Yu. I. Pochivalov

The structure and microhardness evolution in submicrocrystalline molybdenum processed by severe plastic deformation followed by annealing

Abstract A possibility to form submicrocrystalline structure in molybdenum using severe plastic deformation treatment by torsion under high pressure (HPT) at elevated temperatures has been studied. Quantitative parameters of grain–subgrain structure have been obtained by optical microscopy and transmission electron microscopy methods. Thermal stability of microstructure and mechanical properties have been studied. It is established that HPT results in the formation of a submicrocrystalline structure in Mo and in disappearance of residual porosity. The average grain size of HPT-Mo is 0.2 μm. The formation of submicrocrystalline structure enhances significantly (by 2.4 times) the microhardness of Mo relative to that in the as-received (before HPT treatment) state. It is found that grain growth begins at 1173 K and develops intensively at T ⩾1273 K.

Nanostructuring of surface layers and production of nanostructured coatings as an effective method of strengthening modern structural and tool materials

The high efficiency of improvement of macromechanical characteristics of structural and tool materials by nanostructuring their surface layers or due to the application of nanostructured coatings has been theoretically and experimentally substantiated. A loaded solid is considered as a multilevel system in which the surface layers are an independent subsystem. Special attention was paid to the surface-layer (coating)-substrate system, in which a staggered distribution of normal and tangent stresses and related different-scale stress concentrators appears.

Effect of structural states in near-surface layers of commercial titanium on its fatigue life and fatigue fracture mechanisms

Changes of structural states in near-surface layers of α-titanium strongly affect its fatigue life and fatigue fracture mechanisms. In α-titanium subjected to alternate bending a sliding mode crack develops, resulting in slight delamination of the material. Hydrogenation of α-titanium surface layers preserves their sliding mode cracking but greatly enhances their delamination. Nanostructuring of α-titanium surface layers increases their nanohardness and elastic modulus and causes opening mode cracking. The fatigue life of the material after surface hydrogenation decreases three times, and after surface nanostructuring, it increases four times.

Rotational deformation modes in near-boundary regions of grain structure in a loaded polycrystal

Experiments were performed to study the defect structure formation in a thin surface layer of 20Cr18Ni10Ti steel in frictional wear. It is shown that plastic deformation constrained by a friction pair is inhomogeneous and displays a vortex rotational character. A method of excitable cellular automata is developed for a theoretical description of the formation of vortex structures in multiscale polycrystalline structures. Numerical experiments demonstrate the formation of rotational deformation modes on different structural scales—modulation of local moments of forces along the grain boundary and fragmentation of initial structural elements in initial conglomerates of self-consistently deformed grains in the field of couple stresses.

Grain boundary migration initiated by diffusion

A review is presented of work related to two new processes which arise in crystalline materials when impurities diffuse along grain boundaries from the surface of the material: diffusion-initiated grain boundary migration and recrystallization. We analyze the conditions under which DIGM occurs, the kinetics of the process, its driving forces, and also the changes in the grain fine structure and the near-grain regions, as well as the nucleation of new grains on the migrating boundaries, caused by an uncompensated impurity atom flux. We consider the mechanisms for DIGM. It is shown that not one of the mechanisms proposed to describe DIGM is capable of explaining all the experimentally observed properties of grain boundary migration under DIGM conditions. We note that changes in the grain boundary structure caused by the impurity atoms diffusing along it are due to diverse grain-boundary processes, which have important technological implications.

Scale invariance of structural transformations in plastically deformed nanostructured solids

The scale-invariant mechanical behavior of a nanostructured solid is associated with plastic distortion as a major mechanism of nano- and microscale structural transformations. Active grain boundary sliding in a deformed material (microscale) within its highly developed planar subsystem (nanograin boundaries) causes a progressive increase in lattice curvature and plastic distortion of atoms which produces nonequilibrium vacant sites in the nanostructure. The motion of nonequilibrium point defects in nanostructure curvature zones provides conditions for noncrystallographic plastic flow, dissolution or dispersion of initial phases, and formation of nonequilibrium phases in a deformed material. The possibility of reversible structural phase transformations in the presence of high lattice curvature opens the way to greatly increase the fatigue life of surface nanostructured polycrystalline materials.

The Effect of Severe Plastic Deformation on the Structure and Mechanical Properties of Al–Mg–Li Alloys

The structural-phase state and mechanical properties of commercial aluminum alloys produced by severe plastic deformation are studied and compared to the initial polycrystalline state. This kind of treatment is found to give rise to the formation of a homogeneous ultrafine-grained structure with second-phase particles occurring predominantly along grain boundaries. With this structure, the strain-temperature and strain-rate intervals wherein the superplastic properties of the alloys under study are observed are shifted to lower temperatures and higher rates.

Wear of steel with ultrasound-induced nanostructuring of the surface layer. Part 1. Mechanical properties and wear resistance

The structural modification and change of wear resistance on friction with boundary lubrication is considered for 110Γ13 steel with and without impact ultrasound treatment. The deformational relief formed on the lateral surface of the samples in compression and in frictional loading is found to be the same. On that basis, the factors responsible for the change in wear resistance are discussed.

Scientific basis for cold brittleness of structural BCC steels and their structural degradation at below zero temperatures

The paper considers the physics of cold brittleness of structural bcc steels and methods of reducing the ductile-brittle fracture temperature. A complex study was performed to examine the degradation of structural phase state of pipe steel 09Mn2Si from the main gas pipeline of Yakutia after long-term (over 3 0 years) operation. Important regularities of degradation of pearlite colonies with carbide precipitation on ferrite grain boundaries were revealed. This phenomenon is associated with brittle fracture of gas pipelines. It is shown that the low-temperature kinetic processes in main pipelines which define the degradation of their structure and properties are related to interstitial athermal structural states in the zones of local crystal structure curvature. This is a fundamentally new, as yet unknown, mechanism. Pipe steels in warm rolling acquire a longitudinal textured band structure with alternating bands of initial ferrite grains and bands of fine grains with carbide precipitates formed during lamellar pearlite degradation. This type of structure allows for a shift of ductile-brittle transition temperature down to -80°C and ductility δ = 22% at this temperature. The production of high-curvature vortex structure in pipe steel surface layers results in a 3.5-fold increase in their service life.

Modification of the Structure of Low-Carbon Pipe Steel by Helical Rolling, and the Increase in Its Strength and Cold Resistance

The paper reports the investigation results on the microstructure and mechanical properties of low-carbon pipe steel after helical rolling. The processing of the steel leads to the refinement of ferritic grains from 12 (for the coarse-grained state) to 5 μm, to the strengthening of ferrite by carbide particles, a decrease in the total fraction of perlite grains, a more uniform alternation of ferrite and perlite, and the formation of regions with bainitic structure. The mechanical properties of the steel have been determined in the conditions of static and dynamic loading in the range of test temperatures from +20 to–70°С. As a result of processing, the ultimate tensile strength increases (from 650 to 770 MPa at a rolling temperature from 920°С) and the viscoplastic properties at negative temperatures are improved significantly. The ductile–brittle transition temperature of the rolled steel decreases from–32 to–55°С and the impact toughness at the test temperature–40°С increases eight times compared to the initial state of the steel.