This application is based on U.S. Provisional Patent Application No. 60/616,664, filed Oct. 7, 2004, entitled, Nitrogen-Modified Titanium and Method of Producing Same, and is incorporated herein by reference.
BACKGROUND OF THE INVENTION
 1. Technical Field
 The present invention relates in general to a titanium alloy and, in particular, to a nitrogen-modified titanium alloy and method of producing it.
 2. Description of the Related Art
 Metal objects are currently produced by thermomechanical processes, which include casting, rolling, stamping, forging, extrusion, machining, and joining operations. Multiple steps are required to produce a finished article. These conventional operations often require the use of heavy equipment and molds, tools, and dies. For example, a typical process sequence required to form a small cylindrical pressure vessel might include casting an ingot, heat treating and working the casting to homogenize it by forging or extrusion or both, then machining a hollow cylinder and, separately, end caps from the worked ingot and, finally, welding the end caps to the cylinder.
 Conventional production methods are subtractive in nature in that material is cut away from a starting block of material to produce a more complex shape. Subtractive machining methods are deficient in many respects. Large portions of the starting material are reduced to waste in the form of cuttings. These methods produce waste materials, such as metal cuttings, oils, and solvents, which must be further processed for purposes of reuse or disposal. The articles produced are contaminated with cutting fluids and metal chips. They require cutting tools, which wear and must be periodically reconditioned and ultimately replaced. Fixtures for use in manufacturing must be designed, fabricated, and manipulated during production.
 When a part is unusual in shape or has internal features, machining is more difficult. Choosing the machining operations to be used and the sequence of operations requires a high degree of experience. A number of different machines are needed to provide capability to perform the variety of operations, which are often required to produce a single article. Sophisticated machine tools require a significant capital investment and occupy a good deal of space. Use of the invention in place of subtractive machining provides solutions to these problems and disadvantages.
 Another difficulty with conventional machining techniques is that many objects must be produced by machining a number of parts and then joining them together. Producing parts separately and joining them requires close tolerance machining of matching parts, provision of fastening means, such as threaded connections, and welding together of components. These operations involve a significant portion of the cost of producing an article, as they require time for design and production as well as apparatus for performing them.
 Titanium has been used extensively in aerospace and other manufacturing applications due to its high strength-to-weight ratio. To increase the usefulness of titanium, various titanium alloys have been produced, many being tailored to provide desired characteristics. However, the equilibrium solute levels (as measured in wt. %) in conventionally processed titanium alloys are below that which maximizes the beneficial effect of the solute.
 For example, in concentrations over 500 ppm, nitrogen is typically considered a contaminant in titanium alloys. At levels higher than 500 ppm, the tensile strength increases greatly with a corresponding drop in tensile ductility. Additionally, solidification cracking can be a serious problem at high nitrogen levels. It is this embrittling effect that prohibits the use of nitrogen as a significant alloying agent.
 Titanium alloys typically exhibit low wear resistance due to their low hardness. Under certain circumstances, titanium also can be subject to chemical corrosion and/or thermal oxidation. Prior art methods for increasing the hardness of titanium alloys are surface modification techniques only. A hard face coating is a discrete surface layer and is subject to delamination. Current methods are also subject to macro and micro cracking of the surface-hardened layer. For example, U.S. Pat. Nos. 5,252,150 and 5,152,960 disclose titanium-aluminum-nitrogen alloys. These patents disclose an alloy that is formed through a solid-state reaction of titanium in a heated nitrogen atmosphere. The alloy is formed in a melt with aluminum to create the final alloy product.
 To increase the amount of solute levels in the alloys, rapid solidification processes (RSP) can be used. In these processes, a rapid quenching is used in freezing the alloy from a molten state, the solutes remaining in desired phases. After quenching, diffusion may allow for dispersion throughout the material and agglomeration at nucleation sites, further improving the characteristics of the alloy. While this type of process is widely used, the resulting product is typically in powder, flake, or ribbon forms, which are unsuitable for manufacturing applications requiring material in bulk form. Thus, an improved titanium alloy and process for producing the same would be desirable for many practical applications.
SUMMARY OF THE INVENTION
 The present invention comprises a system, method, and apparatus that uses a liquid-state reaction between a titanium molten pool and a fraction of gaseous nitrogen in the atmosphere. This design increases mechanical strength and hardness through nitrogen alloying. In addition, a direct manufacturing technique involving rapid solidification processing is used rather than conventional casting techniques that require bulk melting of solid-state nitrided powder.
 By utilizing rapid solidification techniques, solubility levels can be increased resulting in alloys with unique mechanical and physical properties that are unattainable through conventional processing methods. The present application documents trials performed using laser-powder deposition techniques on commercially pure titanium (Cp-Ti) in atmospheres of varying argon/nitrogen content. The results show that significant strengthening is achieved without cracking in atmosphere concentrations as high as approximately 10% nitrogen. Very high hardness values indicate that this material has valuable applications as a hard face coating on titanium structures and in functionally graded materials.
