High conductivity magnesium alloy

Abstract

A castable, moldable, or extrudable magnesium-based alloy that includes one or more insoluble additives. The insoluble additives can be used to enhance the mechanical properties of the structure, such as ductility and/or tensile strength. The final structure can be enhanced by heat treatment, as well as deformation processing such as extrusion, forging, or rolling, to further improve the strength of the final structure as compared to the non-enhanced structure. The magnesium-based composite has improved thermal and mechanical properties by the modification of grain boundary properties through the addition of insoluble nanoparticles to the magnesium alloys. The magnesium-based composite can have a thermal conductivity that is greater than 180 W/m-K, and/or ductility exceeding 15-20% elongation to failure.

Claims

1. A magnesium-based composite comprising a base metal and a plurality of nanoparticles, said base metal is formed of a magnesium or magnesium alloy, said base metal includes at least 70 wt. % magnesium, said insoluble nanoparticles have a melting point that is greater than a melting point of said base metal, said insoluble nanoparticles have a solubility of less than about 5% in said base metal, said insoluble nanoparticles constitute at least 0.1 vol. % of said magnesium-based composite, said insoluble nanoparticles are located within 200 nm of grain boundaries or dislocations in said magnesium-based composite, said insoluble nanoparticles have an average thermal conductivity of above about 180 W/m-K, said nanoparticles cause said magnesium-based composite to have at least a 30% increase in thermal conductivity as compared to said thermal conductivity of said base metal.

2. The magnesium-based composite as defined in claim 1, wherein said nanoparticles include a plurality of particles selected from the group consisting of carbon, graphene, diamond, copper, silver, aluminum, beryllium, gold, tungsten, SiC, AlN, BeO, BN, and TiB.sub.2.

3. The magnesium-based composite as defined in claim 2, wherein said nanoparticles include copper powder and/or carbon nanotubes.

4. The magnesium-based composite as defined in claim 3, wherein said nanoparticles have a thermal conductivity of greater than 180 W/m-K.

5. The magnesium-based composite as defined in claim 4, wherein said nanoparticles constitute about 0.1-20 vol. % of said magnesium-based composite.

6. The magnesium-based composite as defined in claim 5, wherein at least 50% of said nanoparticles have an average particle size or have at least one dimension that is no more than about 400 nm.

7. The magnesium-based composite as defined in claim 1, wherein said nanoparticles have a thermal conductivity of greater than 180 W/m-K.

8. The magnesium-based composite as defined in claim 1, wherein a weight percent of said base metal is greater than a weight percent of said nanoparticles.

9. The magnesium-based composite as defined in claim 1, wherein said nanoparticles constitute about 0.1-20 vol. % of said magnesium-based composite.

10. The magnesium-based composite as defined in claim 1, wherein at least 50% of said nanoparticles have an average particle size or have at least one dimension that is no more than about 400 nm.

11. The magnesium-based composite as defined in claim 1, wherein said base metal is a magnesium alloy selected from the group consisting of AM series alloy, AZ series alloy, LPSO series alloy, WE series alloy, ZE series alloy, ZK series alloy, ZM5 series alloy, or ZW series alloy.

12. The magnesium-based composite as defined in claim 1, a thermal conductivity of said base metal is no more than 156 W/m-K.

13. The magnesium-based composite as defined in claim 6, a thermal conductivity of said base metal is no more than 156 W/m-K.

14. A magnesium-based composite comprising a base metal and a plurality nanoparticles, said base metal is formed of a magnesium or magnesium alloy, said base metal includes at least 70 wt. % magnesium, a thermal conductivity of said base metal is no more than 156 W/m-K, said insoluble nanoparticles have a melting point that is greater than a melting point of said base metal, said insoluble nanoparticles have a solubility of less than about 5% in said base metal, said insoluble nanoparticles constitute about 0.1-20 vol. % of said magnesium-based composite, said nanoparticles include a plurality of particles selected from the group consisting of carbon, graphene, diamond, copper, silver, aluminum, beryllium, gold, tungsten, SiC, AlN, BeO, BN, and TiB.sub.2, said insoluble nanoparticles are located within 200 nm of grain boundaries or dislocations in said magnesium-based composite, said insoluble nanoparticles have an average thermal conductivity of above about 180 W/m-K, said nanoparticles cause said magnesium-based composite to have at least a 30% increase in thermal conductivity as compared to said thermal conductivity of said base metal, at least 50% of said nanoparticles have an average particle size or have at least one dimension that is no more than about 400 nm.

