Abstract
Annealed foil specimens of Polycrystalline Copper and Copper-Nickel alloys containing 5, 10 and 30 at. % Ni and also a commercial alloy (Constantan) with an approximate composition of 47 at. % Ni were cyclically deformed under conditions of constant total strain amplitude. Electrical resistance changes were measured during cyclic deformation at room temperature, for total strain amplitudes between +/- 0.4 x 10[-3] and +/- 3.8 x 10[-3]. For all compositions except the commercial alloy, measurements were also made at liquid nitrogen temperature after fatigue at that temperature. Annealing studies were used to interpret the results and to determine the migration energies of point defects produced during cyclic deformation. Values of flow stress were determined after cyclic deformation at the highest strain amplitude, as a function of Ni concentration and for the commercial alloy, over a range of grain sizes. The results are compared with those previously obtained for rod specimens and discussed in terms of solute and dislocation hardening. A universal relation between hardening and the number of cycles has been found to fit all the alloys at the high amplitude. A study has been made of the effect of increasing Ni concentration and strain amplitude on the persistent slip bands formation, using optical and scanning electron microscopy. Threshold values of the saturation plastic strain, below which these bands do not appear, were estimated for the 5 and the 10 at. % Ni alloys and compared with pure Cu. These results have been used to interpret the cyclic hardening measurements. Previous workers have suggested that the break-up of Ni clusters, by cyclic deformation is the major contribution to the resistance change, a smaller contribution being attributable to the introduction of defects. The different contributions have been quantitatively estimated for the 5 and the 10 at. % Ni alloys.