Recrystallization and Grain Growth of Cold-Rolled Gold Sheetengineering.snu.ac.kr/pdf/2005(22)/2005_CJH... · 2014-07-29 · Recrystallization and Grain Growth of Cold-Rolled Gold

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). Goldwith some dopants (Be, Ca, and La) was used in this research and its recrystallization temperaturewas 320 °C. Isothermal annealing experiments at 400 °C, 500 °C, and 600 °C were carried out forthe 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 sur-face. Shear texture components remained on the surface for 2 hours at 400 °C and were consumedby other recrystallized grains after 24 hours at 400 °C. Microstructure and texture evolution duringin-situ annealing at 400 °C were investigated from the cold-rolled state to the fully recrystallized stateusing EBSD. Most of the newly, recrystallized grains came from the deformed �-fiber regions andconsisted 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) anddeformation temperature are prevalent controlling factors.[1–4]

The formation of cold-rolling textures in fcc metals has beenreviewed by Hirsch and Lücke.[1,2] Without twinning, thegeneral cold-rolling texture components for medium-highSFE materials are Copper {112}�111�, S{123}�634�,Brass{110}�112� and Goss{110}�001�. In particular,metals with the Copper-type deformation texture havebeen extensively studied with the purpose of understandingmicrostructure and texture evolution during rolling andannealing.

The annealing process consists of recovery, recrystallization,and grain growth.[5,6] During recovery, stored energy is releasedwithout high-angle grain boundary (GB) migration. Recrys-tallization is also driven by stored deformation energy but isaccompanied by high-angle GB migration. Grain growth alsoinvolves the migration of grain boundaries, and the drivingforce for that is the reduction of the GB area.

The recrystallization textures of fcc metals also have beeninvestigated in many articles.[7–11] Most articles have focusedon the formation of recrystallized cube {100}�001� fromthe Copper-type deformation texture. Although the orientationdistribution function (ODF) for many wrought fcc metalsand alloys of medium-high SFE shows little indication ofcube component, it develops remarkably during recrystalli-zation. It has been known that the recrystallization textureis determined mainly by the orientation and growth rate ofthe nuclei.

Sometimes, the deformation texture is similar to that ofrecrystallization. 316L austenitic stainless steel (17.75 wtpct Cr, 12.6 wt pct Ni, 2.38 wt pct Mo) shows similar texture

components in both deformation and recrystallization.[12]

Cold rolling textures of 316L show mainly the Brass ori-entation with a spread toward the Goss orientation. The reten-tion of the Brass orientation is attributed to oriented nucleationand to the inhibition of further selectively oriented growth bya strong solute effect of Molybdenum. In Mo-free austeniticstainless steels (18 wt pct Cr, 9 wt pct Ni), however, thegeneral fcc rolling texture changes into different texturesafter annealing. Some shear textures formed on the surfaceshow the retention of deformation texture or more compli-cated recrystallized behaviors during annealing.[13,14] Consid-ering that surface shear is formed in a relatively narrowlayer, the recrystallization kinetics for the surface and forthe center of materials are different from each other.

Although extensive studies have been carried out on rollingand recrystallization textures for fcc metals and alloys, studieson gold are rare. Kitagawa reported that the texture develop-ment of gold leaf (0.1-�m thickness) fabricated by thetraditional pack and hammering method was (001) texture.[15]

This is different from the general fcc plane compressiontexture, and they pointed out that it comes from cross-slip.

High-purity gold (99.9999 wt pct Au) is too soft andunstable to obtain good properties for industrial use, i.e.,bonding wire. The annealing and recrystallization tempera-ture for pure gold is in the range of 150 °C to 200 °C, andit has been reported that highly deformed pure gold willshow recovery and recrystalization at room temperature.[16]

Therefore, bonding wire commonly has various dopants inthe parts per million (ppm) level in order to control theannealing response and to obtain better thermal and mechan-ical properties. Impurities, even at these low levels, areimportant for controlling the final mechanical properties andmicrostructures of gold and other materials by raising therecrystallization temperature and preventing graingrowth.[17–20] Microstructure, texture, and mechanical prop-erties of gold wires were reported during annealing.[21–24]