15. The magnesium-based composite as defined in claim 14, wherein said nanoparticles include copper powder and/or carbon nanotubes.

16. The magnesium-based composite as defined in claim 15, wherein said base metal is a magnesium alloy selected from the group consisting of AM series alloy, AZ series alloy, LPSO series alloy, WE series alloy, ZE series alloy, ZK series alloy, ZM5 series alloy, or ZW series alloy.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a Plot of Maxwell Model calculation for ratio of magnesium-based composite thermal conductivity versus magnesium thermal conductivity for various particle sizes and loadings of diamonds (1500 W/m-K) from 20 to 400 micron in size.

(2) FIG. 2 is a photograph of a tabletop ultrasonic casting unit for dispersing nanoparticles into magnesium.

(3) FIG. 3 is a photomicrograph of a typical AZ91 cast structure showing the Mg—Al primary (alpha) and the alpha plus gamma eutectic phases.

(4) FIG. 4 is a photomicrograph of a rapidly quenched AZ91 showing the uniform distribution of the eutectic along primary alpha grain boundaries.

(5) FIG. 5 is a graph illustrating thermal conductivity of the magnesium-based composite verses the particle size of the nanodiamond particles in the magnesium-based composite.

(6) FIG. 6 is an illustration of the microstructure of a Mg-CNT composite.

(7) FIG. 7 illustrates an ultrasonication process to disperse nanoparticles in molten metal magnesium material.

(8) FIG. 8 is a picture of the addition of nanoparticles to a molten magnesium alloy.

(9) FIG. 9 is graph illustrating time-temperature curves generated for diffusivity measurement of a magnesium component containing 3 wt. % CNT.

(10) FIG. 10 is a graph illustrating tensile yield strengths for magnesium, magnesium alloys and magnesium-based composites that include nanoparticles.

DESCRIPTION OF THE INVENTION

(11) The present invention is directed to a cast or wrought magnesium-based composite incorporating nanoparticle modifiers, and method for manufacture of such magnesium-based composite. The magnesium-based composite has improved thermal, physical, and mechanical properties as compared to prior art magnesium alloys. The magnesium-based composite includes a modification of grain boundary thermal or sliding resistance through the addition of nanoscale fillers to the high strength magnesium alloys. The magnesium-based composite has a 15% or greater improvement in thermal conductivity, strength, or strain to failure compared to the base material.

(12) The magnesium-based composite includes magnesium or a magnesium alloy having at least one insoluble phase in discrete form that is disbursed in the base metal or base metal alloy. The magnesium-based composite is generally produced by casting. The discrete insoluble particles include nanoparticles that have a different galvanic potential from the magnesium or a magnesium alloy. The discrete insoluble particles are generally uniformly dispersed through the magnesium or a magnesium alloy using techniques such as, but not limited to, thixomolding, stir casting, mechanical agitation, electrowetting, ultrasonic dispersion and/or combinations of these methods; however, this is not required. In one non-limiting process, the insoluble particles are uniformly dispersed through the magnesium or a magnesium alloy using ultrasonic dispersion. Due to the insolubility and difference in atomic structure in the melted magnesium or a magnesium alloy and the insoluble particles, the insoluble particles will be pushed to the grain boundary of the mixture of insoluble particles and the melted magnesium or a magnesium alloy as the mixture cools and hardens during casting solidification. Because the insoluble particles will generally be pushed to the grain boundary, such feature makes it possible to engineer/customize grain boundaries in the magnesium-based composite to control the dissolution rate of the magnesium-based composite. This feature can be also used to engineer/customize grain boundaries in the magnesium-based composite through traditional deformation processing (e.g., extrusion, tempering, heat treatment, etc.) to increase tensile strength, elongation to failure, and other properties in the magnesium-based composite that were not achievable in cast metal structures that were absent insoluble particle additions. Because the amount or content of insoluble particles in the grain boundary is generally constant in the magnesium-based composite, and the grain boundary to grain surface area is also generally constant in the magnesium-based composite even after and optional deformation processing and/or heat treatment of the magnesium-based composite, the corrosion rate of the magnesium-based composite remains very similar or constant throughout the corrosion of the complete magnesium-based composite.

