In-Situ Annealing of Severe Plastic-Deformed OFHC Copper A. VORHAUER, S. SCHERIAU, and R. PIPPAN A pure oxygen free high conductivity (OFHC) copper is subjected to severe plastic deformation by a well-defined high-pressure torsion process at ambient temperature. The change in micro- structure of samples deformed to different strains, up to e = 64, is investigated in situ, during annealing at 170 °C, within a scanning electron microscope at large magnifications. The spatial distribution of nucleation sites changes significantly with increasing strain from nucleation at triple junctions and grain boundaries to a random distribution of sites for von Mises equivalent strains beyond e = 4. The resulting mean size of recrystallized grains is about 6.75 times larger than the mean microstructural size of the corresponding as-deformed state. For strains larger than e = 16, the recrystallized microstructure appears to be independent of preceding strain. A detailed investigation of the nucleation of recrystallized grains following very large strains shows that certain microstructural elements are favored as nuclei. Possible mechanisms that would account for the observation are proposed, whereby microstructural inhomogeneities, which might be present in the microstructure of the as-deformed state, were particularly taken into account. DOI: 10.1007/s11661-007-9409-x Ó The Minerals, Metals & Materials Society and ASM International 2008 I. INTRODUCTION 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-struc- tured 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, indispens- able. 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 inhomo- geneities in the deformed microstructure. After conven- tional 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, micro- bands, transition bands, shear bands, or pre-existing grain boundaries. [15–18] An increase in plastic deforma- tion 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 rela- tively 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. EXPERIMENTAL A. Materials and Materials Processing The material investigated in this study is OFHC copper provided by Buntmetall Amstetten GmbH. [43] The chemical analysis of the material is given in Table I. An annealing for 2 hours at 650 °C was performed on the rod-shaped base material in order to remove existing A. VORHAUER and S. SCHERIAU, Materials Science Engineers, and R. PIPPAN, Professor, are with the Christian Doppler Labora- tory for Local Analysis of Deformation and Fracture, Erich Schmid- Institute of Materials Science, Austrian Academy of Sciences, Jahnstrasse 12, 8700 Leoben, Austria. Contact e-mail: scheriau@ unileoben.ac.at Manuscript submitted August 11, 2006. Article published onlined February 12, 2008 908—VOLUME 39A, APRIL 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A