Microstructure Evolution in Deformed Copper and Nickel

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Microstructure Evolution in Deformed Copper and Nickel Peri Landau1, Roni Z. Shneck1, Guy Makov2, and Arie Venkert2 1 Materials Engineering, Ben- Gurion University, P.O.Box 653, Beer Sheva, 84105, Israel 2 Department of Physics, NRCN, Beer Sheva, 84190, Israel ABSTRACT The combined effect of strain and temperature on the microstructure and detailed internal structure of dislocation boundaries was systematically studied in compressed pure polycrystalline copper and nickel and compared to the microstructure of compressed polycrystalline aluminum. Below 0.5Tm the microstructure of Cu and Ni consists of dislocation cells, however, only in Cu second generation microbands are formed. In Cu and Ni, the dislocations inside the boundaries rearrange themselves from tangles to ordered arrays of parallel dislocations following interplay between strain (requirement for cross slip) and temperature (dislocation mobility and ease of cross slip). The ordered detailed structure is similar to that observed in Al deformed at room temperature and lower strain levels. The amount of strain and temperature applied to Cu and Ni in order to achieve the same detailed structure formed in Al depends on the stacking fault energy (SFE) of the metal- higher strain and temperature as the SFE is lower. INTRODUCTION Patterning of dislocations into boundaries and cells is common to fcc metals following plastic deformation. Many characteristics of this phenomenon have been revealed by the extensive research devoted to the microstructure following plastic deformation [1-12]. A small amount of strain is required for dislocations to rearrange into dislocation boundaries (DBs) delineating cell blocks (CBs) [1-7]. The DBs form misorientations between adjacent cells that increase with increasing strain. The cell size decreases with increasing strain and depends on the SFE [3]. Several types of boundaries are distinguished including incidental DBs (IDBs), dense dislocation walls (DDWs) and geometrically necessary boundaries (GNBs) [1-5]. Increase in strain causes splitting of DDWs to form first generation microbands (MB1). Another microstructural feature, found in medium and low SFE fcc metals [10] (such as: copper, brass and aluminum alloys) is the second generation microbands (MB2). The MB2 produce shear offsets intersecting MB1 and grain boundaries [4-6, 8-10]. They consist of double dislocation walls approximately 0.1-0.4┬Ám apart and are formed on {111} planes after more than ~10% deformation. Their density increases with increasing strain and they tend to cluster into groups [9]. The MB2 were observed in copper following rolling, compression and tension, and also in nickel following torsion and rolling [11-12], but not as frequently as they were in copper [4]. MB2 were not observed in deformed aluminum [4]. The detailed structure of dislocation walls has been discussed in a few reports. In aluminum the dislocation boundaries formed during plastic deformation were found to consist of networks of parallel dislocations (at room temperature [13