WO1988003573A2 - Isothermal process for forming porous metal-second phase composites and porous product thereof - Google Patents

Abstract

A method for the in-situ precipitation of second phase materials, such as ceramic or intermetallic particles, in a substantial volume fraction of solvent metal matrix. The invention involves the propagating reaction of the second phase-forming constituents in a solvent metal medium to provide a porous composite of finely-dispersed second phase particles in the metal matrix. Exemplary materials include titanium carbide or titanium diboride in an aluminum matrix.

Description

  ISOTHERMAL PROCESS FOR FORMING POROUS METAL-SECOND PHASE  COMPOSITES AND POROUS PRODUCT THEREOF    <U>Background of the Invention</U>    The present invention comprises a process for the preparation  of porous metal second phase composite material using an in-situ  precipitation technique involving a propagating reaction wave which  is substantially isothermal in the plane of the wave front, and the  porous products of that process. In one embodiment, a second phase,  such as a ceramic material or an     intermetallic,    is formed directly  in a relatively large volume fraction of metallic or     intermetallic     solvent matrix, which substantially encapsulates the second phase.

    The second phase can comprise a ceramic, such as a     boride,    carbide,  oxide, nitride,     silicide,    sulfide,     oxysulfide    or other compound, of  one or more metals the same as or different than the solvent matrix  metal. Of special interest are the     intermetallics    of     aluminum,    such  as the     aluminides    of titanium, zirconium, iron, cobalt, and nickel.  In the present invention, the second phase is contained in a solvent  matrix metal or     intermetallic,    typically in the form of a porous  composite, which can be introduced into a molten host metal bath to  disperse the second phase throughout the host metal.

   Cooling yields  a final metal matrix having improved properties due to, for example,  uniform dispersion of the second phase throughout the final metal  matrix, and fine grain size. Either the solvent matrix metal or the  host metal, or both, may constitute an alloy of two or more metals,  and the solvent metal may be the same as, or different than, the  host metal. The solvent metal should be soluble in the host metal,  or capable of forming an alloy or     intermetallic    therewith.  



  For the past several years, extensive research has been devoted  to the development of metal-second phase composites, such as           aluminum    reinforced with fibers, whiskers, or particles of carbon,       boron,    silicon carbide, silica, or al     umi        na.    Metal-second phase  composites with good high temperature yield strengths and creep  resistance have been fabricated by the dispersion of very fine (less  than 0.1 micron) oxide or carbide     particles        throughout    the metal or  alloy matrix of composites     formed,    utilizing powder     metallurgy     techniques.

   However, such composites typically suffer     from    poor  ductility and fracture toughness, for reasons which. are explained  below.  



  Prior art techniques for the production of metal-second phase  composites may be     broadly    categorized as powder     metallurgical          approaches,        molten    metal techniques, and internal     dxidation      processes.

   The powder metallurgical type production of   dispersion-strengthened composites would ideally be accomplished by       mechanically    mixing metal powders of approximately 5     micron    diameter  or less with an oxide or carbide powder (preferably 0.01 micron to  0.1     micron).    High speed blending techniques or conventional  procedures, such as     ball    milling, may be used     to    mix the powders.  Standard powder     metallurgy    techniques are then used to form the  final composite.

   Conventionally,     however,    the ceramic. component is  large,     i:e.    greater than 1     micron,    due to a lack of availability,  and high cost, of very small particle size materials, because their       production    is energy intensive, time consuming and     capital     intensive. Furthermore, production of very small particles  inevitably leads to     contamination    at the particle surface, resulting  in contamination at the     particle-to-metal    interface in the  composite; which in turn compromises the mechanical properties  thereof.

   Also, in many cases where the     particulate    materials are  available-in the desired size, they are extremely hazardous due to  their     pyrophoric    nature.  



  Alternatively,     molten    metal     infiltration    of a continuous  skeleton of     the    second phase material has been used to-produce  composites. In some cases, elaborate particle coating techniques  have been developed to     protect    ceramic particles     from    molten metal  during molten     metal        infiltration    and to improve bonding between the  metal and ceramic. Techniques such as this have been developed to      produce silicon carbide-aluminum composites, frequently referred to  as     SiC/Al    or     SiC    aluminum.

   This approach is suitable for large       particulate    ceramics (for example, greater than 1 micron) and  whiskers. The ceramic material, such as silicon carbide, is pressed  to     form    a compact, and liquid metal     i    s forced into the packed bed to  fill the     intersticies.    Such a technique is illustrated in U.S.  Patent No. 4,444,603 to     Yamatsuta    et al, hereby incorporated by  reference.

   Because this technique necessitates molten metal  handling and the use of high pressure equipment, molten metal  infiltration has not been a practical     process    for making  metal-second phase composites, especially for making composites  incorporating     submicron    ceramic particles where press size and  pressure needs would be excessive and unrealistic.  



  The presence of oxygen in ball-milled powders used in prior art  powder metallurgy techniques, or in     molten    metal infiltration, can  result     i    n a deleterious layer, coating, or contamination such as  oxide at the interface of second phase and metal. The existence of  such layers will inhibit interfacial bonding between the second  phase and the metal matrix, adversely effecting ductility of the  composite. Such weakened interfacial contact may also result     i    n  reduced strength, loss of elongation, and facilitated crack       propagation.     



  Internal oxidation of a metal containing a more reactive  component has also been used to produce dispersion strengthened  metals, such as copper containing internally oxidized aluminum. For  example, when a copper alloy containing about 3 percent aluminum is  placed in an oxidizing atmosphere, oxygen may diffuse through the  copper matrix to react with the aluminum, precipitating alumina.  Although this technique is limited to relatively few systems,  because the two metals must have a wide difference in chemical  reactivity, it has offered a possible method for dispersion  hardening. However, the highest possible concentration of       dispersoids    formed in the resultant dispersion strengthened metal is  generally insufficient to impart significant changes in properties  such as modulus, hardness and the like.  



  In U.S. Patent No. 2,852,366 to Jenkins, hereby incorporated by  reference, it is taught that up to 10 percent by weight of a metal      complex can be incorporated into a base metal or alloy. The patent  teaches blending, pressing, and sintering a mixture of a base metal,  a compound of the base metal and a non-metallic     complexing    element,  and an alloy of the base metal and the     complexing        metal.    Thus, for  example, the reference teaches mixing powders of nickel, a  nickel-boron alloy, and a nickel-titanium alloy, pressing,

   and       sintering    the mixed powders to form a coherent body in which a  stabilizing     unprecipitated    "complex" of titanium and     boron    is  dispersed in a nickel matrix. Precipitation of the complex phase is  specifically avoided.  



  In U.S. Patent No. 3,194,656, hereby incorporated by reference,       Vordahl    teaches the formation of a ceramic phase, such as     TiB2     crystallites, by melting a mixture of eutectic o r near eutectic  alloys. It is essential to the process of     Vordahi    that at least one  starting ingredient has a melting point substantially lower than  that of     the    matrix metal of the desired final alloy. There is no  disclosure of the initiation of an exothermic localized second  phase-forming reaction forming a moving isothermal wave     front    at or  near the melting point of the matrix metal.  



       Bredzs    et al; in U.S. Patent Nos.     3,415,6'97;    3,547,673;  3,666,436; 3,672,849; 3,690,849; 3,690,875; and 3,705,791, hereby  incorporated by reference, teach the preparation of     cermet    coatings,  coated substrates, and alloy ingots, wherein an exothermic reaction  mechanism forms an in-situ precipitate dispersed in a metal matrix.

         Bredzs    et al     rely    on the use of alloys having a depressed melting       temperature,    preferably eutectic alloys, and thus do not initiate a       moving    localized second     phase-forming        exothermic    reaction at or near  the melting temperature of the matrix metal.  



  DeAngelis, in U.S. Patent Nos. 4,514,268 and 4,605,634, hereby  incorporated by reference, teaches reaction sintered     cermets    having  very fine grain size. The method taught involves the dual effect of  reaction between and     sintering        together    of     admixed    particulate       reactants    that are shaped and heated at temperatures causing an       exothermic    reaction to occur, be substantially completed. The  reaction products are sintered together to form ceramic-ceramic  bonds by holding the reaction mass at the high temperatures      attained. Thus, this reference relates to a product with sintered  ceramic bonds, suitable for use in contact with molten metal.  



       Backerud,    in U.S. Patent No. 3,785,807, hereby incorporated by  reference, teaches the concept of preparing a master alloy for  aluminum-containing titanium     diboride.    The patentee dissolves and  reacts titanium and boron in molten aluminum at a high temperature,  but requires that titanium     aluminide    be crystallized at a lower  temperature around the titanium     diboride    formed.  



  In recent years, numerous ceramics have been formed using a  process termed "self-propagating high-temperature synthesis"     (SHS),     that involves an     exothermic,    self-sustaining reaction which  propagates through a mixture of compressed powders, generally under  externally applied pressure, to form dense products. The     SHS          process    involves mixing and compacting powders of the constituent  elements, and locally igniting a portion of a green compact with a  suitable heat source. The source can be electrical impulse, laser,       thermite,    spark, etc.

   On ignition, sufficient heat is released to  support a self-sustaining reaction, which permits the use of sudden,  low power initiation of high temperatures when using relatively low  concentrations of binder, rather than bulk heating over long periods  at lower temperatures. Exemplary of these techniques are the  patents of     Merzhanov    et al, U.S. Patent Nos. 3,726,643; 4,161,512;  and 4,431,448 among others, hereby incorporated by reference.  



