US20070256764A1

US20070256764A1 - Method of producing nanostructured metals using high-intensity ultrasonic vibration

Info

Publication number: US20070256764A1
Application number: US11/211,887
Authority: US
Inventors: Qingyou Han; Cailu Xu; Xiaogang Jian
Original assignee: UT Battelle LLC; University of Tennessee Research Foundation
Current assignee: UT Battelle LLC; University of Tennessee Research Foundation
Priority date: 2005-08-25
Filing date: 2005-08-25
Publication date: 2007-11-08

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

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

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

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>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.

OBJECTS OF THE INVENTION

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

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.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration showing a test arrangement for testing the present invention.
FIG. 2 is an approximately 3× magnified photograph showing the tip of a cone specimen that was deformed in accordance with the present invention.
FIG. 3 a is a scanning electron microscopy (SEM) image of the microstructure at the deformed tip shown in FIG. 2.
FIG. 3 b is a transmission electron microscopy (TEM) image of the microstructure at the deformed tip shown in FIG. 2.
FIG. 4 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.
FIG. 4 b illustrates another embodiment of a continuous method of carrying out the present invention.
FIG. 5 a illustrates an embodiment of a continuous method of carrying out the present invention using a roll feed.
FIG. 5 b illustrates another embodiment of a continuous method of carrying out the present invention using a roll feed.
FIG. 6 a is a graph representing applied forces in an ultrasonic processing method.
FIG. 6 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.

DETAILED DESCRIPTION OF THE INVENTION

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.
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

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.
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.
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.
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.
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 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.
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 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.
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.
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).
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.
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.

Claims

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

2. A method of producing a nanostructured article in accordance with claim 1 wherein said external force comprises at least one of the group consisting of external compressive force, magnetic force, and shear force.

3. A method of producing a nanostructured article in accordance with claim 1 wherein said vibration comprises ultrasonic vibration.

4. A method of producing a nanostructured article in accordance with claim 1 wherein said body of material comprises a metal.

5. A method of producing a nanostructured article in accordance with claim 1 wherein said external force and said vibration combine to produce alternating compressive stresses in the body of material.

6. A method of producing a nanostructured metal article comprising simultaneously subjecting a metal body to external compressive force and ultrasonic vibration to produce a desired nanostructure in the metal body.

7. A method of producing a nanostructured metal article in accordance with claim 6 wherein said external compressive force and said ultrasonic vibration combine to produce alternating compressive stresses in the metal body.

8. A method of producing a nanostructured metal article in accordance with claim 6 wherein said external compressive force comprises a severe plastic deformation process.

9. A method of producing a nanostructured metal article in accordance with claim 6 wherein said severe plastic deformation process comprises equal-channel-angular-extrusion.

10. A method of producing a nanostructured metal article in accordance with claim 6 wherein said ultrasonic vibration is applied at a frequency in the range of 1 Hz to 150 MHz and at a power intensity greater than 200 W.

11. A method of producing a nanostructured metal article in accordance with claim 10 wherein said ultrasonic vibration is applied at a frequency in the range of 10 kHz to 25 kHz, and at a power intensity in the range of 500 W to 2000 W.

12. Apparatus for processing a body of material comprising 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.

13. Apparatus for processing a body of material in accordance with claim 12 wherein said vibrator is disposed in contact with said means for applying external force.

14. Apparatus for processing a body of material in accordance with claim 12 wherein said vibrator is supported by said means for applying external force.

15. Apparatus for processing a body of material in accordance with claim 12 wherein said external force comprises at least one of the group consisting of external compressive force magnetic force, and shear force.

16. Apparatus for processing a body of material in accordance with claim 12 wherein said vibrator comprises an ultrasonic vibrator.

17. Apparatus for processing a metal body comprising 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.

18. Apparatus for processing a metal body in accordance with claim 17 wherein said ultrasonic vibrator is disposed in contact with said means for applying compressive force.

19. Apparatus for processing a metal body in accordance with claim 17 wherein said ultrasonic vibrator is supported by said means for applying compressive force.