Due to the strong elastic anisotropy in gold (E�111�/E�100�

� 115 GPa/42 GPa), the change in elastic modulus was usedto detect texture change during recrystallization. Recently,Cho et al. reported recrystallization and grain growth of goldbonding wire during isothermal annealing at 300 °C and 400 °Cusing electron backscatter diffraction (EBSD).[25]

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 36A, DECEMBER 2005—3415

JAE-HYUNG CHO, Research Associate, is with the Sibley School ofMechanical and Aerospace Engineering, Cornell University, Ithaca, NY14853.H.-P. HA, Senior Researcher, is with the Metal Processing Center,Korea Institute of Science & Technology, Seoul 130-650, Korea. K.H. OH,Professor, is with the School of Materials Science & Engineering, Collegeof Engineering, Seoul National University, Seoul 151-744, Korea. Contacte-mail: [email protected]

Manuscript submitted June 28, 2004.

3416—VOLUME 36A, DECEMBER 2005 METALLURGICAL AND MATERIALS TRANSACTIONS A

In cold-rolled gold sheet, the authors found that deformationtextures consist of the general �-fiber (Brass-S-Copper) andsome shear textures on the surface. Annealing textures havethe same �-fiber in addition to cube and random orientation.The rolling and recrystallization textures and the microstruc-ture of gold sheets were analyzed in detail. X-ray diffraction(XRD) and EBSD were used to investigate the texture andmicrostructure evolution during annealing.

II. EXPERIMENTS

A. Material and Sample Preparation

The gold for this research was fabricated for gold bond-ing wire and its purity was more than 99.99 pct. It has someintentional dopants of Be, Ca, and La, less than 50 parts permillion (ppm) by weight. Those dopants are added to increasethe strength of gold bonding wire. The existence of dopantsin gold can affect recrystallization and grain growth eventhough they are at the ppm level. The initial specimen hada 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 anneal-ing experiments followed by XRD and EBSD measurements.

B. Annealing Experiments

The recrystallization temperature of the gold was inferredfrom the measured hardness profile of 85 pct RA sheet afterisothermal annealing for 1 hour. In order to investigate tex-ture evolution from deformation to recrystallization, two cat-egories of annealing experiments were carried out. First,annealing temperature effects were examined at tempera-tures of 400 °C, 500 °C, and 600 °C. In addition, quasi in-situ recrystallization processes were examined from theas-deformed state (98 pct RA) to full recrystallization accord-ing to annealing time. The annealing temperature for thequasi in-situ experiments was fixed at 400 °C, and the anneal-ing times were 5 minutes, 18 minutes, 60 minutes, 4 hours,and 24 hours. One specimen with a reference line was takenand used for the repeated annealing and EBSD measure-ments. The measuring position varied slightly, even with areference 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-rolledspecimens using the XRD method with Cu K�. The XRDspecimens were measured without polishing, leaving thesurface shear region undisturbed. The ODF of the crystal-lite was calculated using the WIMV* method,[26] assuming

*The WIMV with an automated conditional ghost correction suggestedby Matthies and Vinel is a method of ODF reproduction from pole fig-ures. It is based on the analysis of the structure of the exact solution ofthe central problem, on the analytical properties of the ghosts problem, andon the use of the most constructive elements of earlier reproduction activ-ities by Williams and Imhof. With reference to these authors, it bears theacronym WIMV.

orthorhombic sample symmetry. Such symmetry requiresthe 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 byREDS (Reprocessing of EBSD Data in SNU).[27] The mechan-ically polished EBSD specimens were cleaned with ionmilling. The operating voltage was 20 kV and the probe cur-rent was 4 nA. The step size of the EBSD scans was 2 �m.