(13) The magnesium-based composite can be designed to corrode at the grains in the magnesium-based composite, at the grain boundaries of the magnesium-based composite, and/or the location of the insoluble particle additions in the magnesium-based composite depending on selecting where the insoluble particle additions fall on the galvanic chart. For example, if it is desired to promote galvanic corrosion only along the grain boundaries, a magnesium-based composite can be selected such that one galvanic potential exists in the base metal or base metal alloy where its major grain boundary alloy composition will be more anodic as compared to the matrix grains (i.e., grains that form in the casted base metal or base metal alloy) located in the major grain boundary, and then an insoluble particle addition will be selected which is more cathodic as compared to the major grain boundary alloy composition. This combination will cause corrosion of the material along the grain boundaries, thereby removing the more anodic major grain boundary alloy at a rate proportional to the exposed surface area of the cathodic particle additions to the anodic major grain boundary alloy. The current flowing in the grain boundary can be calculated by testing zero resistance current of the cathode to the anode in a solution at a desired solution temperature and pressure that includes the magnesium-based composite. Corrosion of the magnesium-based composite will be generally proportional to current density/unit area of the most anodic component in the grain boundary and/or grains until that component is removed. If electrical conductivity remains between the remaining components in the grain boundary, the next most anodic component in the grain boundary and/or grains will next be removed at a desired temperature and pressure.

(14) Galvanic corrosion in the grains can be promoted in the magnesium-based composite by selecting a base metal or base metal alloy that has at least one galvanic potential in the operating solution of choice (e.g., fracking solution, brine solution, etc.) where its major grain boundary alloy composition is more cathodic as compared to the matrix grains (i.e., grains that form in the casted base metal or base metal alloy), and an insoluble particle addition is selected that is more cathodic as compared to the major grain boundary alloy composition and the base metal or base metal alloy. This combination will result in the corrosion of the magnesium-based composite through the grains by removing the more anodic grain composition at a rate proportional to the exposed surface area of the cathodic non-soluble particle additions to the anodic major grain boundary alloy. The current flowing in the magnesium-based composite can be calculated by testing zero resistance current of the cathode to the anode in a solution at a desired solution temperature and pressure that includes the magnesium-based composite. Corrosion of the magnesium-based composite will be generally proportional to current density/unit area of the most anodic component in the grain boundary and/or grains until that component is removed. If electrical conductivity remains between the remaining components in the grain boundary, the next most anodic component in the grain boundary and/or grains will next be removed at a desired temperature and pressure.

(15) If a slower corrosion rate of the magnesium-based composite is desired, two or more insoluble particle additions can be added to the magnesium-based composite to be deposited at the grain boundary. If the second insoluble particle is selected to be the most anodic in the magnesium-based composite, the second insoluble particle will first be corroded, thereby generally protecting the remaining components of the magnesium-based composite based on the exposed surface area and galvanic potential difference between second insoluble particle and the surface area and galvanic potential of the most cathodic system component. When the exposed surface area of the second insoluble particle is removed from the system, the system reverts to the two previous embodiments described above until more particles of second insoluble particle are exposed. This arrangement creates a mechanism to retard corrosion rate with minor additions of the second insoluble particle component.

(16) The rate of corrosion in the magnesium-based composite can also be controlled by the surface area of the insoluble particle. As such, the particle size, particle morphology, and particle porosity of the insoluble particles can be used to affect the rate of corrosion of the magnesium-based composite. The insoluble particles in the magnesium-based composite can optionally have a surface area of 0.001 m.sup.2/g-200 m.sup.2/g (and all values and ranges therebetween). The insoluble particles in the magnesium-based composite optionally are or include non-spherical particles. The insoluble particles in the magnesium-based composite optionally are or include nanotubes and/or nanowires. The non-spherical insoluble particles can optionally be used at the same volume and/or weight fraction to increase cathode particle surface area to control corrosion rates without changing composition. The insoluble particles in the magnesium-based composite optionally are or include spherical particles. The spherical particles (when used) can have the same or varying diameters. Such particles are optionally used at the same volume and/or weight fraction to increase cathode particle surface area to control corrosion rates without changing composition.

