| Ultrafine-grained (UFG) materials produced by equal channel angular pressing (ECAP) have recently received considerable attention owing to their unique and advantageous mechanical properties. Studies on the structural stability of such UFG materials in various services are of particular importance for their practical engineering applications. However, although there have been extensive investigations on many kinds of room-temperature mechanical properties of UFG materials, our knowledge of their high-temperature mechanical properties still remain less. UFG copper produced by ECAP was selected for the target material in the present work, and its structural instability under the condition of annealing was examined and the compressive deformation and damage behaviors of this material were investigated at different temperatures and at different strain rates.The DSC response curve as function of annealing time at different annealing temperatures was measured. No clear exothermic reactions were found in the DSC response curve during the maintenance of temperature at 373 K,423 K,473 K and 573 K, respectively. UFG Cu was annealed at 398 K for different times, and the change of the hardness of UFG copper with annealing time was measured. It is found that, as the annealing time increases, the hardness decreases obviously, and the material softening and recrystallization take place. Therefore, it can be inferred that under annealing at a certain temperature, UFG copper exhibits, to some extent, a structural instability, i.e., recrystallization and grain coarsening, the process of which may happen gradually at a low developing rate, so that the DSC response curve cannot reflect normally such a softening process.The testing temperature has a strong effect on the plastic deformation and damage behavior of UFG Cu under uniaxial compression. For example, the increase in testing temperature leads to general decreasings of yield stress and steady flow stress, and meanwhile, it promotes grain coarsening and thereby strain softening as the temperature is below recrystallization. Compressive deformation damage features are also strongly dependent on the temperature. Small-and large-scale cracks were observed to form along the shear bands (SBs) at room temperature. At temperatures higher than room temperature but lower than recrystallization temperature, some microvoids with various sizes were detected along the shear direction, and SBs tends to disappear. At temperatures above recrystallization, SBs have disappeared completely, and many grains become greatly coarsened and dislocation slip begins to occur within some coarsened grains. In this case, microvoids or cracks were found to nucleate at grain boundaries (GBs). GB cracking even takes place at a comparatively low strain rate (1.0×10-3/s). With decreasing strain rate, the yield stress and steady flow stress decrease, and the above-described damage phenomenon becomes more pronounced.Microstructural observations demonstrate that, with increasing temperature, grains tend to grow up similarly at both strain rates of 1.0×10-3/s and 1.0×10-2/s. However, the higher the strain rate, the larger the maximum grain size available for the coarsened grain is, but more remarkable the localization of grain coarsening becomes. In contrast, at a comparatively lower strain rate, many more grains become coarsening on the whole. This should be the reason why the yield stress and steady flow stress decrease with decreasing strain rate.The effect of dynamic annealing on the plastic deformation and damage behavior during compressive testing is limited, if compared with the effect of high-temperature deformation. The high-temperature compressive deformation behavior should be contributed to the joint actions of the applied compressive stress and adopted temperature (i.e. dynamic annealing).
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