To identify a grain in EBSD data, it is necessary to deter-mine its perimeter (grain boundary) and the average orien-tation within it. Misorientation angles between adjacentpixels are used for grain identification (ID). Any two adja-cent pixels with a grain ID angle smaller than the cut-offvalue are considered part of the same grain. Most deformedor recrystallized grains have subgrain structures, and theiroverall structures can be described by the misorientationmeasures calculated over a set of pixels contained within thegrain. There are three types of misorientation measures com-monly used in a grain. First, grain orientation spread (GOS)expresses the magnitude of misorientation among all pixelsin a grain. Second, scalar orientation spread (SOS) iscalculated between each pixel and the average orientation.Grain average misorientation (GAM) is the quantitycalculated from adjacent pixels only, and gives informationabout the nearest neighbor correlations. The GAM value isgenerally smaller than GOS or SOS. Considering Pi as anorientation at a point (xi) and Pj as another orientation atadjacent point (xj) in a grain, the GAM can be calculatedwith misorientation angle, which is given for two adjacentorientations at Pi and Pj,

[1]

where

Here, S is the symmetry operator belonging to the appro-priate crystal class concerned and the subscripts in rotationP refer only to position. To calculate the grain size, the num-ber of data points or pixels in a grain is calculated, and usingthe known pixel step size, the grain area is determined.The most convenient measure of grain size from grain areais the equivalent circle diameter.[28]

III. RESULTS

Figure 1 shows the hardness distribution of gold sheetwith 85 pct RA achieved after isothermal annealing. Therecrystallization temperature of the gold used in this research(99.99 pct) is around 320 °C. Considering that pure gold(99.999 pct) recrystallizes at 200 °C, it is higher by morethan 100 °C.

Texture is described by an ODF (2 section) determinedfrom XRD and EBSD. The rolling and recrystallizationtextures for gold are represented by two continuous fiber ori-entations, �-fiber (Goss-Brass) and �-fiber (Brass-S-Copper).

umis � min c acos a trace ((Pi

# Pj�1) # S) � 1

2b d

GAM �a

n

iu

misi

n


Fig. 1—Hardness variations for cold-rolled gold (85 pct RA) duringisothermal annealing at each temperature for 1 h. Gold was provided bythe MK Electron Co., R&D Center.

Fig. 2—Key texture components in Euler space (2 � 45 deg section).

A schematic representation of these fibers in Euler space andmajor texture components are also found in other work.[2]

Figure 2 shows some major orientations in the 2 � 45 degsection of Euler space.

A. Annealing Temperature Effect on Recrystallization

In order to investigate the annealing temperature effecton recrystallization, gold sheets with 98 pct RA were usedfor annealing at 400 °C, 500 °C, and 600 °C for 2 and24 hours. Figure 3 shows ODFs (2 � 45 deg section) fromXRD. Brass, S, Copper, and cube components are prevalentin the ODFs. Rotated cube and Goss orientations havehigher ODF values than other components. The rotatedcube orientation is typical of shear texture in fcc materials,and the Goss is typical of plane strain compression. As theannealing time increases from 2 to 24 hours, the Brass, Sand Copper orientations increase and other componentsdecrease. As the annealing temperature increases, the ODFvalues at the Brass and S orientations also increase. Whencomparing the ODF values at the cube orientation attemperatures of 400 °C, 500 °C, and 600 °C, the ODF at500 °C is greatest.

The volume fraction of each texture component for therolling 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 fromthe exact texture position. In this research, 15 deg wasused as a cutoff. The considered texture components areBrass, S, Copper, Goss, cube, rotated cube {100}�011�,rotated Goss{110}�110�, {111}�112�, {112}�110�,{122}�411�, and {111}�011�. The cube component istypical of recrystallization texture in fcc metals. Rotatedcube 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 integralof each ODF over Euler space is unity.

The rotated cube {100}�011� increases during anneal-ing at 400 °C after 2 hours and decreases after 24 hours

(a)

(d) (e) ( f )

(b) (c)

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.


Fig. 4—Volume fraction of texture components of annealed gold sheetin 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.

Fig. 5—Grain size calculated from EBSD measurements during annealingat different temperatures and times: (a) 2 h and (b) 24 h.

