Abstract:
According to typical inventive practice, a first metallic material is poured into a mold including a bottom inside surface having regularly arrayed rises (truncated spherical convexities). The molten first metallic material cools and solidifies to include a surface correspondingly having regularly arrayed dents (truncated spherical concavities). The resultant “inner casting” is removed from and repositioned in the mold so that the inner casting&#39;s dent-laden surface faces upward. Ceramic spheres are placed in the dents. A second metallic material (having a higher melting point than the first metallic material) is poured into the mold with the inner casting and spheres in place. The molten second metallic material cools and solidifies as an “outer casting” surrounding the inner casting and the spheres. The resultant integral armor structure includes the inner casting, the outer casting, and the spheres, each sphere embedded partially in the inner casting and partially in the outer casting.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to ballistic armor systems, more particularly to composite ballistic armor systems that include a metallic matrix and one or more metallic or non-metallic elements contained therein. 
     Military armor applications include land, air and sea vehicles, stationary structures, and personnel. The need for lighter weight and more effective armor plating for protecting various military vehicles is ongoing, especially as enemy munitions become increasingly powerful. Protection of the vehicles and their occupants is needed against impact by a projectile such as a ballistic body (e.g., small arms fire) or an explosive fragment (e.g., shrapnel from a bomb blast). Conventional metal vehicle armor systems basically consist of metal alloy plates, principally steel. These conventional armor systems are becoming prohibitively heavy in order to protect vehicles from increasingly formidable attack capabilities. 
     A metal matrix composite (MMC) material is a composite material having a metallic matrix and one or more elements, metallic or non-metallic, contained in the metallic matrix. One approach that has been considered for constructing an armor system that is both strong and lightweight involves the utilization of one or more hard solid elements and a relatively lightweight metallic material (elemental metal or metal alloy) as a matrix material for containing the elements. Generally speaking, according to the theory of operation of a metal matrix composite armor system, an element or elements contained in a metallic matrix serve to absorb the energy of an impinging projectile by dissipating the energy into a volume surrounding the penetration point. 
     For instance, hard spheres (e.g., ceramic spheres of uniform size) have been considered for embedment within a lightweight metal such as an aluminum alloy. For optimal efficiency of energy dissipation of an impinging projectile, the embedded spheres should be arranged in a regular array so that the spheres are not in contact with each other, and so that there is a good bond between the spheres and the metal matrix. Fabrication of a metal matrix composite armor system containing spherical elements has been problematical insofar as achieving these objectives. 
     Aluminum oxide (commonly called “alumina”), silicon carbide, boron carbide, and titanium carbide are ceramic materials that are known to be suitable for armor applications. These conventional armor ceramics have been used in conventional practice of armor systems. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, an object of the present invention is to provide a metal matrix composite armor system that is durable and lightweight and that affords effective performance in resisting projectile impact. 
     A further object of the present invention is to provide an efficient and cost-effective method of producing such a metal matrix composite armor system. 
     According to a typical inventive method for making a composite armor system, a mold includes a base portion having a mold surface characterized by plural elevations. A liquid first metallic material is poured into the mold. The first metallic material has a lower melting point than has the material (typically metallic, e.g., steel) of which the mold is composed. The base portion is heated for a period of time during and after the pouring of the liquid first metallic material into the mold. Typically, the heating of the mold is ceased several minutes after the pouring of the first metallic material is commenced and completed. The poured first metallic material gradually cools after the heating of the mold is ceased, as the mold gradually cools. A first casting is removed from the mold. The first casting is composed of the first metallic material, solidified. The first casting has a casting surface characterized by plural depressions corresponding to the plural elevations of the base portion&#39;s mold surface. 
     The first casting is positioned in the mold upon the mold surface so that the first casting&#39;s casting surface faces upward. Plural embedment elements are placed in the depressions of the casting surface. A liquid second metallic material is poured into the mold with the first casting thus positioned upon the mold surface. The second metallic material has a lower melting point than has the first metallic material. The base portion is heated for a period of time during and after the pouring of the liquid second metallic material into the mold. The second metallic material is permitted to infiltrate, with heating persisting for a suitable period of time to promote coverage and bonding among all of the adjacent surfaces of second metallic material, the first casting, and the embedment elements. Typically, the heating of the mold is ceased several minutes after the pouring of the second metallic material is commenced and completed. The poured second metallic material gradually cools after the heating of the mold is ceased, as the mold gradually cools. An integral structure is removed from the mold. The integral structure includes the first casting, the embedment elements, and a second casting. The second casting is composed of the second metallic material, solidified. 