  In most     SHS    processes, the product is a ceramic, that may be  relatively dense for use as a finished body, or may be crushed for  use as a powder raw material. In a few instances, binders, such as  metal, have been included with the compressed powders, but typically  constitute 10 percent or less by weight of the mixture, and almost  invariably less than 30 percent. At these levels, the binder acts  as a ductile consolidation aid     to    fill in porosity during the       exothermic    ceramic     production    reaction, and to increase the product  density.

   The dense products according to the teachings of the       Merzhanov    et al. patents are restricted to binder concentrations  below about 30 percent by mass, to preserve wear-resistance and  hardness, and     porosities    below 1 percent to avoid impairing  operating performance.

   Further, the     SHS        process,    even in the      presence of metal, occurs at higher temperatures     than    those employed  in the present invention, and is .not isothermal as is the present  invention because significantly lower metal concentrations are       employed.    Thus, the     SHS        process    yields sintered ceramic particles,  having substantial variation in size.  



  In U.S. Patent No.     3,726,643,    there is taught a method for       producing    high-melting refractory inorganic compounds by mixing at  least one metal selected from Groups IV, Y, and     -VI    of the Periodic  System with a non-metal, such as carbon, boron, silicon, sulfur, or       liquid    nitrogen, and     heating'the    surface of the mixture to     produce    a       local    temperature adequate to initiate a combustion process. In  U.S.

   Patent No:. 4,161,512, a     process    is taught for preparing  titanium carbide by ignition of a mixture consisting of 80-88 _       percent    titanium and 20-12 percent carbon,     resulting    in an       exothermic    reaction of the mixture under conditions of       layer-by-layer    combustion. These references deal with the  preparation of ceramic     materials,    absent a binder.  



       More    particularly, U.S. Patent No. 4,431,448 teaches  preparation of a dense, hard alloy by intermixing powders of _  titanium,     boron,-carbon,    and a Group     I-B    binder metal o r     alloy,    such  as an     alloy    of copper or silver, compression of the mixture, local  ignition thereof to initiate the exothermic reaction of titanium  with     boron    and carbon, and propagation of the ignition, resulting     i    n  an     alloy    comprising titanium     di    do     ri    de, titanium carbide, and up to  about 30 percent binder metal.

   Upon completion of the exothermic  reaction the resulting     solid-liquid    reaction mass is subjected     to     compression until a     porosity    of below 1 percent is     obtained.    This  reference, however;     i    s limited     to    the use of Group     I-B    metals and  alloys, such as copper- and silver; as binders. As     mentioned,          products    made by this method are dense and concentration of the  binder is restricted to less     than    30 percent to preserve wear  resistance and hardness.  



  Several     intermetallic    self-sustaining reactions have been  studied     theoretically    to     determine    propagation rates, as reported in  the following two articles:     A.P.        Hardt    and     P.Y.        Phung,   <U>Propagation</U>  <U>of</U>     Gasless   <U>Reactions in Solids - I: Analytical Study of Exothermic</U>           Intermetallic   <U>Reaction Rates</U>, Combustion and Flame 21, 77-78 (1973);

    and     A.P.        Hardt    and     R.W.        Halsinger,   <U>Propagation of</U>     Gasless   <U>Reactions</U>  <U>in Solids - II. Experimental Study of Exothermic</U>     Intermetallic     <U>Reaction Rates</U>, Combustion and Flame 21, 91-97 (1973). Compressed  shapes were studied with some binder to cohere the compact.  Experimentation concerned exothermic condensed phase reactions and  suggested the desirability of low heat transfer to     permit    heat  accumulation in the reaction zone in order to allow reaction  propagation.

   Small particle size reactants were also said to be  desirable to permit a high rate of mass transfer, which allows the  reaction to go to completion spontaneously. Thus, heat capacity,  heat of reaction, and particle size were reported to be important  factors. Results showed that increased concentrations of binder  were undesirable, particularly concentrations exceeding 30 percent  by weight, which retarded ignition and prolonged propagation,  contrary to the present invention.  



  U.S. Patent No. 4,540,546 to Giessen et al, hereby incorporated  by reference, teaches a method for rapid solidification processing  of a multiphase alloy. In this process, two starting alloys react  in a mixing nozzle in which a "Melt Mix Reaction" takes place  between chemically     reactable    components in the starting alloys to  form     submicron    particles of the resultant compound in the final  alloy.

   The mixing and chemical reaction are performed at a  temperature which is at or above the highest     liquidus    temperature of  the starting alloys, but which     i    s also substantially below the  1     i        qui        dus    temperature of the final alloy, and as close to the     sol        i        dus     temperature of the final alloy as possible. While  dispersion-strengthened alloys can be     produced    by this technique,  there appear to be a number of inherent difficulties. First,  processing is technically complex, requiring multiple furnaces.

    Second, efficient mixing is important if fine dispersions are to be  consistently produced. Lastly, very high degrees of superheat will  be required to completely dissolve the rapid solidification alloying  elements in order to produce high loading of     dispersoid,    which  necessarily accentuates particle growth, for example, in composites  containing 10-20 percent     dispersoid.         The present invention overcomes the disadvantages of the prior  art: More particularly, the present invention     permits     simplification of procedures and equipment compared to the prior  art.

   For example, the present process obviates need for multiple  furnaces and mixing and control     equipment    because all of the  reactive constituents-of the second phase are present in a single  reaction mass, in the presence of large concentrations of solvent       metal:    The present invention also overcomes the need for forming  multiple melts of components at very high     melting    temperatures.  Further, high loading composites can be prepared without the  necessity of achieving high levels of superheat in holding  furnaces.

   Applicants' invention also provides for a cleaner  particle/metal interface compared with conventional     metal:-ceramic     composites made by techniques using, for example, separate metal and  ceramic powders, because the reinforcing particles are formed  in-situ and encapsulated with solvent metal. Moreover, the porous       products    formed can be dissolved to make uniform dispersions of  substantially     unagglamerated    particles in a matrix, with     controlled     volume fractions of second phase materials. With these facts in  mind, a detailed description of the invention follows, which  achieves advantages over known processes.

      <U>Summary of the Invention</U>  It is an object of the present invention to provide an  inexpensive method for forming     porous    composite materials,  containing finely dispersed second phase, such as a     particulate          ceramic,        intermetallic    material, or     mixtures    thereof; in metal,  metallic alloy, or     intermetallic    matrices.  



  The present invention     produces    a porous composite comprising a  relatively concentrated second phase dispersion in a solvent metal  matrix, which may be the same or different than the final metal  matrix desired. This concentrated composite may be utilized to form  improved final metal matrix composites of lower second phase  concentration, having substantially uniform dispersion and uniform  particle size distribution, by admixture with a molten bath of the  desired     host    metal, metal alloy or     intermetallic    matrix material, or      by admixture with solid host metal, metal alloy, or     intermetallic,     followed by heating to a temperature above the melting point of the  host metal.  



  For purposes of simplifying further description, the matrix of  the porous composite material produced directly by the method of the  present invention shall be referred to as the "solvent metal  matrix," while the metal with which the porous composite may be  admixed shall be referred to as the "host metal." The     metal    of the  final composites resulting from such admixture may be referred to as  "final metal matrix". In each instance, the     word    "metal" shall  encompass the alloys and     interme        tallic    compounds thereof. Further,  the solvent metal may encompass not only metals in which the second  phase-forming constituents are soluble, but also such metals in  combination with other metals.

   Other metals may include those     i    n  which said constituents are not soluble, but in which said solvent  metal is soluble, or which are soluble in said solvent metal. Thus,  "solvent metal" may refer to a combination of solvent metals and       nonsolvent    metals.  



  It is a further object of this invention to provide a     method     for dispersion hardening of metals and alloys. The present  invention relates     to    the preparation of a porous composite, said       porous    composite comprising a second phase particulate in a solvent  metal matrix, such as titanium     dibo    ride o r     titanium    carbide in an  aluminum matrix, using a moving, substantially isothermal,

   wave       front    effecting localized in-situ precipitation of second phase  material     i    n relatively large volume fractions of solvent metal to  make porous products which can be used to form materials having  substantially uniform distribution of second phase material. The  term "relatively     large    volume fractions of solvent metal" as used       herein    shall refer to the presence of at least 10 percent solvent,  preferably more than 20 volume percent, and most preferably more  than 30 percent solvent metal.

   When the volume percentage of second  phase material exceeds 70 percent, the second phase particles will  generally be in contact with each other purely by geometric  considerations, i.e. there is less than 30 volume percent free space  in a close-packed array of     dispersoids.    Accordingly, the incidence      of interparticle sintering increases substantially as the volume  fraction of the second phase increases above 70     volume    percent in  some systems. Thus, it is preferred that the second phase  constitute less than about 70 volume percent of the composite.

    The present invention relates to a process for the localized       in-situ    precipitation of up to less than about 90 percent by volume  of a second phase material in a solvent metal matrix,     wherein    the  second phase can comprise a ceramic, such as a     boride,    carbide,  oxide, nitride,     siTicide,        oxysulfide,    or     sulfide    of a metal the same  as or other than the solvent metal matrix.

   It has been found that  by mixing the constituents or     elements    of the desired second phase  material with a solvent metal, and locally heating to a temperature  at which substantial diffusion and/or dissolution of the reactive  elements into the solvent metal can occur, typically at o r close     to     the melting point of the solvent metal, a moving localized solvent  assisted isothermal reaction, which is always exothermic, can be  initiated. This solvent assisted reaction results in the extremely  rapid formation and dispersion of finely divided particles of the  second phase material in relatively high concentrations of the  solvent metal matrix material.  



  It is an object of the present invention to provide a     process     for     forming    metal-second phase composite materials having a  relatively uniform dispersion of second phase particulate throughout  large volumes of solvent matrix metal.