(17) The strength of the magnesium-based composite can optionally be increased using deformation processing and a change dissolution rate of the magnesium-based composite of less than about 20% (e.g., 0.01-19.99% and all values and ranges therebetween), typically less than about 10%, and more typically less than about 5%.

(18) The ductility of the magnesium-based composite can optionally be increased using insoluble nanoparticle cathodic additions.

(19) The magnesium-based composite can optionally include chopped fibers. These additions to the magnesium-based composite can be used to improve toughness of the magnesium-based composite.

(20) The magnesium-based composite can have improved tensile strength and/or elongation due to heat treatment without significantly affecting the dissolution rate of the magnesium-based composite.

(21) The magnesium-based composite can have improved tensile strength and/or elongation by extrusion and/or another deformation process for grain refinement without significantly affecting the dissolution rate of the magnesium-based composite. In such a process, the dissolution rate change can be less than about 10% (e.g., 0-10% and all values and ranges therebetween), typically less than about 5%, and more typically less than about 1%.

(22) Particle reinforcement in the magnesium-based composite can optionally be used to improve the mechanical properties of the magnesium-based composite and/or to act as part of the galvanic couple.

(23) The insoluble particles in the magnesium-based composite can optionally be used as a grain refiner, as a stiffening phase to the base metal or metal alloy (e.g., matrix material), and/or to increase the strength of the magnesium-based composite.

(24) The insoluble particles can optionally be dispersed throughout the magnesium-based composite using ultrasonic means, by electrowetting of the insoluble particles, and/or by mechanical agitation.

(25) The magnesium-based composite can optionally be used to form all or part of a device for use in hydraulic fracturing systems and zones for oil and gas drilling, wherein the device has a designed dissolving rate. The magnesium-based composite can optionally be used to form all or part of a device for structural support or component isolation in oil and gas drilling and completion systems, wherein the device has a designed dissolving rate.

(26) High conductivity nanoparticles (e.g., carbon (carbon nanotubes and nano-diamond particles), copper etc.) can be added to the magnesium-based composite to increase thermal conductivity of the magnesium-based composite by 100% or more via segregation and concentration in the eutectic alpha plus gamma phase, as well as segregation to subgrain boundaries and other lattice defects.

(27) By adding high conductivity nanoparticles (e.g., 0.507 vol. %), either ex situ (blending), or formed in situ (e.g., from Cu additions, Ag additions, etc.), a significant increase of the thermal conductivity of the magnesium-based composite can be achieved as compared to an alloy absent such additions. This same phenomenon is not observed in single-phase magnesium (e.g., pure magnesium), but only in multiphase alloys where segregation to the eutectic region and to the phase interfaces is observed.

(28) The nanoparticles are selected from the group consisting of fullerenes (including multi-walled and single-walled carbon nanotubes, graphene, nanodiamonds, buckeyballs); inert ceramics, including submicron and nanoparticles (including nanotubes, platelets, and flakes) of W, SiC, AlN, BeO, BN; and/or TiB.sub.2, high thermal conductivity MAX phase materials; and/or Cu, Ag, Al, Be, and/or Au compounds. At least about 30% of the nanoparticles generally have dimensions of less than about 200 nm. The nanoparticles generally constitute about 0.1-15 vol. % of the magnesium-based composite. The nanoparticles generally have at least one dimension below about 400 nm, and at least about 30% of the nanoparticles generally have dimensions of less than about 200 nm. The nanoparticles generally have a thermal conductivity of greater than about 140 W/m-K.

(29) The micron-sized particles (when used) can include one or more materials selected from the group consisting of diamond; heat-treated carbon fiber; SiC particles, fibers or whiskers; heat-treated graphite; AlN; BN; and/or other high thermal conductivity, thermally-stable material. The size of the micron-sized particles (when used), is about 10-300 microns, and the micron-sized particles generally have a high thermal conductivity that is greater than about 180 W/m-K. The micron-sized particles (when used), constitute about 1-45 vol. % of the magnesium-based

EXAMPLE 1

(30) An AZ91D magnesium alloy having 9 wt. % aluminum, 1 wt. % zinc and 90 wt. % magnesium was melted to above 700° C. About 2 vol. % nano iron particles and about 2 vol. % nano graphite particles were added to the AZ91D magnesium alloy using ultrasonic mixing. The melt was cast into steel molds. The iron particles and graphite particles did not fully melt during the mixing and casting processes. The material dissolved at a rate of 2 mg/cm.sup.2-min in a 3% KCl solution at 20° C. The material dissolved at a rate of 20 mg/cm.sup.2-hr in a 3% KCl solution at 65° C. The material dissolved at a rate of 100 mg/cm.sup.2-hr in a 3% KCl solution at 90° C. The dissolving rate of magnesium-based composite for each these test was generally constant.