     The inventive fabrication method described in the two preceding paragraphs represents a kind of “dual casting” methodology. In accordance therewith, a first (inner) metallic component of the inventive armor product is cast using a mold. The mold, together with the first (inner) metallic component and plural embedment elements, is subsequently used again to cast a second (outer) metallic component, thereby forming the inventive armor device comprising the first (inner) metallic component, the embedment elements, and the second (outer) metallic component. According to an alternative inventive approach to making an inventive armor device, the inventive practitioner does not cast the first (inner) metallic component. Rather, the inventive practitioner provides the first (inner) metallic component by first obtaining a metallic (e.g., titanium) plate that is smooth on both faces, and then creating indentations in one of the faces, such as via embossing or another known technique for “dimensionalizing” a smooth metal surface. 
     The present invention&#39;s integral armor structure is typically configured, in terms of its proportions, as an armor “plate” having small through-plane thickness relative to its in-plane length and width. The inventive integral structure represents a composite armor system including metallic matrix material and plural embedment elements embedded in the metallic matrix material. The present invention&#39;s composite armor system includes a layered configuration whereby the embedment elements are situated at an interface between the first casting and the second casting. To enhance the strength (e.g., delamination resistance) of the integral structure, the second casting should be rendered completely exteriorly with respect to the first casting and the embedment elements. Each embedment element of the composite armor system is partially embedded in the first casting and partially embedded in the second casting. According to frequent inventive practice, the embedment elements are spherical. Each spherical embedment element is embedded in the first casting between approximately one-third and one-half of its diameter, and is embedded in the second casting between approximately one-half and two-thirds of its diameter. 
     The present invention lends itself to varied practice in several respects. Numerous metals and metal alloys can be used for the mold material, the first (inner) casting material, and the second (outer) casting material. The present invention&#39;s mold is typically made of steel, but can be made of another (typically, metallic) suitable material. The mold can be designed and constructed to be re-usable by an inventive practitioner. Good casting materials should be used for the first (inner) and second (outer) castings, especially for the second (outer) casting; generally, there are many metallic materials that are known to be suitable casting materials, and these can be considered for inventive practice. Steel is a preferred material for the mold, but other suitable mold materials can be used. A preferred first (inner) casting material is a titanium alloy. A preferred second (outer) casting material is an aluminum alloy (e.g., A356). A typical aluminum alloy is lightweight and strong, and not many other metallic materials meet both criteria as well. Some aluminum alloys and some other alloys are precipitation-hardened, and thus may represent a stronger metallic material. 
     In inventive testing, the present inventor used a titanium alloy as the first (inner) casting material and A356 aluminum alloy as the second (outer) casting to fabricate a small inventive prototype exhibiting excellent material properties. An aluminum alloy and a titanium alloy may afford combined attributes of light weight and strength. Another option for the first (inner) casting material that may be suitable for some inventive embodiments is Al-25% Mn alloy, an aluminum alloy composed of twenty-five percent manganese; however, a titanium alloy has less porosity and hence may be more suitable than an Al-25% Mn alloy. Steel (an alloy of iron and carbon) may be another option for the first (inner) casting material, but its drawback may be its heavy weight. 
     Particularly important in inventive practice are the requirements for selecting materials that are suitable in terms of the relative melting temperatures of the materials. The mold material must have a higher melting temperature than the first (inner) casting material. The first (inner) casting material must have a higher melting temperature than the second (outer) casting material. Similarly, the embedded element material (e.g., ceramic) must have a higher melting temperature than the second (outer) casting material. The first (inner) casting material should not have any low temperature eutectic point. Materials should be selected to suit the particular armor applications for which the inventive embodiments are intended. Generally speaking, the metallic casting materials should be strong and lightweight. Materials should be selected in terms of compatibilities, not only with respect to melting temperatures, but also to promote wetting of solid casting materials by liquid casting materials. No pyrophoric materials (e.g., magnesium) should be used in inventive fabrication; a pyrophoric material is commonly regarded, in a general sense, as a material that automatically or spontaneously ignites or bursts into flames on contact with or exposure to air or another oxygen-containing substance. 
     The term “wetting” is conventionally understood to refer to contact between a liquid material and a solid material. Wetting is associated with intermolecular interactions between the liquid and solid materials that are brought together. Generally speaking, the amount of wetting relates to the contact angle between the liquid-gaseous interface and the solid-liquid interface. The smaller is the contact angle, the greater is the wetting. Furthermore, the greater is the wetting, the greater is the tendency of the liquid to spread over a larger area of the solid surface, and hence the better is the adherence (bonding) between the liquid material and the solid material. A high degree of wetting—and hence, of adherence/bonding—is desirable in the first casting process and especially in the second casting process of the inventive fabrication methodology. In the present invention&#39;s first casting process, extensive wetting is preferred of the mold by the liquid first casting material. In the present invention&#39;s second casting process, extensive wetting is preferred of the solid first casting material and the solid spheres, by the liquid second (outer) casting material. 