   The process comprises  localized ignition     to    cause     in-situ    precipitation of at least one  second phase material in a solvent metal matrix by contacting  reactive second     phase-forming    constituents, in the presence of a  solvent metal, at a     localized    temperature at which sufficient  diffusion of the     constituents    into the solvent metal occurs locally  to initiate a moving     isothermal    reaction of the constituents to       produce    a porous composite material.  



  It is also an object of the present invention to provide a  method for the production of porous     metal-second    phase composite  material, the method comprising precipitating at least one second  phase material in a substantial volume fraction of solvent metal by  locally igniting reactive second     phase-forming    constituents, in the      presence of a substantially     nonreactive    solvent metal in which the  second     phase=forming    constituents are more soluble than the second  phase material,

   at a temperature at which sufficient diffusion of  the reactive second phase-forming constituents into the  substantially     nonreactive    solvent metal occurs to cause a  substantially isothermal propagating second phase-forming reaction  of the constituents; to thereby precipitate second phase particles  in the solvent metal so as to produce finely divided second phase  particles in the solvent metal matrix.  



  The invention further relates to a method for the     production    of       porous    metal-second phase composite materials, the method comprising  precipitating at least one second phase material in a substantial       volume    fraction of solvent metal by locally igniting reactive second  phase-forming     constituents,    in the presence of a substantially       nonreactive    solvent metal in which the second phase-forming       constituents    are more soluble than the second phase, at a local       temperature    at which sufficient diffusion of the constituents into  the solvent metal occurs,

       to    cause a substantially isothermal  propagating reaction of the reactive second phase-forming  constituents to increase the temperature to a temperature exceeding  the melting     temperature    of the solvent metal, to precipitate the  second phase in the solvent metal matrix.  



  The invention further relates     to    a method for dispersion of  second phase     dispersoids    in a metallic matrix, the method comprising  forming a reaction mixture of reactive second phase-forming  constituents in the presence of a substantial volume fraction of at  least two metals, at least one of which acts as a solvent metal in  which the second phase-forming constituents are more soluble than  the second phase     dispersoids,    raising the temperature of the  reaction mixture locally to a temperature at which sufficient  diffusion of the second phase-forming constituents into the lowest  melting solvent metal occurs to initiate a substantially isothermal       reaction    of the constituents,

   whereby the exothermic-heat of  reaction of the constituents causes the temperature of the reaction  mixture to exceed the melting point of the highest melting metal,       permitting    propagation of the reaction and dispersion of the second  phase     dispersoid    in a metal matrix.

        The invention further relates to a     method    for dispersion of  second phase     dispersoids    in a solvent metal matrix, the method  comprising     forming    a reaction mixture of     reactive    second:

    phase-forming constituents in the presence of a substantial volume  fraction of at least     two    metals, at least one of which acts as a  solvent     metal    in which second phase-forming     constituents    are more  soluble than the second phase     dispersoids,    raising the temperature  of the reaction mixture locally to a temperature at which sufficient  diffusion of the second phase-forming constituents into the lowest  melting solvent metal occurs to initiate a     substantially    isothermal       reaction    of the     constituents,

      whereby the exothermic heat of  reaction of the constituents causes the temperature of the reaction  mixture     to    exceed the melting point of the lowest melting point       metal,    permitting propagation of the reaction and dispersion of the  second phase     dispersoid    in a mixed     metal    matrix.  



  The invention further relates to a     method    for dispersion of at  least one     intermetallic    material in a metallic matrix.  



  The invention further relates     to    a method for dispersion of at  least one ceramic material in a metallic matrix.  



  The invention further relates     to    a     method    for dispersing       dispersoid    particles of an     intermetallic    material and a ceramic  material' in a     metal,    metal     alloy,    or     intermetallic    matrix:

    The invention further     relates        to    a porous mass comprising a  dispersion of     in-situ    precipitated insoluble second phase particles  in a solvent metal matrix     produced    by propagating a locally ignited  substantially isothermal exothermic reaction of second phase-forming  constituents in the presence of a substantial-volume fraction of  solvent metal in which the constituents are more soluble than the  second phase.  



  <U>Description of the Preferred</U>     Embodiments     The present invention     relates    to a novel technique for  preparing useful metal-second phase composites. Novelty resides in  the process for preparing a porous solvent metal matrix-second phase  master concentrate suitable for use as an intermediate in the       formation    of a dense composite product.

   Because the process     relies         upon the     production    of second phase particles which are dispersed  and substantially insoluble with respect     to    the solvent matrix  metal, the     porous    composite matrix can be dissolved in another metal  to yield dense composite products having a uniform second phase  dispersion: Resultant composites may be     remel        ted,    facilitating  subsequent processing. The technique comprises the preparation of a  master porous composite containing discrete     dispersoid    particles  each substantially encapsulated or enveloped by solvent matrix  metal.

   Thus, the discrete     dispersoid    particles are not bonded to  each other in the concentrate. This novel technique relies upon  substantial concentrations of high thermal conductivity solvent  matrix     metal    to establish an isothermal wave     front    which propagates  from a place of local ignition of reactive components.

       More     particularly, a     green    compact of compressed reactive components,  typically shaped in the form of a rod, is ignited at one end, and  the substantially isothermal wave front moves along the rod to  precipitate the substantially insoluble second phase material       in-situ    in the relatively high concentration solvent     metal    to form  the aforementioned porous composite.  



  An advantage of the present invention is that such porous  composites may;     i    n turn; be utilized via an     admixture    process     to     introduce the second phase into a host metal in controlled fashion.  Thus, a concentrate may be prepared in the form of a     porous     composite having, for example, a high percentage of a second phase,  such as a ceramic, e.g. titanium     diboride,    in a solvent matrix  metal, such as aluminum.

   This porous composite may then be added to  a     molten    host metal, metal     alloy    or     intermetallic    bath, (which  molten metal may be the same or different from the matrix metal of  the     porous    concentrate) to achieve a final composite having the  desired loading of second phase.

   Alternatively, the porous  composite may be     admixed    with solid host metal, metal alloy or       intermetallic,    and then heated to a temperature above     the    melting  point of the host metal: In the following discussion, admixture  with a "host metal" or "host metal bath" should be understood to  apply equally to each of the different embodiments indicated     above.     



  The melting     point    of the solvent metal must be below the  temperature of the host metal, and there     must    be sufficient      miscibility of the     two    molten metals to insure alloying,  dissolution, or combination.

   For example, titanium can be  reinforced by precipitating titanium     diboride    in aluminum, and  subsequently introducing the titanium     diboride-aluminum    composite       into    molten titanium     to    dissolve the aluminum matrix of the porous  composite; thus     forming    an aluminum-titanium matrix having titanium       di        bo        ri    de dispersed therein.     Simi    T     arly,    lead can be reinforced -by  precipitating titanium     diboride    in aluminum and admixing the  composite with molten lead.

     In certain instances, the "host metal" may comprise material  other than conventional metals, metal alloys or     intermetailics.    The  host metal may, for example, be a dispersion strengthened metal such  as metal containing finely dispersed erbium oxide,     thoria,    alumina,  etc., or a metal-second phase     composite.    It is     important    in these  cases that the preexisting dispersion be stable in the molten metal  for the time/temperature required for introducing the desired     porous     composite material of the present invention.

   The advantage of  utilizing a material containing a second phase dispersion as the  host metal is that a     bimo        dal    distribution of second phase types,  shapes, amounts, etc. may be     obtained.    An example would be the use  of an aluminum matrix containing     a_dispersion    of essentially       equiaxed        TiB2    particles,     to    which a porous composite of the  present invention dispersed' therein having needle shaped     TiN     particles is added: A combination of dispersion strengthening and  high temperature creep resistance is obtained.

   In accordance with  the     foregoing    discussion, it must be     understood    that suitable "host  metal," o r "host     metal,    metal alloy o r     i        ntermetaT    1     i    c" matrices  encompass the types of materials discussed above containing  preexisting second phase dispersions.  



  The present invention encompasses several features that run  directly contrary     to    the prior art wisdom in materials science, and  particularly in the field of metal-second phase composites. These  features may be     more    clearly understood when considered in the  context of the prior art     SHS        processes,    particularly those     $HS     processes employing minor     amounts    of binder     metal.    Firstly, this  prior art is directed to the     formation    of dense products. Porous      products are not considered desirable and have not been  investigated.

   The perspective of the prior art is to develop  finished dense products,     i.e.    products that are ready for machining  or other metal     working    or ready for use in fabricating manufactures.  



  Secondly, the prior art looks to the use of a metal matrix as a  binder. Contrary to the prior art, the present invention looks to  the metal matrix as a solvent in which     dispersoid    particles that are  substantially insoluble relative to the solvent metal matrix are  dispersed. More particularly, the prior art utilizes relatively low  concentrations of binder, whereas the present invention utilizes  relatively high concentrations of metal matrix. Prior art materials  utilize quantities of metal which fill voids among typically  sintered ceramic particles to     densify    the composite.  



  Furthermore, prior art concentrations preclude encapsulating  the     dispersoid    particle with binder. The concentrations utilized in  the present invention, however, are sufficient to provide for  substantial enveloping or encapsulation of the substantially  insoluble     dispersoid    particles in the solvent     metal    matrix. This  feature is advantageous over prior art in which preformed ceramic  powders are combined with metals, because it inhibits the formation  of deleterious coatings o r layers on the particles.

   These coatings  or layers, such as oxide layers, are frequently present in prior art  metal matrix composites, and are believed to negatively affect  physical properties of materials and inhibit further processing of  products     formed    therefrom.  