(a)

(b)

(Figures 4(a) and (d)). However, {122}�411�, which isanother shear-related orientation, already decreases afterannealing at 400 °C for 2 hours. Those two shear orienta-tions maintain similar volumes during deformation, as shownin Figure 4 (98 pct RA). Unlike the rotated cube and the{122}�411�, Brass, S, Copper, and cube orientationsincrease with annealing time at 400 °C. The rotated cubeand {122}�411� also disappear at higher temperatures of500 °C and 600 °C for 2 hours (Figures 4(b) and (c)). TheODFs at 500 °C and 600 °C show the typical �-fiber andcube orientations. Those ODFs show minor differencesbetween 2 and 24 hours and the shear textures are negligi-ble. It is thought that the shear components such as rotatedcube and {122}�411� on the surface were eliminated bynewly recrystallized grains after full annealing conditions;i.e., under higher annealing temperature or longer annealingtime, although they were retained in the beginning of anneal-ing process.

Grain sizes of the Brass, S and Copper orientations forannealed gold sheets, as shown in Figure 3, were measuredindirectly using EBSD and the results are shown in Figure 5.The grain sizes increase with annealing temperature. Anneal-ing temperature affects grain size more than annealing time.The grain sizes of the Brass, S and Copper components for2 hours are similar to those for 24 hours. The average grainsize of all grains is about 10 �m during annealing at 600 °Cfor 2 hours, and it increases up to 20 �m for 24 hours. Con-sequently, orientations other than Brass, S, and Copper typ-ically grow in size after 2 hours.

B. In-Situ Recrystallization

The quasi in-situ evolution of microstructure and texturewas investigated using 98 pct cold-rolled gold sheet duringisothermal annealing at 400 °C. Microstructural evolutionwas examined for the as-rolled, the partially-recrystallized,and the fully-recrystallized sheets.

It is necessary to use some measure for separation of defor-mation and recrystallization regions, especially for the par-tially-recrystallized sheet. A grain ID angle and SOS wereused for the analysis of partially-recrystallized interstitial-free steel (IF steel)[30] and showed reasonable results. Becausethe recrystallized grains generally have a smaller orientationgradient inside the grain than the deformed grains, it is pos-sible to find an appropriate grain ID angle and misorientation

measure to separate deformation regions from recrystallizedones. In this research, grain ID angle and GAM were usedfor that purpose. Figure 6 shows several combinations ofgrain ID angle and GAM to distinguish the deformedgrains from the recrystallized ones in the as-rolled and the

Fig. 6—Recrystallization and deformation regions can be identified by agrain ID angle and GAM. The fully recrystallized and as-rolled gold sheetswere used for this purpose. A grain ID angle of 8 deg and GAM of 1 deggive 90 pct accuracy in distinguishing deformation texture componentsfrom the as-rolled sheet and recrystallization texture components from thefully-recrystallized sheets, respectively.


fully-recrystallized gold sheets, respectively. As the grainID angle increases, one grain has more pixels inside thegrain 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 recrys-tallization. Conversely, a larger GAM favors recrystallization,whereas a smaller GAM favors deformation. An appropriatecombination is found using a grain ID angle of 8 deg anda GAM of 1.0 deg, giving 90 pct accuracy. These valueswere used in this work.

Figure 7 depicts inverse pole figure (IPF) maps fromEBSD for gold sheet for various annealing times at 400 °C.The IPF map after 5 minutes (Figure 7(b)) still looks sim-ilar to that of the as-rolled state (Figure 7(a)). The IPF mapafter 18 minutes shows different microstructure and tex-ture from Figures 7(a) and (b). Most regions are subdividedafter 18 minutes, and equiaxed grains appear throughout theentire measurement region. After 60 minutes, 4 hours, and24 hours, grains are growing gradually. The orientation colorkey is shown in the right bottom side of the figure. Theblack lines in Figures 7(c) through (f) delimit four grainregions and are provided for cross comparison. Althoughthis experiment was not an ideal in-situ process, it was stillpossible to keep track of the microstructural evolution duringannealing.

Figure 8 shows IPF maps with the recrystallized Brass(green) and cube (red) orientations highlighted after anneal-ing times of 5, 18, and 60 minutes. The Copper and S ori-entations are not shown, but their distributions are similarto those of the Brass. The prominence of the �-fiber recrys-tallization regions is explained by nucleation processes inthe deformed �-fiber regions: the prevalence of �-fiber ori-entations in the deformed regions produces a high frequencyof �-fiber nuclei or subgrains. As annealing begins, certainsubgrains with Brass, S, or Copper orientations undergo sub-grain growth. The driving force for subgrain growth is theenergy stored in the subgrain structure, and the number oflow-angle grain boundaries decreases. In addition to the�-fiber orientations, the cube orientation also increases withannealing. It is thought that the cube consumes its neighborsdue to energy advantage over deformed regions.

