Patent Publication Number: US-2005126666-A1

Title: Method for preparing ultrafine-grained metallic foil

Description:
STATEMENT REGARDING FEDERAL RIGHTS  
      This invention was made with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates generally to metallic foil and more particularly to a method for preparing ultrafine-grained metallic foil.  
     BACKGROUND OF THE INVENTION  
      Extremely high rolling strain (reduction in thickness) is needed to produce thin metal foil from much thicker, precursor sheet stock. A reduction in thickness of, for example, 98% is required in order to produce 20-micrometer (μm) thick metal foil from 1 millimeter-thick metal sheet stock. Work hardening prevents a metal sheet stock workpiece from being cold rolled to such a high strain without intermediate annealing. Metals and alloys with high ductility are desirable because they can be cold rolled to a high plastic strain before intermediate annealing becomes necessary before any additional cold rolling.  
      Plastic deformation by rolling, extrusion, drawing, and other conventional techniques often increase the strength of metals and alloys, but decrease their ductility [6-9]. Less conventional severe plastic deformation (SPD) techniques, such as Equal Channel Angular Pressing (ECAP) (sometimes called Equal Channel Angular Extrusion (ECAE)) and High Pressure Torsion (HPT), can increase the strength of metals and alloys while maintaining good ductility [6,11-17]. The theory of ECAE is described by V. M. Segal, V. 1. Reznikov, A. E. Drobyshevskiy, and V. I. Dopylov in “Plastic Working of Metals by Simple Shear,” Russian Metallurgy, vol. 1, pp. 99-105, (1981). Other papers and patents by Segal, the pioneer of the ECAE method, and others describe the use of ECAE to process metals, alloys, plastics, and other materials into rods and plates. Some procedures involve multiple extrusions with billet rotation between subsequent extrusions, which may be followed by forging or cold rolling.  
      Materials processed by SPD have enhanced superplasticity at low temperature and high strain rate [18-24]. These remarkable properties are associated with the unique ultrafine-grained (UFG) structures in metals and alloys produced by SPD techniques. Nanostructured copper (Cu), iron (Fe), and Ti produced by SPD techniques have an average grain size of 60-200 nanometers (nm) [25]. While the upper bound average grain size of nanostructured materials is generally about 100 nm, the above materials with average grain sizes of greater than 100 nm are also classified as nanostructured materials because they generally also have subgrains with low misorientation angles, dislocation cell structures [26-28], and the coherent crystallite domains as measured by X-ray analysis, which are usually smaller than 100 nm [29].  
      Titanium (Ti) is corrosion-resistant and biocompatible with human tissue, and has been used in medical implants and other devices [1-5]. Titanium foil, a desired material for a variety of applications such as hearing aids, has been manufactured traditionally by repetitive cold rolling of Ti sheets, with intermediate annealing at 650-700° C. to remove strain hardening. As a result of the annealing, the strength of Ti foil does not increase significantly with cold rolling. Ultrafine-grained high-strength titanium produced by severe plastic deformation/cold rolling/annealing has been described in U.S. Pat. No. 6,399,215 to Yuntian T. Zhu et al. entitled “Ultrafine-Grained Titanium for Medical Implants,” which issued on Jun. 4, 2002, incorporated by reference herein. According to the &#39;215 patent, titanium billets are first subjected to severe plastic deformation by Equal Channel Angular Extrusion (ECAE), then subjected to cold rolling with optional annealing of the extruded/cold-rolled workpiece. High strength titanium plates, rods, and screws were prepared and used as implants to reinforce damaged bones and missing teeth. While foil is mentioned generally as another additional possible structure, specific details for preparing ultrafine-grained, high strength foil were not described.  
      A method for preparing high-strength, ultrafine-grained metal foil is desirable.  
      Therefore, an object of the present invention is to provide a method for preparing ultrafine-grained, high-strength metal foil.  
      Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.  
     SUMMARY OF THE INVENTION  
      In accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention includes a method for preparing ultrafine-grained metal foil. The method includes preparing an ultrafine-grained metal billet having a thickness; cold rolling the ultrafine-grained billet until the thickness of the billet is reduced by at least 40%; annealing the cold rolled billet; cold rolling the annealed billet until the thickness of the billet is reduced by at least 70%; annealing the billet again; cold rolling the billet again until the thickness is reduced to at least 70%; annealing the cold rolled billet, and then cold rolling the annealed billet to produce ultrafine-grained metal foil.  
      The invention also includes a method for preparing ultrafine-grained titanium foil. The method involves preparing an ultrafine-grained metal billet having a thickness; thereafter cold rolling the ultrafine-grained billet until the thickness of the billet is reduced by about 75%; thereafter annealing the cold rolled billet at a temperature of about 350° C.; thereafter cold rolling the annealed billet again until the thickness of the billet is reduced by at least another 75%; annealing the billet again at a temperature of about 350° C.; thereafter cold rolling the billet again until the thickness is reduced by at least 75%; thereafter annealing the cold rolled billet again at a temperature of about 350° C.; and thereafter cold rolling the annealed billet to produce ultrafine-grained titanium foil.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiment(s) of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings: In the Figures:  
       FIG. 1   a  shows a transmission electron micrograph (TEM) of titanium (Ti) foil with a thickness of 20 μm that has been cold rolled from 2.65-mm thick, ultrafine-grained Ti stock.  
       FIG. 1   b  shows the electron diffraction pattern of the foil of  FIG. 1   a.    
       FIG. 2   a  shows the bright field TEM micrograph of Ti foil in processing state D, in which a 2.65 mm coarse-grained Ti stock was cold-rolled to a thickness of 20 μm.  
       FIG. 2   b  shows the dark field TEM micrograph of the Ti foil of  FIG. 2   a.    
       FIG. 2   c  shows an electron diffraction pattern from the framed area in  FIG. 2   a.    
       FIG. 2   d  shows an electron diffraction pattern from the framed area in  FIG. 2   b.    
       FIG. 3   a - c  show graphical representations of the yield strength, ultimate strength, and elongation to failure, respectively, of Ti sheets processed from UFG Ti and coarse-grained Ti in different processing states.  
       FIG. 4   a  shows a true stress-strain curve of Ti foil in processing state  1  (75% rolling strain starting from UFG Ti).  
       FIG. 4   b  shows a true stress-strain curve of Ti foil in processing state  3  (98.1 rolling strain starting from UFG Ti, annealed at 350° C. for 1 hour); and  
       FIG. 4   c  shows a true-stress-strain curve of Ti foil in processing state C (98.1% rolling strain, starting from coarse-grained Ti).  
    
    
     DETAILED DESCRIPTION  
      The invention includes a method for preparing ultrafine-grained (UFG), high strength metallic foil. The method was demonstrated by converting an ultrafine-grained titanium billet into ultrafine-grained titanium foil.  
      Coarse-grained Ti and UFG Ti were used as the initial material stocks; both are commercially pure Ti, and include certain minor amounts of impurities such as oxygen (0.2 weight percent), hydrogen (0.002 weight percent), nitrogen (0.04 weight percent), carbon (0.07 weight percent), iron (0.1 weight percent iron), silicon (0.01 weight percent), and aluminum (0.28 weight percent).  
      Reference will now be made in detail to the present preferred embodiments of the invention. The grain size and structure of a coarse-grained titanium billet was refined to an ultrafine-grain size by subjecting the coarse-grained billet to repeated extrusion through a warm Equal Channel Angular Extrusion (ECAE) die followed by cold rolling and annealing. The workpiece billet was rotated by 90 degrees clockwise around its longitudinal axis between passes through an Equal Channel Extrusion (ECAE) die for a total of 8 passes. Both the die and the work-piece were preheated to 450° C. The temperature dropped to about 400° C. after 8 passes. The procedure for preparing an ultrafine-grained titanium billet has already been described elsewhere, for example in U.S. Pat. No. 6,399,215 to Yuntian T. Zhu et al. entitled “Ultrafine-Grained Titanium for Medical Implants,” which issued on Jun. 4, 2002, incorporated by reference herein, and in references [10] and [11].  
      The initial dimensions of the Ti workpieces described herein that were subsequently cold rolled, were about 2.65 millimeters (thickness) by 5 millimeters (width) by 40 millimeters (length). These workpieces were prepared from larger titanium billets by machining larger titanium billets that were subjected to Equal Channel Angular Extrusion. For purposes of the invention, these machined materials are also referred to as billets (or as stock), and workpieces cold rolled from them that have not yet become foil are referred to as sheets. While a relatively thin stock (2.65 millimeters in thickness) was used to demonstrate the invention, it should be understood that thicker or thinner stock could also be used.  
      For purposes of the invention, a thickness reduction of 15-5% as used for each rolling pass using an industrial rolling mill for foil production. Intermediate low temperature annealing at 350° C. for 1 hour was carried out at various thickness reductions (see Table 1) to relieve work hardening for further rolling.  
      Microstructures were characterized using an electron microscope EM-125K at an accelerating voltage of 125 kV. Tensile mechanical properties were measured using a PV-3012M mechanical testing machine equipped with an automatic recording of the load-displacement curves. Three to four samples were tested for each processing state. Two sample geometries and sample preparation methods were used for samples from Ti sheets with various thicknesses. For Ti sheets with thickness larger than 600 μm, the tensile samples were cut by electric-spark into a dog-bone shape with gage dimensions of 10 mm by 2 mm by 0.5 mm. The sides of the samples were first polished by sand paper and then by electropolishing. For thinner Ti foil, samples were cut to have gage dimension of 5 mm by 2 mm by ‘h’ mm, where ‘h’ is the thickness of the Ti foil. Special arbors were used to cut the samples.  
      Table 1 lists processing conditions, thicknesses, and grain sizes of Ti workpieces that include the ultrafine-grained (UFG) Ti stock produced by processing ECAP route Bc for 8 passes, and also the corresponding sheet and foil produced from the UFG Ti stock according to one embodiment of the invention. The average grain/subgrain size was calculated by averaging 100 measurements. The initial UFG Ti stock produced by ECAP has an average grain size of 350 nm (see [10-13] for additional details related to the microstructures of UFG Ti).  
      The first 75% rolling strain (thickness reduction) led to additional microstructure refinement with the average grain/subgrain size reduced from 350 nm to 150 nm. Further rolling and intermediate annealing did not further refine the microstructure. It has been reported [10-13] that the nanostructured Ti processed by ECAP and further cold deformation does not recrystallize at annealing temperatures below 400° C. Therefore, the intermediate annealing steps only relieved the work hardening by recovery, i.e. the rearrangement and dislocation density reduction without grain growth, which explains why the grain size did not increase after each intermediate annealing.  
                           TABLE 1                                   Average       State       Thickness   Grain       Number   Processing Conditions   (μm)   Size (nm)                                                UFG   ECAP, route Bc, 8 passes   2650   ˜350       1   ECAP + cold rolling by 75%   660   ˜150       1a   State 1 + annealing at 350° C. for 1 h   660   ˜150       2   State 1a + cold rolling by 75%   165   ˜150       2a   State 2 + annealing at 350 C. for 1 h   165   ˜150       3   State 2a + cold rolling by 70%   50   ˜150       3a   State 3 + annealing at 350 C. for 1 h   50   ˜150       4   State 3a + cold rolling by 60%   20   ˜150                  
 
       FIG. 1   a  shows a typical Transmission Electron Microscopy (TEM) micrograph from a Ti foil in the processing state  4 . In this processing state, the initial UFG Ti stock with a thickness of 2.65 mm has been processed into Ti foil with a thickness of 20 μm. It can be seen that the microstructure is relatively homogeneous although some larger grains also appear. The average grain size is about 150 nm.  FIG. 1   b  shows the electron diffraction pattern of the foil of  FIG. 1   a . The large number of diffraction spots almost form continuous rings, indicating that a large fraction of the grain boundaries are high angle boundaries [30-33]. The sharp contrasts between grains in  FIG. 1   a  also indicate high angle boundaries between most grains.  
      Table 2 lists the fragment and grain sizes of coarse-grained Ti stock and the titanium sheet and foil produced from that stock by cold rolling and intermediate annealing. Table 2 includes the term “fragments”. In Table 2, fragments are defined as subgrains that formed when initial large grains were refined by large plastic deformation. Grains in Table 2 are defined as large crystallites with visible substructures and sizes in the range of several micrometers. These definitions of fragments and grains in Table 2 are for the convenience of discussions on the microstructural heterogeneity, and are more accurate for a processing state with low rolling strain, e.g. state A in Table 2, than for a processing state with high accumulated rolling strain, e.g. state D. As shown in Table 2, the fragment size and grain sizes decrease with increasing rolling strain, but the size decreases are not great. However, plastic deformation usually increases the misorientations across fragment boundaries even when the average fragment sizes no longer decrease [22]. Therefore, more subgrain fragments became grains with high angle grain boundaries with increasing rolling strain.  
                       TABLE 2                       State Number   Processing Conditions   Grain/Fragment Size (μm)                  Coarse grained   Hot rolling   ˜15       A   Cold rolling by 75%   Fragment size: 0.1-0.5               Grain size: 5-10 (fraction: 30%)       Aa   State A + annealing at 350° C. for 1 h   Fragment size: 0.1-0.5               Grain size: 5-10 (fraction: 30%)       B   State Aa + cold rolling by 75%   Fragment size: 0.1-0.5               Grain size: 5-10 (fraction: 30%)       Ba   State B + annealing at 350 C. for 1 h   Fragment size: 0.1-0.5               Grain size: 5-10 (fraction: 30%)       C   State Ba + cold rolling by 70%   Fragment size: 0.1-0.3               Grain size: 4-7 (fraction: 20%)       Ca   State C + annealing at 350 C. for 1 h   Fragment size: 0.1-0.3               Grain size: 4-7 (fraction: 20%)       D   State Ca + cold rolling by 60%   Fragment size: 0.1-0.3               Grain size: 4-7 (fraction: 20%)                  
 
      In contrast to the microstructure of the Ti sheets produced by cold rolling the ECAP-processed UFG Ti (see Table 1 and  FIG. 1 ), the microstructure of the Ti sheets produced by cold rolling coarse-grained Ti is heterogeneous, as evidenced in  FIG. 2 .  FIG. 2   a  and  FIG. 2   b  show the bright field and dark field TEM micrographs of an area in a sample in processing state D. Most of the area shows fine structures. However, areas with large grains also exist.  FIG. 2   c  is an electron diffraction pattern from the framed area in  FIG. 2   a . It shows that the diffraction spots formed a semi-continuous circle, indicating that a large fraction of fragments in the framed area in  FIG. 2   a  have become grains. However,  FIG. 2   d , which is an electron diffraction pattern from the framed area in  FIG. 2   b , shows only elongated diffraction spots that indicate that the whole framed area is from one grain with some distortion. As listed in Table 2, a significant fraction (20%) of the observed area remains coarse-grained with grain sizes in the range of from 4 μm to 7 μm in Ti foil (at processing state D) produced from coarse-grained Ti.  
      The above results indicate that one advantage of producing Ti foil from UFG Ti stock is smaller, homogeneous grain structures. Such a microstructure should translate into better mechanical properties, as discussed below.  
       FIG. 3  compares the mechanical properties of Ti sheets that were produced by cold rolling UFG Ti with those that were produced by cold rolling coarse-grained Ti. It is clear from  FIG. 3   a  and  FIG. 3   b  that at all processing states, the Ti sheets produced from the UFG Ti have significantly higher yield strength and ultimate strength. Similar to earlier observations [12 and 13], cold rolling further increased the strength of ECAP processed UFG Ti.  
       FIG. 3   c  shows that Ti sheets produced from the UFG Ti have higher elongation to failure (ductility) at all rolling strains/thickness reductions. The coarse-grained Ti, with a grain size of 15 μm, has a high elongation to failure of 22.6%. The first round of rollings (state A) reduced the elongation to failure of coarse-grained Ti to less than 5%. The elongation to failure decreased further at higher rolling strains (processing state C and D), which is consistent with previous reports [6-9] that the ductility of metals and their alloys decrease with rolling strain. The UFG Ti processed by ECAP route Bc for 8 passes has an elongation to failure of about 5%, which is lower than our previous results [33]. However, the elongation to failure initially increased with cold rolling strain (processing state  1 , 2 , and  3 ), although it decreased at the final processing state  4 . This decrease could be partially due to surface defects, which have a greater effect on the mechanical properties of thinner Ti foil. Note that the scatter in the measured strength values and elongation to failure values are largest in processing state  4 , at which the Ti sheet is thinnest (20 μm), which is consistent with the effect of surface defects.  
       FIG. 4  shows the tensile true stress-strain curves of Ti sheets in several processing states. The true stress-strain curves were calculated from the engineering stress-strain curves by assuming a uniform deformation along the gage length. As a result, after the necking started, the true stress in the necking section will be higher than the value shown in the figure.  FIG. 4   a  shows that the Ti sheet from cold rolling a UFG Ti sheet for 75% thickness reduction (curve  1 ) has an initial work hardening at strains less than 4% and then a smooth decrease in stress because necking quickly developed and failed the sample. Annealing at 350° C. for one hour significantly improved the ductility (curve  2 ). Curve  2  in  FIG. 4   a  also shows a modest work hardening before the strain reached 7.3% and then a slow gradual decrease in stress for a large strain range before failure, indicating a good resistance of the sample to fracture after necking. The modest work hardening in curve  2  was caused by the accumulation of dislocations. Annealing at 350° C. lowered the dislocation density through recovery, thus allowing dislocation accumulation during the tensile testing. The work hardening helped to effectively resist necking, which postponed necking to larger strain, and consequently led to significant increase in the elongation to failure.  
       FIG. 4   b  shows that Ti foil produced by cold rolling a UFG Ti sheet for 98.1% thickness reduction exhibit a small work hardening with a constant work hardening rate (∂σ/∂ε) over a wide strain range in both as-processed state (curve  1 ) and after annealing at 350° C. for 1 hour (curve  2 ). Such a work hardening is in sharp contrast with the previously reported [14] tensile stress-strain curves of nanostructured Ti processed by ECAP and cold rolling, but at a lower rolling strain of 73%, which showed that the stress peaked soon after yielding and then decreased relatively quickly. The maintenance of high stress level over a large strain range provides the Ti foil with higher toughness, an advantage for its structural applications. It is obvious that extremely large rolling strain achieved during practice of the invention has changed the nanostructure of the material, which contributes to the observed mechanical behavior of the Ti foil. However, it is not clear which particular nanostructure is responsible for the unique mechanical behavior, i.e. the constant work hardening rate and the good ductility observed in  FIG. 4   b . One structural difference between the Ti foils corresponding to  FIG. 4   b  and those in reference [14] is the shape of the grains and the fraction of high angle grain boundaries. The nanostructured titanium of [14] includes elongated grains [yes and a large fraction of low angle grain boundaries and subgrains. The extremely large rolling strain in foil corresponding to  FIG. 4   b  produced smaller, equiaxed grains (not shown here) with higher fraction of high angle grain boundaries similar to  FIG. 1   a.    
       FIG. 4   c  shows the true stress-strain curves of Ti foil produced by cold rolling coarse-grained Ti for 98.1% thickness reduction, the same rolling strain as the Ti foil used for  FIG. 4   b . The stress-strain curves of  FIG. 4   c  show both lower strength and ductility and earlier necking than the curves of  FIG. 4   b , again demonstrating the advantage of using UFG Ti as the starting material for producing Ti foil.  
      It had been anticipated that a mixture of nanosized grains and micron-sized grains would provide the nanomaterials with higher ductility because of the strain hardening in the large grains [34]. However, the mixture of very fine grains and 20% large micrometer-sized grains of the titanium foil produced according to the conditions used did not render the Ti foil with higher ductility.  
      We have demonstrated the advantage of UFG Ti stock in producing Ti foil by cold rolling with intermediate annealing. Compared with coarse-grained Ti, the UFG Ti resulted in more homogeneous nanostructure, higher strength, larger homogeneous deformation under tension, and higher ductility when it was further rolled into thin sheets and foil. Thus, it has been shown that ECAP followed by cold rolling produces metal foil with superior mechanical properties.  
      The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. For example, while titanium metal was used to demonstrate the invention, it should be understood that the invention is expected to be useful for preparing ultrafine-grained foil from other metals such as copper, nickel, aluminum, zinc, zirconium, tantalum, chromium, molybdenum, iron, cobalt, platinum, silver, gold, and other metals, and alloys of these metals as well. For these materials, some of the processing conditions may be different from the conditions used for producing ultrafine-grained titanium foil, without departing from the scope of the invention. The intermediate annealing temperature may be higher or lower than 350° C., for example, and the annealing time may be greater than or less than an hour, depending on the metal or metal alloy used. It is expected that annealing at a temperature of from about 100° C. to about 500° C. would be required for most metals. With regard to the number of passes through the equal angular extrusion die to produce an ultrafine-grained billet, 8 passes through the die is preferred, but it is expected that anywhere from 1 to 12 passes through the die may also result in grain refinement. With regard to the cold rolling steps, while the thickness was reduced to about 75% between each annealing as described herein for titanium foil, it is expected that a reduction in thickness of at least 40% would be adequate, but it should be understood that for less of a reduction in thickness between annealing steps, in order to arrive at foil, one or more additional sequences of reduction in thickness by cold rolling followed by annealing and/or a thinner original billet would be required. In any case, in view of the results obtained and described herein, these additional sequences of cold rolling and annealing will reduce the thickness without significantly affect the grain size.  
      The embodiment(s) were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.  
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