N. V. Nikitina

Evolution of crystallization structure during deep deformation and annealing of single crystals of austenitic carbon steel

The distribution of alloying elements, impurities, and secondary phases in axial and interaxial regions was studied for single crystals of the austenitic carbon steel, 04Kh17N14M3BG2, with the classical cross-shaped dendritic structure. It was shown that the interaxial regions are enriched with chromium, molybdenum, and carbon and contain precipitates of Cr23C6 and Mo2C. The chemical and phase inhomogeneities, connected with dendritic crystallization, are stable with respect to the effect of deep deformation and annealing, and in the finished product evolve into bands with a different content of impurities and second phase particles.

Microplastic deformation of polycrystals during cyclic loading

A model of microplastic deformation of polycrystals during zero-start cyclic loading with tensions lower than the yield strength is proposed according to which during cycling, thermally activated movement of dislocations occurs under conditions of stress relaxation. Based on this model and the statistical theory of polycrystalline microdeformation, the accumulation of microplastic deformation is theoretically described as a function of the number of loading cycles and the stress amplitudes. It is theoretically proved that in the cycling process the microplastic deformation that accumulates over one cycle decreases as the number of cycles increases; up to the macroscopic elastic limit it is independent of the stress amplitude, and then sharply increases. Agreement of the theory with experimental data for spring alloys is observed in the density of mobile dislocations, which decreases during cycling.

Stress relaxation in the region of microplastic deformation of polycrystals

Stress relaxation equations are derived to predict the relaxation capacity of a material on the basis of studies of microplastic deformation under static loading. The approach was checked experimentally on spring steels LANKMts, ÉI702, ÉP637, and 50KhFA.

Elastic lattice distortion in the statistical theory of multicomponent substitutional solid solutions with short-range order

The statistical theory of atomic distributions in multicomponent substitutional solid solutions with short-range order is considered, taking into account the various sizes of atomic components. The effect of elastic lattice distortions on the atomic distribution is estimated, and the dependence of the interatomic distance on alloy composition and amount of short-range order is calculated.

Study on the mechanism of hardening polycrystals of solid solutions Cu-Ni, Cu-Zn, Cu-Al, and Cu-Ge at microdeformation stage

The stress-strain curves were obtained at the stage of microdeformation of polycrystals of the solid solutions Cu-Ni, Cu-Zn, Cu-Al, and Cu-Ge. It was demonstrated that the shape of the stress-strain curves below the yield point depends on the concentration, the distribution of atoms of alloying elements, the properties of the components of the alloy, and on the size of grain. The dependence of the modulus of strain hardening on the composition of the alloys and on the grain size was analyzed. It was concluded that the magnitude of the modulus of strain hardening at the initial stage of microdeformation is determined by the character of the involvement of the individual grains in plastic deformation.

Resistance to dislocation motion in copper-based solid solutions with a statistically disordered atomic distribution

The stress σF due to friction forces in copper-based solid solutions was determined. Under the conditions of the procedure used to measure σF, on the basis of the half-wave hysteresis with polycrystalline samples, the value of d: σF = σF0 + KFd−(1/2) where σF0 is the resistance to dislocation motion in an alloy having an infinite grain size, and KF is a constant. It is shown that σF0 is governed by the interaction of moving dislocations with impurity atoms in the case of a statistically disordered atomic distribution. A study was made of the effects of various factors on σF and of the nature of the changes in σF0 caused by alloying.

Frictional forces and resistance to the beginning of plastic deformation in Cu-Al solid solutions

Extension curves have been obtained for Cu-Al solid solutions in the microdeformation range 10−6-10−3. It is shown that the dependence of the resistance to microdeformation on the degree ɛ of this deformation is described by σ = σ00 + Aɛ1/2, where σ00 is the resistance to the beginning of plastic deformation, and A = const. The stresses σF due to frictional forces are determined from the unloading curves. The dependences of σ00 and σF on the Al concentration c in the alloy are studied. The σ00 = f(c) and σF = φ(c) dependences are analyzed in the light of current ideas regarding the nature of impurity strengthening. The grain size is shown to affect the resistance to microdeformation in Cu-Al alloys.

Strengthening effect of short-range order esf ternary face-centered cubic substitutional solutions

The increase produced by short-range order in the resistance to the movement of dislocations τsro in ternary f. c. c. substitutional solid solutions was studied. A general expression for τsro was derived taking into account the correlation on the first three coordination spheres and the change in the entropy associated with the passage of dislocations. The expressions obtained were used for qualitative calculation of the concentration dependence of τsro for several ternary systems.