2. Brief Description of the Prior Art Materials having fine-scale microstructures, that is grain sizes on the order of a few micrometers (microns) possess unique and technologically attractive properties.

Recently, however, research has focused on reducing grain size from the micrometer to the nanometer range. Such nanostructured materials have diameters in the range of about 2 to about 100 nm (1 nm = 10 angstroms), or on the order of molecular dimensions. A feature of nanostructured materials is the high fraction of atoms (up to

50%) that reside at grain or particle boundaries. It is often possible to design unique features and properties at the atomic level into such materials by subtle manipulation of the reaction environment. One feature of particular interest is pore structure and packing density of nanophase materials at the atomic level. Nanostructured materials often have superior chemical and physical properties in comparison to conventional counterparts having the same chemical composition.

Thus, considerable advantages accrue from the substitution of nanostructured materials for conventional microstructured materials in a wide variety of applications.

These include superior strength, wear resistance, and fracture toughness in martensitic steel; the reduced sintering temperature for powder consolidation and the onset of superplasticity in nanostructured ceramics; improved ductility in ceramic/metal composites (cermets); and reduced thermal conductivity for thermal barrier coatings.

Nanostructured metals, metal alloys and metal carbides in particular are expected to have improved fracture toughness, as well as improved resistance to wear, corrosion, and erosion (see, for example, the examples set forth in Nanostructured Materials, Volume 1, 1992. Such materials also have potential as advanced catalysts.

Certain nickel/chromium (Ni/Cr) alloys, for example, have high strength, excellent hot corrosion resistance and superior wear properties, making them attractive candidate materials for a wide range of high temperature applications, for example, as wear resistant coatings in the hot sections of gas turbine and diesel engines.

Nickel/chromium carbide (Ni-CrxCy) alloys are also of interest. At low chromium (Cr) concentrations, Ni/Cr alloys are fcc substitutional solid solutions (y-phase). At higher chromium concentrations which exceed the solubility limit in nickel (Ni), the equilibrium structure is a two-phase mixture of fcc Ni/Cr solid solution (y-phase) and bcc chromium (a-phase). The corresponding chromium carbide-rich alloys contain a variety of carbide phases: Cr23C6, Cr6C, Cr2C or Cr3C2. Alloys of commercial interest have a high volume fraction of Cr3 C2 carbide (typically greater than about 60 mole %) dispersed in a Ni/Cr solid solution.

Nanostructured Ni/Cr and Ni-Cr3C2 alloys have not been reported.

Microstructured Ni/Cr and Ni-Cr3C2 alloys, however, have produced by melting and

casting methods. Rapid solidification processing technologies, including powder atomization and melt spinning, have more recently achieved finer and more homogeneous structures. However, in order to produce materials with ultrafine or nanoscale structures, alternative processing routes are required. Accordingly, there remains a need in the art for a chemical syntheitc method for the production of nanaostructrued metals, metal alloys, and metal carbides.

Summarv of the Invention: The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by the chemical synthetic method of the present invention for the production of nanostructured metals, metal alloys and metal carbides, including metals such as nickel, chromium, cobalt, iron, molybdenum, tungsten, and vanadium, methal alloys such as nickel/chromium, iron/titanium, iron/nickel, tungsten/cobalt, iron/chromium/molybdenum, and metal carbides such as nickel chromium carbide, nickel titanium carbide, molybdenum titanium carbide and iron titanium carbide. In accordance with the present invention, a starting solution is provided comprising metal salts in a suitable solvent. Optionally, the composition of the starting solution may be adjusted to yield a variety of metal alloy nanostructured powders, thus allowing tailoring of the chemical, structural and performance characteristics of the final powders. A reducing agent is then added to the solution. Preferably, the reducing agent is atomized into the starting solution via a pressurized nozzle, allowing precise control of the nucleation and growth of the nanostructured metal or metal alloy powder. The as-synthesized metal or metal alloy powder is isolated by filtration and washed, passivated with deoxygenated mineral oil, and stored under inert gas. The powder may be heat treated in a high-temperature, environmentally controlled furnace.