  Next, the prior art seeks dense products involving     self-bonding     of ceramic particles. For example, prior art techniques seek  sintering of particles, rather than encapsulation thereof to inhibit  bonding of the particles. These kinds of irreversible techniques  are directly contrary to the present invention, which utilizes a  reversible technique, that is, an encapsulation technique which  inhibits bonding of     dispersoid    particles and facilitates further  processing.  



  Additionally, the higher concentrations of metal matrix,  typically greater than about 30 volume percent, utilized in the  present invention enhance the heat transfer characteristics of      reactant combinations and cause a     more    uniform linear reaction  rate. Additionally, there is a reduction in particle size because  the maximum temperature attained is lower than attained in the prior  art because of the additional heat capacity of the contained metal,  and because of the more rapid quench rate resulting from the higher  thermal conductivity of said metal: Another advantage is in spatial  temperature uniformity, hence uniformity in size distribution of the       dispersoid    particles in the matrix material. Prior art techniques  result in larger particles that are agglomerated and/or sintered.

    The smaller particle size and uniformity in     distribution    of  particles achieved by the present invention result in improved       properties    of final composite products.  



  Another     feature    of the present invention which distinguishes  from the prior art is the formation of an isothermal wave     front     which     promotes    the     uniformity    of particle size of     dispersoid     particles in a cross section of the product produced. The  isothermal character results     from    the selection of a high thermal  conductivity solvent metal matrix; in     combination    with  concentrations of the solvent metal sufficient to     acMeve    the  isothermal character across the material     to    be reacted.  



  The combination of these features permits manufacture of  composite materials suitable for processing via the admixture  process to produce materials whose properties may be tailored to  suit the demands of particular uses. This: admixture     process    takes  advantage of the fact that "poor quality" intermediate composites  are recovered.     Such    composites would have heretofore been regarded  as useless. For example, in the preparation of ceramic bodies by       SHS,    a limiting feature in the process as a means for     producing     useful ceramic shapes or parts, has been the inherently poor  physical quality of the body typically formed by the       self-propagating    synthesis.

   Accordingly, attempts have been made to  enhance the quality of such bodies by techniques such as elevated  pressures at temperature to cause diffusion, sintering, and       densi        fi    canon. In contrast, such properties as friability, low  strength, and porosity have been found, surprisingly,     to    be       advantageous    in the process herein disclosed.      A feature in the admixture process is that molten metal may be  used to advantage in the production of composites, even     though    it is  well known in the art that molten metal should be specifically  avoided in the fabrication and utilization of metals, ceramics and  composites.

   Thus, for example, the infiltration of molten metals  into conventional polycrystalline     metals    results in grain boundary       dehesion,    facilitates crack propagation, and hence causes  brittleness. As a consequence, there have traditionally been  problems, for example, with the containment of     molten    metal in  metallic containers (of higher melting point)     because    of progressive  loss of strength and integrity (the phenomenon of liquid metal       embrittlement).    Similarly; in the use of ceramics in     molten    metal  contacting applications, service longevity has always been a problem  owing to molten metal attack, even with the most chemically inert  and resistant materials.

   Thus, for example, the containment of  molten     aluminum    by titanium     dibo    ride has been a long standing, and  still commercially unresolved problem,     owfing    to penetration of the  molten metal along the ceramic grain boundaries where reaction takes  place with contaminants. Progressive penetration and reaction  ultimately lead to loss of     intergranular    cohesion, mechanical  weakness, and disintegration:  The presence of molten metal is equally disadvantageous in the  manufacture and use of metal-second phase composites, where it has  been regarded of paramount importance     to    avoid the introduction of  molten metal. Several examples are known to illustrate the type of  problems that can arise.

   In the preparation of composites of     SiC    in  A1, precautions must be taken, such as proprietary coating  techniques, to avoid     prolonged    direct contact of the molten metal  and particulate (or the ceramic skeleton in the case of     molten    metal  infiltration).

   Absent such precautions, the metal and ceramic react       together,    a process that obviously diminishes the amount of  particulate reinforcement, but also generates reaction products that  may render the composite extremely susceptible to subsequent  corrosion.     Analagous    problems occur when attempts are made to weld  the     SiC/Al    because, as the melting temperature of the matrix metal  is exceeded, the same-harmful reactions occur.

   In the case of           thoria-dispersed    (TD) nickel, -the     composite    is produced via solid  powder metallurgical techniques, as opposed to liquid metal (ingot       metallurgy),    because the     thoria    ceramic tends to segregate, and even  rise. to the surface of the melt, because of surface tension  effects.: As with     SiC/A1;    welding is again a problem     because    of the  presence of liquid     metal,    this     time    giving rise to the above-noted  segregation.  



  It     would    thus be expected that the combination of poor quality  metal-second phase preforms with     molten    metal would not lead to the  recovery of a useful product. However, it has surprisingly been  found that employing: these features in the admixture process  invention yields unexpected and quite     unobvious    benefits, yielding  products that had'     heretofore    been unattainable using prior art  techniques.  



  In addition to the novel and beneficial processing features  alluded     to    above, other. advantages derive     from    the isothermal,  propagating in-situ second phase precipitation process of the  present invention, such as clean coherent interfaces between the  metal and second phase. Moreover, the admixture     process    allows  these advantages to be achieved, while avoiding the shortcomings  below, that are inherent to     in-situ    precipitation of a second phase  in metal.

   Thus, for the     production    of fine precipitates, the  process must, by necessity, avoid prolonged heating at elevated  temperatures, which results in particle     growth.    For this reason,  relatively high concentrations of     dispersoid    precursor are preferred  in order that the brief duration of exothermic heat be sufficient to  complete the     in-situ    formation     process.    In the case of higher  concentrations, excessive heat is experienced, hence sintering and  agglomeration of particles results. In the case of lower     dispersoid     concentrations, the amount and time of external heat that must be  applied to complete the reaction are such that particle growth may  be a problem.

   Thus, the range of second phase loadings that may be       recovered    in a     product        i's    constrained by these criteria.     However,     when the admixture process     i's    used, the constraint disappears       because@the    particle formation     process    may be conducted under the  circumstances that most effectively lend themselves     to    the           production    of second phase of the desired morphology, size, type and  other characteristics, without regard to loading level.

   As an  example, the optimum second phase loading range may be used in the  initial propagating isothermal second phase formation process. This  preformed     porous    composite may then be combined with molten host  metal     i    n variable amounts, to provide full latitude in     dispersoid     concentration in the recovered final metal matrix.  



  The present invention is directed     to    a novel process for the  in-situ precipitation of fine particulate second phase materials,  such as ceramics or     intermetallics,    typical of which are refractory  hard metal     borides    or     aluminides,    within metal, alloy, and       intermetallic    systems, to produce a solvent metal-second phase       compo-site    suitable for use as a master concentrate in the admixture  process. However, the process described may also be used for  introducing larger particles of a second phase material into molten  host metal, up to the point at which such larger particles result in  component     embrittlement,    or loss of ductility, etc.

   The improved  properties of the novel final composites offer weight-savings in  stiffness limited applications, higher operating temperatures and  associated     energy    efficiency improvements, and reduced wear in parts  subject to     erosion.    A specific use of such material is in the  construction of turbine engine components, such as blades.  



  In this context, it should be noted that the final metal-second  phase products of the present invention are also suitable for use as  matrix materials, for example, in long-fiber reinforced composites.  Thus, for example, a particulate reinforced aluminum composite of  the present invention may be used in conjunction with long     SiC    or  carbon fibers to enhance specific directional properties while  retaining high transverse modulus. Typical fabrication routes for  such materials include diffusion bonding of thin     layed-up    sheets,  and molten metal processing.  



  A method is taught whereby the second phase forming elements  are caused to react     i    n a solvent metal to form a finely-divided  dispersion of the second phase material in the solvent metal  matrix. In accordance with the present invention, the second  phase-forming constituents most easily combine at or about the      melting temperature of the solvent metal, and the     exothermic    nature  of this reaction causes a very rapid temperature elevation or     spike,     which can have the effect of melting additional metal,  simultaneously promoting the further reaction of the second  phase-forming constituents.  



  In systems where the reactive elements have substantial       diffusivity    in the solid matrix metal, the     reaction    may be initiated  at temperatures well below the melting point of the matrix     metal.     Thus, a solid state     initiation    is possible, wherein a liquid state  may or may not -be achieved.  



  Exemplary of suitable second phase ceramic     precipitates    are the       borides,    carbides, oxides, nitrides,     silicides,    sulfides, and       oxysulfides    of the elements which are reactive to form ceramics,  including, but not limited to,     transition    elements of the third to  sixth     groups    of the Periodic Table.

   Particularly useful       ceramic-forming    or     intermetallic        compound-forming    constituents  include     aluminum,        titanium,    silicon, boron, molybdenum, tungsten,  niobium, vanadium, zirconium, chromium, hafnium, yttrium, cobalt,  nickel, iron magnesium, tantalum, thorium, scandium, lanthanum, and  the rare earth elements.     Particularly    useful:

   additional       intermetallic-forming    elements include copper, silver, gold, zinc,  tin, platinum, manganese,     lithium    and     beryllium.    Preferred second  phase materials include titanium     di        bo        ri:    de,     titanium    carbide,  zirconium     di        bori    de, zirconium carbide, zirconium     di        sil        icide,    and  titanium nitride.  



  As the solvent metal, any metal capable of dissolving or  sparingly dissolving the constituents of the second phase, and  having a lesser     capability    for     dissolving    the second phase  precipitate may be used. Thus, the solvent metal     component    must act  as a solvent for the specific reactants, but not for the desired  second phase     precipitate.    The solvent metal acts primarily as a  solvent in the process of the present invention, and the  constituents of the second phase precipitate have a greater affinity  for each other than either has for the solvent     metal.    Additionally,  it is important that the second phase-forming reaction releases  sufficient energy for the reaction to go substantially to      completion.