     In both the first and second casting processes, the heat should continue to be applied for several minutes after pouring, so that the metallic casting material remains molten for several minutes after pouring, thereby ensuring bonding of all surfaces; at an appropriate point, the heat can be turned off so that the mold gradually cools down. In other words, the mold should be heated for a suitable period for the first casting process, and re-heated for a suitable period for the second casting process. The temperature of the mold should be at or near the melting point of the metallic casting material used to pour into the mold. 
     In order to optimize bonding, inventive practice frequently prefers that the second (outer) casting totally surround the first (inner) casting. If the first (inner) casting material and the second (outer) casting material merely describe discrete adjacent layers, with no surrounding of the first (inner) casting material by the second (outer) casting material, the risk of delamination will be greater. During the second casting process, some of the second (outer) casting material, which is typically very fluid, flows around the first (inner) casting and between the first (inner) casting and the mold; that is, the second (outer) casting material “crawls” below the first (inner) casting and above the topside dimpled surface of the mold. Enough second (outer) casting material should be poured to completely cover/coat the spheres and leave some degree of thickness above the spheres. 
     The desired thickness of the second (outer) casting material above the embedment elements may depend on the contemplated armor application of the inventive armor product. Generally in the case of spherical embedment elements, in furtherance of bonding of the liquid second (outer) casting material to surfaces of the spherical elements and the solid first (inner) casting, inventive practice calls for a thickness, above the spherical elements, of the second (outer) casting material that is in the approximate range between one-quarter and one-third of the diameter of the spherical elements. The inventive practitioner can weigh the armor-related benefit of additional above-spheres thickness of the second (outer) casting material, versus the detriment thereof in terms of the additional weight associated with the additional volume of the second (outer) casting material. 
     It is emphasized that an inventive armor product following the second casting process can be subjected to further inventive processing, such as being machined and/or shaped and/or bent and/or combined with another structure, to suit one or more contemplated armor applications. Of particular note, two or more inventive armor structures can be combined to form a larger inventive armor device comprising the smaller inventive armor structures. For instance, plural inventive armor structures, each characterized by planar layers including a planar layer of embedment elements, can be stacked to form a multi-layered armor device having plural planar layers of embedment elements. Additionally or alternatively, plural inventive armor structures can be arranged side-by-side to form an armor device having a larger planar area, thus presenting a larger strike face for defending against projectiles. 
     The embedment elements can be made of any hard and relatively tough material, such as a metallic material, a polymeric material, a glass material, or a ceramic material. Examples of suitable ceramic materials include silicon carbide (SiC), boron carbide (BC), titanium carbide (TiC), aluminum oxide (Al 2 O 3 ), boron nitride (BN), etc. Ceramic balls suitable for inventive practice are commercially available, primarily manufactured for bearing applications. For instance, for his inventive testing the present inventor obtained hard silicon nitride (SiN) spheres from Saint Gobain Ceramics CERBEC® USA, East Granby, Conn. 06026, www.cerbec.com. 
     According to frequent inventive practice, prior to being placed in the indentations, the embedment elements are coated with another material to provide a surface that promotes wetting by the second (outer) metallic casting material. For instance, a silver surface can be provided for the embedment elements to promote bonding of the embedment elements to the second (outer) casting material, e.g., a suitable aluminum casting alloy. For instance, the present inventor obtained (from the aforementioned manufacturer Saint Gobain Ceramics) ceramic (SiN) spheres, each having a mirror-smooth surface. The present inventor placed the SiN spheres, along with boron carbide powder (B 4 C, −325 mesh) in a ball mill, and milled the SiN spheres and BaC powder together for several hours. The B 4 C powder was found, with some searching by the present inventor, to be the only material available that was harder than the SiN of the spheres. As a result of the milling, the surfaces of the ceramic spheres were abraded. The ceramic spheres were then coated with silver in accordance with the method disclosed by the present inventor at U.S. Pat. No. 5,091,362, issued date 25 Feb. 1992, invention title “Method for Producing Silver Coated Superconducting Ceramic Powder,” incorporated herein by reference. The method of Ferrando U.S. Pat. No. 5,091,362 involves decomposition of a silver-containing compound to form a thin uniform coating of silver metal on the surface of a particle. 
