Microstructure changes in ultrafine-grained nickel processed by high pressure torsion under ultrasonic treatment

Commercially pure nickel was processed by high pressure torsion (HPT) and subjected to ultrasonic treatment (UST) with different amplitudes of compression-tension stresses in the zone of stress antinode of a standing wave. It was found that microstructure parameters such as the dislocation density, low- and high-angle grain boundary fractions, microhardness, and the stored excess energy as well, non-monotonically depend on the ultrasound amplitude. A structure relaxation leading to a reduction of internal stresses and stored energy and increase of the fraction of high-angle boundaries was observed at some intermediate amplitudes of the oscillating stress. The maximum relaxation effect was observed in the samples after UST with the amplitude of 60MPa. Possible mechanisms of the influence of ultrasound on the microstructure of deformed materials are discussed.