A bstractThe features of the deformation bands (DBs) in copper single crystals under cyclic straining were surveyed. A simple model was proposed to account for the formation of DBs. In this model, both crystal rotation and dislocation density variation induced by cyclic straining were considered. The gradual lattice rotation caused by tension and compression irreversibility is the main driving force for the formation of DBs. The drastic release of mobile dislocations resisted by the primary slip bands is the direct trigger for the formation of DBs. From this analysis, one may understand why the formation of DBs is easier in copper single crystals with double-and multiple-slip orientations than that with singleslip orientation. At the same time the e ect of the dislocation avalanche factor on the formation of DBs was discussed.
In order to reveal the e ect of grain boundaries (GBs) on cyclic deformation, the cyclically saturated dislocation patterns within grains and in the vicinity of GBs in a copper bicrystal with a large-angle GB and a copper multicrystal containing small-angle GBs have been observed by the electron channelling contrast technique in scanning electron microscopy. The observations for the multicrystal show that dislocation walls and persistent slip bands (PSBs) can transfer through the small-angle GBs. However, in the copper bicrystal with a large-angle GB perpendicular to the stress axis, PSBs can only form within the component grain with a relatively higher Schmid factor and cannot pass through the GB. From the experimental observations, the e ect of GBs on cyclic deformation behaviour is discussed. § 1. IntroductionSingle-crystal copper oriented for single slip exhibits three di erent regions in its cyclic stress± strain curve (CSSC) over a wide range of plastic strains and the saturation resolved shear stress of the plateau region (region B) maintains a constant value in the range 28± 30 MPa (Mughrabi 1978, Cheng andLaird 1981). Correlation between the saturation dislocation patterns and the three regions in the CSSC has been well established (Mughrabi 1978, Laird et al. 1986). Dislocation structures induced by cyclic deformation are generally observed by transmission electron microscopy (TEM). TEM investigations require thin foil specimens and therefore the bulk specimen has to be destroyed. It is impossible to study the evolution of dislocation structure during the deformation of a single bulk specimen by TEM. In addition, TEM requires tedious specimen preparation and permits only a relatively small specimen area to be investigated.Recently, the electron channelling contrast (ECC) technique in scanning electron microscopy (SEM) has been applied to study the dislocation patterns in cyclically deformed metals such as nickel (Schwab et al. 1996, Bretschneider et al. 1997), copper (Melisova et al. 1997, Li et al. 1998) and stainless steel (Zauter et al. 1992. In comparison with TEM, the SEM ECC technique has shown many attractive features. This technique has been found to be extremely suitable for studying the dislocation arrangements over a large specimen area and at some special sites, for example in the vicinity of grain boundaries (GBs), within deformation bands (Li et al. 1998) and ahead of cracks. It is well established that the saturation plateau observed in the CSSC is associated with the localization of the plastic deformation in
Cyclic deformation behaviour of a [4 15 20Š-‰18 2 7] copper bicrystal with a tilting Sˆ19b grain boundary (GB) was investigated in the axial plastic strain range 1:5 £ 10 ¡4 -2:13 £ 10 ¡3 at room temperature in air. The primary slip planes (111) within the G 1 ‰4 15 20] and G 2 ‰18 2 7] grains in the bicrystal were designed to be coplanar in order to reveal the interaction of slip bands with the GB. The results show that the cyclic stress-strain curve of the bicrystal displays a plateau region with axial saturation stresses of 61.6± 63.5 MPa over the applied strain range. This result is similar to that for a single-slip-oriented copper single crystal, indicating that the GB has little eOE ect on the saturation stress. After cyclic deformation, the surface morphology of the bicrystal exhibits the following features. Firstly, only the primary slip system was activated in both grains. Secondly, the primary slip bands on the four surfaces of the two grains show a good one-to-one relationship across the GB, indicating that surface slip bands can transfer through it. Thirdly, secondary slip systems were not activated, even in the vicinity of the GB. Dislocation patterns of the bicrystal were observed by the electron channelling contrast technique. The two-phase structure of persistent slip bands (PSBs) and matrix (or veins) formed in both grains. The ladder-like PSBs were observed to transfer through the GB continuously on one surface of the bicrystal but piled-up at the GB on the other surface, showing a discontinuous dislocation distribution next to the GB. Several kinds of interaction between dislocations and the GB were observed on the common slip plane. The cyclic stress± strain response and the interactions between dislocations, PSBs and the GB are discussed.
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