     According to many preferred inventive embodiments, the embedment elements are spherical. For instance, spherical embedment elements can all be congruent (geometric spheres with equal diameters), and can be arranged in a regular pattern to be embedded thusly. Nevertheless, multifarious shapes, sizes, and distributions of the embedment elements can be inventively selected and effected, with the ultimate armor objectives (such as deflections of particular projectiles) kept in mind by the inventive practitioner. For instance, embedment elements of varying shapes and/or sizes can be used within a single array of embedment elements essentially describing a single geometric plane. If embedment elements all of the same shape (e.g., spherical) are used, they can be of the same size or different sizes. Additionally or alternatively, the coplanar embedment elements can be arranged in any of a variety of one-dimensional patterns. Moreover, instead of spherical, the embedment elements can be prolate spheroidal (e.g., egg-shaped elements having parallel longitudinal axes) or cylindrical (e.g., short rod-shaped elements having co-planar longitudinal axes). 
     It is generally preferred inventive practice that the embedment elements be spaced apart (i.e., not touch each other) when they are placed in the indentations of the first (inner) casting, so that they will be spaced apart accordingly when they are completely embedded in dichotomized metallic materials via the second inventive casting process. At least slight separations between the embedded elements are preferred, because the inventive armor product will thus be more effective in defending against projectiles. For instance, if spherical embedded elements are at least slightly separated from each other, they will transfer energy directly from one ball to another upon impact by a projectile. Therefore, the design of the original mold, particularly with respect to its “pimpled” surface, is significant. The protuberances of the original mold&#39;s bottom inner surface should be arranged in such a way that the embedded elements, when placed in the first (inner) casting&#39;s indentations corresponding to the mold&#39;s protuberances, are completely separated from each other. 
     Other objects, advantages and features of the present invention will become apparent from the following detailed description of the present invention when considered in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein like numbers indicate same or similar parts or components, and wherein: 
         FIG. 1  is a top plan view of an embodiment of a “pimpled” mold in accordance with the present invention. The mold includes a base section having a pimpled horizontal upper surface. 
         FIG. 2  is a cross-sectional elevation view of the mold shown in  FIG. 1 , taken essentially through a geometric plane indicated by section line  2 - 2  in  FIG. 1 . Also shown in  FIG. 2  are symbolic representations of heating elements associated with the base section of the mold. 
         FIG. 3  is a diagram, including the view of  FIG. 2 , illustrating the pouring of a “first” liquid metallic material into the mold in accordance with an embodiment of an armor fabrication method of the present invention. 
         FIG. 4  is a view, similar to the views of  FIG. 2  and  FIG. 3 , of the mold and the dimpled “first” metallic casting (i.e., the first metallic material, solidified). 
         FIG. 5  is a cross-sectional elevation view, sectioned essentially through the dimples, of the first metallic casting shown in  FIG. 4 . As shown in  FIG. 5 , the first metallic casting&#39;s dimpled side faces downward. 
         FIG. 6  is an elevation view of the first metallic casting, similar to the cross-sectional view of  FIG. 5 .  FIG. 6  illustrates vertical peripheral grooves provided in the edgewise periphery of the first metallic casting, and also shows the first metallic casting&#39;s interior “dimples” in transparency. 
         FIG. 7  is the view of  FIG. 5 , flipped (upturned) so that the first metallic casting&#39;s “dimpled” side faces upward. The cross-sectional view of  FIG. 7  is taken essentially through a geometric plane indicated by section line  7 - 7  in  FIG. 9 . 
         FIG. 8  is a geometric planar profile representative of the cross-sectional view of  FIG. 7 . 
         FIG. 9  is a top plan view of the first metallic casting shown in  FIG. 7 . 
         FIG. 10  is the view of  FIG. 2  (of the mold) together with the view of  FIG. 7  (of the first metallic casting). As illustrated in  FIG. 10 , the first metallic casting is situated, dimpled side horizontal and up, atop the pimpled horizontal upper surface of the base section of the mold. The cross-sectional view of  FIG. 10  is taken essentially through a geometric plane indicated by section line  10 - 10  in  FIG. 11 . 
         FIG. 11  is a top plan view of the mold-plus-casting assembly shown in  FIG. 10 . 
         FIG. 12  is the view of  FIG. 10 , additionally showing placement of spherical elements in the dimples of the first metallic casting. The cross-sectional view of  FIG. 12  is taken essentially through a geometric plane indicated by section line  12 - 12  in  FIG. 14 . 
         FIG. 13  is a diametric cross-sectional view of one of the spherical elements shown in  FIG. 12 , in particular illustrating a ceramic core and a metallic (e.g., silver) coating. 