 The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the present invention, taken in conjunction with the appended claims and the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
 So that the manner in which the features and advantages of the invention, as well as others which will become apparent, are attained and can be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiment thereof which is illustrated in the appended drawings, which drawings form a part of this specification. It is to be noted, however, that the drawings illustrate only an embodiment of the invention and therefore are not to be considered limiting of its scope as the invention may admit to other equally effective embodiments.
 FIG. 1 is a schematic, perspective view of a portion of a solid freeform fabrication device.
 FIG. 2 is a schematic front view of the device of FIG. 1 during fabrication of a part.
 FIG. 3 is a flowchart of one embodiment of a method of the present invention.
 FIG. 4 is a series of optical and electron micrographs depicting various structures of a composition of matter constructed in accordance with the present invention;
 FIG. 5 is a plot of atmospheric nitrogen versus nitrogen absorbed and hardness in the composition of matter constructed in accordance with the present invention;
 FIG. 6 is a series of optical micrographs depicting various structures of a composition of matter constructed in accordance with the present invention; and
 FIG. 7 is a series of optical micrographs depicting various structures of a composition of matter constructed in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
 The present invention is directed to a novel composition of matter comprising a titanium alloy and a method for producing the alloy. This new alloy is ideally suited for use in applications, such as aerospace applications, that require a combination of high strength and low density. To enable formation of this new composition of matter, one method of producing the alloy utilizes a solid freeform fabrication (SFF), or direct deposition, device to achieve rapid cooling and solidification while forming a bulk part.
 The alloy of the present invention requires a rapid solidification process (RSP) to retain the desired metastable phases, and a method of direct manufacturing that results in rapid solidification is shown in the figures. FIG. 1 is a schematic, perspective view of a portion of a SFF device 11, such as is available from Optomec Design Company, Albuquerque, N. Mex., and sold under the trademark LENS™ (Laser Engineered Net Shaping).
 Device 11 comprises a high energy density heat source, such as a laser beam 13. Other forms of heat sources may include, for example, electron beams and arcs, as illustrated at step 301 in FIG. 3. The laser beam 13 may be formed by various laser types and delivered to the desired location by fixed or fiber optics. Beam 13 acts as the heat source for melting a feedstock, such as a metallic powder. Other types of feedstock may include wire, for example. The feedstock is delivered through one or more guide nozzle(s) 15 (four shown), as depicted at step 305 in FIG. 3. The feedstock or powder exits nozzles 15 through an outlet 17 at the lower end of each nozzle 15.
 In one embodiment, the controls for beam 13 or heat source and nozzles 15 are mounted to a movable platform, as depicted in step 303 in FIG. 3. In the laser embodiment, the controls utilize optics to direct the laser beam 13. The platform is computer-controlled to position the beam 13 and nozzles 15 in a desired location for each section or layer of the part being formed. These portions of the method are illustrated at step 307 in FIG. 3. Device 11 is shown as having four nozzles 15 located at 90° increments in an array having a selected radius from, and being centered on, beam 13. Though shown with four nozzles 15, device 11 may have more or fewer nozzles 15, and the nozzles 15 may be arranged in various orientations.
 To form a part using the device 11, the feedstock metal is delivered into and through the nozzles 15. As shown in FIG. 2, when the powdered metal 19 is used as the feedstock, the metallic powder is entrained in an inert gas, typically argon, for delivery via the nozzles (step 305, FIG. 3). The feedstock metal is carried out of the exit 17 of each nozzle 15 and directed at a point where the stream(s) of the metal 19 converge with the heat source or beam 13. The laser beam 13 melts the metal 19 (step 309, FIG. 3), forming a molten pool on substrate 21. The metal 19 is simultaneously exposed to a gaseous alloying element (such as nitrogen). As the platform for the beam 13 and the nozzles 15 is moved (step 311, FIG. 3), the pool rapidly cools and solidifies as an alloy. When the heat source or beam 13 is moved away, a continuous line of deposited alloy 19 forms a portion of part 23. Device 11 is used to form adjacent, side-by-side layers to form the width of the part, and is used to form adjacent, stacked layers to create the height of part 23.
 In one experiment, five different argon/nitrogen atmospheric combinations were evaluated in addition to a baseline 100% Ar CP-Ti. Custom mixed bottles of argon and nitrogen were mixed with the following ratios (Ar/N2): 96/4, 93/7, 90/10, 85/15, and 70/30. Cp-Ti specimens were then laser deposited in each gas composition. Prior to deposition, an amount of the desired composition was purged through the system to ensure a homogeneous mixture at the target concentration. Another amount of the desired composition was used to keep the chamber at operating pressure and as a carrier gas for the powder delivery system.
