Features of change of V-4Ti-4Cr alloy hardness during microstructure evolution under severe plastic deformation

Features of changes of microhardness and nanohardness of V–4Ti–4Cr alloy at different stages of microstructure transformation during severe plastic deformations by torsion under pressure are presented. Microstructure features and mechanisms of its transformation affecting the hardness of studied alloy are discussed. Local temperature increase and activization of relaxation processes of nonequilibrium nanostructure states with high (hundreds of degrees/µm) elastic curvature of the crystal lattice are considered as main factors that define nonmonotonic character of changes in the microhardness at the stage of formation and evolution of the two-level structural states. In alloy under study at a value of true logarithmic deformation e ≥ 6.6 the formation of areas consisting of nanocrystals several nanometers in size with a high density of large-angle boundaries and elastic curvature of the crystal lattice hundreds of degrees/µm was found. Hardness of the material (Hnano ≈ E/16) differs little from its theoretical (limit) hardness

Theoretical analysis of features of dipole and quadrupole configurations of partial disclinations in nanocrystals of metals

The results of theoretical analysis of fields of local internal stresses and their gradients of nonequilibrium substructures in metal materials during the formation of nanostructural states in conditions of intensive deformation are presented. Possibility of use of disclinational approach within the continual theory of defects for description of the above mentioned states at nanoscale structural level is shown. Dipole and quadrupole configurations of wedge partial disclinations are considered. It's shown that the size of local internal stresses and their gradients can reach P ≈ E/50 and ∂P/∂x ≈ 0.02 nm-1 respectively. The calculating experiment demonstrates that high values of local internal stresses and their gradients in the field of elastic distortions can be described by multipole configurations of partial disclinations like dipole/quadrupole. The problem of the nanodipole configuration of partial disclinations and its disintegration on quadrupoles system is considered

Stress fields and energy of disclination-type defects in zones of localized elastic distortions

This paper studies theoretically the elastically deformed state and analyzes deformation mechanisms in nanocrystals in the zones of localized elastic distortions and related disclination-type defects, such as dipole, quadrupole and multipole of partial disclinations. Significant differences in the energies of quadrupole and multipole configurations in comparison with nanodipole are revealed. The mechanism of deformation localization in the field of elastic distortions is proposed, which is a quasi-periodic sequence of formation and relaxation of various disclination ensembles with a periodic change in the energy of the defect

Effect of thermomechanical treatment modes on structural-phase states and mechanical properties of metastable austenitic steel

The features of the structural-phase states and mechanical properties of metastable austenitic steel after thermomechanical treatments have been investigated. It is shown that low-temperature and subsequent deformation in the temperature range 300–773 K contributes to the direct (γ → α′)-martensitic transformation. The combination of low-temperature, subsequent warm deformation at 873 K and annealing at 1073 K leads to the direct (γ → α′)- and reverse (α′ → γ)-martensitic transformations. As a result of thermomechanical treatments submicrocrystalline two-phase structural states with high strength properties (σ0.1 ≈ 1160–1350 MPa) are formed

Features of deformation localization in stable austenitic steel under thermomechanical treatment

Features of structural states of Fe-18Cr-14Ni-Mo austenitic steel after thermomechanical treatment, including low-temperature and warm rolling deformation, were investigated by means of transmission electron microscopy. It is shown that mechanical twinning in multiple systems and strain localization bands contribute to grain fragmentation with the formation of the submicrocrystalline austenitic structure. These bands lie in the miсrotwin structure, have high-angle (≈60°–90°, 〈110〉) misorientations of the crystal lattice relative to the matrix and localize significant (up to ≈1) shear strain. In areas of the bands, structural states with high (tens of deg/μm) curvature of the crystal lattice and high local internal stresses are observed. The internal structure of the bands is presented by nanoscale fragments of austenite and α′-martensite. The presence of specific misorientations and fragments of martensite means that the formation mechanism of localized deformation bands are direct plus reverse (γ → α′ → γ) martensitic transformations with the reverse transformation follows by an alternative path. These structural states provide high strength properties of steel: the yield strength is up to 1150 MPa

Nanostructural states in Nb-Al mechanocomposite after conbined deformation treatment

Nanostructural states were investigated, that were formed in Nb-Al system-based mechanocomposite after combined deformation treatment that includes mechanical activation in a planetary ball mill and subsequent consolidation by torsion under pressure on Bridgman anvils. The formation of the layered structure, consisting of Nb and Al nanobands with width from several to several tens of nanometers was revealed. The structural states with high elastic curvature of crystal lattice and high level of local internal stresses found in Nb and Al subgrains were investigated by transmission electron microscopy

The features of microstructure and mechanical properties of austenitic steel after direct and reverse martensitic transformations

The features of structural states of metastable austenitic steel after thermomechanical treatments, including low-temperature deformation, warm deformation and subsequent annealing are investigated. It is shown that under these conditions the direct (γ → α′) and reverse (α′ → γ) martensitic transformations occur and submicrocrystalline structural states are formed. The proposed thermomechanical treatment allows varying the strength and plastic properties of austenitic steel in a wide range. The strength of steel in submicrocrystalline state is 4–6 times higher than its original value

Evolution of microstructure and microhardness of dispersion-hardened V-Cr-Zr-W alloy during deformation by torsion under pressure

Results of the study of the microstructural evolution and microhardness changes of dispersion-hardened V–Cr–Zr–W alloy under severe deformation during torsion on Bridgman anvils are presented. Typical structural states and mechanisms of their formation are revealed for basic evolution stages as well as appropriate microhardness values are determined. It was shown that at true logarithmic strain values (e) in the range 0.7 ≤ e 2, the anisotropic submicrocrystalline structure is observed and the formation of two-level nanostructural states was found within grains. In the strain range (e) from 3 to 6.6, submicrocrystal sizes hardly change, but changes of two-level nanostructural state parameters are observed: the nanofragment size decreases and values of elastic curvature of the crystal lattice increases

Influence of temperature on microstructure parameters and microhardness of dispersion-hardened V–Cr–Zr–W alloy after deformation by torsion under pressure

Study of microstructure transformation and microhardness changes of dispersion-hardened V–Cr–Zr–W alloy after severe plastic deformation by torsion on Bridgman anvils and subsequent heat treatments was conducted. Basic stages of relaxation processes were revealed: at 800°C recovery processes take place and primary recrystallization begins; at 900°C primary recrystallization intensifies; in range of 950–1050°C collective recrystallization processes activate; at 1200°C secondary recrystallization starts. Microhardness measurement and comparison of its values with structural states features were conducted. Strengthening mechanisms and their contribution at various stages of defect substructure relaxation are discussed. It is shown that increase of thermal stability of V–Cr–Zr–W alloy microstructure is a consequence of the formation of high density of thermally stable Zr (O–N–C)-based nanoparticles