Cyclic softening of ultrafine grain copper

Ultrafine grain copper produced by severe plastic deformation (SPD) was tested in tension-compression fatigue under strain control. The fatigue 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 softening in all tests performed. The softening increased with increasing fatigue life at high strain ranges (Δεp=2×10−2−5×10−3). Below Δεp=2×10−3 softening began to decrease accompanied by a remarkable heterogeneity in the dislocation substructure. It is likely that the regions with the finest dislocation substructure appear `hardʹ to dislocation motion and, hence, are more difficult to soften by the to-and-fro motion of dislocations during cyclic deformation. Finally, observations of the sample surfaces revealed damage in the form of parallel cracks and extrusions reminiscent of persistent slip band (PSB) activity, particularly for samples tested at lower strain ranges Δεp≈1×10−3.