EXAMPLE 2

(31) Carbon nanotubes and/or finely divided copper nanoparticle powder were added to pure magnesium and an AZ91 magnesium alloy (having 9 wt. % aluminum, 1 wt. % zinc and 90 wt. % magnesium) when in molten form. The AZ91 magnesium alloy was melted to above 700° C. Insoluble nanoparticles in the form of carbon nanotubes (multiwall, high thermal conductivity) were added to the molten AZ91 magnesium alloy. The insoluble carbon nanotubes were added by consolidating the carbon nanotubes into a magnesium rod by mechanically blending the carbon nanotubes with magnesium powder and then cold pressing the mixture of carbon nanotubes and magnesium powder into a rod. The rod containing the carbon nanotubes was fed/inserted into the molten AZ91 magnesium alloy. The insoluble carbon nanotubes were dispersed in the molten AZ91 magnesium alloy by ultrasonic mixing wherein the rod was directed into the ultrasonic sweet spot to melt the rod at a melt temperature of 700° C. The carbon nanotubes constituted about 3 vol. % of the formed magnesium-base composite. The average particle size of the carbon nanotubes was less than 300 nm.

(32) The copper nanoparticle powder was added to the molten AZ91 magnesium alloy by consolidating the copper nanoparticle powder with magnesium powder and then cold pressing the mixture of copper powder and magnesium powder into a rod. The rod containing the copper nanoparticle powder was fed/inserted into the molten AZ91 magnesium alloy. The insoluble copper nanoparticle powder was dispersed in the molten AZ91 magnesium alloy by ultrasonic mixing wherein the rod was directed into the ultrasonic sweet spot to melt the rod at a melt temperature of 700° C. The copper nanoparticles constituted about 3 vol. % of the formed magnesium-base composite. The average particle size of the copper nanoparticles was less than 300 nm. When both carbon nanotubes and copper nanoparticles were added, the carbon nanotubes constituted about 2 vol. % of the formed magnesium-base composite and the copper nanoparticles constituted about 2 vol. % of the formed magnesium-base composite.

(33) A 10 lb. casting of the magnesium-based composite in accordance with Example 2 was prepared in a steel permanent mold having a 3″ diameter. After casting, the cast materials were extruded into ½″ rods for mechanical and thermal testing at an extrusion temperature of about 340° C. Table I illustrates the results of the Mg-CNT, AZ91-CNT and AZ91-Cu-CNT composites formed in accordance with the present invention as compared with casting formed of pure magnesium and AZ91 magnesium alloy.

(34) TABLE-US-00001 TABLE 1 Comparative Results of Magnesium-Based Composite Thermal Ultimate Tensile Tensile Yield Material Conductivity Strength (ksi) (ksi) Mg 156 27.1 14.7 AZ91 108 49.5 41.7 Mg—Cu 179 38.4 35.5 Mg-CNT 89 35.5 26.8 AZ91-CNT 204 49.8 41.9 AZ91-Cu-CNT 271 52.3 44.6

(35) The results in Table 1 illustrate that the AZ91-CNT and AZ91-Cu-CNT composites had a greater thermal conductivity, tensile strength, and yield strength than the pure magnesium and AZ91 magnesium alloy that was absent the nanoparticle additions. The Mg-CNT composite had a thermal conductivity that was less than the thermal conductivity of pure magnesium, but had a greater tensile strength and yield strength than pure magnesium.

(36) Alternative alloy systems developed that demonstrate the mechanical property improvements and propensity for high thermally and electrically conductive magnesium-based composites were researched. These alloys were formed in a similar manner as the alloy of Example 2. The AZ91 magnesium alloy was substituted for magnesium alloys of AXM4304, AX50, ZMS616, WEK430, ZWK120, ZWE111 and ZWEK1450.

