Cyclic response and substructure evolution of copper polycrystals
Studies on the cyclic response and the substructure evolution of copper polycrystals have been carried out in the high-cycle fatigue regime. The effects of ramp-loading and microstructure have been investigated using various mechanical tests and electron microscopy. Investigations on the influence of loading mode on the cyclic response of fine-grained polycrystalline copper and the associated dislocation structures showed that the saturation behavior under constant load control, for two sets of specimens, with and without initial ramp-loading, exhibits strong differences in the "intermediate" range of stress amplitudes, i.e., from 70 to 98 MPa. Within this range the ramp-loading mode promotes a gradual substructure evolution which leads to localization of slip in primary systems and the formation of persistent slip bands (PSBs), whereas conventional loading leads to the formation of elongated cells and multiple sets of wall structures (e.g., labyrinth structure), both intimately associated with multiple slip conditions. In studying ramp-loading as a mechanical pre-treatment it is found that the well-defined matrix structure inherited from the very efficient cyclic hardening during the ramp-treatment promotes very uniform and homogeneous structures of primary dislocations, i.e., PSBs and wall structure, from grain to grain. These structures favor strain localization, and thus lower hardening rates in the cyclic stress-strain curves (CSSC) of ramp-treated copper polycrystals are obtained. However, plateau-like behavior was not observed. The presence of a weak but noticable $\langle 111\rangle$-$\langle 100\rangle$ "hard" texture in the ramp-treated specimens studied here suggests that the observation of a plateau in the CSSC of polycrystals may be very sensitive to texture. Microstructure, expressed in terms of a complex factor of combined grain size and texture, showed a very significant effect in the cyclic response of copper at amplitudes where structures which localize deformation are expected to be present. Initial homogeneous multiple slip promotes faster substructure evolution into cell structure, which accounts for the low levels of localization of deformation observed in the CSSC of coarse-grained copper. Finally, a polycrystalline model based on treating differently oriented grains as composite material is shown to account qualitatively for most of the results obtained in the present investigation.
Llanes, Luis Miguel, "Cyclic response and substructure evolution of copper polycrystals" (1992). Dissertations available from ProQuest. AAI9233346.