J. Phys. Colloques 32 (1971) C2-144-C2-144
MODULUS CHANGES IN STRAIN-CYCLED COPPERR. DEN BUURMAN
Laboratorium voor Metaalkunde Technische Hogeschool Delft, Nederland This work is part of the research programme of the foundation F. O. M., sponsored by Z. W. O.
The shear modulus (G) of copper is heavily reduced by cold working at low temperatures as has been shown by Druyvesteyn et al. . The effect, the so-called ƊG-effect, was attributed to dislocations introduced by the deformation as described by Mott  and Friedel . When an elastic stress is applied, the dislocations bow out in their glide planes causing an extra elastic strain and thus a decrease of the modulus. This modulus defect was reduced by annealing at higher temperatures which was ascribed to the pinning of dislocations by point defects. Many investigators studied these effects after unidirectional deformation  but analogous studies on cyclically deformed materials are much less numerous. In the present work the decrease of the shear modulus due to strain controlled cyclic torsion was measured on rods of polycrystalline pure copper cycled at various amplitudes. After some 5 x 104 cycles at 78 °K, G reached a saturation value qualitatively in accordance with for instance the behaviour of the flow stress vs. number of cycles in fatigued metals. The saturation value of - ƊG/G increased with strain amplitude up to about 4 % for a strain amplitude of 4.2 x 10-4. The isochronal recovery behaviour was analogous to that observed after unidirectional deformation. Three discrete steps were present, called IIa, IIb, and III centered at 133 °K, 193 °K and 273 °K respectively. After cyclic deformation step III was considerably smaller. Cycling at room temperature and at 78 °K caused about the same ƊG-effect, if measured at 78 °K. As the point defects produced by the deformation are mobile at room temperature, this implies that point defect pinning does not occur when the dislocations move to and fro during cyclic deformation. The dependence of the modulus effect on the measuring temperature is in accordance with the observations of Druyvesteyn and Blaisse  after unidirectional deformation.