The Development of Vacancies during Severe Plastic Deformation

Abstract

A high density of lattice defects is introduced to materials by severe plastic deformation (SPD). Numerous experimental techniques, in particular electron microscopy, X-ray, electron and neutron diffraction, etc. are employed to characterize the evolution of microstructure and defects with strain introduced to the material by SPD. These techniques concentrate mainly on the investigation of planar (grain boundaries) and line defects (dislocations). On the other hand, point defects, namely vacancies and their agglomerates are investigated in less detail. Positron annihilation spectroscopy (PAS) proved to be an effective method for the investigation of point defects and dislocations in ultra-fine grained (UFG) materials. This study summarizes the results of the investigation of lattice defects in UFG metals with fcc (Al, Ni, Cu), bcc (Fe, Nb, W) and hcp (Mg, Ti) structure prepared by high pressure torsion (HPT). Two techniques of PAS were employed (i) positron lifetime spectroscopy (LT) allowing to characterize the type and concentration ratio of lattice defects in the severely deformed material and (ii) Doppler broadening (DB) of annihilation radiation providing analysis of the homogeneity of the UFG structure and spatial distribution of defects. The latter technique was complemented by mapping of microhardness distribution throughout the surface of the HPT specimens. The LT studies revealed that HPT straining at room temperature introduced not only dislocations but also a high concentration of vacancies. A significant fraction of deformation-induced vacancies disappeared by diffusion to sinks at grain boundaries. Remaining vacancies agglomerated into vacancy clusters. The average size of vacancy clusters differs in various metals and is affected by the activation energy for migration of vacancies in the given material. The analysis of DB of positron annihilation radiation and its correlation with microhardness distribution indicated that dislocation density tends to saturate with strain. On the other hand, the spatial (lateral) distribution of vacancy clusters remains non-uniform even in samples subjected to a high number of HPT revolutions. The average size of vacancy clusters increases with radial distance from the centre of the sample due to the increasing production rate of vacancies.

High‐pressure torsion of SiO2 quartz sand: Phase transformation, optical properties, and significance in geology

“…It is then concluded that HPT processing can induce oxygen vacancies in SiO 2 in good agreement with the earlier reports on the formation of vacancies in the HPT‐processed TiO 2 , 11 ZnO, 12 TiO 2 ‐ZnO, 13 Al 2 O 3 , 14,15 BaTiO 3 16 and ZrO 2 , 17 LiTaO 3 18 and GaZnON 22 . The formation of vacancies by HPT processing is usually attributed to the effect of strain on continuous formation of vacancies and to the effect of pressure on suppressing the migration and annihilation of vacancies 3,70–72 . The formation of oxygen vacancies by HPT suggests a possible pressure‐temperature‐strain‐based mechanism for the formation of vacancies in SiO 2 ‐based minerals and sands 73–75 …”

Improved Photocatalytic Hydrogen Evolution on Tantalate Perovskites CsTaO3 and LiTaO3 by Strain-Induced Vacancies

“…It is then concluded that HPT processing can induce oxygen vacancies in CsTaO3 and LiTaO3 in good agreement with the earlier reports on the formation of oxygen vacancies in the HPT-processed BaTiO3 [37], TiO2 [30], ZnO [31], Al2O3 [33,40] and TiO2-ZnO [32]. The formation of vacancies by HPT processing is due to the strain effect on continuous generation of vacancies and the pressure effect on suppressing the migration and annihilation of vacancies [46][47][48].…”

Power Law Breakdown in the Creep in Single-Phase Metals

“…Early modeling by Mecking et al [12] suggested that vacancy supersaturation by deformation in the power-law regime was unlikely, but suggested that supersaturation may occur within PLB. More recently, important modeling and experiments by others [13][14][15] also suggested that supersaturation may occur with significant plasticity within PLB. Wu and Sherby [16] however, subsequently suggested that internal stress, as did Nix and Ilschner, albeit from different sources, explains PLB behavior and appeared to abandon vacancy superstation effects.…”

“…If we assume just volume diffusion at ambient temperature for silver, which appears to achieve steady-state after strains greater than 3.0 at ambient temperature, the predicted vacancy concentration (based on the usual equations [27]) is roughly 10 −17 . With substantial plasticity (e.g., >0.3) the latest experimental and theoretical estimates [12][13][14][15]28] of the vacancy concentration at ambient temperature after large strain deformation in metals is of the order of 10 −3 to 10 −5 (roughly a factor of 10 13 higher than the equilibrium concentration). This suggests that the low-temperature diffusion coefficient may be relatively high giving rise to higher than expected creep rates leading to PLB.…”

“…(4) The changes in the diffusion coefficient from D sd to D p with large strain plasticity within the PLB regime may independently contribute to the observation of PLB. Another point must be made and that is references [14,15,28] verify vacancy supersaturation at ambient temperature; other, somewhat higher temperatures within PLB were not checked. Of course, higher temperature x-ray diffraction experiments are more difficult than those at ambient temperature.…”