CYCLIC SOFTENING OF ULTRAFINE GRAIN COPPER

Ultrafine grain copper produced by severe plastic deformation (SPD) wa s tested in tension-compression fatigue under strain control. The fati gue life of SPD Cu follows the Coffin-Manson law, though it appears to be shorter than that of coarse grain Cu. SPD Cu showed significant so ftening in all tests performed. The softening increased with increasin g fatigue life at high strain ranges (Delta epsilon(p) = 2 x 10(-2) - 5 x 10(-3)). Below Delta epsilon(p) = 2 x 10(-3) softening began to de crease accompanied by a remarkable heterogeneity in the dislocation su bstructure. It is likely that the regions with the finest dislocation substructure appear 'hard' to dislocation motion and, hence, are more difficult to soften by the to-and-fro motion of dislocations during cy clic deformation. Finally, observations of the sample surfaces reveale d damage in the form of parallel cracks and extrusions reminiscent of persistent slip band (PSB) activity, particularly for samples tested a t lower strain ranges Delta epsilon(p) approximate to 1 x 10(-3). (C) 1998 Elsevier Science S.A. All rights reserved.