Abstract:
A method of producing a nanostructured article includes simultaneously subjecting a body of material to external force and vibration to produce a desired nanostructure in the body of material.

Description:
[0001]     The United States Government has rights in this invention pursuant to contract no. DE-AC05-00OR22725 between the United States Department of Energy and UT-Battelle, LLC. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to methods of producing nanostructured materials, and more particularly to methods of producing nanostructured materials, especially metals (including alloys and metal-matrix composites), wherein a combination of external force, especially compressive force, and vibration, especially ultrasonic vibration, is used to process solid material to produce improved nanostructures therein.  
       BACKGROUND OF THE INVENTION  
       [0003]     Nanostructured materials offer unique and entirely different mechanical, electrical, optical, and magnetic properties compared with conventional microstructured or millimeter-scaled materials. For example, the hardness of nanocrystalline copper is known to increase with decreasing grain size; nanostructured copper having 6 nm grains can have as much as five times the hardness of conventionally prepared copper. Another example is nanostructured Al—Ni—In alloys, which are known to exhibit a tensile strength (σ f &gt;1200 MPa) greater than conventional high-strength aluminum alloys. Nanostructured M50 steel is more fatigue and fracture resistant than conventional M50 steel that is widely used in the aircraft industry as the main-shaft bearings in gas turbine engines.  
         [0004]     Conventional methods for producing nanostructured materials include gas atomization, ball milling followed by consolidation, and rapid solidification. Such processes tend to be expensive and prone to contamination. Recent approaches for producing nanostructured materials include severe plastic deformation. Equal Channel Angular Extrusion (ECAE) is one of the methods that use severe plastic deformation to produce nanostructured materials but it is an expensive method for producing nanostructured materials.  
       OBJECTS OF THE INVENTION  
       [0005]     Accordingly, objects of the present invention include the provision of methods of processing metal bodies to produce desired nanostructures therein. Further and other objects of the present invention will become apparent from the description contained herein.  
       SUMMARY OF THE INVENTION  
       [0006]     In accordance with one aspect of the present invention, the foregoing and other objects are achieved by a method of producing a nanostructured article that includes simultaneously subjecting a body of material to external force and vibration to produce a desired nanostructure in the body of material.  
         [0007]     In accordance with another aspect of the present invention, a method of producing a nanostructured metal article that includes simultaneously subjecting a metal body to external compressive force and ultrasonic vibration so that a desired nanostructure is produced in the metal body.  
         [0008]     In accordance with a further aspect of the present invention, apparatus for processing a body of material includes means for applying external force to a body of material in combination with a vibrator disposed for simultaneously applying vibration to the body of material to produce a desired nanostructure in the body of material.  
         [0009]     In accordance with yet another aspect of the present invention, apparatus for processing a metal body includes means for applying external compressive force to a metal body in combination with an ultrasonic vibrator disposed for simultaneously applying ultrasonic vibration to the metal body to produce a desired nanostructure in the metal body.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic illustration showing a test arrangement for testing the present invention.  
         [0011]      FIG. 2  is an approximately 3× magnified photograph showing the tip of a cone specimen that was deformed in accordance with the present invention.  
         [0012]      FIG. 3   a  is a scanning electron microscopy (SEM) image of the microstructure at the deformed tip shown in  FIG. 2 .  
         [0013]      FIG. 3   b  is a transmission electron microscopy (TEM) image of the microstructure at the deformed tip shown in  FIG. 2 .  
         [0014]      FIG. 4   a  illustrates an embodiment of a continuous method of carrying out the present invention.  
         [0015]     Like elements in the figs. are called out with like numerals.  
         [0016]      FIG. 4   b  illustrates another embodiment of a continuous method of carrying out the present invention.  
         [0017]      FIG. 5   a  illustrates an embodiment of a continuous method of carrying out the present invention using a roll feed.  
         [0018]      FIG. 5   b  illustrates another embodiment of a continuous method of carrying out the present invention using a roll feed.  
         [0019]      FIG. 6   a  is a graph representing applied forces in an ultrasonic processing method.  
         [0020]      FIG. 6   b  is a graph representing applied forces in a combined ultrasonic and compression processing method in accordance with the present invention.  