Figure 9(a) shows the proportions of the number of grainsof each type (deformed or recrystallized) at various orien-tations for each annealing time. The proportions were cal-culated from the IPF map in Figure 7. Each IPF map hasapproximately the same mapping area and contains about4000 to 7000 grains depending on annealing time. Overallthe proportion of newly recrystallized grains increased atthe expense of the deformed grains. After 18 minutes, theproportions of recrystallized grains for the �-fiber orientationshave become equal to those of the deformed �-fiberorientations. After 60 minutes, the Brass and S orientationsappear to have saturated. The proportions of Copper andcube components are lower than those of the Brass and Sorientations, but continue to increase with annealing time.Figure 9(b) shows the proportions of both recrystallized anddeformed grains monotonically increasing up to 80 pct after4 hours.

Figure 10 shows the volume fraction of grain type (deformedor recrystallized) as a function of annealing time. After18 minutes, about 65 pct of grains are recrystallized, andafter 60 minutes, more than 80 pct grains are recrystallized.

The recrystallization volume fraction continues to increasegradually up to 24 hours. Considering our criterion for dis-tinguishing recrystallized grains from deformed ones, i.e.,grain ID angle and GAM, it is likely that most regions haverecrystallized after 60 minutes, and the volume fraction ofrecrystallized grains is near saturation.

The volume fractions of nine texture components are shownin Figure 11. As-rolled sheet has 65 pct volume of �-fiber.Other texture components or random orientations make thebalance. The total volume of nine texture componentsdecreases with annealing time in Figure 11(a). It is closelyrelated to the decrease of the total volume fraction of the�-fiber. Instead, random orientations except those nine com-ponents increase. As annealing time increases, the volumeof the deformed regions (Figure 11(b)) decreases and the vol-ume of the recrystallized regions increases (Figure 11(c)).

Figure 12 shows the misorientation angle distributioncalculated from IPF maps in Figure 7. Although a grain IDangle of 8 deg is considered for IPF maps, the misorientationangle distribution is shown down to 2 deg. The loss of low-angle boundaries clearly shows during recrystallization.Misorientation angle fraction peaks around 2 deg in theas-rolled sheet, and it is closely related to grain subdivisionduring deformation. This peak in the as-rolled sheet decreasesafter 18 minutes. The peak decreases up to 4 hours annealing,and it increases slightly again after 24 hours. The increaseafter 24 hours is due to the substructure induced by graingrowth. After 18 minutes, other high misorientation angledistributions are found from 50 to 60 deg. High misorientationangle fraction peaks around 60 deg and is related to the�-fiber. Two variants of the Brass or Copper orientationsoften contact each other and have 3 or 60 deg �111�boundary in Table I. High misorientation angles of around50 deg are also found between cube and Brass, cube andCopper, or cube and S orientations, as seen in Table I. Thecube orientation surrounded by �-fiber provides the high mis-orientation angle of 49 deg with the S component and of57 deg with the Brass or the Copper orientation. Therefore,it seems that the increase of high-angle misorientation resultsfrom those major recrystallization texture components. Whenconsidering the trends of misorientation angle distributionaround 60 deg with respect to annealing time, the fractionincreases up to 4 hours but decreases after 24 hours.

It is interesting to investigate the spatial distribution of ori-entations. Here, we focused on cube and Brass orientationsand their adjacent orientations. These two orientations aretypical of recrystallization textures. Figures 13(a) and (b)show which orientations surround each of the cube and Brassorientations in terms of volume fraction. The volume fractionof the cube orientation is less than 0.5 pct and that of theBrass is 35 pct in the IPF map in Figure 11(a) for the as-rolledstate. Although the Brass orientation has the most volumefraction in the deformed matrix, there is more copper orien-tation than Brass around the cube. The Copper orientationis also found to be more than the S or Goss component aroundthe cube. As annealing time increases, the Copper orientationis not dominant anymore around the cube. The volume frac-tions of the �-fiber adjacent to the cube orientation decreasefrom 70 pct to 40 to 50 pct, and random orientations occupythe remaining volume in Figure 13(a). The increase of ran-dom orientations and the decrease of �-fiber are similarlyfound in both Figures 11(a) and 13(a). The Brass orientation


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 minfor the Brass orientation; and (d) 5 min, (e) 18 min, and ( f ) 60 min for the cube orientation.

