Investigations On Dislocation Structures As Well As Their Thermal Stability Of Fatigued Copper Single Crystals With Different Orientations

As a typical face-centered cubic (fcc) crystal, a great deal of efforts have been invested in understanding the cyclic deformation and dislocation structures of Cu single crystals with different orientations, and some comprehensive results have been well established. However, the dislocation structures of some double-slip-oriented Cu single crystals still need to be further investigated. In particular, knowledge of the dislocation microstructures in deformation bands of Cu single crystals with various orientations remains much less. In addition, studies on the thermal stability of dislocation structures in fatigued Cu single crystals are also rarely reported. For exploring the above-menioned three aspects of subjects, systematic investigations were performed in the present work by using the electron channeling contrast (ECC) technique in scanning electron microscopy (SEM) and transition electron microscopy (TEM), and some new information have been achieved on understanding of fatigue microstructures as well as their thermal stabilities.The dislocation structures of fatigued [017] critical double-slip-oriented Cu single crystals are strongly dependent upon the applied plastic strain amplitude. As the applied plastic strain amplitude γp1is below the quasi-plateau region in the cyclic stress-strain (CSS) curve of the [017] crystal, irregular labyrinth structures formed. As γp1falls into the quasi-plateau in the CSS cueve, a special "two-phase" dislocation structure, i.e., persistent slip band (PSB) ladder-like structures and matrix labyrinth structures, was observed. When γp1beyond the quasi-plateau region, two types of deformation bands denoted DBI and DBII distribute in the whole specimens. The dislocation microstructures in DBI and DBII consist mainly of well-developed regular labyrinth structures and densely-aligned dislocation walls, respectively, and the microstructures at the intersection region of DBI and DBII comprise dislocation walls together with a number of dislocation cells. Further TEM observations reveal that dislocation walls evolve gradually into elongated dislocation cells at some inhomogeneous regions of strain concentration at γp1=3.0×10-3. As for [112] conjugate double-slip-oriented Cu single crystals, even as γp1is below the plateau region, have PSB ladder-like structures begun to appear. Veins structures and PSB ladder-like structures with various channel widths along the primary slip direction were observed on the specimen surfaces as γp1is within the plateau region. When γp1is beyond the plateau region, dislocation walls form in both deformation bands (DBs) and the matrix.DBs in fatigued [41841]-,[017]-,[233]-,[223]-,[112]-and [011]-oriented Cu single crystals were systematically examined, and it is interesting to find that two types of DBs denoted DBI and DBII can be clearly seen on the crystal surfaces. DBI develops roughly along the primary slip direction, and DBII makes a certain angle with the primary slip direction. Significantly, the dislocation structures in these two types of DBs are differently dependent on the crystal orientation. For instance, the dislocation structures in DBI are strongly dependent on the crystal orientation, which can be probably composed of different dislocation structures, i.e., PSB ladder-like structures, labyrinth structures, dislocation walls, and dislocation cell etc. In contrast, the dislocation structures in DBII are almost independent upon the crystal orientation, and they are mainly composed of dislocation walls. Meanwhile, the dislocation walls in DBs often make a certain angle with the ones in the matrix, indicating that the local crystal rotation appears between DBs and matrix.A group of Cu single crystal specimens oriented typically as [41841] for single slip and [017],[233] and [223] for double slip were cyclically deformed firstly at different γp1, and then annealed at different temperatures for30min. The results show that the dislocation structures have undergone an obvious process of revovery at300℃; however, at500℃and800℃, the violent recrystallization take place in all oriented crystals and a large number of annealing twins appear. The formation of the annealing twins and recrystallization are strongly dependent on the annealing temperature, the applied plastic strain amplitude and the accumulated plastic strain. With the plastic strain amplitude and accumulated plastic strain increasing, the degree of strain concentration increased significantly, which provides the required local strain energy for the initiation of twins and recrystallization. The DSC measurements show that the recrystallization process and the formation process of twins should be a gradually-developing process, instead of a suddenly-forming process.