         FIG. 14  is a top plan view of the mold-plus-casting-plus-spheres assembly shown in  FIG. 12 . In other words,  FIG. 14  is the view of  FIG. 11 , additionally showing placement of spherical elements in the dimples of the first metallic casting. 
         FIG. 15  is a diagram, including the view of  FIG. 12 , illustrating the pouring of a “second” liquid metallic material into the mold-plus-casting-plus-spheres assembly in accordance with an embodiment of an armor fabrication method of the present invention. 
         FIG. 16  is a view, similar to the views of  FIG. 12  and  FIG. 15 , of the mold-plus-casting-plus-spheres assembly in combination with the “second” metallic casting (i.e., the second metallic material, solidified). 
         FIG. 17  is a view, similar to the view of  FIG. 16 , of an embodiment of the present invention&#39;s composite armor system. The cross-sectional view of  FIG. 17  is taken essentially through a geometric plane indicated by section line  17 - 17  in  FIG. 18 . The inventive composite armor system shown in  FIG. 17  is a product of inventive fabrication method steps including those illustrated in  FIG. 1  through  FIG. 17 . 
         FIG. 18  is a top plan view of the inventive composite armor product shown in  FIG. 17 . 
         FIG. 19  is a bottom plan view of the inventive composite armor product shown in  FIG. 17 . 
         FIG. 20  is a partial and enlarged view, similar to the view of  FIG. 17 , of an inventive composite armor product embodiment that is bent in order to conform to a particular curved surface, such as of a vehicle characterized by surface contours. 
         FIG. 21  and  FIG. 22  are diagrams that include the view of  FIG. 17 , rotated ninety degrees.  FIG. 20  and  FIG. 21  illustrate two opposite orientations of an inventive composite armor product embodiment with respect to an impinging projectile. 
         FIG. 23  is a perspective view of a cross-bored metallic block in accordance with another mode of inventive practice. As illustrated in  FIG. 23  through  FIG. 25 , parallel horizontal channels and parallel vertical channels (which are narrower than the horizontal channels) intersect each other at “drop-in” locations suitable for placement of spherical elements (which are narrower than the horizontal channels but wider than the vertical channels). 
         FIG. 24  and  FIG. 25  are the same cross-sectional elevation view, sectioned essentially through one of the horizontal channels shown in  FIG. 23 . As illustrated in  FIG. 24  and  FIG. 25 , each spherical element can be placed by pushing it (and/or causing it to roll) along a horizontal channel so that the spherical element arrives and remains at a drop-in location. Each drop-in location is defined by the intersection of a horizontal channel and a vertical channel. 
         FIG. 26  is a cross-sectional plan elevation view of an inventive composite armor system embodiment that differs from the inventive composite armor system embodiment depicted in  FIG. 17 . The inventive composite armor systems of  FIG. 17  and  FIG. 26  are also made via different inventive fabrication methodologies. The inventive composite armor system shown in  FIG. 26  is a product of inventive fabrication method steps including those illustrated in  FIG. 22  through  FIG. 26 . The inventive composite armor system shown in  FIG. 26  is an integrated product that includes the cross-bored metallic block, the spherical elements, and the casting, wherein the casting is both infiltrative and circumscriptive of the cross-bored metallic block and the spherical elements. The cross-sectional view of  FIG. 26  is taken essentially through a geometric plane indicated by section line  26 - 26  in  FIG. 27 . 
         FIG. 27  is a cross-sectional plan view of the bored metallic block shown in  FIG. 21  with spherical elements distributed therein such as depicted in  FIG. 23 . Shown in transparency are the drop-in locations upon which the spherical elements rest, one spherical element per drop-in location. 
         FIG. 28  is a view, similar to the view of  FIG. 27 , of a bored metallic block with spherical elements distributed therein such as depicted in  FIG. 26 . The drop-in locations portrayed in  FIG. 27  are arrayed differently from the drop-in locations portrayed in  FIG. 28 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1  and  FIG. 2 , steel tray-like mold  40  includes a horizontal base plate portion  41  and four vertical wall portions  42 . The inside surfaces of mold  40  include the “pimpled” upper surface  43  of horizontal base plate portion  41  and the respective smooth (even) side surfaces  44  of vertical wall portions  42 . The pimpled upper surface  43  of base plate portion  41  is characterized by a regular pattern of congruent elevations  45 , each of which describes the geometric shape of a sphere that is horizontally truncated below its apex, at or above its horizontal planar bisector. Associated with mold  40  (for instance, coupled with base plate portion  41 ) are heating devices  50 . 
