A. D. Korotaev

Evolution of defect substructure of metal alloys at microscopic and mesoscopic level under torsion

Abstract Transmission electron microscopy was used to investigate the reorientation of crystal lattice during the formation of ultrafine-grained (UFG) copper, nickel, and an alloy of Ni–18% Al–8% Cr–1% Zr–0.15% B (at.%) under severe plastic deformation by equal-channel angular (ECA) pressing and twisting at a high quasi-hydrostatic pressure. The crystal lattice was found to transform into a UFG state; it is fragmented at the nano-, micro-, and mesoscale levels. Possible mechanisms for the reorientation of the crystal lattice under deformation at the micro- and mesoscale level are discussed.

Crystal-lattice distortions during mechanical twinning of the B2 phase of titanium nickelide via the mechanism of local reversible martensitic transformations

Tensor of distortions upon the formation of a {113} deformation twin by the mechanism of a combined (forward-plus-reverse) (occurring on an alternative transformation system) martensitic transformation in the B2 phase of titanium nickelide has been analyzed theoretically in the approximation of small deformations using the model of martensitic transformations based on the concept of “freezing” of cooperative thermal vibrations of atoms in close-packed atomic planes in metals. It is shown that this mechanism satisfactorily describes not only the reorientation angle but also the habit plane of the twin.

Defect substructure and local internal stresses inherent in plastic flow at mesolevel

Abstract This paper is concerned with the characteristics of defect substructures associated with plastic flow at the mesolevel. The important features are high curvature of the crystal lattice such that the local internal stress could reach the theoretical shear strength of the crystal and high stress gradient up to G/5 μm −1 giving rise to stress moments. The foregoing is characteristics of the deformation of high-strength materials.

Physics of the plasticity and fracture of high-strength crystals

Systematic studies of the mechanism of plastic deformation, strain hardening, and fracture of high-strength single crystals of heterophase alloys based on copper and austenitic stainless steels with nitrogen are reported. It is shown that the attainment of high resistance to the motion of dislocations results in the appearance of new mechanical behavior: strong orientation dependence of the critical shear stresses, a change in the deformation mechanism from slip to twinning, loss of mechanical flow stability at early stages in deformation, and a transition from viscous to brittle fracture.

Phase transformations in Mo under simultaneous implantation of metal and gas ions

Abstract Phase transformations of molybdenum under ion implantation with metal and gas ions have been studied by means of transmission electron microscopy. It is shown that, as a result of ion mixing of the surface-adsorbed gas elements of the ion source, thin (⩽ 100 nm) solid layers of the interstitial phases may be formed in the ion-implanted layers. The mechanisms of formation of the structure found are discussed by investigating the crystallographic details of phase transformations.

Peculiarities of structural phase and elastic stress states of superhard TiN-based nanocomposite coatings

A new concept of designing nanocomposite coatings is proposed. The concept consists in microstructural self-organization through simultaneous nucleation of islands of different mutually insoluble or slightly soluble phases at the stage of coating formation. Physical principles on which to select compositions of the coatings were developed and were experimentally verified on multicomponent nanocomposite coatings. With a Sprut magnetron arc plasma complex, superhard (Hμ > 40 GPa) multicomponent nanocomposite coatings of the system Ti-Al-Si-Cr-Ni-Cu-O-C-N were obtained. The peculiarities of structural phase and elastic stress states of the multicomponent coatings before and after annealing at a temperature of up to 1000 °C were studied by transmission electron microscopy, X-ray diffraction analysis, microhardness measurements and scratch tests. The study reveals a wide range of lattice bending-torsion (up to 200° μm−1) of nanosized (less than 30 nm) coherent scattering regions in the two-level coating structure and of individual (up to 15 nm) TiN nanocrystals. Annealing of the coatings causes the two-level grain structure to relax with the formation of TiN-based nanocrystals of size less than 30–40 nm and with a decrease in lattice bending-torsion down to 40°–50° μm−1. Comparative analysis of acoustic emission signals and tracks of the multicomponent and TiN coatings in scratch tests points to an increase in fracture ductility in the multicomponent coatings.

Defect microstructure in titanium nitride submicrocrystals

Transmission electron microscopy has been applied to the defect structure of titanium nitride in the submicrocrystalline (SMC) state produced under conditions of deviation from equilibrium in ion-plasma synthesis. The submicrocrystals contain a new type of defect substructure having a continuum disclination density up to 2.5 rad/µm2. Direct structure methods give evidence for a high density of partial disclinations at the SMC grain boundaries in the nitride phase. A novel method has been used to examine substructures having a high defect density, which has been used to estimate the partial disclination density at the submicrocrystal boundaries. The origin of this highly defective state and the effects of it on the properties of SMC materials is discussed.

Substructure formation in high-strength dispersely strengthened alloys

ConclusionsAnalysis of new experimental laws of plastic flow observed in high-strength alloys with dispersional strengthening (such as the formation of substructure with high crystal-lattice curvature, high-temperature localization of deformation from the earliest stages, with reorientation of the localized-shear zones and the adjacent undeformed structural elements) leads to the conclusion that deformational point defects play an important role in the realization of collective deformational modes in the high-strength state.In conditions of high nonequilibrium concentration, deformational point defects, first, permit the inclusion of quasi-viscous diffusional mechanisms of crystal-lattice reorientation by point-defect drift in the local fields of high inhomogeneous stress and, second, by facilitating dislocational deformation mechanisms, may lead to local weakening of the shear zones, localization of the plastic flow, and stability loss, in particular, as a result of mutually consistent autocatalytic defect multiplication.

Equipment and methods of surface modification of the microstructure and properties of metals by adsorption assisted ion implantation

Abstract Data on phase and structural transformations in the surface layers of metallic alloys as a function of the conditions of ion implantation are reviewed. It is noted that the ion mixing of the surface-absorbed active elements from the implantational gas medium plays an important role in the formation of the element and phase composition of the ion-doped layers. The most important treatment parameters determining the relative role of ionic mixing in structural-phase transformations in the ion-doped layer are: the elementary composition and pressure of the implantational gas medium, the target temperature, the atomic weight of the implanted ions, and the reactivity of these ions and the target ions to the absorbed elements (C, N, O). The formation of high-energy defect structures and the dispersion of the implanted-layer crystal structure in the strong internal-stress fields generated by the highly nonequilibrium solid solutions and in the course of phase transformation are considered. New possibilities for microstructural modification during high-dose implantation are identified with the formation of high-energy defect (including nanocrystalline) structures; heterophase, completely amorphized and other phase-structural states and their combinations in iondoped layers of metallic alloys. Taking advantage of these possibilities makes it possible to reduce by an order of magnitude the implanted dose necessary to upgrade the performance of the surface, and to raise the productivity and lower the cost of the ionbeam technological treatment.

Modification of the microstructure of TiN-based columnar coatings in indentation zones

The columnar structure of titanium nitride-based coatings deposited by magnetron sputtering is studied. The structure-phase state of the coatings is analyzed after deposition and in the indentation zone. The type of dislocation structure, the grain size, the subgrain size, the misorientations at boundaries, and their change during coating growth are determined. The detected decrease (several tens of percent) in the coating thickness under an indenter indicates plastic deformation of a coating. On a microscopic level, this deformation manifests itself in an increase in the density, the misorientation, and the nonequilibrium of boundaries.