Formation and stability of the ultrafine-grained structure of commercial-purity copper deformed at 80 K

Abstract

The structure and microhardness of commercially pure copper have been studied after cryogenic (80 K) high-pressure torsion performed at an angle of the anvil rotation from 15° to 10 revolutions. The retardation of dynamic softening processes due to impurity dragging allows us to establish the stages of the structural change in commercially pure copper upon deformation, as compared to high-purity copper, in which recrystallization rapidly develops upon heating to room temperature, significantly distorting the deformed structure and decreasing the microhardness of the deformed copper. Two stages of deformation have been observed. The stages change at a true strain of e = 7.3. Dislocation slip and mechanical twinning are the main structureforming mechanisms at the first stage. No mechanical twins are found at the second stage. The second stage is characterized by misoriented microcrystallites which play the role of recrystallization centers upon heating up to room temperature. The average microcrystallite size is 0.1-0.2 μm. Microcrystallites provide a low thermal stability of the structure. Some grains in the structure formed at the second stage of deformation can grow up to several microns in 1-2 days; the fraction of the recrystallized structure is 20 %. Holding for 3 years almost completes the recrystallization; the maximum size of recrystallized grains is 100 μm. Recrystallization at room temperature develops slowly in the structure with deformation twins: the early signs of recrystallization are observed 1.5 years later after the end of the deformation; and the fraction of the recrystallized structure does not exceed 10 %.