Investigations of Fatigue Dislocation Structures in Differently Oriented Copper Single Crystals Using SEM-ECC Technique
|Course||Materials Physics and Chemistry|
|Keywords||SEM-ECC technique copper single crystal dislocation structures persistent slip band deformation band|
The cyclic deformation behavior and the fatigue dislocation structures of copper single crystals have attracted substantive interests in many decades. Recently, the cyclic deformation behavior of differently oriented copper single crystals have been investigated systematically, but it is still far from complete understanding of the microstructures in copper single crystals oriented for double-and multiple-slip. Accordingly, in the present dissertation, some copper single crystals with representative orientations were adopted, i.e.,  single-slip-oriented crystals with a maximal schmid factor of 0.5,  conjugate double-slip-oriented crystals and  coplanar double-slip-oriented crystals located on different sides of the standard stereographic triangle, and  multiple-slip-oriented crystals. The fatigue dislocation structures in crystals with the orientations selected above were observed and analyzed using electron-channelling contrast technique in scanning electron microscopy (SEM-ECC), and some new research findings have been achieved as follows.According to the observations of the dislocation structures in cyclically saturated copper single crystals oriented towards , it is found that the persistent slip bands (PSBs) observed in  crystals might exhibit different dislocation patterns as the applied plastic strain amplitudeγpl increases, i.e. typical ladder structures, coarse slip bands consisting of some single PSB with some dislocation cells, and the narrow PSB being in the process of transformation to labyrinth structures and cell structures. However, the occurrence of dislocation cells in PSBs at so lowγpl of 3.3×10-4 and 1.3×10-3 shows that dislocation cells may exist not only at highγpl, but also at comparatively lowγpl.The experimental results of  and  crystals show that PSBs formed on the surfaces of  crystals are composed of irregular or regular dislocation cells aligned along the primary slip plane, whereas atγpl= 6.0×10-4, PSBs comprising irregular dislocation cells exist on the surfaces of  crystal. Actually, some elongated cell structures have tended to occur by transforming from a mass of vein structures even at lowγpl of 1.3×10-4 and 3.4×10-4, demonstrating that the cell structure could form in single-slip crystals cycled at low plastic strain amplitudes, and that it is not only characteristics under high strain amplitudes. Moreover, the dislocation microstructures in deformation bands (DBs) formed in the  crystal were observed comprising irregular wall structures and cell structures, and different dislocation microstructures appeared in DBs of  crystals, such as wall-like features, dislocation walls, elongated cell structures and dense dislocation cells. Apparently, the DBs formed in the above differently double-slip-oriented crystals may exhibit different dislocation configurations.For the  crystal, it should be pointed out that this oriented crystal could finally enter into a saturation state during cycling at a very lowγpl of 8.8 x 10"5, and the relevant saturation dislocation structures are mainly composed of labyrinth-like vein structures. As ypi increases to 4.0 x 10-4, no clear saturation stage can be found, but the corresponding dislocation structures are characterized by two kinds of distinctive configurations, i.e. dislocation walls and misoriented cells. Interestingly, these misoriented dislocation cells are strictly aligned along the primary slip plane (111), constructing a unique PSB structure. Moreover, there is a locally distinctive region comprising some highly-misoriented cells, having a recrystallization-like feature, in the whole structure of PSB cells.The SEM-ECC observations above have greatly enriched the understanding of micro-configurations in PSBs and DBs. Obviously, PSBs might exhibit different dislocation features depending upon the crystallographic orientation and the applied plastic strain amplitude, while the microstructures in DBs might depend strongly upon the crystallographic orientation, the applied plastic strain amplitude, the accumulated plastic strain, the strain localization and so on.