In-Situ SEM/EBSP Analysis during Annealing in a Pure Aluminum foil for Capacitor M. Kobayashi 1,2,a Y. Takayama 1,b,H. Kato 1,c and H.

Transcription

1 Materials cience Forum Online: IN: , Vols , pp doi:1.428/ 7 Trans Tech Publications, witzerland In-itu EM/EBP Analysis during Annealing in a Pure Aluminum foil for Capacitor M. Kobayashi 1,2,a Y. Takayama 1,b,H. Kato 1,c and H. TODA 2,d 1 Department of Mechanical ystems Engineering, Utsunomiya University, Utsunomiya, Tochigi , Japan 2 Department of Production ystems Engineering, Toyohashi University of Technology, Toyohashi, Aichi , Japan a m-kobayashi@sp-mac4.pse.tut.ac.jp, b takayama@mech.utsunomiya-u.ac.jp, c katoha@cc.utsunomiya-u.ac.jp d toda@tutpse.tut.ac.jp Keywords: orientation, recrystallization, grain growth, textural evolution, scanning electron microscopy/ electron backscattered diffraction pattern (EM/EBP) technique. Abstract. In-situ EM/EBP analysis has been performed during the evolution of the cube texture in a pure aluminum foil. In general, foils for capacitor are manufactured in an industrial process of casting, homogenizing, hot rolling, cold rolling (CR), partial annealing (PA), additional rolling (AR) and final annealing (FA). The foil samples after CR or AR in the process were analyzed by the EM/EBP technique at a constant temperature which was step-heated repeatedly by 1-K from a room temperature to 623K or 598K. In a CRed sample, cube ({1}<1>) grains begin to grow preferentially at 53K to cover the sample. On the other hand, in a sample subjected to PA at 53K and AR, cube grains coarsened rapidly and preferentially at more than 533K in contrast to other oriented small grains remaining their sizes. Further, intragranular misorientation analysis revealed that the misorientation, which corresponds to dislocation density or strain, was much smaller in cube grains than in ({123}<634>) and ({112}<111>) ones. Introduction It is well known that the sharp cube texture is evolved in pure aluminum foils for capacitors in the interest of capacitance augmentation [1]. The foils are often produced in an industrial process patented by Company Pechiney [2], that is, hot rolling, heavily cold-rolling, partial annealing (PA), additional rolling (AR) and final annealing (FA). Numerous studies on the cube texture formation were performed extensively in the production process from the hot rolled sheets to the final annealed foils up to date [1, 3-8]. It was understood in the previous studies that cube-oriented ({1}<1>) nuclei formed during the hot rolling or cold rolling, they became stable grains in partial annealing process, and in final annealing process the cube grains were able to grow preferentially with strain-induced grain boundary migration (IBM) driven by difference in stored energy introduced by additional rolling process. It was mentioned that cube grains were less affected than other oriented grains by deformation of additional rolling [8-11] or were able to recover earlier than other oriented ones [9, 12]. The purpose of the preset study is to reveal how the cube grain has an advantage in comparison with the other oriented grains during microstructural evolution by using in-situ scanning electron microscopy/ electron backscattered diffraction pattern (EM/EBP) analysis [13]. Application of the analysis enables us to find the process where the cube grain gets priority in growth and a critical temperature where it does. Further, effect of PA and AR on preferential growth of the cube grain is investigated from comparison of conditions with and without them. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania tate University, University Park, UA-4/3/16,21:28:35)