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.


of interest to the total number of pixels along all grainboundaries. For the as-rolled state, about 1 pct of all grainboundary pixels surround the cube orientation (Figure13(c)). In fact, the cube orientations are found less thanother �-fiber orientations in the as-rolled state, and thenumber of pixels adjacent to the cube orientation are small.

Fig. 11—Volume fraction evolution of texture components in the differ-ent regions during annealing at 400 °C.

Fig. 12—Misorientation angle distribution during annealing at 400 °C.

is surrounded by other �-fiber orientations, the Copper andthe S, as seen in Figure 13(b). The volume fractions of theCopper and S orientations make up 60 pct, with the Gossorientation also contributing. As annealing time increases,the volume fractions of the Copper, S, and Goss decrease, andcube and random orientations increase around the Brass.

The pixel fraction in Figures 13(c) and (d) shows howmany pixels contact the grain of interest. The pixel frac-tion is related to the periphery of the grain and is definedby the ratio of the number of pixels adjacent to the grain

(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 grainsfor deformation and recrystallization.

Fig. 10—Volume fraction evolution during annealing at 400 °C.


IV. DISCUSSION

In the cold-rolled gold sheet, the rotated cube and its twinorientations are found on the surface region in addition toother �- and �-fiber orientations. Although the cube orien-tation is considered to be the typical recrystallization texture

Table I. Misorientations between Typical Texture Components Expressed as Axis/Angle Pairs

1 2 3 4 5 6 7 8 9

1 60.0 35.6 35.6 19.4 19.4 53.7 35.3 56.6 �111� �80605� �56080� �685943� �435968� �126278� �101� �597724�

2 35.6 35.6 53.7 53.7 19.4 35.3 56.6 �58060� �80460� �127862� �781262� �684359� �110� �245977�

3 60 19.4 51.5 51.5 54.7 56.6 �111� �726035� �765735� �577631� �110� �245977�

4 51.5 19.4 19.4 54.7 56.6 �573176� �603572� �723560� �101� �597725�

5 38.2 50.2 43 48.6 �111� �436464� �823745� �525664�

6 38.6 43.0 48.6 �663766� �374582� �566452�

7 43 48.6 �824537� �526456�

8 45 �001�

9

Notes: 1: 54.74 45 0 (Brass), 2: 35.26 90 45 (Brass), 3: 0 35.26 45 (Copper), 4: 50.77 65.9 63.43 (Copper), 5: 31.02 36.7 26.57 (S), 6: 62.97 57.6971.56 (S), 7: 37.13 74.49 56.31 (S), 8: 0 90 45 (Goss), and 9: 0 0 0 (cube).

(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 interestto the total number of pixels on the grain boundaries: (a) volume fractions of orientations adjacent to the cube, (b) volume fractions of orientations adja-cent to the Brass, (c) pixel fractions of boundaries adjacent to the cube, and (d) pixel fractions of boundaries adjacent to the Brass.

(a)

As annealing time increases, the number of pixels adja-cent to the cube orientation increases up to 15 pct. Thevolume fraction of recrystallized Brass is less than that ofdeformed brass, and the percentage of pixels surroundingthe Brass orientation decreases during annealing, as shownin Figure 13(d).


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 orienta-tions comprise a weak {100} fiber on the surface due to sheardeformation. The existence of the shear layer on the surfaceis inferred from the difference in the volume fractions ofthe shear orientations between XRD (Figure 4(a)) and EBSD(Figure 11(a)) measurements. The XRD specimen wasannealed and measured without surface polishing, and exhib-ited shear texture components on the surface during theannealing process. The EBSD specimen was annealed aftermechanical polishing and ion milling, and contained a rela-tively small surface shear layer during annealing. The �-fiberis found more in the interior than on the surface.