     With reference to  FIG. 3  through  FIG. 9 , the interior surfaces (including upper surface  43  and side surfaces  44 ) of mold  40  are coated, as appropriate, with a mold release agent (e.g., zirconium oxide or zirconia). Heating devices  50  serve to extremely raise the temperature of mold  40  and thereby facilitate casting processes in accordance with the present invention. Heating devices  50  are activated to prepare for a first inventive metallic casting process. The melting point of mold  40  must be higher than the melting point of the first metallic casting material  100 , which is designated herein “ 100 L” when in liquid form, and “ 100 S” when in solid form. 
     As illustrated in  FIG. 3  and  FIG. 4 , hot liquid titanium or titanium alloy material  100 L is poured into mold  40 . For convenience, the titanium or titanium alloy is referred to herein simply as “titanium.” Enough molten metallic material  100 L should be poured not only to completely cover the pimpled upper surface  43 , but also to provide an additional thickness of the molten metallic material  100 L above the elevations  45 . The amount poured of the molten metallic material  100 L, which determines the additional thickness of the solidified metallic material  100 L, may depend on the contemplated application(s) of the completed inventive armor  500 . 
     Mold  40  should be heated via heating devices  40  to a temperature at or near the melting point of the first metallic casting material  100  for a suitable period of time (e.g., for several minutes) to ensure complete settling of the first liquid metallic casting material  100 L within mold  40 . Several minutes after the first metallic material  100 L is poured, the heating devices  50  are inactivated. The molten titanium  100 L is permitted to cool and solidify for several hours to form a first inventive metallic casting  100 S, which is a solid titanium piece. 
     First metallic casting  100 S is removed from mold  40 . First metallic casting  100 S is a metallic plate having two opposite faces, namely, a smooth (even) surface  101  and a “dimpled” surface  102 . Dimpled surface  102  represents a kind of “egg crate” configuration. Dimpled surface  102  is characterized by a regular pattern of congruent depressions  105 , each of which describes the geometric shape of a sphere that is horizontally truncated above its nadir, at or below its horizontal planar bisector. The congruent depressions  105  of dimpled surface  102  correspond to the congruent elevations  45  of pimpled surface  43 . 
     Before first metallic casting  100 S is situated in an inverted horizontal position within mold  40 , an optional and sometimes preferred embellishment in inventive practice is to machine vertical grooves  110  (such as shown in  FIG. 6 ) around the periphery  109  of first metallic casting  100 S. Grooves  110  will serve as flow channels for facilitating the downward gravitational flow of the second liquid metallic casting material  200 L, during a second inventive metallic casting process. 
     Now referring to  FIG. 10  through  FIG. 17 , the interior surfaces (including upper surface  43  and side surfaces  44 ) of mold  40  are coated again, as appropriate, with a suitable mold release agent (e.g., zirconium oxide or zirconia). The first metallic casting  100 S is positioned in mold  40  in an inverted orientation—i.e., with the depressions  105  facing upward, as shown in  FIG. 7  through  FIG. 11 . In other words, first metallic casting  100 S is inverted vis-à-vis its orientation when cast in mold  40 , as shown in  FIG. 4  and  FIG. 5 . The periphery  109  of the first metallic casting  100 S abuts the inwardly facing side surfaces  44  of the mold  40 &#39;s vertical wall portions  44 . 
     Spherical elements  300  are placed in the upward facing depressions  105  of the first metallic casting  100 S, one spherical element  300  per depression  105 . Spherical elements  300  should be characterized by an at least slightly smaller diameter than are the depressions  105 , in order that the spherical elements can be placed in the depressions  105  and remain in place. Preferably for many inventive embodiments, spherical elements  300  are slightly smaller in diameter than depressions  105  in order that the spherical elements fit snugly when placed in the depressions  105 . Frequently preferred inventive practice utilizes spherical elements  300  each having a ceramic core  301  and a silver coating  302  such as depicted in  FIG. 13 , the silver coating having been provided in accordance with the afore-noted methodology taught by Ferrando U.S. Pat. No. 5,091,362. 
     As shown in  FIG. 2  through  FIG. 5 , elevations  45  geometrically constitute a truncated sphere having slightly less than one-half of the diameter of an entire sphere. Since the depressions  105  of first metallic casting  100 S are cast from the elevations of mold  40 , depressions  105  likewise geometrically constitute a truncated sphere having slightly less than one-half of the diameter of an entire sphere, as shown in  FIG. 5 ,  FIG. 7 ,  FIG. 8  and  FIG. 10 . Therefore, as shown in  FIG. 12  and  FIG. 15  through FIG.  17 —and there is some approximation here because each spherical element  300  is shown to be slightly smaller than its corresponding depression  105 —each spherical element  300  is recessed within a depression  105  to a corresponding depth of slightly less than one half of the diameter of the spherical element  300 . According to typical inventive practice, each spherical element  300  is recessed within a depression  105  to a depth in the approximate range between one-third and one-half of the diameter of the spherical element  300 . As the present invention is frequently practiced, congruent spherical elements  300  are all recessed within their corresponding depressions  105  to the same or approximately the same depth. In accordance with the spacing of the mold  40 &#39;s elevations  45  and hence of the first metallic casting  100 S&#39;s depressions  105 , the spherical elements  300  when placed in the depressions  105  are spaced apart from each other. 
