Impact strength tests of nickel samples exposed to equal-channel angular pressing (ECAP) were carried out. The influence of annealing and ultrasonic treatment with stress amplitudes of 20, 50 and 100 MPa on the impact fracture of ultrafine grained nickel processed by ECAP was shown.
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.
The results of the study on the microstructure and properties of solid state joints of titanium sheets obtained by ultrasonic welding (USW) with frequency 20 kHz and vibration amplitude 20 μm during 2 and 3 s under a 6 kN clamping force are presented. The thermo-mechanically affected zone is observed along the defect-free weld line (joint). It was found that the length and width of this zone increased with the welding time. In the thermo-mechanically affected zone, a significant grain growth takes place, a change in the texture is observed and maxima near 60° and 90° in the grain boundary misorientation distributions appear. The maximum lap shear failure load of the joints obtained by USW of 2 and 3 s durations are 1998±122 and 2506±226 N, respectively. There is a change in the failure mode from interface debonding after 2 s USW to nugget pull-out for the welding time of 3 s.
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