In another embodiment of the present invention, the above nanostructured metal or metal alloy powder is reacted with a carbon source using gas phase carburization techniques. After carburization, the obtained powder is dispersed in distilled, deoxygenated water to remove by-products from the nanostructured cermet powder.

The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.

Brief Description of the Drawings Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: FIGURE 1 is XRD patterns for nanostructured, crystalline Ni/Cr alloys of different compositions after heat treatment (all at 600 OC, except for the 80 mole % Ni/Cr alloy, which was heat treated at 700 "C.) FIGURE 2 is a graph showing particle size of the nanostructured nickel (y-phase) and chromium (a-phase) as a function of alloy composition.

FIGURE 3 is (a) a bright-field TEM micrograph and (b) corresponding electron diffraction pattern of the nanostructured 20 mole % nickel in Ni/Cr alloy.

FIGURE 4 is a histogram showing particle size distribution of the 20 mole % nickel in Ni/Cr alloy obtained by computer image analysis of the TEM micrograph of FIGURE 3(a).

FIGURE 5 is XRD patterns showing the effect of heat treatment on an 80 mole % nickel in Ni/Cr alloy powder.

FIGURE 6 is XRD patterns showing the effect of heat treatment on an 20 mole % nickel in Ni/Cr alloy powder.

FIGURE 7 is a prior art equilibrium phase diagram of the Ni/Cr alloy system.

FIGURE 8 is a graph comparing the lattice parameters of the nanocrystalline alloys of the present invention with the prior art data lattice parameters of Ni/Cr alloys.

FIGURE 9 is an XRD pattern of the nanocrystalline 20 mole % Ni-Cr3C2 composite powder.

FIGURE 10 is (a) a bright-field TEM micrograph and (b) corresponding electron diffraction pattern of the 20 mole % Ni-Cr3C2 composite powder.

FIGURE 11 is a histogram showing particle size distribution obtained from computer image processing of the TEM micrograph of the 20 mole % Ni-Cr3C2 composite powder.

Detailed Description of the Invention As will be explained in more detail below, nanostructured metals and metal alloys are synthesized in accordance with the present invention by first providing a starting solution comprising at least one metal salt in a suitable solvent. Optionally, the composition of the starting solution may be adjusted by addtion of additional metal precursors to yield a variety of nanostructured metal alloy powders, thus allowing tailoring of the chemical, structural and performance characteristics of the final powders. A reducing agent is then added to the starting solution, preferably by atomization via a pressurized nozzle, allowing precise control of the nucleation and growth of the nanophase powder. A nanostructured powder is formed, and the as-synthesized powder is washed and filtered, passivated with deoxygenated mineral oil, and stored under inert gas. The powder may be heat treated in a high-temperature, environmentally controlled furnace.

In the first step of the method a starting solution is provided, prepared from at least one metal salt and a solvent such as toluene, methanol, and/or water. Suitable metal salts are those which will yield the desired metal or metal alloy upon reduction, including but not limited to chlorides and nitrate of transition metals, for example, Ni, Cr, and other transition metals such as Co, V, W, Fe, Mo, Mn, Cu, Ti, and the like.

Where required, the compositions of the starting solutions are adjusted to yield a range of nanostructured metal alloy powders, with tailored chemical, structural, and performance characteristics. The adjustment includes but is not limited to the addition of a soluble salt of the second, third or greater metal, or the addition of acid or base to the starting solution. Suitable alloying metals include but are not limited to metals, transition metals such as Ni, Cr, Co, V, W, Fe, Mo, Mn, Cu, Ti, and the like, and semiconductor metals known in the art.

A reducing agent is then introduced into the starting solution or adjusted starting solution. The process is preferably carried out by atomizing the reducing agent into the metal salt solution via a pressurized nozzle or ultrasound, or adding the solution dropwise into the metal salt solution. Thus, the nucleation and growth of the nanophase powder can be precisely and reproducibly controlled. Suitable reducing agents include lithium or sodium triethylborohydride, or lithium or sodium borohydride. Alternatively, the metal salt may be atomized into a solution of reducing agent.

