Microstructure of ECAE-Processed Copper after Long-Term Room-Temperature Storage
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THE strong demand for materials with advanced properties has in recent years stimulated the development both of new materials and of new processing techniques. In response to this demand several novel deformation techniques have emerged, which have resulted in a large number of publications reporting unusual physical and mechanical properties. Equal channel angular extrusion (ECAE)[1,2] is one such technique and has attracted significant attention. A large amount of research work on the microstructure resulting from ECAE processing has been accumulated and these materials are now being considered for commercial applications. Successful commercialization of ECAE-processed materials will, however, require not only the development of a suitable microstructure in the as-processed condition, but also the ability to maintain this microstructure over significant periods of time. It is known that microstructures of heavily deformed metals are sometimes unstable even at room temperature. For example, heavily-drawn thin wires of pure copper used in electrical and electronic products can recrystallize during aging at room temperature,[3,4] which leads to a deterioration in their mechanical properties. To stabilize the deformation structure, changes in the chemical composition or in the thermomechanical history are usually required. Data in the literature regarding the
O.V. MISHIN, Senior Scientist, is with the Center for Fundamental Research: Metal Structures in Four Dimensions, Materials Research Department, Risø National Laboratory for Sustainable Energy, Technical University of Denmark, DK-4000 Roskilde, Denmark. Contact e-mail: [email protected] A. GODFREY, Professor, is with the Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, P.R. China. Manuscript submitted March 26, 2008. Article published online October 1, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
microstructural stability of ECAE-deformed copper are very inconsistent. While commercially pure copper after multipass ECAE is often considered stable at temperatures below 100 C,[5,6] there have been several communications[7–10] describing large grains and roomtemperature partial recrystallization after ECAE processing. To evaluate the long-term room-temperature stability of ECAE-processed copper, samples kept at room temperature for a period of 8 years have been re-investigated following a transmission electron microscope (TEM) analysis of their microstructures in the as-deformed condition. A sample extruded with 90 deg rotations between passes (route Bc) was originally used for a TEM investigation of boundary misorientations in the as-processed condition.[11,12] The as-deformed microstructure of this sample did not contain coarse recrystallized grains, but it was very heterogeneous; both the local morphology and the fraction of high angle boundaries (HABs) varied significantly in different regions of the sample. A TEM investigation was also carried out on a sample prepared using route A
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