Effect of ultrasonic treatment on the structure and microhardness of ultrafine grained nickel processed by high pressure torsion

Abstract

Effect of ultrasonic treatment (UST) on the microstructure and microhardness of ultrafine grained nickel processed by high pressure torsion (HPT) was studied. With this aim, samples after HPT were subjected to oscillating compression-tension stresses with amplitudes 15, 30, 45, 60 and 90 MPa in the zone of strain antinode of an ultrasonic instrument. The microstructure of initial and ultrasonically treated samples was studied by X-ray diffraction analysis, transmission electron microscopy and electron back-scatter diffraction (EBSD). Dependences of average dislocation density, internal strains and microhardness on the ultrasound amplitude were obtained. At lower amplitudes (15, 30 MPa) UST leads to some increase of microhardness, internal stresses and dislocation density. At amplitude 60 MPa a considerable reduction of all mentioned characteristics as compared to the initial state occurs, while with a further increase of the amplitude to 90 MPa they exhibit a back increase. An increase of the fraction of high-angle boundaries without a significant change in the average grain size also occurs after UST. The regularities observed are explained as follows. With an increase of ultrasound amplitude, generation and accumulation of dislocations in the grains occur first. Then the processes of their passing through grain boundaries and rearrangement are activated that results in an increase of misorientations of the grains, more effective screening of dislocation stress fields and stress relaxation. With further increase of the stress amplitude processes of generation and accumulation of dislocations prevails over the processes of their annihilation and rearrangement, so that the relaxation effect weakens and diminishes.

Nonequilibrium grain boundaries in bulk nanostructured metals and their recovery under the influences of heating and cyclic deformation. Review

“…This work allowed one to predict many qualitative effects related to UST such as the relaxation of GBs, internal strains, and increase of the fraction of high-angles boundaries. All these predictions are confirmed by experimental data presented in [20,24,25].…”

“…Multiplication of dislocations during UST of ordinary materials is observed at stress amplitudes less than 10 MPa [81], therefore, in UFG metals dislocation generation can be expected at stresses of several dozens of megapascals. Increase of internal strains in experiments reported in [24,25] occurs at about 30 MPa. Therefore, the observed behavior of an increase of internal stresses followed by relaxation with an increase of the UST amplitude can be explained by the interaction of lattice dislocations and NGBs.…”

“…their relaxation occurred. Later studies of HPT-processed Ni have shown that the effect of UST is not monotonous: first an increase of the GB energy, rms strains, microhardness occurs with an increase of the UST amplitude, then all these characteristics decrease indicating on the GB relaxation [24,25]. Studies using ECAP-processed Ni revealed an uncommon effect of UST on mechanical properties: at stress amplitudes 50 ÷ 90 MPa it resulted in a simultaneous enhancement of both the elongation to failure and ultimate strength at room temperature tensile tests [22].…”

Energy Stored during High Pressure Torsion of Pure Metals

“…Среди многочисленных процессов ИПД для получения объемных наноструктурных материалов наиболее часто используются равноканальное угловое прессование (РКУП) [3] и кручение под высоким давлением (КВД) [4]. Первоначально считалось, что процессы ИПД протекают при температуре окружающей среды, которая соответствует низкой гомологической температуре, обычно T < 0.3 • T M , для большинства металлических материалов, таких как Cu [5], Ni [6,7], Ti [8], Zr [9] и др.…”

A double-gaussian waveguide for ultrasonic treatment of metals

“…Recent studies on the influence of UST on these materials discovered a number of effects related to the relaxation of their highly nonequilibrium structure such as the reduction of internal stresses, refinement of the structure of grain boundaries, enhancement of the thermal stability of the microstructure, of ductility and impact toughness without a loss of the strength etc. [11][12][13][14][15][16].…”