In -Situ Annealing of Severe Plastic-Deformed OFHC Copper
- PDF / 1,407,493 Bytes
- 11 Pages / 593.972 x 792 pts Page_size
- 73 Downloads / 214 Views
TRODUCTION
METALS and alloys consisting of very fine grains (below 1 lm) are of special interest, because they can exhibit excellent properties.[1–7] Besides the conventional methods for producing such fine microstructures, such as rapid solidification, powder metallurgy, and vapor condensation methods,[8] it is now well established that nano- or at least submicrometer-grained microstructures can also be obtained by applying large strains at low homologous temperatures to metallic materials.[3,9,10] Many different techniques for severe plastic deformation are known,[11–14] which all provide nearly infinite strains without failure of the material. However, the resulting small crystallites produced with these techniques contain a huge amount of stored energy in the form of lattice defects in their as-processed state. Most severely deformed materials, processed at ambient temperature, exhibit low tensile ductility and must be annealed to obtain an optimum balance of strength and ductility. Furthermore, long-term applications of these fine-structured materials at temperatures above the processing temperature would inevitably cause unpredictable recrystallization or at least annealing phenomena.[5] It is clear that this would lead to undesirable changes in the material properties. Thus, a transformation of the severely deformed state into a microstructure consisting of fine recrystallized grains is, in many cases, indispensable.
To achieve the desired microstructure following recrystallization of the as-deformed state, a detailed knowledge of the early stages of recrystallization—the nucleation ‘‘event’’—is necessary. It is known that nucleation of recrystallized grains originates at inhomogeneities in the deformed microstructure. After conventional deformations, these regions of inhomogeneity may be initial grain boundaries or inhomogeneities induced by the plastic deformation itself. Among others, these are single dislocation cells and subgrains, microbands, transition bands, shear bands, or pre-existing grain boundaries.[15–18] An increase in plastic deformation leads to a significant rise in the number of such inhomogeneities. Consequently, the density of preferred nucleation sites increases with the amount of preceding strain, reflected in smaller recrystallized grain sizes. The methods of severe plastic deformation are relatively new. Despite the huge number of publications that are related to this topic, the recrystallization behavior of severely deformed materials in their as-processed state (without any preaging) is rarely investigated. The present work is focused on the nucleation behavior during recrystallization of severely deformed oxygen free high conductivity (OFHC) copper. The results of in-situ investigations performed within a scanning electron microscope at large magnifications and possible underlying mechanisms are discussed. II.
A. VORHAUER and S. SCHERIAU, Materials Science Engineers, and R. PIPPAN, Professor, are with the Christian Doppler Laboratory for Local Analysis of Deformation and Fracture, Erich Sc
Data Loading...
