INVESTIGATION OF THE INFLUENCE OF JUNCTION DISCLINATIONS ON PLASTIC FLOW STRESS OF POLYCRYSTALS

Plastic deformation of polycrystalline solids is accompanied by the appearance of linear rotational-type mesodefects at grain boundary ledges and triple junction of grains, such as starin induced junction disclinations. Junction disclinations generate long-range spatially inhomogeneous fields of elastic stresses, which significantly influence on the structure formation, strain hardening and fracture of materials. In present work a comparative analysis of the contributions of junction disclinations of different sign and strength to the plastic flow stress of a polycrystal is performed. The results of calculationsshow, that when a pile-up of lattice dislocations passes through the elastic field of disclinations, general regularities are observed.Regardless of the sign of disclination, it has a retarding effect on the plastic shear. The equilibrium distributions of the linear density and the density of the Burgers vector of dislocations pile-upretarded by the elastic field of disclination are calculated.It is shown that the largest number of dislocations is concentrated not in the pile-up head, as in classical dislocation pile-upsstoped near impenetrable barriers, but in its central part. The dependences of the critical stress of the passage of the head dislocation of the pile-up through the force barrier of disclination are calculated depending on the strength and sign of disclination, the number of dislocations in the pile-up, and the distance between the disclination and the slip plane of lattice dislocations.It is shown that the change in the sign of disclination significantly influences on the form of the equilibrium distribution of dislocations along the length of the pile-up, but practically does not affect the value of the critical shear stress. It is shown that for a fixed number of dislocations in the pile-up, the critical shear stress increases with the distance between the slip plane and disclination. Thus, when plastic deformation is localized, the greatest strengthening effect from the elastic field of junction disclination is achieved not near the boundary, but far from it.

DIFFUSION NUCLEATION OF CAVITIES IN GRAIN JUNCTIONS OF SUBMICROCRYSTALLINE MATERIALS

The conditions of diffusional cavity nucleation in submicrocrystalline materials processed by the methods of intensive plastic deformation (equal-channel angular pressing, multiaxial forging, high pressure torsion, etc.) are analyzed. To date, the question of the mechanism of nucleation of cavities in such materials remains debatable due to the fact that the processing of materials by the methods of intensive plastic deformation is carried out at high hydrostatic pressures that prevent the appearance of pores. The possibility of diffusive nucleation of nanopores in the region of triple junctions of grains containing negative strain-induced wedge disclinations, generating high tensile stresses in the vicinity of triple junctions, comparable in magnitude to external hydrostatic pressure, is shown. Such junction disclinations inevitably occur at the grain junctions due to the heterogeneity of the plastic deformation through the ensemble of polycrystal grains. It is shown that an important condition for the nucleation of cavities is not only the presence of high internal tensile stresses from junction disclinations, but also an extremely high concentration of nonequilibrium strain-induced vacancies characteristic of submicrocrystalline metals, comparable in values to the vacancy concentration, at temperatures close to solidus. The influence of the strength of junction disclinations, the value of external hydrostatic pressure and the degree of supersaturation of the material by nonequilibriumstrain-induced vacancies on the rate of diffusional nucleation and the volume of critical pore nuclei is analyzed. It is established that in order to effectively suppress the process of pore formation in the grain boundary triple junctions, it is necessary to apply an external hydrostatic pressure that compensates for internal elastic fields from junction disclinations.

Computer Simulation of the Interaction of Junction Disclinations in Nanomaterials with Grain Boundary Vacancies

Junction disclinations are important elements of the structure of nanostructured metals produced by severe plastic deformation (SPD). Effect of these defects on the formation energy of vacancies in grain boundaries (GBs) is studied by means of atomistic computer simulations. Estimates based on the calculations of vacancy formation energies suggest that at least two orders of magnitude increase of the GB diffusion coefficient can be expected due to junction disclinations in nanostructured metals.