     
    
       [0021]     For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawings.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     Solid materials subjected to vibration, especially high intensity ultrasonic vibration, undergo alternating tensile and compressive stresses and/or strains. Under the influence of such alternating forces, beneficial vacancies and dislocations are induced but the material is subject to fatigue failure due the tensile forces. The basic concept of the present invention is to simultaneously use external force (force applied to the exterior of a work-piece), preferably external compressive force, to the work-piece (a metal body, for example) subject to vibration. Because of the composite nature of the forces/stresses, the alternating tensile/compressive stresses are modified into alternating compressive forces, reducing pernicious tensile forces and preventing materials from undergoing fatigue failure. Shear forces and even some tensile forces may remain and even may be beneficial to the invention.  
         [0023]     External force can be applied by any means, such as, for example, compressive force, magnetic force, and combinations of the foregoing. External compressive force can be applied to a metal body by any of the various and sundry known methods of metalworking such as, for example, extrusion, swaging, hammering, pressure, forging, etc.  
         [0024]     Vibration, preferably ultrasonic vibration, can be applied to the metal body by any vibrator, preferably an ultrasonic vibrator, capable of producing sufficiently intense vibration, and can be applied directly to the work-piece or indirectly, such as through the body of an extrusion die, magnet, anvil, or press ram.  
         [0025]      FIG. 1  shows a schematic illustration of a test of the method. A specimen work-piece having a conical tip with a length of an integral multiple of a half wave-length of the ultrasonic wave is connected to an ultrasonic horn and an ultrasonic generator. High-intensity ultrasonic energy is then injected into the tip of the cone. Alternating compressive stresses are thus generated at the tip of the cone specimen partly due to ultrasonically induced stresses and strains and partly due to the weight of the ultrasonic horn/generator assembly. As a result, severe plastic deformation occurs at the tip of the specimen.  
       EXAMPLE I  
       [0026]     A metal cone specimen was subject to ultrasonic energy as described above.  FIG. 2  shows the tip of the deformed cone specimen. The sharp tip of the cone becomes umbrella-shaped.  FIG. 3   a  is a SEM image of the microstructure at the deformed tip and  FIG. 3   b  is a TEM image of the grains in the deformed region. The grain sizes in the deformed tip are about 100 nm.  
         [0027]     The method described above can be adapted and modified into a continuous process for the production of wires having nanostructured grains.  FIGS. 4   a  and  4   b  illustrate some embodiments of continuous methods of carrying out the present invention to form wires using ultrasonic vibration and external forces to cause severe plastic deformation of the work-piece, and use dies to collect the deformed material. Further information relating to severe plastic deformation methods can be found in U.S. Pat. No. 6,895,975 issued on May 24, 2005 to Chaudhury, et al. entitled “Continuous Severe Plastic Deformation Process for Metallic Materials”, the entire disclosure of which is incorporated herein by reference. Some embodiments of the invention use a die similar to the type of die used in equal-channel-angular-extrusion (ECAE) processes.  
         [0028]      FIG. 4   a  shows a die  20  having a die channel  22  with a sharp corner  24  for causing severe plastic deformation of the work-piece (metal body, for example)  28 , usually a wire. An ultrasonic vibrator  14  is shown in contact with the feeding end of the work-piece  28 . Ultrasonic vibration is injected into the work-piece  28  as it is pushed through the die  20  to produce bulk nanostructured wire.  
         [0029]      FIG. 4   b  shows a die  30  having a die channel  32  with a sharp corner  34  for causing severe plastic deformation of the work-piece  28 , usually a wire. An ultrasonic vibrator  14  is shown in the die and in contact with the work-piece  28  as it passes through the die channel  32 . Ultrasonic vibration is injected into the work-piece  28  as it is forced through the die  30  to produce bulk nanostructured wire.  
         [0030]     Nanostructured wire produced by the present invention is free from contaminants such as oxidation and surface contamination that usually occurs that use ball milling and rapid solidification. Moreover, nanostructured wire produced by the present invention is free from porosity formation that occurs in methods that use condensation of small particles or droplets.  
         [0031]     In accordance with the present invention, vibration at an ultrasonic frequency is operably applied at a frequency in the range of 1 Hz to 150 MHz, preferably in the range of 10 kHz to 25 kHz, and at a power intensity greater than 200 W, preferably in the range of 500 W to 2000 W. The duration of ultrasonic processing can be anywhere in the range of 0.1 second to 20 minutes. Once the beneficial results of ultrasonic processing are achieved, continued subjection of the process material is not deleterious, therefore duration is not considered to be a critical parameter.  