   While a large number of combinations of matrices and       di        spersoi        ds    may be envisioned, the choice of in-situ precipitated  phase (ceramic or     intermetallic)    in any one given matrix, is limited  by these criteria.  



  Suitable solvent matrix     metals    include aluminum, nickel,  titanium, copper, vanadium, chromium, manganese, cobalt,     iron,     silicon, molybdenum, beryllium, silver, gold, tungsten, antimony,  bismuth, platinum, magnesium, lead, zinc, tin, niobium, tantalum,  hafnium, zirconium, and alloys of such     metals.     



  The host metal may be any metal in which the second phase  precipitate is not soluble, and with which the second phase does not  react during the time/temperature regime involved in the     admixture     process, subsequent fabrication, and/or recasting. The host metal  must be capable of dissolving or alloying with the solvent metal,  and must wet the porous composite. Thus, the host metal may be the  same as the solvent metal, an alloy of the solvent metal, or a metal  in which the solvent metal is soluble.

   When alloys are utilized,  one may substantially retain the beneficial     properties    of the  alloys, and increase, for example, the modulus of elasticity, high       temperature    stability, and wear resistance; although some loss of  ductility may be encountered in certain soft alloys.     Further,    final  metal matrix composites prepared     from    the solvent metal matrix  materials of the present process may be fabricated in conventional  fashion, by casting, forging, extruding, rolling, machining, etc.,  and may also be remelted and recast while retaining substantial  uniformity in second phase particle distribution and retaining fine  second phase particle size, fine grain size, etc., thereby  maintaining associated improvements in physical properties.

    



  The degree of porosity of the porous composite can be varied by  procedures such as vacuum degassing or compression applied prior to,  during, or subsequent to initiation of the second phase-forming  reaction. The degree of vacuum applied and temperature of the       degassing    step is determined purely by the kinetics of evaporation  and diffusion of any absorbed moisture or other gases. High vacuum  and elevated temperatures aid the degassing operation.

   In the case  of titanium, aluminum, and     boron    mixtures, however, the pre-reacted      compact must not be exposed to temperatures above 300 C for  prolonged periods of time, as this will induce the     volatilization    of  some components and induce the formation of titanium al     umini    de by  solid state diffusion. This is undesirable because it forms as  large-plates, which     are    detrimental to mechanical properties, and  also reduces the chemical driving force for the formation o f the  titanium     dibori    de.

   Nonetheless, conversion of titanium al     umini    de to       titanium-diboride    in the presence of boron and aluminum can occur  slowly if the components are held at temperatures above the melting  point of aluminum.  



  When vacuum degassing is applied prior to reaction; lower  porosity is obtained. When vacuum is applied during reaction, the  compact typically expands, resulting in a significant increase in       po        ro        si        ty.     



  Absent the degassing step, the composite formed may be  relatively     porous,    and lower in density than the matrix metal. In  such a state, this material may be of a high second phase  concentration and may be added to a measured volume of matrix metal  (either the same or different from the matrix in which the       dispersoid    was- first formed) to achieve a     specifically        desired     second phase volume fraction. Relatively high concentration of the  second phase in the solvent metal matrix may be achieved while  retaining substantially uniform dispersion of discrete second phase       particles    within the solvent metal matrix.  



  In     preparing    the porous composite materials, degassing of the  powders of the     reactant    mixture may not be necessary, and, in fact,  it may be advantageous not to     degas    the     powders,    because a porous  product tends to be advantageous in the subsequent addition to a  host metal. It may even be desirable, in some instances, to  incorporate a porosity enhancer such as a low boiling point metal,  for example, magnesium in the     initial    reactant mixture, the enhancer  volatilizing during     the    in-situ     reaction,    thereby increasing the  porosity of the resultant composite.  



  As formed, the second phase particles of the porous composite  are     protected        from    oxide or other deleterious covering layers which  form on prior art ceramic powders. The in-situ     formed    second phase,      such as ceramic, of the present invention, uniformly dispersed  within a solvent matrix metal, may be introduced into a molten host  metal bath to     redisperse    the second phase particles of the porous       composite        throughout    the host metal.

   The     molten    host     metal    of the  bath may be of such composition that in-situ precipitation of the  desired second phase could not occur within the bath, or could occur  only with difficulty. Thus, metals other than the solvent-matrix  metal may be provided with a uniform dispersion of second phase  particles of     submicron    and larger size.

   The molten host metal may  also be the same as the solvent metal matrix of the porous  composite, but of so great a volume, as compared     to    the porous       composite,    that     in-situ    second phase precipitation     would        be     difficult to effect or     control:The    concentration of the second phase  in the     porous        composite    need not be large, however.  



  It is believed that the prior art suggestions of introduction  of fine second phase particles directly to a     molten    metal bath are  technically difficult and     produce    metal products having less  desirable     properties    upon solidification due     to    a     deleterious    layer,  such as an oxide,     which    forms on the surface of each second phase  particle at the time of or prior to introduction into the molten  metal bath:

   The second phase particles of the present invention,  being formed in-situ, do     not        possess    this deleterious coating or       layer.    Thus; the present invention     may    lead to metal     products     having unexpectedly superior     properties.     



  Three basic reaction modes to make porous composite have been       identified    in accordance with the present invention. In the first       mode,    the starting materials constitute individual powders of each  of the solvent metal and the individual     constituents    of the second  phase to be formed.

   For example, a mixture of aluminum, titanium,  and boron may be compacted into a rod and ignited locally to cause  an isothermal, propagating reaction wave     front    to consume the  elements and to form a dispersion of titanium     diboride    in an  aluminum matrix:  In the second     made    of the invention, individual alloys may be  reacted, one such alloy comprising an alloy of the solvent metal  with one of the constituents of the second phase; and the other      comprising an alloy of the same solvent     metal,    or another metal with  which the solvent metal readily alloys, with the other constituent  of the second phase.

   As an example of using two alloys of a common  metal, a mixture of aluminum-titanium alloy with aluminum-boron  alloy may be compacted     into    a rod and ignited locally to cause an  isothermal, propagating reaction wave front to consume the elements  and to form a dispersion of titanium     diboride    in aluminum. This  alloy-alloy reaction     route    may, in some cases, be relatively slower  than the elemental route, yet     may    offer economic advantages because  the alloys utilized can be cheaper than the elemental powders.  



  The third reaction mode constitutes a combination, or  intermediate, of the first two modes discussed above. Thus, one may       react    a premixed alloy containing one reactive. species and the  solvent matrix     metal,    with an elemental powder of the second  reactive species, such as combining an aluminum-titanium alloy with  elemental boron powder, compacting into a     rod,    and igniting locally  to cause an     isothermal,    propagating reaction wave front to consume  the elements and to form a dispersion of titanium     di        bori    de in an       aluminum    matrix.

   This reaction mode may be relatively more  expensive than the alloy-alloy reaction     mode,    but offers a more  rapid reaction,     which    in turn permits formation of finer particle  precipitates than obtainable by the alloy-alloy route.     However,    the       alloy-elemental    powder reaction mode could be     relatively    less  expensive, although slower,     than    the elemental powder mode, in most  cases.  



  It should be noted in performing the process of the present  invention that certain criteria must be met in order to produce the  desired porous composite: Firstly, the heat generated by the  initial local reaction of the second phase-forming constituents must  be sufficient to allow the reaction wave front to propagate through  the reaction mass. In addition, the heat source, such as  inductively heated graphite, should supply sufficient local heat to  initiate the second     phase-forming    reaction by, for example, locally  melting solvent metal.  



  Both of the above criteria-have a significant impact on the  feasibility of different composite. forming     reactions        performed    in      accordance with the present invention because the relatively high  volume fractions of solvent metal in the reaction mass absorb heat  and therefore tend to quench the reaction. For this reason, it can  be important to preheat the reactant mass prior to local initiation  of     reaction.    Preheating     may    thus permit certain non-propagating  reactions to     propagate,    or, in the alternative, allow reactions to  propagate at higher solvent metal concentrations.

   Other advantages  to preheating include the ability to remove     adsorbed    gases from the  reaction mass prior to initiation, and the attainment of higher  maximum reaction temperatures that permit the second phase-forming       reaction    to go substantially to completion.  



  The resultant intermediate composite from any of the reaction  modes mentioned above, typically a porous concentrate, may be  subsequently combined with additional metal in the admixture  procedure. As described earlier, this procedure yields dense  composites with superior properties that combine the beneficial  effects of in-situ precipitated     dispersoids    and molten metal  processing to achieve the required loading of second phase. In one  embodiment of the present invention the in-situ second phase  formation process and the admixture procedure are performed  sequentially without a break, or, in the alternative, are made to  occur almost simultaneously. Such a procedure has obvious  advantages in that intermediate materials handling operations are  eliminated, and just one apparatus may be used for both processes.

    Typical of such a combined second phase formation and admixture  procedure     would    be the preparation of a compacted rod of elemental       boron,    titanium and aluminum powders, followed by suspending the rod       i    n such a manner as     to    dip the end of the rod into a bath of molten  aluminum. The self     propagating,    substantially isothermal reaction  could then be allowed to consume the second phase-forming  constituents before the reacted compact     i    s admixed with the molten  metal by releasing the suspension means.

   Alternatively, the rod  could be immersed into the molten metal essentially concurrently  with the second phase-forming reaction by more rapid release of the  suspension means. In the extreme, the rod might be replaced with  compacted briquettes, for example, that are simply dropped into the           molten    metal. Accordingly, the self-propagating, substantially       isothermal    reaction might take place in the briquette while  submerged in the melt, thereby causing essentially     simultaneous     formation and dispersion of second phase     dispersoids.     