(37) These alloys were also developed as 10 lb. castings, but cast into 1″ and 2″ ingot sizes. After casting, the parts were extruded into <½″ rods for mechanical characterization using an 8:1 extrusion ratio and varying extrusion processing parameters. The ZMS alloys were also put through a double-aging process post extrusion and the LPSO phase alloys were solution treated prior to extrusion.

(38) TABLE-US-00002 TABLE 2 Mechanical Properties of Alternative Alloy Systems Tensile Ultimate Tensile Yield Elongation to failure Material (ksi) (ksi) (%) AXM4304 50.5 43.6 3.5 AX50 38.1 23.6 18 ZMS616 50.5 47.1 11 WEK430 33.3 21.3 15 ZWK120 34.5 22.0 20 ZWE111 32.3 18.7 25 ZWEK1450 38.5 22.1 18

(39) FIG. 1 is a Plot of Maxwell Model calculation for ratio of magnesium-based composite thermal conductivity versus magnesium thermal conductivity for various particle sizes and loadings of diamonds (1500 W/m-K) from 20 to 400 micron in size. FIG. 1 illustrates that at least 60 microns are needed to have significant effect due to interface resistance between electron conductor (Mg, metal), and phonon conductor (carbon, ceramic, intermetallic, diamond). FIG. 1 also illustrates that the addition of fine particles as a well-mixed composite into the magnesium or other light alloys (aluminum, etc.) leads to a reduction or no change in thermal performance of the magnesium-based composite.

(40) FIG. 2 is a photograph of a tabletop ultrasonic casting unit for dispersing nanoparticles into magnesium. This unit can be used to obtain an initially uniform dispersion in a single-phase molten metal mixture of nanoparticles and dispersoids. Typically, this is done under various degrees of superheat, from about 675-725° C. for magnesium, or from 25-150° C. degrees of superheat, and typically from about 50-100° C. degrees of superheat for light metals.

(41) FIG. 3 is a photomicrograph of a typical AZ91 cast structure showing the Mg—Al primary (alpha) and the alpha plus gamma eutectic phases. Segregation of ex situ- and in situ-added high conductivity particulates to the subgrain boundaries and eutectic liquid resulted in elimination of microstructural defects and an enhancement of strength and conductivity of the interfacial and interphasic regions.

(42) FIG. 4 is a photomicrograph of a rapidly quenched AZ91 showing the uniform distribution of the eutectic liquid along primary alpha grain boundaries. By changing solidification and nucleating grains more homogenously, or through the use of other techniques, a more uniform distribution of the eutectic liquid can be obtained; thus, alloying lower concentrations of additives can be used to achieve the desired results in accordance with the present invention.

(43) It is surmised that the next generation of missile airframes and spacecraft thermal management systems could be designed using a magnesium-based composite with the combination of the lowest density, highest thermal conductivity, and highest strength. The cast or wrought magnesium-based composite incorporating nanoparticle modifiers can be formed by a highly scalable, low cost process that advances the state-of-the-art of metal matrix thermal conductors to reach a theoretical goal of 578 W/mK (up to 270 W/mK achieved), a density less than aluminum (1.7 g.Math.cc achieved), and a yield strengths over 30 ksi (≈207 MPa, 42KSI achieved at 8:1 extrusion ratio). The addition of high conductivity reinforcements is limited due to interfacial resistance, requiring large particles to achieve significant improvements. Thermal conductivity of the magnesium-based composite versus diamond particle size (Type I, k=1500 w/m-K) is illustrated in FIG. 5. The system is a magnesium-diamond system that includes 20 vol. % diamonds. By adding high conductivity nanofibers in accordance with the present invention, interfacial resistances can be reduced or eliminated. This concept is illustrated in FIG. 6, illustrating the high conductivity phases connected with “short circuit” nanotubes. FIG. 6 is an illustration of the microstructure of a Mg-CNT composite. In the past, the incorporation of high conductivity carbon nanofibers into metals and the production of highly grain-refined metals (nanostructured metals) has been found to improve mechanical and thermal properties of materials, with limited technical and commercial success. The majority of these efforts have focused on powder metallurgy solid state approaches, which are expensive and have poor scalability, not to mention significant safety concerns in magnesium alloys. The present invention focuses on enabling the production of metal matrix nanocomposites through advanced dispersion casting techniques, combining the production of engineered nanocomposite feedstocks with the addition of acoustic and mechanical energy to control nanophase dispersion and chemistry.