2 Materials cience Forum Vols Experimental Procedure amples were 99.99% pure aluminum foils for capacitors, which contain with 8ppm Fe, 8ppm i and 5ppm. The foils were produced by the process consisting of casting, homogenizing, hot rolling and cold rolling with a reduction of 97%. Furthermore, they were partially annealed (PAed) at 53K and 17% additionally rolled (ARed). The CRed and PA/ARed samples were of 132μm and 11μm thickness, respectively. The crystallographic orientation analyses were performed repeatedly in the same position of samples during heating by the EM/EBP technique using HITACHI -35H EM / TL OIM systems. For such in-situ EM/EBP analyses, a specially designed sample holder with a built-in ceramic heater was used. The temperature of the sample was controlled by the heater current. First, the temperature was raised from room temperature to a given one, which was 493K for a cold rolled (CRed) and 523K for a PAed and ARed (PA/ARed) samples, at a constant heating rate of.16/k. Next, after a temperature which was 1 to K lower than the given one was held to stabilize grain structure (or suppress grain boundary migration), the EM/EBP analysis was carried out for about 8min. Immediately after the analysis the temperature was raised to a 1 or K higher given one. Then, the EM/EBP analysis was performed at a 1 to K lower temperature, and raising the temperature. In the same way the foil sample was analyzed at a constant temperature which was step-heated repeatedly by 1-K from a room temperature to 623K or 598K. The standard analyzed area was 3 6μm2 and the standard step size of 3.μm. Grain interior misorientation was measured with a step size of.5μm. Results and Discussion Orientation maps obtained at each temperature by the in-situ EM/EBP analyses in the cold rolled sample are displayed in Fig. 1. Although ({123}<634>) oriented area covers more than a half of the sample at (a) 493K, cube oriented grains multiply gradually with increasing temperature. This means the gradual conversion from rolling texture to cube texture. grains are evidently observed in (a) 493K map though their number and size are not so large. As temperature rises, not only cube grains grow on the surface but also some of them appear from inside of the foil. An arrowed cube grain in Fig. 1(d), which appears from inside and beside is very close to ideal orientation, coarsens extremely later. grains appearing from inside grow and remain to an equivalent size to the cube grains. These facts are probably related to the size effect, that is, the grains appearing from inside are larger in three dimensions than those on surface. Figure 2 shows orientation maps representing change of microstructure in the PA/ARed sample. Fairly large equiaxed grains of to 3μm are observed in (a) as received sample. The grains involve numbers of cube ones. The cube grains probably grew during partial annealing (PA) [14]. The microstructure does not change markedly until temperature reaches the PA temperature of 53K. At (c) 533K the cube grains become larger rapidly while the other grains remain small. The cube grains grew likely with a driving force of consuming the stored energy of unrecrystallized fine grains. [15] At (e) 543K and later, the cube grains cover extended area of the sample instantaneously. Microstructure where the same oriented grains occupy nearly whole area includes lots of low angle and irregular boundaries. It is noted that the microstructure is smaller in grain size compared with that of CRed sample. Changes in texture components against the temperature during heating in both of the samples are compared in Fig. 3. In CR sample, fraction of the cube grains increases gradually to about 8%. The competitive orientation slightly increases in fraction after about % decrease at 57K. Another orientation ({112}<111>) lowers in fraction slowly toward near zero at 613K. On the other hand, the cube fraction of PA/ARed sample shows a drastic increase about 533K while both competitive orientations and decrease in fraction to zero. Evolution of cube orientation in the PA/ARed sample is regarded as sensitive to temperature owing to stored energy of the additional rolling

3 364 THERMEC 6 compared with that in the CRed one, though difference in heating conditions between both samples should be considered for comparison. (a) 493K (477K-478K) (e) 568K (545K-546K) (b) 53K (486K-488K) (f) 593K (57K-572K) (c) 523K (55K-56K) (g) 613K (578K-581K) (d) 543K (524K-528K) (h) 623K (66K-69K) RD TD Fig. 1 Orientation maps obtained at each temperature by the in-situ EM/EBP analyses in a cold-rolled (CRed) foil.

4 Materials cience Forum Vols (a) (e) 543K (535K-536K) (b) 523K (5K-53K) (f) 558K (547K-549K) (c) 533K (517K-5K) (g) 573K (573K-568K) (d) 533K Cooling (h) 598K (588K-599K) RD TD Fig. 2 Orientation maps obtained at each temperature by the in-situ EM/EBP analyses in a 17% additionally rolled foil after partial annealing at 53K (PA/ARed). Changes in numbers of grains of each texture component as a function of temperature is displayed in Fig. 4. In both samples, the total number of grains decreases gradually during microstructural evolution. The number of cube grains in contrast increases steadily to keep a fixed level above 543K for CRed sample. For PA/ARed sample, the number also increases until 543K, and then decreases slightly. The increase in the number of cube grains for both samples in lower temperature range means that numbers of cube grains appear from inside to surface as described above. Further, the decrease in the number reveals that even growing cube grains can be consumed in the process of cube