  It is particularly     to    be noted that the prior art teaches that  the combination of elemental metal powders, or alloy powders,       particularly    of a coarse particulate size, would typically yield       intermetallic:

          compounds.    In fact, conventional techniques for  forming     intermetallics    involve, for example, reacting a mixture of  titanium and aluminum, to     form    titanium     aluminide,    and a mixture of  boron and aluminum to form aluminum     di        boride.    Thus, one would  expect that a mixture comprising powders of titanium,     aluminum,    and  boron would yield an aggregate agglomeration of titanium al     umi        nide,          aluminum        dibori    de, and possibly,

   titanium     di        boride.    In contrast,  the present invention provides for the formation of essentially just  one finely dispersed precipitate from the two reactive components in  a matrix of the third component. It is important that the second  phase precipitate material not be soluble in the solvent metal,  while the constituents' of the second phase, individually, are at  least sparingly soluble in the solvent metal. Thus, the     exothermic     dispersion reaction mechanism depends upon a certain amount of each  second phase-forming constituent dissolving and diffusing in the .

    solvent     metal,    and while in solution (either liquid or solid     state),          reacting        exo        thermically    to form the insoluble ceramic, which  precipitates rapidly as a fine     particulate.    The solvent metal  provides a medium in which the reactive elements may diffuse and  combine. Once the initial reaction has occurred, the heat released  by the     exothermic    reaction may cause additional solvent metal to       melt,    thereby enhancing diffusion of reactive components in the  solvent     metal,    and completing the reaction.  



  The cool-down period following initiation of the reaction and  consumption of the reactive constituents is believed important to  achieving very small particle size, and limiting particle     growth.     



  It is known that at high temperatures, it is possible for the second  phase particles to grow, or sinter together. This should also be  avoided,     i    n     most    cases, because of the negative effect of large      particle sizes on ductility. The cool-down or quenching of the  reaction is, in a sense, automatic, because once the second  phase-forming constituents are completely reacted, there is no  further energy released to maintain the high temperatures achieved.  However, one may control     the    rate of cool-down to a certain extent  by     control    of the size and/or composition of     the    mass of material  reacted.

   That is,     large    thermal masses absorb more energy, and cool  down more slowly, thus permitting     growth    of larger particles, such  as may be desired for greater wear resistance, for example, for use  in cutting tools. Fast cooling may be achieved, for example, by  placing the reaction mass on a water-cooled copper     substrate.    This  avoids the- contamination typically obtained with refractory  substrates such as alumina.  



  Initiation of the reaction is accomplished by local heating of  a portion of the reaction mass, rather than heating of the entire  mass. .Localized heating may be achieved by electrical impulse,       thermite    spark, laser, etc. The preferred method is inductive  heating of a graphite     susceptor.     



  While it is unnecessary to actually reach the melting  temperature to initiate the reaction, a temperature where localized  melting occurs must be achieved, or where substantial diffusion of  the     reactive    species in the solvent metal can occur. In some cases,  as temperature increases it is possible for the starting  constituents to diffuse     into    the solvent matrix metal, forming an  alloy therewith having a lower melting temperature than the matrix  metal. Thus, reaction initiation temperature is lowered.  



  Regarding impurities, the solvent metal may be alloyed in  conventional manner, while in the reactive     constituents,    large  amounts of alloying elements or impurities may cause problems in  certain instances. For example, the presence of large amounts of  magnesium in     boron    may inhibit the formation of titanium     dibo    ride in  an aluminum matrix by forming a magnesium-boron complex on the  surface of the boron particles, thus limiting diffusion of the boron  in the matrix. However, the presence of magnesium in the aluminum  does not have this effect.

   That is,     boride    forming materials in the       boron    itself may inhibit the desired dissolution or diffusion of the      boron and its subsequent     reaction    to form titanium     diboride.     Likewise, thick oxide. films around the starting constituent powders  may also act as barriers to diffusion and reaction. Extraneous  contaminants, such as absorbed water vapor, may also. yield  undesirable phases such as oxides or     hydrides,    or the powders     may    be  oxidized to such an extent that the reactions are influenced.  



  It is noted that undesirable compounds which may be formed     from     the reaction of one     constituent    and the solvent metal during the       porous    composite formation process can be essentially eliminated in  some instances by the addition of     more    of the other constituent.  



  For example, titanium     aluminide    formation in the titanium       dibori        de-alumi        num    porous composite may be substantially eliminated  by adding boron above     stoichiometric    proportion prior to initiation  of the second phase-forming     reaction.    The boron can be in the form  of elemental boron, boron alloy or boron halide.

   It     i    s also noted  that in the admixture process, wherein composite material of the       present    invention is added to a molten host metal,     undesirable     compounds formed in     the    composite material by the reaction of one  constituent and the solvent metal     may    be     introduced        into    the melt:  These undesirable compounds may be     essentially    eliminated by adding  an additional amount of another constituent to the molten host  metal.

   For example,     titanium    al     umi        ni    de formed in: a titanium       diboride-aluminum        composite    material may be essentially removed from  a     host        aluminum    melt by adding additional     boron        to    the melt. Such a  boron addition also provides the benefit     that    any free titanium,  which can adversely affect the viscosity of the melt for casting  operations is converted to titanium     diboride.     



  It is also to be noted that, in accordance with the present  invention, the complex precipitation of a plurality of systems may  be caused. Thus, it is possible to precipitate complex phases, such  as     Ti(BO.5C0.5),    or     alternatively,    to precipitate plural second  phases, such as a     mixture    of titanium     diboride    and     zirconium          diboride    in an     aluminum    matrix, in accordance with the reaction:  
EMI0028.0041     
  
    Ti <SEP> +Zr+4B+A1 <SEP> <B>--).</B> <SEP> Ti <SEP> B2+ZrB2+Al.

         Substitution of titanium by zirconium or vice versa, is also  possible; yielding complex     borides    of the type     (Ti,Zr)B2.         It is also possible to achieve a low temperature solvent  assisted reaction in a metal matrix which has a high melting  temperature by alloying or admixing the high melting metal with a  lower melting solvent metal. This may allow for easier initiation  and     propagation.     



  In accordance with the present invention, it has been found  that the powders need not be compacted prior to localized firing,  but     doing    so allows easier diffusion and thus easier initiation.  This is due to localized melting, and increased diffusion, which are  possible when the powders are in close proximity.  



  The starting powders must be     protected        from    extensive oxidation  due     to    exposure to the atmosphere, as this will restrict the  diffusion of the components     into    the solvent metal matrix, and the  reaction should preferably be carried out under an inert gas to  minimize oxidation at high temperatures.  



  In accordance with the present method, particle     growth    of the  second phase can be controlled. As is known in the art, the  elevated temperatures produced as, for example, by the exothermic       reaction,    will     remain    higher and subside more slowly for a large  mass of material than for a smaller mass. These conditions of high  temperature for long periods of time favor particle growth of  ceramics. Thus, the formation of relatively small volume porous  composites of in-situ formed ceramic will facilitate quicker cooling  and limit particle growth of the ceramic phase.  



  The particle size of the second phase reaction     product    is  dependent upon heat-up rate, reaction temperature, cool-down rate,  crystallinity and composition of the starting materials.  



       Appropriate    starting powder sizes may range from less than 5     microns     to more than 200     microns.    For economic reasons, one may normally  utilize     larger    particle size powders. It has been found that the  particle size of the     precipitated    second phase in the matrix may  vary     from    less than about 0.01 microns to about 5     microns    or larger,  dependent upon such factors as those discussed above.  



  It has been found that some specific reactant     properties    have a  greater impact than powder particle size on the particle size of the  second phase produced. For example, the use of amorphous     boron        may         result in the precipitation of a finer particle size     titanium          diboride    than does the use of crystalline boron in an otherwise  comparable mixture. The precipitation of specific particle size  second phase may be selectively controlled by proper control of  starting composition, temperature of reaction, and cool-down rate.  



  In selecting the constituents and the solvent matrix     metal    for  the composite materials produced by the above described process, it  is important that the formed second phase material have a low  solubility in the molten mass, for example, a maximum solubility of  5 weight percent, and preferably 1 percent or less, at     the     temperature of the molten host metal. Otherwise,     significant     particle     growth    in the second phase material may be experienced over  extended periods of time. For most uses of composite materials, the  size of the second phase particles should be as small as possible,  and thus particle growth is undesirable.

   When the solubility of the  formed second phase material in the molten mass is low, the     molten     mass with dispersed second phase particles can be maintained in the  molten state for a considerable period of time without     growth    of the  second phase: particles. For example, a molten mass of aluminum  containing dispersed titanium     diboride    particles can be-maintained  in the molten state for three to four hours without appreciable  particle growth.  



  One advantage of the admixture process is that the use of  porous composite,     particularly        that    having a high loading of second  phase material, permits one     to    simply make a single batch of porous  composite material. One may then     produce    a wide variety of final  composites having different second phase loadings.

   Additionally,  with the admixture procedure,     i    t     i    s possible to form the second  phase material in a matrix metal which is conducive     to    the formation  of     particles    of a desired type, size, and morphology, and thereafter  incorporate the particles in a host     metal    in which such particles  cannot otherwise be     produced.     



  A further advantage of the use of the admixture concept is the  fact that in the in-situ precipitation of second phase material in a  solvent metal matrix, the particle size of the second phase material  appears to be     related    to the loading level of the second phase      material. For example,     i    n titanium     di        bo        ri        de-aluminum    composites,  particle size decreases with higher concentration, up to about 40-60  percent second phase material, and then the particle size increases  as the concentration approaches 100 percent.

