Structural change during cold rolling of electrodeposited copper

Transcription

1 Materials Science Forum Vols (2007) pp online at (2007) Trans Tech Publications, Switzerland Structural change during cold rolling of electrodeposited copper X. Huang a, Q. H. Lu b, M. L. Sui c, D. X. Li d, N. Hansen e Center for Fundamental Research: Metal Structures in Four Dimensions, Materials Research Department, Risø National Laboratory, DK-4000, Roskilde, Denmark a xiaoxu.huang@risoe.dk, b qhlu@imr.ac.cn, c mlsui@imr.ac.cn, dxli@imr.ac.cn, e niels.hansen@risoe.dk Keywords: nanometer twins, electrodeposition, cold rolling, shear bands, lamellar structure. Abstract. Copper sheet samples composed of nanometer scale lamellar twins was produced by electrodeposition. The coherent lamellar twin boundaries were within 20 of being parallel to the sheet plane in more than 60% of the grains. The electrodeposited sample was cold rolled to 30 and 85% reductions in thickness and the structural evolution during cold rolling was examined by transmission electron microscopy (TEM) and high resolution TEM (HRTEM). Extensive activity of partial dislocations along twin boundaries and of perfect dislocations within twins (in particular in coarse twins >100nm) were identified. Moreover, it was found that shear banding occurred, which locally destroyed the lamellar twin structure. A dislocation structure developed within the shear bands, and such a structure evolved with strain and gradually replaced the lamellar twin structure. After 85% deformation, a large volume fraction of the lamellar twin structure was replaced by a lamellar dislocation structure characteristic of high strain rolling where the lamellar dislocation boundaries are almost parallel to the rolling plane. It was also found that the structural scales are coarser in the lamellar dislocation structure than in the initial lamellar twin structure. Introduction Plastic deformation and the evolution of a nanostructure are studied along two different routes: (1) the plastic deformation is applied to a sample to produce a nanostructure in a coarse grained material; (2) the plastic deformation is applied to a nanostructured material to explore its deformation behavior. This work relates to the second aspect and aims at examining the structural evolution during cold rolling of an electrodeposited copper composed of nanometer scale lamellar twins [1]. Experimental Copper sheet samples with a thickness of about 1.8mm were prepared by means of electrodeposition technique using an electrolyte of CuSO 4 and a substrate of Ti. The as-deposited Cu had a purity of about at%. The density of the as-deposited sample was determined to be 8.91±0.03 g/cm3, which agrees well with the theoretical density (8.96 g/cm3) of pure Cu with a purity of 99.4±0.3%. Positron annihilation spectroscopy measurements showed no vacancy-cluster-like volumes, indicating that the sample represents a fully dense material. The deposited sheets were cold rolled to 30 and 85% reductions in thickness in a two-high rolling mill with 75mm diameter rolls. The texture of the as-deposited sample was measured by electron backscatter diffraction (EBSD) over a large area ( µm 2 ) using a step size of 1µm. The initial structure in the as deposited sample and the deformation structures at different rolling strains were characterized with both conventional TEM and HRTEM. TEM thin foil samples were prepared from the longitudinal plane (perpendicular to the transverse direction of the rolled sample) to reveal the details of deformation microstructures [2]. TEM was carried out in a JEM2000FX electron microscope operating at 200kV and HRTEM was conducted in a JEM2010 electron microscope operating at 200kV and in a JEM 3000F electron microscope operating at 300kV. 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 the publisher: Trans Tech Publications Ltd, Switzerland, (ID: /11/06,09:54:53)