The reduction of metal halides to form metal clusters, for example, is represented by the following equation: MClx + xNaBEt3H - M + xNaCl + xBEt3 + (x/2)H2 where M = Ni for x = 2 and M = Cr for x = 3. According to previous studies on the reduction of similar metal halides, this reaction either may result from direct hydride transfer to form a metal hydride intermediate, which subsequently reductively eliminates hydrogen, or occurs directly by an electron-transfer mechanism. Metal halides may also be reduced by sodium borohydride to form nanostructured metals and metal alloys. In this instance, however, reactions are carried out in an aqueous medium instead of an organic solvent.

The next step comprises isolation and washing of the nanostructured suspensions using deoxygenated water or other solvents. The high surface area reactive powder is then passivated with deoxygenated mineral oil and stored under an inert gas.

Where appropriate, the powder may be heat treated in a high-temperature environmentally-controlled furnace.

Some high surface area metal powders obtained from this reduction process are extremely air and moisture sensitive such as nickel-based alloys. Initial experiments to purify nanostructured nickel metal powders from by-products such as NaCl used extensive washing with deoxygenated water, which was followed by the application of a protective coating of deoxygenated mineral oil. XRD analysis of the washed powders indicated that the powders were highly disordered or amorphous, since only very broad nickel and chromium peaks were observed. Attempts to convert the powders into

crystalline form via high temperature heat treatment resulted in the formation of a greenish powder, which consisted of nickel and Cr2O3 phases. During the course of the heat treatment it was observed that once the furnace temperature reached 60 "C, a sudden temperature increase up to 300 "C occurred over a few minutes, even though the heating rate was only set at a few degrees per minute. Based on these observations and analytical results, it is hypothesized that the O-H groups of water molecules are initially physisorbed into the metal surface. Upon slow heating, these O-H groups are gradually transformed into chemisorbed species, which then react with the metal powders to form Cr2O3, instead of being evaporated away in the heat treatment process.

To avoid this oxidation problem, the air and moisture sensitive precipitated powders are coated with deoxygenated mineral oil before washing. In this case, C-H groups derived from the mineral oil and residual toluene are first physisorbed onto the metal surfaces. These physisorbed C-H groups gradually transform into chemisorbed species during heat treatment, and then react with the metal surfaces to form thin layers of protective carbide phase. Since these layers are extremely thin, they are not detectable by XRD analysis. However, ICP analysis indicates that there is appreciable carbon content in such sample. Importantly, the initial heating rate during heat treatment of these materials should be extremely slow below 300 "C. Otherwise, the C-H groups will be partially evaporated, and this will create problems of slow burning of the sample upon exposure to air after treatment. Thus, it appears that the transformation of the physisorbed C-H groups into chemisorbed species is very important for passivation of certain metals and alloys, such as Ni/Cr alloys.

To obtain nanostructured metal/ceramic composites, the obtained metal or metal alloys are reacted with a carbon source using gas phase carburization. Such gas phase carburization is well-known in the art. After carburization, the powder is dispersed in distilled oxygenated water to remove by-products from the nanostructured cermet powder.

Examples of the materials which may be synthesized by the above methods include nanostructured transition metals, for example nickel, chromium, and other transition metals such as Co, V, W, Fe, Mo, and the like, nanostructured transition

metal alloys such as Ni/Cr, Fe/Ti, Fe/Ni, W/Co, Fe/Cr/Mo, and the like, as well as nanostructured nickel chromium carbide and nanostructured iron titanium carbide.

The invention will be further illustrated by the following non-limiting examples.

EXAMPLES Nickel chloride (NiCl2), chromium chloride (CrCl3) and a 1.0 M toluene solution of sodium triethylborohydride (NaBEt3H) were obtained from Aldrich Chemical, Inc., Milwakee, WI. The toluene solvent was first dried over calcium chloride, refluxed, distilled, and then deoxygenated. All manipulations were carried out under an atmosphere of ultrahigh purity nitrogen (99.99% purity) using standard Schlenk techniques, because of the oxygen and moisture sensitivities of the starting compounds and the resulting nanostructured powders.

The nanostructured Ni/Cr alloy and Ni-Cr3C2 powders were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and inductively coupled plasma spectroscopy (ICP).

