In -Situ Annealing of Severe Plastic-Deformed OFHC Copper

Abstract

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 microstructure 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.

The Limits of Refinement by Severe Plastic Deformation

“…[29] The occurrence of the saturation of the flow stress is in good agreement with the saturation in the structural refinement. [25,31] In Figure 9 the size of the granular structural elements determined from back scattered electron micrographs shows a similar increase with increasing temperature as Figure 7 in Cu. This correlates very well with the decrease of the flow stress with increasing SPD temperature.…”

“…In principle one could imagine two very different types of reasons for the occurrence of the steady state, a change in the dominant deformation mechanism or a balance in the structural restoration process. [31] For both of them exist different explanations, which cannot be discussed here. Only few comments are listed in the following.…”

High-pressure torsion applied to nickel single crystals

“…However, the applied pressure influences the friction between the dies and the excess material and therefore contributions to the measured torque. For a more detailed description of the high pressure torsion process can be found in [10,11]. Due to the torsion exerted on the small metallic discs the initial orientation of the single crystal is rotated around the torsion axis as a function of the disc height and the twist angle θ.…”

Adiabatic heating and the saturation of grain refinement during SPD of metals and alloys: experimental assessment and computer modeling

“…Steady-state behavior has been reported [33] in SPD processing of body-centered cubic Armco Fe by HPT in the range 0.08-0.4T M . The flow stress was estimated from the applied torque which depended on deformation parameters and, in this temperature regime, an increase in deformation temperature or decrease in deformation rate resulted in a decrease in the flow stress as well as the strain at onset of the steady-state.…”