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
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