2 5014 THERMEC 2006 Results and discussion Texture and structure in the as-electrodeposited sample. Fig. 1 shows the inverse pole figure of the normal direction (ND) to the electrodeposited sheet. It is seen that a strong ND//<111> fiber texture has been developed during the electrodeposition process. Fig. 1. Inverse pole figure showing the direction of the normal to the electrodeposited sheet. Fig. 2 HRTEM image viewed in the longitudinal section of the aselectrodeposited sample showing the initial lamellar twin structure. The straight boundaries are coherent twin boundaries (see insert) that are approximately perpendicular to the ND. Microstructural observations showed that the as-electrodeposited sheet is composed of columnar grains elongated in the ND. The grain size was in the range of sub-micrometer to a few micrometers. Within each grain a high density of lamellar twins were observed. Fig. 2 illustrates a HRTEM image of the lamellar twins within a columnar grain. The lamellar boundaries are coherent twin boundaries and in most grains they are approximately perpendicular to the ND, in agreement with the formation of the ND//<111> fiber texture. A low density of steps or ledges is seen at the coherent twin boundaries. The spacing between the twin boundaries were measured and more than 90% of the measured values were in the range from a few nanometers to 100nm although some larger spacings (>100nm) were also found. Structural evolution during cold rolling. After 30% cold rolling, TEM and HRTEM observations revealed three characteristic microstructural features:

3 Materials Science Forum Vols (1) Dislocation activity along twin boundaries. Fig. 3 shows a TEM observation at a relatively low magnification. It is seen that the straight line feature of twin boundaries is maintained but the twin boundaries are no longer smooth and exhibit a complex morphology. HRTEM observations revealed that a high density of ledges developed along the twin boundaries caused changes in their appearance. As seen in Fig. 4, the ledges are often only a couple of nanometers high, and the distance between the ledges is in the range of a few nanometers to about 20nm. These ledges are formed as a result of accumulation of Shockley partial dislocations. These observations suggest an extensive activity of Shockley partial dislocations during the deformation. Frank partial dislocations were also observed, as a product of reaction of a Shockley partial with a perfect dislocation from the twin [3]. One example is shown in Fig. 4. Fig. 3. Dislocation activities at twin boundaries and within twins observed in a 30% cold rolled sample (see text for details). (2) Dislocation activity within twins. Activity of perfect dislocations was also identified within the twins and a twin size effect was observed (Fig. 3 and Fig. 4). In the fine twins (width<100nm), individual dislocation lines that extend through the twin were observed. Fig. 4 shows the identification of activation of a perfect dislocation in a 6nm wide twin, in agreement with the observation of dislocations in a nanostructured copper [4]. In the coarse twins (width>100nm), extensive activities of perfect dislocations result in the formation of dislocation arrays or dislocation boundaries, as indicated by a white arrow in Fig. 3. Different dislocation activities within fine and coarse twins give rise to different deviations of the boundary misorientation from the ideal twin misorientation. Larger deviations were observed across the twin boundaries associated with the coarse twins (up to 8 ) than across those associated with fine twins (<4 ). (3) Shear banding. In some regions in the sample, shear bands were formed, which penetrated through a stack of lamellar twins. An example is shown in Fig. 5, where two sets of shear bands at ±30 to the RD are indicated by arrows. The lamellar twin boundaries are severely bent near the shear bands and they are changed into an orientation parallel to the shear direction. In the central location within some of the shear bands, the initial lamellar twin structure was almost destroyed (Fig. 5b). Note that the structure within the shear bands, Fig. 5b, is coarser compared with the lamellar twin structure. This suggests that the localized shear banding destroys the lamellar twin structure. At 30% deformation, the shear bands occupied an area of about 15% of the entire structure. The occurrence of shear banding in structures containing lamellar twins has been studied in fcc metals and alloys [5-8] where the lamellar twin structures were introduced by deformation. It seems that a narrow spaced lamellar structure favors the formation of shear bands [7,9,10] due to

4 5016 THERMEC 2006 the restriction of homogeneous deformation through the lamellar structure. The present observation of shear bands in the cold-rolled electrodeposited copper agrees well with this suggestion. Fig. 4. HRTEM observation of the twin boundaries in a 30% cold rolled sample. High density of ledges associated with Shockley partial dislocations are seen at the twin boundaries. The inserted Fourier filtered image shows the identification of a Frank partial dislocation. Fig. 5 (a) Shear bands observed in a 30% cold rolled sample. The shear bands interest and one is sheared by the other. The twins near the shear bands are bent in the same direction to accommodate the local deformation. (b) The details of the structure within a shear band. The initial lamellar twin morphology is destroyed. After 85% cold rolling, a composite structure of regions with a lamellar twin structure and regions with a lamellar dislocation structure was observed with the latter being dominant (80% of the entire area). Fig. 6 shows a TEM observation near the interface between the two types of structure. In the lamellar twin structure region, the twin misorientation was identified by electron diffraction. The twin boundary morphology is similar to that observed at low strain (compare with Fig. 3 and Fig. 5). The twin boundary spacing is not much different from the initial value. These