     Heating devices  50  are activated again to prepare for the second inventive metallic casting process. The melting point of mold  40  must be higher than the melting point of both the first metallic casting material  100  and the second metallic casting material  200 . Further, the melting point of the first metallic casting material  100  must be higher than the melting point of the second metallic casting material  200  (which is designated herein “ 200 L” when in liquid form, and “ 200 S” when in solid form). 
     As illustrated in  FIG. 15  and  FIG. 16 , hot liquid aluminum or aluminum alloy material  200 L is poured into the mold assembly  400 , which includes mold  40 , first metallic casting  100 S, and spherical elements  300 . For convenience, the aluminum or aluminum alloy is referred to herein simply as “aluminum.” Enough molten metallic material  200 L should be poured not only to completely cover the dimpled surface  102  and spherical elements  300 , but also to seep around and below the first metallic casting  100 S as well as to provide an additional thickness of the molten metallic material  200 L above the spherical elements  300 . The amount poured of the molten metallic material  200 L, which determines the additional thickness of the solidified metallic material  200 L, may depend on the contemplated application(s) of the completed inventive armor  400 . 
     Mold  40  should be heated via heating devices  50  to a temperature at or near the melting point of the second metallic casting material  200  for a suitable period of time (e.g., for several minutes) to ensure complete flow of the second liquid metallic casting material  200 L within mold assembly  400  and circumscriptive of first metallic casting  100 S and spherical elements  300 ; in particular, complete bonding should be achieved of the second liquid metallic casting material  200 L with respect to the adjoining outside surfaces of the first metallic casting  100 S and the spherical elements  300 . 
     Several minutes after the molten second metallic material  200 L is poured, the heating devices  50  are inactivated. The molten aluminum  200 L is permitted to cool and solidify for several hours to form a second metallic casting  200 S, which is integrated with the first metallic casting  100 S and the spherical elements  300 . As depicted in  FIG. 17  through  FIG. 19 , the first metallic casting  100 S, the spherical elements  300 , and the second metallic casting  200 S together constitute a solid composite piece—more specifically, an inventive ceramic-embedded dual-metal matrix composite system  500 , a device suitable for armor applications. 
     A “straight” (planar) inventive embodiment is depicted in  FIG. 17  through  FIG. 19 . A “curved” (contoured) inventive embodiment is depicted in  FIG. 20 . Both straight/planar and curved/contoured inventive embodiments can be made in accordance with inventive fabrication methodology such as described herein with reference to  FIG. 1  through  FIG. 17 . A curved/contoured inventive embodiment would typically require an additional production phase involving bending or shaping of a straight/planar product of the inventive fabrication methodology. 
     The inventive composite armor system  500 , shown in  FIG. 17  to be removed from the mold  40 , is an integrated product that includes three components, viz., the first metallic casting  100 S, the spherical elements  300 , and the second metallic casting  200 S. Since the second metallic casting  200  component circumscribes (or nearly circumscribes) the first metallic casting  100  component and the spherical elements  300  component, the first metallic casting  100  component and the second metallic casting  200  component may be described as the “inner casting” and the “outer casting,” respectively. 
     Note that the second metallic casting  200 S component of the inventive composite armor system  500  includes an upper second metallic casting layer  521 , a lower second metallic casting layer  522 , and four peripheral second metallic casting layers  523 . The lower second metallic casting layer  521  covers the first metallic casting  100 S&#39;s smooth (even) surface  101 . The upper second metallic casting layer  522  covers: the upper portions of the spherical elements  300 ; the smooth/even portions of the first metallic casting  100 S&#39;s dimpled surface  102  that are between the spherical elements  300 ; the interface between the depressions  105  and the lower portions of the spherical elements  300 . The four peripheral second metallic casting layers  523  cover the first metallic casting  100 S&#39;s periphery  109 . 
