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By the late 1940s, and since then, the continuous development of dislocation theories have provided the basis for correlating the macroscopic time-dependent deformation of metals and alloys—known as creep—to the time-dependent processes taking place within the metals and alloys. High-temperature deformation and stress relaxation effects have also been explained and modeled on similar bases. The knowledge of high-temperature deformation as well as its modeling in conventional or unconventional situations is becoming clearer year by year, with new contemporary and better performing high-temperature materials being constantly produced and investigated. This book includes recent contributions covering relevant topics and materials in the field in an innovative way. In the first section, contributions are related to the general description of creep deformation, damage, and ductility, while in the second section, innovative testing techniques of creep deformation are presented. The third section deals with creep in the presence of complex loading/temperature changes and environmental effects, while the last section focuses on material microstructure–creep correlations for specific material classes. The quality and potential of specific materials and microstructures, testing conditions, and modeling as addressed by specific contributions will surely inspire scientists and technicians in their own innovative approaches and studies on creep and high-temperature deformation.
Grain-boundary displacement, occurring in bicrystals during creep at elevated temperature (350 degrees c), has been measured as a function of the copper content (0.1 to 3 percent) in a series of aluminum-rich aluminum-copper solid-solution alloys. The minimums in stress and temperature, below which grain-boundary motion does not occur, increase regularly with the copper content as would be expected if recovery is necessary for movement. Otherwise, the effects, if any, of the copper solute upon grain-boundary displacement and its rate are too small for identification by the experimental technique employed. It was shown, additionally, that grain-boundary displacement appears regular and proceeds at a constant rate if observed parallel to the stress axis, whereas the motion is seen to occur in a sequence of surges and the rate to diminish with time if the observations are made perpendicular to the stress axis. This is interpreted as further evidence that grain-boundary shearing occurs within a layer of metal of finite thickness and not by sliding upon a single interface.
The prospect of structural failure, in proposed OFHC copper nuclear waste canisters, initiated by intergranular creep cavitation has prompted an investigation into the possibility of cavitation resistance enhancement through grain boundary engineering (GBE). In order to successfully implement GBE, the influence of the grain boundary network structure on creep cavitation must be ascertained. For this purpose, post-creep test characterization, using the electron backscatter diffraction technique, was performed on cavitated and non-cavitated grain boundary and triple junction structures. The implication of the findings were discussed in reference to the Coincident Site Lattice framework for grain boundaries and Bollmann's I-line/U-line model for triple junctions. Grain boundary characterization revealed that creep cavitation occurred primarily along a path of random boundaries. It was also determined that low angle ($\Sigma$ 1) and twin ($\Sigma$ 3) boundaries were the more cavitation resistant than random and other higher $\Sigma$ CSL ($\Sigma$ 5-29b) boundaries. The lack of observed cavitation selectivity between random and $\Sigma$ 5-29b boundaries was attributed to the fact that creep conditions might be too severe. This may also explain the lack of selectivity of U-lines as preferential sites for cavity nucleation observed in this thesis. The findings of this thesis, however, do illustrate that there is grain boundary structural influence on intergranular creep cavitation of OFHC copper.