5 Materials Science Forum Vols observations indicate that such regions have not experienced much deformation. In the lamellar dislocation structure region, the spacing between lamellar dislocation boundaries is significantly larger than the spacing between twin boundaries. Misorientation measurements by Kikuchi diffraction showed that many of the lamellar dislocation boundaries have low to medium angles (3 14 ). An example of misorientation measurement is shown in Fig. 7 where a lamellar structure characteristic of high train rolling is developed. The formation of a characteristic high strain lamellar dislocation structure indicates that after the initial twin structure has been destroyed, further evolution of the structure follows the general pattern [11] established for cold rolling. Fig. 6. TEM image in an 85% cold rolled sample showing a composite of a lamellar twin structure (left to the dashed line) and a lamellar dislocation structure (right to the dashed line). Note the difference in the structure scale between the two types of structure. Fig. 7 (a) TEM image in an 85% cold rolled sample showing a well developed lamellar dislocation structure in a region away from the lamellar twin structure regions. (b) Misorientation measurements for the lamellar dislocation boundaries and a few interconnecting boundaries are in the area marked by the frame in (a). Note the lamellar boundaries have misorientation angles far below the twin misorientation angle (60 ).

6 5018 THERMEC 2006 Conclusion The structural evolution during cold rolling of an electrodeposited Cu sample containing a high density of nanometer lamellar twins has been characterized by TEM and HRTEM, and the main observations are the following: (1) Shockley partials are activated extensively along the twin boundaries, producing a high density of ledges at the twin boundaries. (2) Perfect dislocations contribute to the deformation, with more activity of perfect dislocations in coarse twins (>100nm) than in fine twins (<100nm). (3) Shear banding destroys the lamellar twin structure. At high strain (85% reduction), the deformed structure is dominated by a lamellar dislocation structure typical for cold rolled samples. Acknowledgement The authors gratefully acknowledge the Danish National Research Foundation for supporting the Center for Fundamental Research: Metal Structures in four Dimensions, within which part of this work was performed. This research was supported by National Nature Science Foundation of China (No ). References [1] M. L. Sui, Q. H. Lu, X. Huang, D. X. Li and N. Hansen, in: Evolution of Deformation Microstructures in 3D, edited by C. Gundlach, K. Haldrup, N. Hansen, X. Huang, D. Juul, T. Leffers, Z. J. Li, S. F. Nielsen, W. Pantleon, J. A. Wert, and G. Winther, Risø National Laboratory, Roskilde (2004). p. 533 [2] G. Winther, X. Huang, A. Godfrey and N. Hansen: Acta Mater. Vol. 52 (2004), p [3] W. T. Read, Dislocations in Crytsals,(McGraw-Hill Book Company, Inc., New York, 1953) [4] D. A. Hughes and N. Hansen: Phil. Mag. Vol. 83 (2003), p [5] H. Paul, J. H. Driver, C. Maurice and Z. Jasieński: Mater. Sci. Eng. Vol. A359 (2003), p. 171 [6] H. Paul, J. H. Driver, and Z. Jasieński: Acta Mater. Vol. 50 (2002), p [7] K. Morii and Y. Nakayama: Trans Japan Institute of Metals, Vol. 22 (1981), p [8] Morikawa, and K. Higashida, in: Recrystallization Fundamental Aspects, Proceedings of the 21st International Risø Symposium on Materials Science, edited by. N. Hansen, X. Huang, D. Juul Jensen, E. M. Lauridsen, T. Leffers, W. Pantleon, T. J. Sabin and J. A. Wert, Risø National Laboratory, Roskilde (2000). P. 167 [9] C. Donadille, R. Valle, P. Dervin, and R. Penelle: Acta metall. Vol. 37 (1989), p [10] W. Y. Yeung and B. J. Duggan: Acta metal. Vol. 35 (1987), p. 35 [11] N. Hansen: Metall. Mater. Trans. A. Vol. 32A (2001), p