 Heat treatments were performed on some test samples in order to examine microstructural stability and thermal effects. Microstructural characterization was carried out using optical and scanning electron microscopy. Under equilibrium conditions, the solidification sequence for compositions under 1.2% N, which corresponds to about 7% atmospheric nitrogen, is: L→L β→β→β α→α Ti.s- ub.2N
 And for equilibrium solidification at compositions greater than 1.9% N: L→L α→α β→α→.a- lpha. Ti2N
 This solidification behavior is likely valid under equilibrium conditions and therefore not necessarily valid for laser deposited structures (due to rapid solidification characteristics). Rapid solidification tends to increase solid solubilities, which effectively shifts the phase diagram towards the solute end, thus favoring metastable phase formation. However, microstructural analysis is consistent with the above solidification sequences, though the composition limits may be uncertain. In one embodiment, the Ti alloy contains a weight percentage of N of approximately 0.05% to 3.0%.
 FIG. 4 shows a micrograph series for the 90/10 and 70/30 mixtures of Ar/N2. For the 90/10 mixture (FIGS. 4A, 4B, 4C), the macrostructure (FIG. 4A) is typical of what is seen in conventional Ti alloys (i.e., large prior β grain boundaries with a martensitic α' lath basket weave structure). FIG. 4B shows a backscattered electron SEM image (BSEM) that reveals compositional contrast and indicates that TixN.sub.y compounds might exist in the interlath regions. FIG. 4C shows the 90/10 composition after heat treatment for 1-hour at 1000° C. Here, the TixN.sub.y particles are clearly seen pinning a grain boundaries in a recrystallized microstructure. The particle composition was verified using energy dispersive spectroscopy (EDS) to be Ti2N.
 The 70/30 mixture (FIGS. 4D, 4E, 4F) has a macrostructure that is quite different from the 90/10 composition. FIG. 4D shows an optical micrograph of the as-deposited structure clearly showing dendritic formation of primary α. Closer look via BSEM (FIG. 4E) shows that the interdendritic region likely contains the Ti2N compound. FIG. 4F shows the 70/30 mixture after 1-hour heat treatment at 1150° C. Here again, the Ti2N particles are clearly seen pinning the a grain boundaries though the size of the particles is much larger when compared to those seen in the 90/10 sample (note the micron bars).
 Chemistry results are shown in Table 1. Of interest here is the nearly linear relationship between atmospheric nitrogen and dissolved nitrogen in the as-deposited samples. This relationship is more clearly seen in FIG. 5, as are the plotted superficial hardness values. Here the relationship seems to follow a power-law relationship indicating that significant hardening benefits can be obtained at low concentrations while the effect diminishes at higher concentrations. TABLE-US-00001 TABLE 1 Element CP--Ti 4% N 7% N 10% N 15% N 30% N Nominal ASTM B348 C 0.0880% 0.0980% 0.0670% 0.0870% 0.0640% 0.0910% 0.0910% 0.0800% H 0.0050% 0.0018% 0.0020% 0.0012% 0.0037% 0.0038% -- 0.0150% N 0.0200% 0.6700% 1.7300% 1.3300% 1.9400% 3.4500% 0.0080% 0.0300% O 0.1700% 0.1500% 0.1440% 0.1400% 0.1470% 0.1400% 0.1250% 0.1800%
 Table 2 shows results from mechanical testing of the control CP-Ti specimens and the 96/4 and 90/10 compositions. The samples above 10% suffered cracking that prevented them from being tested. A small amount of nitrogen (as little as 0.1%) may result in gains in ultimate tensile strength on the order of 60% (i.e., as high as 140 ksi), and gains in hardness on the order of 100% (up to 55 HRC). Essentially no ductility was found in any of the nitrogen-modified samples. TABLE-US-00002 TABLE 2 Comp. ID Test Log Temp. UTS 0.2% YS % E % RA Mod. Hard. 10% N4 -- -- -- -- -- -- -- 55 10% N5 980791 RT 33.3 -- -- -- 18.6 55 10% N6 980792 RT 28.4 -- -- -- 18.5 55 AVG 30.9 -- -- -- 18.6 55.0 4% N26 980796 RT 137.9 -- -- -- 17.2 46 4% N27 980797 RT 155.5 -- -- -- 17.3 48 4% N28 980789 RT 139 -- -- -- 17 47 AVG 144.1 -- -- -- 17.2 47.0 CP N21 980793 RT 88 76.7 6.5 9.5 16.7 100 (23) CP N23 980794 RT 88 74.6 23 31 16.7 97 (18) CP N24 980795 RT 81 73.2 5.5 13 16.7 98 (19) AVG 85.7 74.8 11.7 17.8 16.7 98.3 (20.0)
 FIGS. 6 and 7 show the effect of heat treatment on the 90/10 composition. FIGS. 6A-6D show a series of optical micrographs of the sample in the as-deposited condition. Here the layered deposition structure is clearly seen. This structure is likely due to local thermal variation resulting in small differences in the scale of the microstructural features. This inhomogeneity is detrimental to mechanical properties as it provides a path of least resistance for defects to propagate. The series of optical micrographs in FIGS. 7A-7D show the same sample after a β anneal heat treatment at 1000° C. Here the microstructure has recrystallized and eliminated the layered structure seen in the non-heat treated condition. This microstructure might lead to mechanical property improvement, namely ductility.
 While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.