   Thus, for example, if  the smallest possible particle size     wasdesired    in a final composite  having a low-second phase concentration, one could prepare a second  phase-containing concentrate in the 40-60 percent concentration  range of titanium     diboride    to yield the smallest particles     possible,     and thereafter admix the porous composite to the desired second  phase concentration.  



  Thus, according to the invention, the weight concentration of  the solvent metal exceeds 10 volume percent, more preferably exceeds  20 volume percent, and     most    preferably more than 30 percent. Also,  the porosity of the product exceeds 1 percent, more preferably  exceeds 10 percent, and most preferably exceeds 25 percent. The  particle size of the     dispersoid    particles may vary from     about    0.01       microns    to about 5 microns, preferably from 0.1 microns to about 3       microns.     



  The reactants may be formed into any desired conventional  shape. Typically, a rod or cylindrical green compact is used. The  shaped article can be compressed in a manner     known    in the art. Any  shape is useful that facilitates local ignition. In a conventional  manner, an end of a rod is ignited and the isothermal wave     front     moves along the rod to its terminal end. Any conventional means for  ignition can be used.  



  Examples 1 through 6 illustrate the production of titanium       diboride    second phase particles in aluminum matrices and the effects  of atmosphere, compaction pressure, and preheating on reaction  propagation rate.  



       EXAMPLE   <U>1</U>  Titanium,     boron,    and aluminum powders are ball-milled in the  proper     stoichiometric    proportions to provide 60 weight percent  titanium     diboride    second phase in an aluminum solvent matrix. The  mixture is then packed in     gooch    tubing and     isostatically    pressed to  40     ksi,    forming a compact approximately 1     centimeter    in diameter by      5 centimeters long and having a density of 2.39 grams per cubic       centimeter.    The compact is then placed end to end     with    a graphite  rod in a quartz tube under flowing argon.

   The graphite rod is  heated in a radio frequency field which initiates a reaction at the  interface of the compact and' the rod. The reaction propagates: the  length of the compact at a rate of 0.77 centimeters per second.  Analysis of the resultant composite material reveals a dispersion of  substantially     unaggiomerated    titanium     diboride    particles having an  average diameter of approximately 1 micron in an aluminum matrix.  



       EXAMPLE   <U>2</U>  A compact containing titanium, boron, and     aluminum:    is prepared  and reacted as in Example 1, with an additional step of     preheating     the compact     to    500 C prior to initiation of the     reaction.    The  reaction is observed     to    propagate faster than in the     unpreheated     compact at a rate of 1.38 centimeters per second.  



  <U>EXAMPLE 3</U>  A compact containing titanium,     boron,    and aluminum is prepared  and reacted as in Example 2, except that the reaction is     done    in a  vacuum rather than under flowing argon. The reaction propagates at  1.33     centimeters    per second:  <U>EXAMPLE 4</U>  A compact containing titanium, boron, and aluminum is prepared  and reacted as in Example 1, except that the reaction is done in an       atmosphere    of flowing helium rather than argon. The reaction is  observed to propagate at a rate of 0.47 centimeters per second.  



  <U>EXAMPLE 5</U>  A compact     i's    prepared and     reacted    as in Example 1, except that  the     mixture    of titanium, boron and aluminum powders is compacted to  13     ksi    rather than 40     ksi;    yielding a compact     having-a    lower density  of 2.06 grams per cubic centimeter. The reaction propagates at 0.66   centimeters per second.           EXAMPLE   <U>6</U>  A compact containing titanium, boron, and aluminum is prepared  and     reacted    as in Example 5, except that the reaction is done in a  vacuum rather than under flowing argon. The reaction is observed to  propagate at a rate of 0.44 centimeters per second.  



  The following example illustrates the ability to produce a  composite material comprising titanium carbide second phase  particles in an aluminum matrix by the process of the present  invention and the subsequent addition of the composite material to  molten aluminum to produce a composite of lower second phase loading.  



       EXAMPLE   <U>7</U>  239.5 grams of titanium powder, 60.3 grams of carbon black, and  200.2 grams of aluminum powder are ball-milled for 30 minutes,  packed in     gooch    tubing, and     isostatically    pressed to 40     ksi,    forming  a green compact 1 inch     i    n diameter by 12 inches long. The compact  is placed on two water cooled copper rails in a 4 inch diameter  quartz tube under flowing argon. A 1 inch by 1 inch piece of carbon  placed next to one end of the compact is induction heated until an  exothermic reaction     i    s initiated at the end of the compact.

   Power  to the induction unit heating the carbon is turned off and the       reaction    is allowed to propagate the length of the compact. When  cool, the reacted concentrate, comprising 60     weight    percent titanium  carbide second phase particles in an aluminum matrix, is crushed and  slowly added to molten aluminum at 770 C while mechanically  stirring. The melt is maintained at 770 C and stirred vigorously  for several minutes. The     melt    is then fluxed with chlorine gas for  15 minutes, skimmed, and cast. The resultant material contains  approximately 7.5 volume percent titanium carbide second phase  particles in an aluminum matrix.  



  The following example illustrates the     production    of a composite  material comprising titanium     diboride    second phase particles in an  aluminum matrix by the process of the present invention, including  the use of boron above     stoichiometric    proportion. The example also      demonstrates the subsequent introduction of this composite material  into additional aluminum to produce a composite of lower second  phase loading.  



       EXAMPLE   <U>8</U>  207 grams of titanium powder, 106 grams of boron powder (15  weight percent above     stoichiometric    proportion), and 200.2 grams of  aluminum powder are ball-milled for 30 minutes, packed in     gooch     tubing, and     isostatical'ly    pressed to 40     ksi,    forming a green compact  approximately 1 half inch in diameter by<B>12</B> inches long. The  compact is placed on a water cooled copper trough in a 2 inch       diameter    quartz tube under flowing argon. A 1 inch by 1 inch piece  of carbon placed next to one end of the compact is induction heated  until an exothermic reaction is initiated at the end of the  compact.

   Power     to    the induction unit heating the carbon is     turned     off and the     reaction    is allowed to propagate the length of the  compact. When cool, the reacted concentrate is crushed and slowly  added to molten     aluminum    at 770 G while mechanically stirring. The  melt is maintained at 770 C and stirred vigorously for several  minutes. The melt is then fluxed with chlorine gas for 15 minutes,       skimmed,    and cast. The resultant material contains approximately 10  volume percent titanium     diboride    second phase particles having an  average size of 0.9 microns in an aluminum matrix, substantially  free of titanium     aluminide.     



  It is noted that the present invention has a number of  advantages over methods taught by the .prior art. For example, this  invention circumvents the need for     submicron,        unagglomerated     refractory metal     bori    de starting materials, which materials are not       commercially    available, and are often     pyrophoric.    Further, the  present invention yields a porous composite with a second phase  precipitated therein, suitable for     admixture    with a host metal to  achieve a final composite having superior hardness and modulus  qualities over currently employed composites,

   such as     SiC/aluminum.     This admixture process also eliminates the technical problems of  uniformly dispersing a second phase in a     molten    metal, and avoids  the     problem    of oxide or other deleterious layer     formation    at the      second phase/metal interface during processing. Final metal matrix  composites prepared     from    the porous composites of the present  invention also have     improved    high temperature stability, in that the  second phase is not reactive with the metal matrix.

   Further, such  final metal matrix composite can be remelted and recast while  retaining fine grain size, fine particle size, and the resultant  superior physical     properties.     



  It is understood that the above description of the present  invention is susceptible     to    considerable     modification,    change, and  adaptation by those skilled in the art, and such modifications,  changes, and adaptations are intended     to    be considered to be within  the scope of the present invention, which is set forth by the  appended claims.

Claims

<U>Claims</U> 1. A method for the production of porous metal-second phase composite material, the method comprising precipitating at least one second phase material in a substantial volume fraction of solvent metal by locally igniting a mixture comprising reactive second phase-forming constituents in the presence of a substantially nonreactive solvent metal in which the second phase-forming constituents are more soluble than the second phase material,

at a temperature at which sufficient diffusion of the reactive second phase-forming constituents into the substantially nonreactive solvent metal occurs to cause a substantially isothermal propagating second phase-forming reaction-of the constituents to thereby precipitate second phase particles in the solvent metal so as to produce finely divided second phase particles in the metal matrix. 2. The method as set forth in Claim i, wherein the temperature is about the melting temperature of the solvent metal. 3.

The. method as set forth in Claim 1, wherein the reactive second phase-forming constituents and the substantially nonreactive solvent metal are each provided as ind':ividual elements. 4. The method as set forth in Claim 1, wherein the second phase-forming constituents are each provided-as an alloy of the solvent metal. 5. The-method as set forth in Claim 1 wherein the mixture is compressed to a compact prior to local ignition. 6. The method as set forth in Claim 1-, wherein at least one second.phase-forming constituent is provided as an alloy of the solvent metal, and the other second phase-forming constituents are provided as individual elements. 7.

The method as set forth in Claim 1, wherein the particulate second phase is titanium diboride, zirconium diboride, zirconium disilicide, zirconium carbide, titanium carbide, or titanium -nitride. 8. The method as set forth in Claim 1, wherein the second phase is a ceramic. _ 9. The method as set forth in Claim 1, wherein the second phase is an intermetallic. 10. The method as set forth in Claim 1, wherein the concentration of the solvent metal matrix is greater than 20 percent by volume. 11. The method as set forth in Claim 1, wherein the particle size of second phase particles is between about 0.1 and 3 microns. 12.

