Deformation Induced Vacancies with Severe Plastic Deformation: Measurements and Modelling

Abstract

In discussing hardening characteristics in terms of crystalline lattice defects, in most cases the properties and kinetics of dislocations and their arrangement have been considered. However, during plastic deformation also vacancies and/or vacancy type defects are produced in very high densities which are typically close to those of vacancies in thermal equilibrium at the melting point. The effect of high vacancy concentrations on the hardening characteristics is twofold: (i) direct effects by impeding the movement of dislocations (ii) indirect one by inducing climbing and annihilation of edge dislocations leading to softening or even absolute decreases in strength. This paper presents first measurements of deformation induced vacancies in SPD materials which have been achieved by combined evaluation of resistometry, calorimetry and X-ray diffraction. The density of vacancies during and after SPD deformation is found to be markedly higher than in cases of conventional deformation and/or coarse grained material which may be partly attributed to the particular conditions of SPD namely the enhanced hydrostatic pressure as well as the changes in deformation path. It is suggested to make this high vacancy concentration responsible for both dynamic and static recovery and/or recrystallisation processes recently found during and after SPD, being potential reasons for enhanced ductility and superplasticity which only occur with nanomaterials originating from SPD. Recent publications show that in alloys, SPD induced vacancies can also enable the existence of phases which do not appear in the equilibrium diagram.

Surface morphology and deuterium retention in tungsten and tungsten–rhenium alloy exposed to low-energy, high flux D plasma

“…The BDT temperature depends also on the material microstructure and, for W single crystals, on the crystal-lattice orientation [25]. Note that in W materials liable to plastic deformation under D plasma exposure, vacancy-type defects are produced in very high densities which are typically close to those of vacancies in thermal equilibrium at the melting point [20], and the vacancy-type defects are formed in surroundings of D atoms.…”

“…During exposure to the high flux hydrogen plasma, a high hydrogen concentration in the implantation zone stresses the matrix lattice. When the compressive stress induced by the hydrogen supersaturation exceeds the yield stress of the material, fracture deformation and/or plastic deformation occur [8], and both intergranular/intragranular cracks (cavities) [10] and vacancy-type defects (vacancies and vacancy clusters) [20] are generated. Diffusing hydrogen atoms are trapped at the 6 vacancy-type defects forming consequently gas bubble nuclei, and recombine also on the cavity surfaces.…”

Hardening of pure metals by high-pressure torsion: A physically based model employing volume-averaged defect evolutions

“…Ta and also for other metals E A,x is substantially lower than the activation energy for self- strongly influenced and aided by a high density of deformation induced defects such as vacancies and dislocations [38,39,82].…”

Formation of vacancy-type defects in titanium nickelide

“…Based on recent publications, the development of nanoporosity, occurring in Al-, Cu-03017-p. 4 based alloys after equal-channel angular pressing (ECAP), causes the decrease in the long-time durability [55]. Most investigations on the formation deformation vacancies at SPD were conducted predominantly on pure metals [56][57][58], which was under conditions of ambient temperature when vacancy mobility is comparably low.…”