In this experiment, the recrystallization texture componentsare the same as the deformation texture components. Subgraingrowth is one of the mechanisms that preserve the sametexture components during annealing. In the followingsections, we discuss subgrain growth, retention of the rotatedcube, and recrystallization of cube and �-fiber orientationsduring annealing.

A. Dynamic Recovery and Subgrain Growth

Sometimes, nuclei for recrystallization are formed from sub-grain growth and grow into the surrounding deformed regions.The recrystallization texture components formed in this wayare similar to the deformation texture components. Alterna-tively, discontinuous recrystallization comes from dislocation-free grains or nuclei. These dislocation-free grains grow intothe deformed matrix and result in recrystallization textures,which are generally different from deformation textures.

Some measurements indicate that deformation textureschange little through subgrain growth followed by discon-tinuous recrystallization. Gold bonding wire textures of�111� and �100�// normal direction (ND) are examples.[25]

�111� and �100� texture components persisted afterannealing, although their fractions changed. �100� and�111� orientated grains experienced some recovery andsubgrain growth without high-angle boundary migration.In the beginning of annealing, subgrains grew in the draw-ing direction, increasing their aspect ratio. Later in theprocess, the growth expanded laterally, decreasing the aspectratio. The migration of high-angle grain boundaries between�100� and �111� fibers occurred through discontinuousrecrystallization after the early subgrain growth.

In Figure 14, the hardness distribution in cold-rolled goldsheet exhibits a saturation value of around 90 HV after 90 pctRA. The maximum value occurs at 95 pct RA, and thehardness decreases slightly at 97 and 98 pct RA. The decreasein hardness implies the release of the stored energy in thedeformed matrix by dynamic recovery. Dynamic recoverycan decrease the driving force for formation of recrystal-lization nuclei, which expand into the deformed region. Inthis case, subgrain growth plays the dominant role in recrys-tallization early in the annealing process.

B. The Retention of the Rotated Cube

Figure 4(a) shows an increase in the rotated cube orien-tation after annealing at 400 °C for 2 hours. After 24 hours,the remaining shear components were consumed by new

grains (Figure 4(d)). Considering the recrystallizationtemperature of 320 °C (Figure 1) and the thickness of goldsheet (200 �m), it was expected that 2 hours at 400 °C wouldbe sufficient for recrystallization of the gold sheet.

The retention of the shear texture components duringannealing was also found in the rolled commercial purityaluminum and copper,[31] where it was observed in materialwith texture inhomogeneity. This stored energy in thematerial with texture inhomogeneity was also found to bemuch less than that of the material with homogeneous rollingtextures. It implies that the retention of the rotated cube isdue to low stored energy. In our experiment, the rotatedcube orientation was on the surface of the gold sheet alongwith �-fiber orientations. Although the rotated cube wasretained temporarily through low-energy advantage, thenewly-recrystallized Brass, S, Copper and Goss orientationsreplaced it after full annealing. The misorientation anglesbetween the rotated cube and the Brass, S, Copper, or Gossorientations 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 cangrow into the deformation region through energy advantage.Dillamore et al. predicted that transition bands would formand contain the cube orientation.[11] Cube-orientated grainshave also been found experimentally in the transition bandsof deformed copper and aluminum. In cold-rolled copper,extensive recovery occurred in the cube-oriented cells, andshear bands contributed to the sharp cube recrystallizationtexture.[12] The nuclei of the random orientations within theshear bands weakened the cube orientation by destruction ofthe elongated cube nuclei. In aluminum, the cube-orientedgrains were nucleated from transition bands preferentiallylocated in the Copper or ND-rotated Copper orientations.[32]

Similarly, the Copper orientation in gold sheet mainly formsadjacent to the cube orientation, as seen in Figure 13(a).

As pointed out previously, the volume fraction of the cubeorientation varies with the measuring position. The volumefraction of the cube orientation in the gold sheet with surfaceshear layer (Figure 4) changed from 3.1 pct (as-rolled) to3.5 pct (2 hours) to 5.8 pct (24 hours). That of the cube

Fig. 14—Hardness (HV) of cold-rolled gold sheet according to RA: (a) ascast, (b) 75 pct, (c) 90 pct, (d) 92 pct, (e) 95 pct, (f) 97 pct, and (g) 98 pct.