Example 1: Nanostructured Ni/Cr Nanostructured Ni/Cr powders were prepared with the following compositions: 100% Ni, 80 mole % Ni in Ni/Cr, 50 mole %Ni in Ni/Cr, 20 mole % Ni in Ni/Cr and 100 % Cr. In a typical experiment, e.g., for 20 mole % Ni in Ni/Cr, 16.43 gm of CrCl3 (0.1038 mol) and 3.36 gm of NiCl2 (0.0259 mol) were dispersed in 400 mL of deoxygenated toluene to form a purple suspension in a 2-liter, 3-neck flask equipped with a magnetic stirrer and gas inlet/outlet tubes. A stream of N2 was bubbled through the stirred solution for 2 hours at room temperature, then the 400 mL of a 1.0 M toluene solution of NaBEt3H was also slowly added dropwise into the halide mixture via a pressure equalized addition funnel at temperature under a nitrogen atmosphere. A black suspension slowly formed while hydrogen gas evolved. The suspension was stirred for 48 hours at room temperature to ensure complete reduction of the halides. The black powder was filtered and washed with deoxygenated toluene under a stream of nitrogen.

At this stage, no attempt was made to remove the sodium chloride byproduct. The high

surface area reactive powder was passivated with deoxygenated mineral oil and stored in a Schlenk flask under nitrogen. Other compositions of the nanostructured Ni-Cr powders were prepared in a similar manner.

In variation to the above dropwise addition, the 400 mL of a 1.0 M toluene solution of NaBEt3H was atomized to the halide mixture via a pressurized nozzle under a nitrogen atmosphere. In this approach, the nucleation and growth of the nanostructured powder can be precisely controlled.

Heat treatment was performed using a high temperature, environmentally controlled Lindburg tube furnace. About 2 grams of the powder mixture was placed in an alumina boat inside an alumina tube having an inner diameter 32 mm. Prior to heat treatment, the chamber was evacuated to about 10-5 torr (1.33 x 10-3 Pa), flushed with argon repeatedly, and backfilled with argon to near ambient pressure. Heat treatment was carried out at 300-600 "C with a fume gas (97% Ar, 3% H2) flow rate of about 1 L/min. The sample was first slowly heated to 300 "C in 1.5 hours, and then heated to the desired peak temperature at a heating rate of 25 "C/min, with a dwell time at the heat treatment temperature of 1 hour. After heat treatment, the fine powder was suspended in distilled water and then centrifuged to separate the sodium chloride and water from the nanostructured Ni/Cr alloy powder.

FIGURE 1 is XRD patterns of the nanostructured, crystalline Ni/Cr alloy powders of different compositions after heat treatment at 600 "C. Where chromium concentration is below 50 mole %, only nickel is detected, and the peaks correspond to fcc Ni/Cr. Where chromium concentrations are greater than 50 mole %, both nickel and chromium are detected, and the nickel peaks correspond to fcc Ni/Cr, and the chromium peaks to bce chromium. Values of the lattice constants for Ni/Cr and chromium are listed in the Table below. The lattice parameter of the nanostructured, crystalline nickel powder is estimated to be a =3.5264 Å, which agrees well with the known bulk value for nickel (a = 3.5238A). With increasing chromium concentration from 20 to 80 mole %, the lattice parameter gradually increases: a = 3.5437 Å for 20% Cr, a = 3.5457 Å for 50% Cr, and 3.5513 Å for 80% Cr. The lattice parameter of the nanostructured, crystalline chromium powder is determined to be a = 2.8854 A, which again is in good

agreement with the known bulk value for pure chromium (a = 2.8839 Å). As the concentration of nickel increases from 0 to 50 mole %, there is a slight increase of the Cr lattice parameter: a = 2.8908 Å for 20% Ni, and a = 2.8957 A for 50% Ni. Composition Ni lattice Parameter Cr lattice Parameter 100% Ni 3.5264 Å 20% Cr 3.5437 A 50% Cr 3.5457A 2.8957 80% Cr 3.5513A 2.8908A 100% Cr --- 2.8854A The crystallite size of the nanostructured powder was determined from x-ray peak broadening. FIGURE 2 shows the variation in particle size of nanostructured, crystalline Ni/Cr alloy as a function of chromium concentration. The particle size of Ni/Cr (y-phase) varied from 10 nm to 18 nm, being about 10 nm for 100 mole % Ni, 13 nm for 80 mole % Ni in Ni/Cr, 10 nm for 50 mole % Ni in Ni/Cr, and 18 nm for 20 mole % Ni in Ni/Cr. The particle size of chromium (a-phase) is also shown in FIGURE 2. The particle sizes of the a-phase chromium were determined to be 14 nm for 100 mole % Cr, 29 nm for 80 mole % Cr, and 21 nm for 50 mole % Cr.

