Patent Application
20070256764
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.
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 (σf>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.
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.
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.
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.
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.
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.
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.
a is a scanning electron microscopy (SEM) image of the microstructure at the deformed tip shown in
b is a transmission electron microscopy (TEM) image of the microstructure at the deformed tip shown in
a illustrates an embodiment of a continuous method of carrying out the present invention.
Like elements in the figs. are called out with like numerals.
b illustrates another embodiment of a continuous method of carrying out the present invention.
a illustrates an embodiment of a continuous method of carrying out the present invention using a roll feed.
b illustrates another embodiment of a continuous method of carrying out the present invention using a roll feed.
a is a graph representing applied forces in an ultrasonic processing method.
b is a graph representing applied forces in a combined ultrasonic and compression processing method in accordance with the present invention.
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.
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.
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.
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.
A metal cone specimen was subject to ultrasonic energy as described above.
The method described above can be adapted and modified into a continuous process for the production of wires having nanostructured grains.
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.
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.
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.
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.
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.
Referring to
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.
The device shown in
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.
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.
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.
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.
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).
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.
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.
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.
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.