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 thecube orientation after 24 hours does not depend on the initialvolume of the cube directly. It seems that the cube orientationin the surface shear layer is not the major source of therecrystallized cube during in-situ experiment.

D. Recrystallization of �-Fiber

It is generally accepted that the new grains for recrystal-lization do not “nucleate” as totally new grains through atomby atom construction. New grains grow from small regions,recovered subgrains, or cells, which are already present inthe deformed microstructure. The orientation of each newgrain arises from the same orientation present in the deformedstate. After rolling of gold sheet, deformed �-fiber orienta-tions 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 isunderstood by subgrain growth followed by discontinuousrecrystallization as in gold bonding wire. Subgrain growthaccompanies low-angle grain boundary migration. In Figure 12,the misorientation angle distribution for the as-rolled sheet hasa peak around 2 deg. In the beginning of annealing, thelow-angle grain boundary fraction decreased, mainly due tosubgrain growth. After 18 minutes, the low misorientationangle peak continued to decrease, and another peak appearedaround 50 deg. This means that new grains started to growinto the deformed region. The typical recrystallization withnucleation and discontinuous recrystallization already startedafter 18 minutes. Discontinuous recrystallization has beenshown 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 inhibitsnucleation and high-angle grain boundary migration, resultingin the retention of rolling texture. However, this is notthe case for single-phase pure gold sheet (99.99 wt pct).Instead, we can focus on the influence of solute elementson the retention of deformation textures as nuclei. Theaddition of Mo in solid solution in 316L leads to the homog-enization of grain boundary velocity, and {110}�112� and{110}�001� rolling texture components are retainedthrough the discontinuous recrystallization process.[15] Inpure copper, the addition of dilute phosphorus significantlymodifies the annealing texture.[33] It is also known that thehomogenization of grain boundary velocity induced bydopant results in the homogenous volume, grain size, andshape of most annealing textures. Some dopant (Be, Ca, andLa) added to the gold in this research also could affect thegrain boundary properties, considering that random texturesin the gold sheet account for 50 pct of the volume fraction,as shown in Figure 11(a). The dopant effect on recrystal-lization of gold sheet has not yet been investigated.

V. CONCLUSION

In order to investigate the recrystallization and grain growthof cold-rolled gold sheet, microstructure and texture weremeasured with EBSD and XRD after rolling and annealing.

1. The rotated cube {100}�011� and its twin were foundon the surface region in addition to �- and �-fibers afterrolling. The {100} fiber consisted of shear textures onthe surface. The �-fiber was found more inside than onthe surface of the gold sheet.

2. Most shear texture components disappeared in the begin-ning of annealing, except rotated cube. The rotated cubecomponent remained on the surface for 2 hours at 400 °C.It was ultimately consumed by other recrystallized grainsafter annealing for 24 hours at 400 °C.

3. Dynamic recovery or restoration process occurred duringgold sheet rolling. This resulted in subgrain growth earlyin the annealing process.

4. Annealing experiments at 400 °C for 5 minutes, 18 min-utes, 60 minutes, 4 hours, and 24 hours were carried outfor the in-situ investigation of microstructure and textureevolution. Newly-recrystallized grains contain the �-fiber,cube, and other random orientations in the IPF map. Mostof 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 frequencyof recrystallized grains with �-fiber orientations. The cubeorientation in texture in homogeneity will grow into newgrains due to low-energy advantage.

6. The cube orientations are mainly surrounded by the Brass,S, and Copper orientations in both deformed and recrys-tallized states. The Copper orientation is found most oftennear the cube orientation in the beginning of annealing.

ACKNOWLEDGMENTS

This research was supported by the BK21 project of theMinistry of Education & Human Resources Development,South Korea. The authors are also thankful to MK ElectronCo., R&D Center. J.H. Cho appreciates some commentsfrom A.D. Rollett, Carnegie Mellon University, and D.E.Boyce, Cornell University.

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