         [0032]     The amount of the external force should be larger enough to modify the alternating tensile/compressive stresses (forces) induced by the high-intensity ultrasonic vibration into mainly alternating compressive and shear stresses (forces). It is necessary to prevent materials from undergoing fatigue failure under high-intensity ultrasonic vibrations. Generally the external force can be high but not too high to cause dimensional instability or even the failure of the materials to be processed.  
         [0033]     Referring to  FIG. 6   a , a sine wave  60  represents alternating tensile and compressive forces caused by ultrasonic vibration. Line  62  represents zero force, arrow  64  represents tensile force caused by ultrasonic vibration, and arrow  66  represents compressive force caused by ultrasonic vibration. In  FIG. 6   b , external compressive force  68  is applied, so that sine wave  60 ′ is offset below the zero force line  62  and now represents increasing and decreasing (alternating) compressive forces and no tensile forces.  
         [0034]     The bulk grain size obtained by this invention is about 100 nm by passing through the material over the ultrasonic radiator. Using a device similar to ECAE, the material can be processed a few times with further grain size reduction after each pass.  
         [0035]     The device shown in  FIG. 5   a  can be used to assist the ECAE process for material of large cross-section. The die  30  and ultrasonic vibrator  14  are similar to that shown in  FIG. 4   b . Rollers  52  are used to force the metal work-piece  28  through the die channel  32  with sharp corner  34 .  
         [0036]      FIG. 5   b  shows another embodiment of the invention wherein ultrasonic vibration is applied to the die. The ultrasonic vibrator  14  applies ultrasonic vibration to the die body  50 . Rollers  52  are used to force the metal work-piece  28  through the die channel  22  with sharp corner  24 .  
         [0037]     The application of high-intensity ultrasonic vibration brings about two effects: One is the acoustic “softening” of materials (because the dislocations are dislodged and moved by the ultrasonically induced instantaneous stresses/strains) and the other is the reduction of friction forces at the metal/die interface.  
         [0038]     Due to the first effect, the metal to be extruded becomes soft so it will be easier to be extruded using the EACE process. This is also extremely important for materials that are not ductile or that are difficult to be extruded using ECAE process. These materials include Mg metal and alloys, titanium metal and alloys, and other materials with hcp crystal structure.  
         [0039]     Due to the second effect, the forces required to push material through an ECAE die will be greatly reduced. This is also important since it is the friction force that limited the application of the ECAE process. This is especially true for the extrusion of metal of large cross-section, in which the friction force is so high that basically there are no materials tough enough to be used as the die material. The largest aluminum 6061 bar that has been extruded using the ECAE process is only a few square inches in cross-section.  
         [0040]     The two effects described above can be utilized to assist the ECAE process. One embodiment of this invention is to use ultrasonic vibration and transmit the vibration to the interface of the extruded material and the ECAE die (for reducing friction force) and to the extruded material around the sharp corner of the ECAE die (for softening the material).  
         [0041]      FIGS. 5   a, b  shows schematically how ultrasonic vibration can be used to assist the ECAE process. Ultrasonic vibration is applied by the ultrasonic vibrator  14  to the work-piece of extruded material  28  at the corner of the ECAE die, where the shear stress and friction stresses are the largest. Rolls  52  are used to continuously feed the extruded material  28  through the ECAE die  30 . The use of ultrasonic vibration will generally soften the material  28  at the corner  34  of the die  30  and reduce the friction between the extruded material  28  and the die  30 , significantly reducing the amount of applied force necessary to carry out the ECAE process. A significant issue involved in this embodiment of the invention is that the rolls  52  should preferably be positioned at the antinodes where the ultrasonic vibration is at a minimum. Such placement of the rolls isolates the roll feed system from vibration from the extruded material.  
         [0042]     As can be seen in the description above, the ultrasonic vibrator can be disposed in contact with the means for applying compressive force, and can even be supported thereby. Such disposition, although generally preferable is not, however absolutely necessary. It is critical to the invention that the relative disposition of the ultrasonic vibrator and means for applying compressive force be such that the forces generated thereby have a combined effect on the metal body.  
         [0043]     While there has been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications can be prepared therein without departing from the scope of the inventions defined by the appended claims.