(44) The problem of incorporating high-aspect-ratio, high-surface-area particles (including fiber and flake) with controlled and repeatable concentration and distribution into molten metals is a large undertaking, and must factor in the molten metal temperature, composition, and surface tension as well as particle surface area, reactivity, clustering, segregation, and temperature and time-dependent wetting phenomena. Direct feeding of the low-density high-surface-area particles into the melt does not work, as particles burn, float, react with the molten metal, or do not stay in the metal. Other feeding mechanisms attempted in the past (such as auger feeding into the metal, in situ formation, and stir casting) are cost prohibitive and not always scalable. To solve these problems, high-aspect-ratio nanoparticles (carbon nanofibers) in accordance with the present invention are incorporated into a pre-dispersed master-composite that enables safe and reliable feeding into molten magnesium to create a high-strength, high thermal-conductivity magnesium-based composite. As illustrated in FIG. 7, the process can be based on ultrasonication technologies with nanoparticle surface chemistry control to disperse nanoparticles into a metal-compatible binder that can be formed into controlled density pellets or rods similar to grain refiners used commercially. These pellets or rods can be directly inserted into a pool of molten metal. Without fabrication of these magnesium-based composites, the nanoparticles agglomerate at the surface of the molten metal, being burned or removed from the casting with slag, instead of being incorporated into the nanocomposite. The method has resulted in both strength increase (>50%) and improvement in thermal conductivity (>150%) in magnesium using multi-walled carbon nanotubes incorporated at low volume fractions into high strength castable/wrought magnesium alloys. These low-cost methods of achieving high-strength, high-conductivity magnesium-based composites using casting techniques have now been demonstrated, leading to an exceptional balance of properties and cost not achievable in currently available alloys or materials.

(45) After preparation of the master alloy, the high concentration nanocomposites were added to magnesium alloy melts using stircasting in 101b melts and a flux cover as illustrated in FIG. 8. The master alloy was fabricated into rods, which were thrust below the flux cover manually, and mixed using a steel stirring rod. The degree of superheat and mixing procedures were optimized through iterative development to obtain a good dispersion. After casting, ingots were extruded at ratios of 4-12:1, as well as roll-reduced to improve mechanical properties and to improve ductilities from the cast structures. A 175-ton press was used, and extrusion temperatures of 300-400° C. were used to further process the ingots.

(46) Thermal and mechanical testing were completed as a function of nanotube loading, alloy composition, and high conductivity filler loading. A steady state vacuum thermal chamber was used for thermal diffusivity measurement, calibrated to an aluminum baseline. FIG. 9 illustrates time-temperature curves generated for diffusivity measurement. Measured conductivities, depending on alloy composition, diamond and CNT compositions ranged from 145 W/m-K (Mg is 156), to 204-270 W/m-K (depending on alloy). 204 W/m-k was achieved at low CNT loadings in a low cost AZ91 alloy matrix, a nearly 100% increase in conductivity. Strengths of wrought alloys were maintained or slightly improved in the magnesium-based composites, as illustrated in FIG. 10. FIG. 10 illustrates tensile yield strengths for high conductivity magnesium-based composite in accordance with the present invention. Interestingly, the addition of CNT's significantly increased the elongation to failure in magnesium alloys over equivalently processed pure alloy.

(47) The production of wrought magnesium-based composite is highly scalable. The magnesium-based composite can be cast into cast billets and extruded to form a rod product which can be used for the production of magnesium frac balls in widespread use in the oil and gas industry.

(48) It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained, and since certain changes may be made in the constructions set forth without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. The invention has been described with reference to preferred and alternate embodiments. Modifications and alterations will become apparent to those skilled in the art upon reading and understanding the detailed discussion of the invention provided herein. This invention is intended to include all such modifications and alterations insofar as they come within the scope of the present invention. It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention, which, as a matter of language, might be said to fall there between. The invention has been described with reference to the preferred embodiments. These and other modifications of the preferred embodiments as well as other embodiments of the invention will be obvious from the disclosure herein, whereby the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation. It is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims.