Dislocation substructure evolution in torsion of pure copper
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I.
INTRODUCTION
F O U R stages of work hardening are now recognized:m stage I is characterized by "easy glide," where one slip system dominates deformation; stage II is a linear work-hardening region in which a network or carpets of dislocation structures are formed and where similitude prevails;t2m and stage III is a parabolic work-hardening region which occurs when cells cease t o shrink.t4-91 This hypothesis that the cell size stops changing has been challenged by observations in several metals at l a r g e strains, where cells are observed to shrink up to several hundred percent,lj°-~4~ At higher strains, stage IV is usually characterized by a very low work-hardening rate. Generally, dislocation arrangements vary from the equiaxed cell structures observed in the transverse sections of tension specimens at moderate strainstl6-wJ to the elongated ribbon-shaped grains found at l a r g e deformations, in the longitudinal direction, in wire drawing and rolling of metals, tl2-2°l The deformation mode does not affect the dislocation structures and work hardening at low strains. 12~,22j However, the behavior of these structures cannot be extrapolated t o moderate or l a r g e strains. Recent results by Hughes and Nix|231 indicate that there are significant differences in the dislocation structures between torsion and drawing (and rolling) of metals. This present investigation details the evolution of dislocation structures during torsion of copper for strains ranging from 10 t o 440 pct. It also addresses the dependence of the work-hardening rate on the dislocation substructures.
II.
EXPERIMENTAL P R O C E D U R E
The material studied in this investigation was highpurity copper (99.94 pct). Cylindrical tensile specimens, as shown in Figure l(a), were annealed in an argon atmosphere for 2 hours at 620 °C, prestrained to 10 pct C.D. LIU, Research Scientist, and M.N. B A S S I M , Professor, are with the Metallurgical Sciences Laboratory, Department of Mechanical and Industrial Engineering, University of Manitoba, Winnipeg, MB R3T 2N2, Canada. Manuscript submitted February 17, 1992.
METALLURGICAL TRANSACTIONS A
plastic deformation, and then heat-treated for 1 hour at 404 °C in an a r g o n atmosphere. This procedure was used earlier t4"24] and results in a grain size of 42.5 /zm. Each specimen was subjected to torsion with a constant rate of 0.017 radians s -l, which resulted in a maximum shear strain rate of 1 0 -4 S -1 . The angle of twist was measured within an accuracy of 0.035 radians by using a micrometer mounted on the rotary shaft. The resisting torque was measured using a 60-1b capacity compression pressure meter. Four specimens were torqued t o rupture to determine shear stress-shear strain curve and the failure strain. Another two specimens were torqued t o a torsion a n g l e of 0J8 and 0i/2, respectively, where 0s denotes the twist a n g l e of the specimen at failure. Three torsion specimens w i t h different torsion angles were sliced parallel to the longitudinal axis t o obtain transmission electron
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