Disorientations in dislocation structures: Formation and spatial correlation
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During plastic deformation, dislocation boundaries are formed and orientation differences across them arise. Two different causes lead to the formation of two kinds of deformation-induced boundaries: a statistical trapping of dislocations in incidental dislocation boundaries and a difference in the activation of slip systems on both sides of geometrically necessary boundaries. On the basis of these mechanisms, the occurrence of disorientations across both types of dislocation boundaries is modeled by dislocation dynamics. The resulting evolution of the disorientation angles with strain is in good agreement with experimental observations. The theoretically obtained distribution functions for the disorientation angles describe the experimental findings well and explain their scaling behavior. The model also predicts correlations between disorientations in neighboring boundaries, and evidence for their existence is presented.
I. INTRODUCTION
Plastic deformation is maintained by mobile dislocations. After traveling a certain path, they become immobilized. The dislocations stored in the crystal tend to gather into boundaries, and dislocation structures develop with dislocation boundaries of a high dislocation density separating nearly dislocation-free regions. Simultaneously, orientation differences arise between the regions on both sides of a dislocation boundary. The disorientations across the boundaries increase with proceeding deformation. They are directly connected with the dislocations forming the boundary (or more specifically with the excess dislocations), and their occurrence is most appropriately modeled on the basis of dislocation dynamics. II. EXPERIMENTAL OBSERVATIONS
The dislocation structure and the arising disorientations have been investigated in detail in cold-rolled pure aluminum polycrystals for different rolling reductions.1,2 As sketched in Fig. 1, an inspection of individual boundaries by transmission electron microscopy (TEM) allows a morphological distinction between two types of dislocation boundaries:3 curved, thick cell walls with a loose appearance and straight, almost parallel dense dislocation boundaries running along several dislocation cells. The former are assumed to originate by statistical trapping due to mutual capturing of dislocations and termed incidental dislocation boundaries (IDBs), whereas the J. Mater. Res., Vol. 17, No. 9, Sep 2002
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second type of boundaries is assumed to arise from differences in the activated slip systems on both sides of the boundary.3 These geometrically necessary boundaries (GNBs) separate cell blocks. From orientation measurements on either side of the boundary the disorientation angles across the boundaries are evaluated, separately for both boundary types. In general, the average disorientation angles ¯ are higher across GNBs than across IDBs and increase with plastic strain as shown in Fig. 2.4 A normalization of the angles by their respective average ¯ removes (nearly) all differences b
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