A typical TEM micrograph for a 20 mole % Ni in Ni/Cr alloy powder is shown in FIGURE 3 and a particle size distribution as determined by image analysis was performed (see FIGURE 4). The average particle size of the nanocrystalline alloy was 12 nm. Selected area diffraction of the powders showed the presence of fcc nickel and bce chromium.

The effects of heat treatment on the crystallinity and phase formation of the nanostructured powders was also investigated. FIGURE 5 shows XRD results for the 80 mole % nickel in Ni/Cr alloy. The as-synthesized Ni/Cr powder is amorphous. The 300 "C heat treated sample is also amorphous. At 400 "C, the XRD peaks for nickel start to appear, superimposed on a very diffuse background intensity. As the annealing

temperature increases, the diffraction peaks sharpen and the diffuse background decreases. At 500 "C and 600 "C, there is an observable background intensity in combination with very well-defined nickel crystalline peaks. At 700 "C, there is no diffuse background and the nickel phase peaks are sharp; but no chromium phase is detected in this particular composition. The crystallite size of the nickel phase increases from 4 nm to 13 nm when the temperature is increased from 300 "C to 600 "C, and is about 20 nm at 700 "C. At lower annealing temperatures, for example, below 600 "C other broad peaks are also evident. At 400 "C, two very broad peaks are centered at about 20 = 370 and 62 ". Relative intensities of these two peaks decrease as the annealing temperature increases, and they finally disappear at a temperature of 700 "C.

FIGURE 6 shows XRD results for heat treatment of 20 mole % Ni in Ni/Cr.

Once again, the as-synthesized powder is amorphous. After heat treatment at 300 "C, only nickel peaks are detected. At higher temperatures, however, peaks for both nickel and chromium phases are observed. At 500 "C, no amorphous background is observed, but there is phase separation of nickel and chromium. At this temperature, the XRD signal of the nickel phase is strong, but the XRD signal of the chromium phase is weak.

After heat treatment at 600 "C, well-defined XRD peaks of the chromium phase are visible. The crystallite size of the nickel phase increases from 4 nm to 20 nm when the annealing temperature increases from 300 "C to 600 "C, while the chromium phase increases from 2 nm to 15 nm when the temperature increases from 500 "C to 600 "C.

The alloy powders are extremely fine after heat treatment and they can be readily dispersed in a solvent. Even after several months of storage, they show no evidence of settling. After washing in distilled water, they could not be separated from NaCl and water because all of the particles passed through the microporous filter. The separation process, however, may be accomplished by agglomerating the nanoparticles into micron-size agglomerates by centrifugation followed by filtering. Morphological examination of these agglomerates by SEM shows that they are several tens of microns in size. From all of the above results, it is concluded that the powders after centrifugal processing are micron size agglomerates consisting of an assemblage of nanostructured,

crystalline Ni/Cr alloy particles. The agglomerates can be easily broken down into their individual entities via ultrasonic agitation.

The established Ni-Cr phase diagram is shown in FIGURE 7. At a composition of less than 36 mole % Cr only y-phase is at the Ni-rich region, where chromium atoms are substitutional. The a-phase is at the chromium-rich region, where only a very small amount (< 2%) of nickel solute atoms are substitutional. In the nanostructured, crystalline Ni-Cr system, only fcc nickel XRD signals were detected (see FIGURE 1) at 100 mole % Ni and 20 mole % Cr, with an increased lattice parameter for the 20 mole % chromium sample. This data is in agreement with the established phase diagram for conventional micron-sized Ni/Cr, since these compositions are at the single phase field (y-phase). At higher concentrations of chromium, for example, 50 mole % Cr and 80 mole % Cr, both y-phase and a-phase were evident, as predicted by the phase diagram.