5 366 THERMEC 6 Fraction (%) (a)cr Fraction (%) (b)pa53k+ar17% Fig. 3 Changes in fraction of main texture components as a function of temperature in (a) CRed and (b) PA/ARed foils. Number of grain, N (a)cr All (b)pa53k+ar17% 1 4 All Number of grain, N Fig. 4 Changes in number of grains of main texture components as a function of temperature in (a) CRed and (b) PA/ARed foils. texture formation. The numbers of - and -oriented grains decrease with evolving cube texture in both samples. In order to investigate the reason why cube grains grow more preferentially in a PA/ARed sample, a detailed in-situ EBP analysis with a step size of.5μm was conducted to measure grain interior misorientation or kernel average misorientation (KAM) [16]. KAM is defined for a given point as the average misorientation of that point with all of its neighbors, which is calculated with the proviso that misorientations exceeding a tolerance value of 5 are excluded from averaging calculation. From the KAM value the density of geometrically necessary dislocation can be derived [17]. Thus, stored energy can be evaluated by KAM. Figure 5 illustrates distributions of KAM in cube, and grains of the PA/ARed sample at each heating step. Comparing three orientations, the distribution of the cube grain shows a sharp peak at the lowest misorientation angle among them. The misorientation of the KAM peak becomes higher in order of cube, and grains. It is, therefore, understood that the stored energy of the cube grains is lower than that of the other oriented grains, which results in retarding growth of the latter and stimulating more preferential growth of the former. In addition, the KAM distributions shifted to the right side as the temperature rose. This may be due to not only microstructural change but also thermal expansion of the sample.

6 Materials cience Forum Vols ummary In order to reveal how the cube grain has an advantage in comparison with the other oriented grains during microstructural evolution, the in-situ EM/EBP analysis was performed in a pure aluminum foil. The results obtained are summarized as follows. 1. In the CRed sample, when the temperature reached 53K, cube grains started to grow preferentially to cover the sample. The grains appearing from inside tended to grow and remain finally. 2. In the PA/ARed sample, the cube grains coarsened rapidly and preferentially at more than 533K in contrast to the other oriented small grains remaining their sizes. 3. The cube grains in the CRed sample were larger than those in PAed one. 4. The intragranular misorientation analysis revealed that the stored energy of the cube grains was lower than that of the other oriented grains, which resulted in retarding growth of the latter and stimulating more preferential growth of the former. References Fraction(%) Fraction(%) Fraction(%) (a) 483K 53K 523K 533K Kernel average misorientation (degree) (b) K 53K 523K 1 533K Kernel average misorientation (degree) (c) 483K 53K K 533K Kernel average misorientation (degree) Fig. 5 Distributions of KAM for (a), (b) and (c) orientated grains of the PA/ARed sample at each heating step. [1] O. Engler, M.-Y. Huh: Mater. ic. and Eng. A271(1999) [2] Company Phechiney: Japan Patent Office, Examined patent application publication (B2), (1979). [3] F. eki and T. Kamijyo: J. Japan Inst. Light Metals, 48(1998) [4] K. Kajihara, K. Tokuda, Y. ugizaki and Y. eki: J. Japan Inst. Light Metals, 51(1) [5] T. Murakami: J. Japan Inst. Light Metals, 52(2) [6]. Endou and H. Inagaki: J. Japan Inst. Light Metals, 52(2) [7] N. Takata, K. Ikeda, F. Yoshida, H. Nakashima and H. Abe: Mater. Trans., 45(4) [8] M. Kobayashi, Y. Takayama and H. Kato: Mater. Trans., 45(4) [9] A. A. Ridha and W. B. Hutchinson: Acta Metall. 3(1982) [1] I. amajdar and R. D. Doherty: cripta Metal. Mater. 32(1995) [11] I. amajdar and R. D. Doherty: Acta Mater. 46(1998) [12] H. E. Vatne, T. Furu and E. Nes: Mater. ci. and Tech. 12(1996) [13] B. L. Adams,. I. Wright and K. Kunze: Metall. Trans. A 24(1993) [14] D. J. Jensen, cripta metall. mater. 27(1992) [15] H. E. Vatne, T. Furu, R. Ørsund and E. Nes, Acta metar 44(1996) [16].I. Wright, D.P. Field and D.J. Dingley: Electron Backscatter Diffraction in Materials cience, Eds. A.J. chwartz, M. Kumar and B.L. Adams, Kluwer Academic, (), chp.13. [17] Y. Takayama and J.A. zpunar: Mater. Trans., 45(4)

7 THERMEC / In-itu EM/EBP Analysis during Annealing in a Pure Aluminum Foil for Capacitor 1.428/ DOI References [17] Y. Takayama and J.A. zpunar: Mater. Trans., 45(4) Fraction (%) ern el averag e misorien tation (d eg ree) K (b ) 483K 15 53K 523K 1 533K (a) b e 483K 53K 523K 533K Fraction (%) Fraction (%) K 25ern el averag e misorien tation (d eg ree) (c) 483K 53K 523K K Kern el averag e misorien tation (d eg ree) Fig. 5 Distributions of KAM for (a), (b) and (c) orientated grains of the PA/ARed sample at each heating step. 1.23/matertrans