanium is also utilized as a light weight fabrication material. These metals suffer from some drawbacks, however, which limit their usefulness. These include low stiffness (low modulus of elasticity), high thermal coefficient of expansion, and low strength. Some of these drawbacks have been overcome through the use of metal matrix composites of these metals. Typically, ceramics are added to the metals. The primary objectives of these additives have been to increase the modulus of elasticity and to reduce the thermal coefficient of expansion. When fibrous material, such as silicon carbide whiskers, are added, strengthening has been observed. Other added materials include the fibers of boron metal, carbon, aluminum silicate, and aluminum oxide. Still other typical strengthening agents are aluminum oxide particulates, boron carbide and silicon carbide in various forms.  
           [0003]    Of these, only aluminum oxide particulate and silicon carbide particulate have been extensively utilized in the aluminum-based matrix. To add either of these to molten aluminum, a continuous stirring action must be utilized because the specific gravity of the additives are significantly greater than the molten aluminum. Similar problems would be observed with molten magnesium. The continuous stirring causes oxide inclusions and hydrogen to contaminate the melts.  
           [0004]    Furthermore, because of the contamination and the non-uniform nature of the metal matrix composites, remelting (for recycle, etc.) is a problem due to the variability of the resulting feed product.  
         OBJECTS OF THE INVENTION  
         [0005]    Accordingly, it is an object of the present invention to provide a metal matrix composite using molten metal of aluminum, magnesium and titanium wherein a minimum of stirring is required to maintain particles of the additive agent in suspension.  
           [0006]    It is another object of the present invention to provide a metal matrix composite wherein the strengthening agent has a specific gravity similar to that of the molten metal whereby there is little settling of the additive particles during the formation of the metal matrix composite.  
           [0007]    It is a further object of the present invention to provide a metal matrix composite wherein the additive particles increases the ductility of the metal matrix composite.  
           [0008]    Further and other objects of the present invention will become apparent from the description contained herein.  
         SUMMARY OF THE INVENTION  
         [0009]    In accordance with one aspect of the present invention, a new and improved metal matrix composite comprises a metal matrix composite formed from a molten metal selected from the group consisting of aluminum, magnesium, titanium and mixtures thereof, and particles of calcium hexaboride. The amount of calcium hexaboride particles present in the metal is in the range of about 0.1 to about 80 wt. %.  
           [0010]    In accordance with another aspect of the present invention, a new and improved metal matrix composite comprises a metal matrix composite formed from a molten metal selected from the group consisting of aluminum, magnesium, titanium and mixtures thereof, and particles of calcium hexaboride and particles of silicon hexaboride. The amount of particles of calcium hexaboride and particles of silicon hexaboride in the metal is in the range of about 0.1 to about 80 wt. %.  
           [0011]    In accordance with another aspect of the present invention, a new and improved article of manufacture comprises a metal matrix composite. The metal matrix composite comprises a metal selected from the group consisting of aluminum, magnesium, titanium and mixtures thereof, and particles of calcium hexaboride. The amount of particles of calcium hexaboride in the metal is in a range of about 0.1 to about 80 wt. %.  
         DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
         [0012]    A metal matrix composite was fabricated by adding particles of calcium hexaboride to molten aluminum. The calcium hexaboride was supplied by WACKER-CHEMIE of Kempten Germany. Since the specific gravity of the calcium hexaboride is very close to that of aluminum, only a minimal amount of stirring was required to achieve a homogeneous mixture. If heating is accomplished in an induction furnace, a stirring action is automatically achieved. Some mechanical stirring is required under other conditions of heating.  
           [0013]    While a range of compositions of from about 0.1 wt. % to about 80 wt. % of calcium hexaboride can be utilized relative to the aluminum, a range of about 5 to about 40 wt. % is most practical for most applications and was utilized for testing. The calcium hexaboride typically has an average particle size range of about 0.1 to about 200 microns. The resultant metal matrix is light weight has improved strength, increased ductility and reduced thermal coefficient of expansion.  
           [0014]    The addition of the calcium hexaboride to the molten metal was principally utilized in the development of the present invention. However, it will be understood that the invention also includes the blending of the calcium hexaboride particles with powdered aluminum metal and any other alloying constituents prior to melting the mixture.  
           [0015]    In addition a mixture of calcium hexaboride particles and silicon hexaboride particles may be added to the molten metal. A silicon hexaboride prepared by a substantially commercial process can be used and was comparable with that supplied by CERAC of Chicago, Ill. and typically has an average particle size range of about 0.1 to about 200 microns.  
           [0016]    The molten mixture can be cast into a desired shape as a manufactured product, such as horseshoes, memory disk substrates, actuator arms for disk drives or any product which would benefit from the improved mechanical and physical properties of the metal matrix composite of this invention. 
       
    
    
     EXAMPLE 1  
       [0017]    A metal matrix composite horseshoe was made from an aluminum matrix composite. The aluminum matrix composite contained from about 5 to about 10 wt % calcium hexaboride particles having an average particle size of about 75 microns and the remainder being A356 aluminum metal. The resulting horseshoe was light, strong, abrasion resistant and unexpectedly ductile. The horseshoe could be bent at an angle of 45 degrees without damage.  
       EXAMPLE 2  
       [0018]    A metal matrix composite memory disk substrate was made from an aluminum matrix composite. The aluminum matrix composite contained from about 40 wt % calcium hexaboride particles having an average particle size of about 10 microns and the remainder being aluminum metal. The resulting memory disk substrate was light, strong, abrasion resistant and ductile.  
       EXAMPLE 3  
       [0019]    A metal matrix composite actuator arm for a hard drive was made from an aluminum matrix composite. The aluminum matrix composite contained about 30 wt % calcium hexaboride particles having an average particle size of about 50 microns and the remainder being A356 aluminum metal. The resulting actuator arm for a hard drive was light, strong, abrasion resistant and unexpectedly ductile.  
         [0020]    An unexpected increase in ductility was observed in the products made by the addition of calcium hexaboride particles into the metal matrix composites relative to other metal composites.  
         [0021]    Magnesium and titanium have low specific gravities similar to that of aluminum. Accordingly, metal matrix composites of these metals with calcium hexaboride is within the scope of the present invention.  
         [0022]    From the foregoing, it will be understood that improved metal matrix composites of aluminum, magnesium and titanium are achieved by the addition of calcium hexaboride particles. The composition can be easily prepared with a minimum of stirring, and the product can be recycled if desired.  
         [0023]    While there has been shown and described what is at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.