This data illustrates that the experimental findings for the nanostructured, crystalline Ni-Cr system agree very well with the established phase diagram for the conventional microstructured, crystalline Ni-Cr system.

FIGURE 8 is a comparison of the lattice parameters for conventional microstructured, crystalline Ni/Cr alloys as described in Floyd and Taylor, J. Inst. Met., Vol. 80, pp. 169 (1952) and the corresponding nanostructured, crystalline Ni/Cr alloys.

In the experiments of Floyd and Taylor, the alloys of various compositions were heat treated at 1100 "C for six days. The samples were then cooled by either (i) rapid quenching to room temperature, or (ii) slow cooling to room temperature by furnace cooling. The lattice parameter vs. composition plots for both sets of samples had a similar slope up to 45 mole % chromium, which is indicative of the presence of only one phase, the y-phase. For the present nanostructured Ni/Cr alloys, it was found that at low concentrations of chromium, for example, 0% Cr and 20 mole % Cr, the measured lattice parameters agreed well with the data obtained by Floyd and Taylor.

However, a slope change was evident at higher concentrations of chromium. It may be that the nanostructured alloy has already reached its solubility limit around of 29 mole % chromium at a temperature of 600 "C. Upon increasing the concentration of chromium, phase separation into y- and a-phases occurs.

Example 2: Ni-Cr3£2 Powders Nanostructured Ni-Cr3C2 powder can be prepared by the gas phase carburization technique described by L.E. McCandlish, B.H. Kear and B.K. Kim, in Nanostruct.

Mater. Vol. 1, p. 119 (1992) by high temperature fluidized bed reactor at near ambient pressure, herein incorporated by reference in its entirety. However, the process described herein use a vacuum carburization technique, which is routinely used in commercial carburization of steels.

About 2 grams of the nanostructured 80 mole %Cr/20 mole %Ni powder was transferred into a tubular carburization unit equipped with vacuum pumps, pressure gauges, and inlet/outlet gas tubes. The carburization unit was first evacuated to abut 5 m torr, flushed with argon, and backfilled with methane to 200 torr. Carburization was accomplished at a temperature of 880 "C and a pressure of 200 torr. The heating rate and the dwell time at the reaction temperature was 60 C/min and 1 hour, respectively.

After carburization, the powder was dispersed in distilled water and then centrifuged to remove sodium chloride and water from the nanophase cermet powder. As shown by the XRD analysis (FIGURE 9) this alloy composition resulted in the formation of a nanostructured mixture of nickel and Cr3C2.

Morphological examination using SEM reveals that the cermet powder has a porous structure, but detailed information can not be obtained due to the very small particle size. A typical TEM micrograph of the powder is shown in FIGURE 10(a), which reveals a size distribution of particles ranging from a few nanometers to several tens of nanometers. Particle size analysis from the bright field micrograph via an image processing technique, FIGURE 11, indicates that the average particle size of the Ni-Cr3C2 powder is about 34 nm. Using x-ray line broadening analysis, particle size of the nickel phase is shown to be 18 nm, and that of the Cr3C2 phase is 30 nm. This discrepancy in particle size may be attributed to overlapping particles not being taken into consideration in the image analysis technique, and the presence of amorphous regions in the particle grain boundaries. Electron diffraction analysis confirmed the presence of nickel and Cr3C2 phases. The individual particle sizes of the two different phases could not be determined from the phase contrast in the TEM studies.

In summary, a new chemical method for the synthesis of nanostructured Ni/Cr and Ni-Cr3C2 alloys is presented. The method involves reductive decomposition of an organic solution of metal chloride precursors using sodium triethyl borohydride. After co-precipitation of amorphous nanostructured mixed metal (Ni+Cr) powder, low temperature annealing is used to form nanostructured, crystalline Ni/Cr alloy powder, or low temperature gas phase carburization to form nanostructured, crystalline NiCr3C2 cermet powder.

While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.

What is claimed is: