Recrystallization and Grain Growth of Cold-Rolled Gold Sheet

Transcription

1 Recrystallization and Grain Growth of Cold-Rolled Gold Sheet JAE-HYUNG CHO, H.-P. HA, and K.H. OH Recrystallization and grain growth of a cold-rolled gold sheet with 98 pct reduction in area (RA) were investigated with electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). Gold with some dopants (Be, Ca, and La) was used in this research and its recrystallization temperature was 320 C. Isothermal annealing experiments at 400 C, 500 C, and 600 C were carried out for the cold-rolled gold sheet, and recrystallization texture was examined. In the cold-rolled gold sheet, - and -fibers were measured mainly and some shear texture components were found on the surface. Shear texture components remained on the surface for 2 hours at 400 C and were consumed by other recrystallized grains after 24 hours at 400 C. Microstructure and texture evolution during in-situ annealing at 400 C were investigated from the cold-rolled state to the fully recrystallized state using EBSD. Most of the newly, recrystallized grains came from the deformed -fiber regions and consisted of -fiber, cube, and other random orientations. I. INTRODUCTION THE rolling textures of fcc metals have been studied, and it has been found that stacking fault energy (SFE) and deformation temperature are prevalent controlling factors. [1 4] The formation of cold-rolling textures in fcc metals has been reviewed by Hirsch and Lücke. [1,2] Without twinning, the general cold-rolling texture components for medium-high SFE materials are Copper {112} 111, S{123} 634, Brass{110} 112 and Goss{110} 001. In particular, metals with the Copper-type deformation texture have been extensively studied with the purpose of understanding microstructure and texture evolution during rolling and annealing. The annealing process consists of recovery, recrystallization, and grain growth. [5,6] During recovery, stored energy is released without high-angle grain boundary (GB) migration. Recrystallization is also driven by stored deformation energy but is accompanied by high-angle GB migration. Grain growth also involves the migration of grain boundaries, and the driving force for that is the reduction of the GB area. The recrystallization textures of fcc metals also have been investigated in many articles. [7 11] Most articles have focused on the formation of recrystallized cube {100} 001 from the Copper-type deformation texture. Although the orientation distribution function (ODF) for many wrought fcc metals and alloys of medium-high SFE shows little indication of cube component, it develops remarkably during recrystallization. It has been known that the recrystallization texture is determined mainly by the orientation and growth rate of the nuclei. Sometimes, the deformation texture is similar to that of recrystallization. 316L austenitic stainless steel (17.75 wt pct Cr, 12.6 wt pct Ni, 2.38 wt pct Mo) shows similar texture JAE-HYUNG CHO, Research Associate, is with the Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY H.-P. HA, Senior Researcher, is with the Metal Processing Center, Korea Institute of Science & Technology, Seoul , Korea. K.H. OH, Professor, is with the School of Materials Science & Engineering, College of Engineering, Seoul National University, Seoul , Korea. Contact kyuhwan@snu.ac.kr Manuscript submitted June 28, components in both deformation and recrystallization. [12] Cold rolling textures of 316L show mainly the Brass orientation with a spread toward the Goss orientation. The retention of the Brass orientation is attributed to oriented nucleation and to the inhibition of further selectively oriented growth by a strong solute effect of Molybdenum. In Mo-free austenitic stainless steels (18 wt pct Cr, 9 wt pct Ni), however, the general fcc rolling texture changes into different textures after annealing. Some shear textures formed on the surface show the retention of deformation texture or more complicated recrystallized behaviors during annealing. [13,14] Considering that surface shear is formed in a relatively narrow layer, the recrystallization kinetics for the surface and for the center of materials are different from each other. Although extensive studies have been carried out on rolling and recrystallization textures for fcc metals and alloys, studies on gold are rare. Kitagawa reported that the texture development of gold leaf (0.1- m thickness) fabricated by the traditional pack and hammering method was (001) texture. [15] This is different from the general fcc plane compression texture, and they pointed out that it comes from cross-slip. High-purity gold ( wt pct Au) is too soft and unstable to obtain good properties for industrial use, i.e., bonding wire. The annealing and recrystallization temperature for pure gold is in the range of 150 C to 200 C, and it has been reported that highly deformed pure gold will show recovery and recrystalization at room temperature. [16] Therefore, bonding wire commonly has various dopants in the parts per million (ppm) level in order to control the annealing response and to obtain better thermal and mechanical properties. Impurities, even at these low levels, are important for controlling the final mechanical properties and microstructures of gold and other materials by raising the recrystallization temperature and preventing grain growth. [17 20] Microstructure, texture, and mechanical properties of gold wires were reported during annealing. [21 24] Due to the strong elastic anisotropy in gold (E 111 /E GPa/42 GPa), the change in elastic modulus was used to detect texture change during recrystallization. Recently, Cho et al. reported recrystallization and grain growth of gold bonding wire during isothermal annealing at 300 C and 400 C using electron backscatter diffraction (EBSD). [25] METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 36A, DECEMBER

2 In cold-rolled gold sheet, the authors found that deformation textures consist of the general -fiber (Brass-S-Copper) and some shear textures on the surface. Annealing textures have the same -fiber in addition to cube and random orientation. The rolling and recrystallization textures and the microstructure of gold sheets were analyzed in detail. X-ray diffraction (XRD) and EBSD were used to investigate the texture and microstructure evolution during annealing. II. EXPERIMENTS A. Material and Sample Preparation The gold for this research was fabricated for gold bonding wire and its purity was more than pct. It has some intentional dopants of Be, Ca, and La, less than 50 parts per million (ppm) by weight. Those dopants are added to increase the strength of gold bonding wire. The existence of dopants in gold can affect recrystallization and grain growth even though they are at the ppm level. The initial specimen had a brick shape (12-mm wide 12-mm long 10-mm thick) and was cold-rolled. After 98 pct reduction in area (RA), gold sheets with a thickness of 200 m were taken for annealing experiments followed by XRD and EBSD measurements. B. Annealing Experiments The recrystallization temperature of the gold was inferred from the measured hardness profile of 85 pct RA sheet after isothermal annealing for 1 hour. In order to investigate texture evolution from deformation to recrystallization, two categories of annealing experiments were carried out. First, annealing temperature effects were examined at temperatures of 400 C, 500 C, and 600 C. In addition, quasi insitu recrystallization processes were examined from the as-deformed state (98 pct RA) to full recrystallization according to annealing time. The annealing temperature for the quasi in-situ experiments was fixed at 400 C, and the annealing times were 5 minutes, 18 minutes, 60 minutes, 4 hours, and 24 hours. One specimen with a reference line was taken and used for the repeated annealing and EBSD measurements. The measuring position varied slightly, even with a reference line, but the effect seemed negligible. C. XRD and EBSD Measurements Three incomplete pole figures, {111}, {200}, and {220}, were determined on the surface of the 98 pct cold-rolled specimens using the XRD method with Cu K. The XRD specimens were measured without polishing, leaving the surface shear region undisturbed. The ODF of the crystallite was calculated using the WIMV* method, [26] assuming *The WIMV with an automated conditional ghost correction suggested by Matthies and Vinel is a method of ODF reproduction from pole figures. It is based on the analysis of the structure of the exact solution of the central problem, on the analytical properties of the ghosts problem, and on the use of the most constructive elements of earlier reproduction activities by Williams and Imhof. With reference to these authors, it bears the acronym WIMV. orthorhombic sample symmetry. Such symmetry requires the elementary Euler space defined by 0 deg 1 90 deg, 0 deg 90 deg and 0 deg 2 90 deg. To obtain the EBSD pattern, High Resolution EBSD (HR EBSD, JEOL* 6500F with INCA/OXFORD EBSD system) was *JEOL is a trademark of Japan Electron Optics Laboratory Co., Ltd., Tokyo. used for measurement, and the data analysis was made by REDS (Reprocessing of EBSD Data in SNU). [27] The mechanically polished EBSD specimens were cleaned with ion milling. The operating voltage was 20 kv and the probe current was 4 na. The step size of the EBSD scans was 2 m. To identify a grain in EBSD data, it is necessary to determine its perimeter (grain boundary) and the average orientation within it. Misorientation angles between adjacent pixels are used for grain identification (ID). Any two adjacent pixels with a grain ID angle smaller than the cut-off value are considered part of the same grain. Most deformed or recrystallized grains have subgrain structures, and their overall structures can be described by the misorientation measures calculated over a set of pixels contained within the grain. There are three types of misorientation measures commonly used in a grain. First, grain orientation spread (GOS) expresses the magnitude of misorientation among all pixels in a grain. Second, scalar orientation spread (SOS) is calculated between each pixel and the average orientation. Grain average misorientation (GAM) is the quantity calculated from adjacent pixels only, and gives information about the nearest neighbor correlations. The GAM value is generally smaller than GOS or SOS. Considering P i as an orientation at a point (x i ) and P j as another orientation at adjacent point (x j ) in a grain, the GAM can be calculated with misorientation angle, which is given for two adjacent orientations at P i and P j, a i GAM n where u mis min c acos a trace ((P # i P 1 j ) # S) 1 bd 2 Here, S is the symmetry operator belonging to the appropriate crystal class concerned and the subscripts in rotation P refer only to position. To calculate the grain size, the number of data points or pixels in a grain is calculated, and using the known pixel step size, the grain area is determined. The most convenient measure of grain size from grain area is the equivalent circle diameter. [28] III. RESULTS Figure 1 shows the hardness distribution of gold sheet with 85 pct RA achieved after isothermal annealing. The recrystallization temperature of the gold used in this research (99.99 pct) is around 320 C. Considering that pure gold ( pct) recrystallizes at 200 C, it is higher by more than 100 C. Texture is described by an ODF ( 2 section) determined from XRD and EBSD. The rolling and recrystallization textures for gold are represented by two continuous fiber orientations, -fiber (Goss-Brass) and -fiber (Brass-S-Copper). n u mis i [1] 3416 VOLUME 36A, DECEMBER 2005 METALLURGICAL AND MATERIALS TRANSACTIONS A

3 A schematic representation of these fibers in Euler space and major texture components are also found in other work. [2] Figure 2 shows some major orientations in the 2 45 deg section of Euler space. Fig. 1 Hardness variations for cold-rolled gold (85 pct RA) during isothermal annealing at each temperature for 1 h. Gold was provided by the MK Electron Co., R&D Center. Fig. 2 Key texture components in Euler space ( 2 45 deg section). A. Annealing Temperature Effect on Recrystallization In order to investigate the annealing temperature effect on recrystallization, gold sheets with 98 pct RA were used for annealing at 400 C, 500 C, and 600 C for 2 and 24 hours. Figure 3 shows ODFs ( 2 45 deg section) from XRD. Brass, S, Copper, and cube components are prevalent in the ODFs. Rotated cube and Goss orientations have higher ODF values than other components. The rotated cube orientation is typical of shear texture in fcc materials, and the Goss is typical of plane strain compression. As the annealing time increases from 2 to 24 hours, the Brass, S and Copper orientations increase and other components decrease. As the annealing temperature increases, the ODF values at the Brass and S orientations also increase. When comparing the ODF values at the cube orientation at temperatures of 400 C, 500 C, and 600 C, the ODF at 500 C is greatest. The volume fraction of each texture component for the rolling and annealing specimens is shown in Figure 4. Volume fraction can be calculated as in Reference 29. Texture components are assumed to be spherical in shape. The cut-off value is given by a misorientation angle from the exact texture position. In this research, 15 deg was used as a cutoff. The considered texture components are Brass, S, Copper, Goss, cube, rotated cube {100} 011, rotated Goss{110} 110, {111} 112, {112} 110, {122} 411, and {111} 011. The cube component is typical of recrystallization texture in fcc metals. Rotated cube and {111} 011 are shear texture components. {122} 411 is the twin orientation of the rotated cube, which is formed by slip in low-sfe material. The integral of each ODF over Euler space is unity. The rotated cube {100} 011 increases during annealing at 400 C after 2 hours and decreases after 24 hours (a) (b) (c) (d) (e) ( f ) Fig. 3 Recrystallization texture ( 2 45 deg section) of gold sheet with 98 pct RA after annealing. Contours: 1, 2, 3, 5, 10, 20, and 30: (a) 400 C, (b) 500 C, and (c) 600 C for 2 h; and (d) 400 C, (e) 500 C, and ( f ) 600 C for 24 h. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 36A, DECEMBER

4 Fig. 4 Volume fraction of texture components of annealed gold sheet in Fig. 3: (a) 400 C, (b) 500 C, and (c) 600 C for 2 h; and (d) 400 C, (e) 500 C, and (f) 600 C for 24 h. (a) (Figures 4(a) and (d)). However, {122} 411, which is another shear-related orientation, already decreases after annealing at 400 C for 2 hours. Those two shear orientations maintain similar volumes during deformation, as shown in Figure 4 (98 pct RA). Unlike the rotated cube and the {122} 411, Brass, S, Copper, and cube orientations increase with annealing time at 400 C. The rotated cube and {122} 411 also disappear at higher temperatures of 500 C and 600 C for 2 hours (Figures 4(b) and (c)). The ODFs at 500 C and 600 C show the typical -fiber and cube orientations. Those ODFs show minor differences between 2 and 24 hours and the shear textures are negligible. It is thought that the shear components such as rotated cube and {122} 411 on the surface were eliminated by newly recrystallized grains after full annealing conditions; i.e., under higher annealing temperature or longer annealing time, although they were retained in the beginning of annealing process. Grain sizes of the Brass, S and Copper orientations for annealed gold sheets, as shown in Figure 3, were measured indirectly using EBSD and the results are shown in Figure 5. The grain sizes increase with annealing temperature. Annealing temperature affects grain size more than annealing time. The grain sizes of the Brass, S and Copper components for 2 hours are similar to those for 24 hours. The average grain size of all grains is about 10 m during annealing at 600 C for 2 hours, and it increases up to 20 m for 24 hours. Consequently, orientations other than Brass, S, and Copper typically grow in size after 2 hours. (b) Fig. 5 Grain size calculated from EBSD measurements during annealing at different temperatures and times: (a) 2 h and (b) 24 h. B. In-Situ Recrystallization The quasi in-situ evolution of microstructure and texture was investigated using 98 pct cold-rolled gold sheet during isothermal annealing at 400 C. Microstructural evolution was examined for the as-rolled, the partially-recrystallized, and the fully-recrystallized sheets. It is necessary to use some measure for separation of deformation and recrystallization regions, especially for the partially-recrystallized sheet. A grain ID angle and SOS were used for the analysis of partially-recrystallized interstitialfree steel (IF steel) [30] and showed reasonable results. Because the recrystallized grains generally have a smaller orientation gradient inside the grain than the deformed grains, it is possible to find an appropriate grain ID angle and misorientation Fig. 6 Recrystallization and deformation regions can be identified by a grain ID angle and GAM. The fully recrystallized and as-rolled gold sheets were used for this purpose. A grain ID angle of 8 deg and GAM of 1 deg give 90 pct accuracy in distinguishing deformation texture components from the as-rolled sheet and recrystallization texture components from the fully-recrystallized sheets, respectively. measure to separate deformation regions from recrystallized ones. In this research, grain ID angle and GAM were used for that purpose. Figure 6 shows several combinations of grain ID angle and GAM to distinguish the deformed grains from the recrystallized ones in the as-rolled and the 3418 VOLUME 36A, DECEMBER 2005 METALLURGICAL AND MATERIALS TRANSACTIONS A

5 fully-recrystallized gold sheets, respectively. As the grain ID angle increases, one grain has more pixels inside the grain boundary, affecting the GAM or SOS of the grain. A larger grain ID angle favors recognition of deformation, while a smaller grain ID angle favors recognition of recrystallization. Conversely, a larger GAM favors recrystallization, whereas a smaller GAM favors deformation. An appropriate combination is found using a grain ID angle of 8 deg and a GAM of 1.0 deg, giving 90 pct accuracy. These values were used in this work. Figure 7 depicts inverse pole figure (IPF) maps from EBSD for gold sheet for various annealing times at 400 C. The IPF map after 5 minutes (Figure 7(b)) still looks similar to that of the as-rolled state (Figure 7(a)). The IPF map after 18 minutes shows different microstructure and texture from Figures 7(a) and (b). Most regions are subdivided after 18 minutes, and equiaxed grains appear throughout the entire measurement region. After 60 minutes, 4 hours, and 24 hours, grains are growing gradually. The orientation color key is shown in the right bottom side of the figure. The black lines in Figures 7(c) through (f) delimit four grain regions and are provided for cross comparison. Although this experiment was not an ideal in-situ process, it was still possible to keep track of the microstructural evolution during annealing. Figure 8 shows IPF maps with the recrystallized Brass (green) and cube (red) orientations highlighted after annealing times of 5, 18, and 60 minutes. The Copper and S orientations are not shown, but their distributions are similar to those of the Brass. The prominence of the -fiber recrystallization regions is explained by nucleation processes in the deformed -fiber regions: the prevalence of -fiber orientations in the deformed regions produces a high frequency of -fiber nuclei or subgrains. As annealing begins, certain subgrains with Brass, S, or Copper orientations undergo subgrain growth. The driving force for subgrain growth is the energy stored in the subgrain structure, and the number of low-angle grain boundaries decreases. In addition to the -fiber orientations, the cube orientation also increases with annealing. It is thought that the cube consumes its neighbors due to energy advantage over deformed regions. Figure 9(a) shows the proportions of the number of grains of each type (deformed or recrystallized) at various orientations for each annealing time. The proportions were calculated from the IPF map in Figure 7. Each IPF map has approximately the same mapping area and contains about 4000 to 7000 grains depending on annealing time. Overall the proportion of newly recrystallized grains increased at the expense of the deformed grains. After 18 minutes, the proportions of recrystallized grains for the -fiber orientations have become equal to those of the deformed -fiber orientations. After 60 minutes, the Brass and S orientations appear to have saturated. The proportions of Copper and cube components are lower than those of the Brass and S orientations, but continue to increase with annealing time. Figure 9(b) shows the proportions of both recrystallized and deformed grains monotonically increasing up to 80 pct after 4 hours. Figure 10 shows the volume fraction of grain type (deformed or recrystallized) as a function of annealing time. After 18 minutes, about 65 pct of grains are recrystallized, and after 60 minutes, more than 80 pct grains are recrystallized. The recrystallization volume fraction continues to increase gradually up to 24 hours. Considering our criterion for distinguishing recrystallized grains from deformed ones, i.e., grain ID angle and GAM, it is likely that most regions have recrystallized after 60 minutes, and the volume fraction of recrystallized grains is near saturation. The volume fractions of nine texture components are shown in Figure 11. As-rolled sheet has 65 pct volume of -fiber. Other texture components or random orientations make the balance. The total volume of nine texture components decreases with annealing time in Figure 11(a). It is closely related to the decrease of the total volume fraction of the -fiber. Instead, random orientations except those nine components increase. As annealing time increases, the volume of the deformed regions (Figure 11(b)) decreases and the volume of the recrystallized regions increases (Figure 11(c)). Figure 12 shows the misorientation angle distribution calculated from IPF maps in Figure 7. Although a grain ID angle of 8 deg is considered for IPF maps, the misorientation angle distribution is shown down to 2 deg. The loss of lowangle boundaries clearly shows during recrystallization. Misorientation angle fraction peaks around 2 deg in the as-rolled sheet, and it is closely related to grain subdivision during deformation. This peak in the as-rolled sheet decreases after 18 minutes. The peak decreases up to 4 hours annealing, and it increases slightly again after 24 hours. The increase after 24 hours is due to the substructure induced by grain growth. After 18 minutes, other high misorientation angle distributions are found from 50 to 60 deg. High misorientation angle fraction peaks around 60 deg and is related to the -fiber. Two variants of the Brass or Copper orientations often contact each other and have 3 or 60 deg 111 boundary in Table I. High misorientation angles of around 50 deg are also found between cube and Brass, cube and Copper, or cube and S orientations, as seen in Table I. The cube orientation surrounded by -fiber provides the high misorientation angle of 49 deg with the S component and of 57 deg with the Brass or the Copper orientation. Therefore, it seems that the increase of high-angle misorientation results from those major recrystallization texture components. When considering the trends of misorientation angle distribution around 60 deg with respect to annealing time, the fraction increases up to 4 hours but decreases after 24 hours. It is interesting to investigate the spatial distribution of orientations. Here, we focused on cube and Brass orientations and their adjacent orientations. These two orientations are typical of recrystallization textures. Figures 13(a) and (b) show which orientations surround each of the cube and Brass orientations in terms of volume fraction. The volume fraction of the cube orientation is less than 0.5 pct and that of the Brass is 35 pct in the IPF map in Figure 11(a) for the as-rolled state. Although the Brass orientation has the most volume fraction in the deformed matrix, there is more copper orientation than Brass around the cube. The Copper orientation is also found to be more than the S or Goss component around the cube. As annealing time increases, the Copper orientation is not dominant anymore around the cube. The volume fractions of the -fiber adjacent to the cube orientation decrease from 70 pct to 40 to 50 pct, and random orientations occupy the remaining volume in Figure 13(a). The increase of random orientations and the decrease of -fiber are similarly found in both Figures 11(a) and 13(a). The Brass orientation METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 36A, DECEMBER

6 Fig. 7 IPF maps for cold-rolled gold sheet (98 pct RA) after annealing at 400 C according to annealing times. A grain ID angle of 8 deg was used: (a) as-rolled, (b) 5 min, (c) 18 min, (d) 60 min, (e) 4 h, and ( f ) 24 h. Fig. 8 IPF maps for recrystallized Brass and cube orientations according to annealing times (annealing at 400 C): (a) 5 min, (b) 18 min, and (c) 60 min for the Brass orientation; and (d) 5 min, (e) 18 min, and ( f ) 60 min for the cube orientation VOLUME 36A, DECEMBER 2005 METALLURGICAL AND MATERIALS TRANSACTIONS A

7 (a) (b) Fig. 9 Variations of number of grains during annealing at 400 C: (a) proportion of grains for each orientation and (b) proportion of grains for deformation and recrystallization. Fig. 11 Volume fraction evolution of texture components in the different regions during annealing at 400 C. Fig. 10 Volume fraction evolution during annealing at 400 C. is surrounded by other -fiber orientations, the Copper and the S, as seen in Figure 13(b). The volume fractions of the Copper and S orientations make up 60 pct, with the Goss orientation also contributing. As annealing time increases, the volume fractions of the Copper, S, and Goss decrease, and cube and random orientations increase around the Brass. The pixel fraction in Figures 13(c) and (d) shows how many pixels contact the grain of interest. The pixel fraction is related to the periphery of the grain and is defined by the ratio of the number of pixels adjacent to the grain Fig. 12 Misorientation angle distribution during annealing at 400 C. of interest to the total number of pixels along all grain boundaries. For the as-rolled state, about 1 pct of all grain boundary pixels surround the cube orientation (Figure 13(c)). In fact, the cube orientations are found less than other -fiber orientations in the as-rolled state, and the number of pixels adjacent to the cube orientation are small. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 36A, DECEMBER

8 Table I. Misorientations between Typical Texture Components Expressed as Axis/Angle Pairs Notes: 1: (Brass), 2: (Brass), 3: (Copper), 4: (Copper), 5: (S), 6: (S), 7: (S), 8: (Goss), and 9: (cube). (a) (b) (c) (d) Fig. 13 Pixels surrounding cube and Brass orientations. The pixel fraction is defined as the ratio of the number of pixels adjacent to the grain of interest to the total number of pixels on the grain boundaries: (a) volume fractions of orientations adjacent to the cube, (b) volume fractions of orientations adjacent to the Brass, (c) pixel fractions of boundaries adjacent to the cube, and (d) pixel fractions of boundaries adjacent to the Brass. As annealing time increases, the number of pixels adjacent to the cube orientation increases up to 15 pct. The volume fraction of recrystallized Brass is less than that of deformed brass, and the percentage of pixels surrounding the Brass orientation decreases during annealing, as shown in Figure 13(d). IV. DISCUSSION In the cold-rolled gold sheet, the rotated cube and its twin orientations are found on the surface region in addition to other - and -fiber orientations. Although the cube orientation is considered to be the typical recrystallization texture 3422 VOLUME 36A, DECEMBER 2005 METALLURGICAL AND MATERIALS TRANSACTIONS A

9 component in fcc metals and to be unstable under plane strain, some cube orientation is still found on the surface after rolling. It seems that the cube, near cube, and rotated cube orientations comprise a weak {100} fiber on the surface due to shear deformation. The existence of the shear layer on the surface is inferred from the difference in the volume fractions of the shear orientations between XRD (Figure 4(a)) and EBSD (Figure 11(a)) measurements. The XRD specimen was annealed and measured without surface polishing, and exhibited shear texture components on the surface during the annealing process. The EBSD specimen was annealed after mechanical polishing and ion milling, and contained a relatively small surface shear layer during annealing. The -fiber is found more in the interior than on the surface. In this experiment, the recrystallization texture components are the same as the deformation texture components. Subgrain growth is one of the mechanisms that preserve the same texture components during annealing. In the following sections, we discuss subgrain growth, retention of the rotated cube, and recrystallization of cube and -fiber orientations during annealing. A. Dynamic Recovery and Subgrain Growth Sometimes, nuclei for recrystallization are formed from subgrain growth and grow into the surrounding deformed regions. The recrystallization texture components formed in this way are similar to the deformation texture components. Alternatively, discontinuous recrystallization comes from dislocationfree grains or nuclei. These dislocation-free grains grow into the deformed matrix and result in recrystallization textures, which are generally different from deformation textures. Some measurements indicate that deformation textures change little through subgrain growth followed by discontinuous recrystallization. Gold bonding wire textures of 111 and 100 // normal direction (ND) are examples. [25] 111 and 100 texture components persisted after annealing, although their fractions changed. 100 and 111 orientated grains experienced some recovery and subgrain growth without high-angle boundary migration. In the beginning of annealing, subgrains grew in the drawing direction, increasing their aspect ratio. Later in the process, the growth expanded laterally, decreasing the aspect ratio. The migration of high-angle grain boundaries between 100 and 111 fibers occurred through discontinuous recrystallization after the early subgrain growth. In Figure 14, the hardness distribution in cold-rolled gold sheet exhibits a saturation value of around 90 HV after 90 pct RA. The maximum value occurs at 95 pct RA, and the hardness decreases slightly at 97 and 98 pct RA. The decrease in hardness implies the release of the stored energy in the deformed matrix by dynamic recovery. Dynamic recovery can decrease the driving force for formation of recrystallization nuclei, which expand into the deformed region. In this case, subgrain growth plays the dominant role in recrystallization early in the annealing process. B. The Retention of the Rotated Cube Figure 4(a) shows an increase in the rotated cube orientation after annealing at 400 C for 2 hours. After 24 hours, the remaining shear components were consumed by new Fig. 14 Hardness (HV) of cold-rolled gold sheet according to RA: (a) as cast, (b) 75 pct, (c) 90 pct, (d) 92 pct, (e) 95 pct, (f) 97 pct, and (g) 98 pct. grains (Figure 4(d)). Considering the recrystallization temperature of 320 C (Figure 1) and the thickness of gold sheet (200 m), it was expected that 2 hours at 400 C would be sufficient for recrystallization of the gold sheet. The retention of the shear texture components during annealing was also found in the rolled commercial purity aluminum and copper, [31] where it was observed in material with texture inhomogeneity. This stored energy in the material with texture inhomogeneity was also found to be much less than that of the material with homogeneous rolling textures. It implies that the retention of the rotated cube is due to low stored energy. In our experiment, the rotated cube orientation was on the surface of the gold sheet along with -fiber orientations. Although the rotated cube was retained temporarily through low-energy advantage, the newly-recrystallized Brass, S, Copper and Goss orientations replaced it after full annealing. The misorientation angles between the rotated cube and the Brass, S, Copper, or Goss orientations have high values of 46, 38.7, 35.3, and 62.8 deg, respectively. C. Recrystallization of Cube Orientation From the viewpoint of oriented nucleation, cube nuclei can grow into the deformation region through energy advantage. Dillamore et al. predicted that transition bands would form and contain the cube orientation. [11] Cube-orientated grains have also been found experimentally in the transition bands of deformed copper and aluminum. In cold-rolled copper, extensive recovery occurred in the cube-oriented cells, and shear bands contributed to the sharp cube recrystallization texture. [12] The nuclei of the random orientations within the shear bands weakened the cube orientation by destruction of the elongated cube nuclei. In aluminum, the cube-oriented grains were nucleated from transition bands preferentially located in the Copper or ND-rotated Copper orientations. [32] Similarly, the Copper orientation in gold sheet mainly forms adjacent to the cube orientation, as seen in Figure 13(a). As pointed out previously, the volume fraction of the cube orientation varies with the measuring position. The volume fraction of the cube orientation in the gold sheet with surface shear layer (Figure 4) changed from 3.1 pct (as-rolled) to 3.5 pct (2 hours) to 5.8 pct (24 hours). That of the cube METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 36A, DECEMBER

10 orientation in the gold sheet without surface shear layer (Figure 11(a)) varied from 0.3 pct (as-rolled) to 5.5 pct (1 hour) to 5.9 pct (24 hours). The volume fraction of the cube orientation after 24 hours does not depend on the initial volume of the cube directly. It seems that the cube orientation in the surface shear layer is not the major source of the recrystallized cube during in-situ experiment. D. Recrystallization of -Fiber It is generally accepted that the new grains for recrystallization do not nucleate as totally new grains through atom by atom construction. New grains grow from small regions, recovered subgrains, or cells, which are already present in the deformed microstructure. The orientation of each new grain arises from the same orientation present in the deformed state. After rolling of gold sheet, deformed -fiber orientations are prevalent and provide the source of recrystallized -fiber components. The generation of the recrystallized -fiber is different from that of the recrystallized cube. The recrystallization behavior of the -fiber orientation is understood by subgrain growth followed by discontinuous recrystallization as in gold bonding wire. Subgrain growth accompanies low-angle grain boundary migration. In Figure 12, the misorientation angle distribution for the as-rolled sheet has a peak around 2 deg. In the beginning of annealing, the low-angle grain boundary fraction decreased, mainly due to subgrain growth. After 18 minutes, the low misorientation angle peak continued to decrease, and another peak appeared around 50 deg. This means that new grains started to grow into the deformed region. The typical recrystallization with nucleation and discontinuous recrystallization already started after 18 minutes. Discontinuous recrystallization has been shown in Figures 7(c) through (f), in contrast to Figures 7(a) and (b). These trends are also found in gold bonding wire. Sometimes, a second phase or a precipitate inhibits nucleation and high-angle grain boundary migration, resulting in the retention of rolling texture. However, this is not the case for single-phase pure gold sheet (99.99 wt pct). Instead, we can focus on the influence of solute elements on the retention of deformation textures as nuclei. The addition of Mo in solid solution in 316L leads to the homogenization of grain boundary velocity, and {110} 112 and {110} 001 rolling texture components are retained through the discontinuous recrystallization process. [15] In pure copper, the addition of dilute phosphorus significantly modifies the annealing texture. [33] It is also known that the homogenization of grain boundary velocity induced by dopant results in the homogenous volume, grain size, and shape of most annealing textures. Some dopant (Be, Ca, and La) added to the gold in this research also could affect the grain boundary properties, considering that random textures in the gold sheet account for 50 pct of the volume fraction, as shown in Figure 11(a). The dopant effect on recrystallization of gold sheet has not yet been investigated. V. CONCLUSION In order to investigate the recrystallization and grain growth of cold-rolled gold sheet, microstructure and texture were measured with EBSD and XRD after rolling and annealing. 1. The rotated cube {100} 011 and its twin were found on the surface region in addition to - and -fibers after rolling. The {100} fiber consisted of shear textures on the surface. The -fiber was found more inside than on the surface of the gold sheet. 2. Most shear texture components disappeared in the beginning of annealing, except rotated cube. The rotated cube component remained on the surface for 2 hours at 400 C. It was ultimately consumed by other recrystallized grains after annealing for 24 hours at 400 C. 3. Dynamic recovery or restoration process occurred during gold sheet rolling. This resulted in subgrain growth early in the annealing process. 4. Annealing experiments at 400 C for 5 minutes, 18 minutes, 60 minutes, 4 hours, and 24 hours were carried out for the in-situ investigation of microstructure and texture evolution. Newly-recrystallized grains contain the -fiber, cube, and other random orientations in the IPF map. Most of these come from the deformed -fiber regions. 5. The nuclei for -fibers seem to develop by subgrain growth, and their high initial frequency leads to a high frequency of recrystallized grains with -fiber orientations. The cube orientation in texture in homogeneity will grow into new grains due to low-energy advantage. 6. The cube orientations are mainly surrounded by the Brass, S, and Copper orientations in both deformed and recrystallized states. 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