I. A. Ditenberg

On the limiting minimum size of grains formed in metallic materials produced by high-pressure torsion

Abstract-Results are presented concerning the experimental studies of the limiting minimum grain size dmin in nanostructured metallic materials produced by high-pressure torsion in Bridgman anvils. It is shown that important factors that determine dmin in the process of severe plastic deformation are the critical stresses for dislocation shear and the mobility of point defects, which control the paths of these defects in stress fields, the characteristic scales of the microvolumes involved into reorientation processes, and therefore the sizes of grains and subgrains of submicroscopic and nanoscopic scales.

Lattice curvature evolution in metal materials on meso- and nanostructural scales of plastic deformation

The paper generalizes results of electron microscopy studies of structural states with high lattice curvature which arise in a wide class of materials under various conditions of severe plastic deformation: rolling, equal channel angular pressing, mechanical activation in planetary ball mills, and torsion in Bridgman anvils. The states are divided into two types: 1) a substructural state with elastoplastic lattice curvature of tens of degrees per micron due to high density of like-sign excess dislocations; and 2) a state with elastic lattice curvature up to several hundreds of degrees per micron in volumes of several nanometers. Analysis is performed to inquire into the formation of these states, peculiarities of their evolution, and their role in different mechanisms of plastic deformation and formation of nanocrystalline structures.

Microstructural evolution of nickel under high-pressure torsion

The evolution peculiarities of grain and defect structures in nickel under high-pressure torsion were studied by transmission electron microscopy and X-ray diffraction analysis. Lattice reorientation mechanisms characteristic of different stages of plastic deformation were disclosed. The conditions and features of cooperative realization of various structure formation mechanisms under severe deformation were discussed.

Peculiarities of the formation of high-defect states in mechanocomposites and powders of niobium and aluminum under severe deformation in planetary ball mills

The microstructural peculiarities of Nb powder and Nb + Al powder mixtures after mechanical activation in a high-energy planetary ball mill were investigated by transmission electron microscopy. The materials revealed two-level structural states: nanograins of size from 50 to 100 nm that contain subgrains of size ∼20 nm and less with low-angle misorientation boundaries, elastic lattice curvature gCij≈100°…200°μm−1, curl or curvature gradient ∂χij/∂r > 100 μm−2 and high (up to 10E μm−1) local stress gradients (couples). An important factor in the formation of the above states is the capability of nano-objects to reach high elastic lattice curvature, high gradients of this curvature, and high local internal stress gradients at rather low absolute values of the internal stress.

Features of the formation of the submicrocrystalline structural state upon plastic deformation of a V-4Ti-4Cr alloy in Bridgman anvils

Results are presented of electron-microscopic investigations of the main features of the microstructure of an V-4Ti-4Cr alloy after severe plastic deformation by high-pressure torsion in Bridgman anvils. Parameters of the defect structure in the bulk and at the boundaries of grains have been studied using the method of dark-field analysis of discrete and continuous misorientations. Fields of local internal stresses and gradients of these stresses on the submicron level have been analyzed. Mechanisms of the formation of the submicrocrystalline structural state have been discussed.

Effect of severe mechanical alloying on the microstructure parameters of 3Ti + Al mechanocomposites

The microstructure of mechanocomposites produced by mechanical alloying of Ti and Al powder mixtures in high-energy planetary ball mill has been studied by the methods of transmission electron microscopy and X-ray diffraction analysis. The formation of highly defect structural states with high magnitudes of the crystal-lattice curvature and large density of disclinations at the boundaries of submicrocrystals and nanocrystals has been revealed. It is assumed that such a highly defect structural state is an important channel for the accumulation of deformation energy upon mechanical alloying and plays an essential role in phenomena of increasing reactivity of components of mixture systems, abnormal mass transfer, and solid-state interaction of reactants.

Nanodipoles of partial disclinations in the region of localized elastic distortions

Strain localization in the region of elastic distortions is revealed in nickel dynamic recrystallization grains during torsion in Bridgman anvils, which leads to the formation of reorientation nanobands with the elastic lattice curvature equal to hundreds of degrees per micron. It is shown that the nanobands are formed by the motion of partial disclination nanodipoles, i.e., zones of constrained elastic shear and rotations distinguished by extremely high local internal stress gradients.

Microstructural peculiarities of copper and mechanisms of its hardening after mechanical activation and torsion in Bridgman anvils

The paper presents the results of complex research on microstructural peculiarities of copper and mechanisms of its hardening after mechanical activation in planetary ball mills, high-pressure torsion, and combined treatment which includes mechanical activation and subsequent consolidation under high-pressure torsion in Bridgman anvils. The main structural factors responsible for the hardening mechanisms are discussed depending on the pattern and degree of deformation.

Features of the microstructure and mechanical properties of V-Zr-C alloy depending on modes of thermomechanical treatment

Features of the grain and heterophase structure modification for the alloy of V-Zr-C system depending on the modes of thermomechanical treatment are investigated. The formation of structural states with a high density of fine-grained (up to 5 nm) second-phase particles uniformly distributed over the material volume provides a significant increase in the strength characteristics at room and high (800°C) temperatures while maintaining technologically acceptable plasticity. The effect of various factors up on the strength of V-Zr-C alloy is considered.