     Typically during an inventive fabrication process, some liquid second metallic casting material  200 L seeps around the first metallic casting  100 S&#39;s periphery  109  and settles below the first metallic casting  100 S&#39;s smooth (even) surface  101 , eventually covering the entire surface  101 . The peripheral second metallic casting layers  523  and the lower second metallic casting layer  521  layer correspond, respectively, to the lateral downward gravitational flow of the highly fluid second metallic material  200 L around the first metallic casting  100 S, and to the continued flow thereof beneath the first metallic casting  100 S. Also typically during an inventive fabrication process, some liquid second metallic casting material  200 L seeps around and settles below the spherical elements  300 , with the result that some of the upper second metallic casting layer  522  is situated between the depressions  105  and the lower portions of the spherical elements  300 . 
     With reference to  FIG. 21  and  FIG. 22 , in armor application an inventive composite armor system  500  lends itself to either of two basic dispositions relative to a projectile  60 . As portrayed in  FIG. 21 , the inventive composite armor system  500  is oriented with its smooth surface  501  as the strike face. In contrast, as portrayed in  FIG. 22 , the inventive composite armor system  500  is oriented with its dimpled surface  502  as the strike face. 
     Reference now being made to  FIG. 23  through  FIG. 28 , a different mode of inventive practice involves the boring (e.g., drilling) of horizontal and vertical holes (e.g., cylindrical channels)  601  in a solid metallic block  600 . The horizontal set of holes  600   h  and the vertical set of holes  601   v  are each bored at least partially through solid metallic block  600 . The horizontal holes  600   h  describe at least one horizontal geometric plane and have the same horizontal hole diameter. The vertical holes  600   v  describe at least one vertical geometric plane and having the same vertical hole diameter, which is smaller than the horizontal hole diameter. The horizontal holes  600   h  and the vertical holes  600   v  are arranged so as to form intersections, each intersection being of a horizontal hole  600   h  and a vertical hole  600   v . Each horizontal hole  600   h  intersects at least one vertical hole  600   v , and each vertical hole  600   v  intersects at least one horizontal hole  600   v.    
     Plural spherical elements  300  are situated in the horizontal holes  600   h . Each spherical element  300  has a spherical element diameter that is larger than the vertical hole diameter but smaller than the vertical hole diameter. Each spherical element  300  is situated at an intersection (between a horizontal hole  600   h  and a vertical hole  600   v )—for instance rolled and/or pushed along a horizontal hole  600   h —so as to rest upon and partially within a vertical hole  600   v . Each intersection at which a spherical element  300  is placed is referred to herein as a “drop-in location.” As shown in  FIG. 26  and  FIG. 27 , a metallic material  700  in hot, liquid form is cast in association with the bored metallic block  600  and the spherical elements  300 . According to typical inventive practice, the block  600  metallic material and the metallic material  700  are different metallic materials, the latter having a lower melting point than the former. The metallic material  700 S in cooled, solidified form encompasses block  600 , infiltrates horizontal holes  600   h  and vertical holes  600   v , and sets spherical elements  300 . 
     The resultant composite structure is an armor device  800  such as depicted in  FIG. 26  and  FIG. 27 . The armor device  800  shown in  FIG. 28  is inventively produced similarly as the armor device  800  shown in  FIG. 27 . In both  FIG. 27  and  FIG. 28 , the vertical holes  600   v  are shown to be aligned with the horizontal holes  600   v ; however, the vertical holes  600   v  are arranged differently in  FIG. 27  versus  FIG. 28 . As shown in  FIG. 27 , the drop-in locations are aligned in two perpendicular directions. As shown in  FIG. 28 , the drop-in locations are aligned in one direction (along the horizontal channels) and are staggered in the perpendicular direction.  FIG. 27  and  FIG. 28  can be understood to illustrate how the mode of inventive practice illustrated in  FIG. 1  through  FIG. 20  can also lend itself to variation in terms of arrayal of the embedded spherical elements  300 . 
     In inventive testing, the present inventor made a prototype inventive armor structure  800  and observed some casting voids (e.g., shrinkage porosity), in the solidified metallic material  700 S. This problem may be correctable by designing more favorable configurations of blocks  600  having holes  601 , such as being characterized by single-layer arrangements of the spherical elements  300 . More “open” geometries of the holes  600  may also reduce propensities to casting voids. In addition, adjustments of the heating temperatures may reduce such propensities in the inventive armor product  800 . 
     The present invention, which is disclosed herein, is not to be limited by the embodiments described or illustrated herein, which are given by way of example and not of limitation. Other embodiments of the present invention will be apparent to those skilled in the art from a consideration of the instant disclosure or from practice of the present invention. Various omissions, modifications and changes to the principles disclosed herein may be made by one skilled in the art without departing from the true scope and spirit of the present invention, which is indicated by the following claims.