The method asset forth in Claim 1, wherein the metal-second phase composite material has a porosity of greater than 10 percent. 13. The method as set forth in Claim 1, wherein the metal-second phase composite material has a porosity of greater than 25 percent. 14. The method as set forth in Claim 1, wherein the reactive second-phase forming constituents are shaped in the form of a rod. 15. The method as set forth in Claim 1, wherein the reactive second-phase forming constituents are compressed in the shape of a rod. 16. The method as set forth in Claim 1, wherein the second phase particles are substantially encapsulated by solvent matrix metal. 17. The method as set forth in Claim 1, wherein the reaction further comprises an isothermal wave front. 18.

A method for dispersion of a second phase in a metal, metal alloy, or intermetallic matrix, the method comprising steps of: (a) preparing a mixture of reactive second phase-forming constituents and a substantially large volume fraction of nonreactive solvent metal in which the second phase-forming constituents are more soluble than the second phase;

(b) locally heating the mixture to a reaction initiation temperature approximating the melting point of the solvent metal to initiate a substantially isothermal exothermic reaction; and (c) permitting the exothermic reaction to further heat the mixture and to propagate, consuming the second phase-forming constituents, forming a distribution of second phase particles in a metallic matrix. 19. The method as set forth in Claim 18, wherein at least one second phase-forming constituent is a transition metal of the third to sixth groups of the Periodic Table. 20.

The method as set forth in Claim 18, wherein at least one second phase-forming constituent is aluminum, titanium, silicon, boron, carbon, sulfur, tantalum, thorium, yttrium, cobalt, nickel, molybdenum, tungsten, vanadium, zirconium, niobium, hafnium, magnesium, scandium, lanthanum, chromium, oxygen, nitrogen, lithium, beryllium, iron, manganese, zinc, tin, copper, silver, gold, platinum or a rare earth element. 21. The method as set forth in Claim 20, wherein the solvent metal is aluminum, nickel, titanium, copper, vanadium, chromium, manganese, cobalt, iron,: silicon, molybdenum, beryllium, silver, gold, platinum, niobium, tantalum, hafnium, zirconium, mangesium, lead, zinc, tin, tungsten, antimony, bismuth, or an alloy of such metals. 22.

The method as set forth in Claim l'8, wherein the second phase-forming constituents and the solvent metal are provided as i i n dividual elements. 23. The method as set forth 9n Claim 18, wherein the second phase-forming constituents are each provided as an alloy of the solvent metal. 24. The method as set forth in Claim 18, wherein at least one second phase-forming constituent is provided as an individual element, and at least one other second phase-forming constituent is provided as an alloy of the solvent metal. 25.

The method as set forth in Claim<B>18,</B> wherein the particulate second phase is titanium diboride, zirconium diboride, titanium carbide, zirconium di si l i ci de, zirconium carbide, or titanium nitride. 26. The method as set forth in Claim 1'8, wherein the concentration of the solvent metal is greater than 20 percent by volume. 27. The method as set forth in Claim 18; wherein the particle size of second phase particles is between about 0.1 and 3 microns. 28. The method as set forth in Claim 18, wherein the metal-second phase composite material has a porosity of greater than 10 percent. 29.

The method as set forth in Claim 18, wherein-the metal-second phase composite material has a porosity of greater than 25 percent. 30. The method as set forth in Claim 18, wherein the mixture is shaped in the form of a .rod. 31. The method as set forth in Claim 18, wherein the mixture is compressed in the shape of a rod. 32. The method as set forth in Claim 18, wherein the second phase particles are substantially encapsulated by solvent metal. 33. The method as set forth in Claim 18, wherein reaction propagation further comprises an isothermal wave front. 34.

A method for the production of porous metal-second phase composite materials, the method comprising precipitating at least one second phase material in a substantial volume fraction of solvent matrix by locally igniting a mixture of reactive second phase-forming constituents and a substantially nonreactive solvent metal in which the second phase-forming constituents are more soluble than the second phase, at a temperature at which sufficient diffusion of the constituents into the solvent metal occurs, to cause a substantially isothermal propagating reaction of the reactive second phase-forming constituents to increase the temperature to a temperature exceeding the melting temperature of the solvent metal, to precipitate the second phase in the solvent metal. 35.

A method for providing a second phase in a metallic matrix, the method comprising steps of: (a) preparing a mixture of reactive second phase-forming constituents and a substantial volume fraction of nonreactive solvent metal in which the second phase-forming constituents are more soluble than the second phase; (b) locally heating the mixture to a reaction initiation temperature approximating the melting point of the solvent metal to initiate a substantially isothermal exothermic reaction; and (c) permitting the exothermic reaction to further heat the mixture and to propagate, forming a uniform distribution of second phase particles in a metallic matrix. 36.

A method for dispersion of second phase dispersoids in a metallic matrix, the method comprising forming a reaction mixture of reactive second phase-forming constituents in the presence of a substantial volume fraction of at least two metals, at least one of which acts as a solvent metal in which the second phase-forming constituents are more soluble than the second phase dispersoids, raising the temperature of the reaction mixture locally to a temperature at which sufficient diffusion of the second phase-forming constituents into the lowest melting solvent metal occurs to-initiate a substantially isothermal reaction of the constituents,

whereby the exothermic heat of reaction of the constituents causes the temperature of the reaction mixture to locally exceed the melting point of the highest melting metal, permitting propagation of the reaction, and dispersion of-the second phase dispersoid in an alloy.. 37.

A method for dispersion of second phase dispersoids in a metal matrix, the method comprising forming a reaction mixture of reactive second phase-forming constituents in the presence of a substantial volume fraction of at least two metals., at least one of which acts as a solvent metal in which the second phase-forming constituents are more soluble than the second phase dispersoids, raising the temperature of the reaction mixture locally to a temperature at which sufficient diffusion of the second phase-forming constituents into the lowest melting solvent metal occurs to initiate a substantially isothermal reaction of the constituents,

whereby the exothermic heat of reaction of the constituents causes the temperature of the reaction mixture to exceed the melting point of the lowest melting point solvent metal, permitting propagation of the reaction and dispersion of the second phase dispersoids in a mixed metal matrix: 38.

A method for dispersion of at least one intermetallic material in a metallic matrix, comprising steps of: (a) preparing a mixture of reactive intermetallic-forming constituents and a substantial volume: fraction of nonreactive solvent metal, metal alloy, or intermetallic in which the intermetallic-forming constituents are more soluble than the intermetallic material;

(b) locally heating the mixture to a reaction initiation temperature approximating the melting point of the solvent metal, metal alloy or i ntermetal l i c ; and (c) permitting the resultant substantially isothermal exothermic reaction to raise the temperature of the mixture to a temperature exceeding the melting point of the solvent metal, and to propagate, consuming the intermetallic-forming constituents. 39.

A method for dispersion of at least one ceramic material in a metallic matrix, comprising steps of: (a) preparing a mixture of reactive ceramic-forming constituents and a substantial volume fraction of nonreactive solvent metal in which the ceramic-forming constituents are more soluble than the ceramic material; (b) locally heating the mixture to a reaction initiation temperature approximating the melting point of the solvent metal; and (c) permitting the resultant substantially isothermal exothermic reaction to raise the temperature of the mixture to a temperature exceeding the melting point of the solvent metal, and to propagate, consuming the ceramic-forming constituents. _ 40.

A method for dispersing dispersoid particles of an intermetallic material and a ceramic material in a metal matrix, the method comprising steps of: (a) preparing a mixture of reactive intermetallic and ceramic-forming constituents and a substantial volume fraction of nonreactive solvent metal in which the intermetallic-forming constituents and the ceramic-forming constituents are more soluble than the intermetallic or the ceramic material, respectively; (b) locally heating the mixture to a reaction initiation temperature approximating the melting point of the solvent metal;

and (c) initiating a substantially isothermal exothermic reaction to raise the temperature of the mixture to a temperature exceeding the melting point of the solvent metal, and to propagate, consuming the intermetallic-forming constituents and ceramic-forming constituents. 41.

A method for the production of porous metal-second phase composite material, the method comprising forming at least one second phase material in a substantial volume fraction of nonreactive metal by locally igniting a mixture comprising reactive second phase-forming constituents in the presence of a substantially nonreactive metal at a temperature that causes a substantially isothermal propagating second phase-forming reaction of the constituents to thereby form second phase particles in the metal so as to produce finely divided second phase particles in the metal matrix. 42.

A porous mass comprising a dispersion of in-situ precipitated second phase particles in a solvent metal matrix produced by propagating a substantially isothermal exothermic reaction of second phase-forming constituents in the presence of a substantial' volume fraction of solvent metal in which the constituents are more soluble than said second phase. 43. The dispersion as set forth in Claim 42, wherein the second phase particles comprise from about 40 to about 60 percent by weight of the composite formed. 44. The dispersion as set forth in Claim: 43, wherein the particle size of the second phase is from about 0.1 to about 3 microns. 45. The dispersion as set forth in Claim 42, wherein the porosity of the composite exceeds about<B>10</B> percent.

46: The dispersion as set forth in Claim 42, wherein the concentration of the solvent metal exceeds 20 percent by volume. 47. The dispersion as set forth in Claim 42, wherein the second phase particles have a particle size between about.0.1 and 3 microns. 48.: The dispersion as set forth in Claim 42, wherein the second: phase particles are substantially encapsulated by solvent metal. 49. The dispersion as set forth i n Claim 42, wherein the second phase particles are substantially devoid of any coating or layer thereon. 50.

The dispersion as set forth in Claim 42, wherein the second phase particles include titanium diboride or titanium carbide and the solvent metal matrix includes aluminum or an alloy thereof.