V. V. Shpeizman

Determination of elastic moduli of GaN epitaxial layers by microindentation technique

It is demonstrated that Young’s modulus of epitaxial gallium nitride layers can be determined by the microindentation of their growth surface. The technique is based on the solution of the Hertz problem for the elastic indentation of a steel sphere into the studied surface. It is established that the isotropic approximation applied in this case is justified and leads to the satisfactory results. The microhardness measurements of epitaxial layers are carried out.

Pseudoelastic deformation and generation of reactive stresses in a Cu-Al-Ni shape-memory alloy in the temperature range 4.2–293 K

Pseudoelastic deformation and the magnitude of reactive stresses in Cu-14.2% Al-4.5% Ni shape-memory alloy single crystals were studied experimentally in the temperature range 4.2–293 K. It is established that pseudoelasticity and the shape-memory effect are observed in this alloy over the entire temperature range indicated above. It is found that, as the constrained samples are heated at a constant rate from liquid-helium temperature, the reactive stresses increase continuously at temperatures of up to 100 K and then remain constant. When the temperature of preliminary deformation is 77 K, the generation of reactive stresses with an increase in temperature occurs by two stages, which agrees with the multistage behavior of the pseudoelastic-deformation curves of this alloy above the liquid-nitrogen boiling temperature. Using the theory of diffuse martensitic transitions, a quantitative calculation is performed of pseudoelastic-deformation curves and reactive-stress curves over the temperature range 4.2–293 K under conditions of two-stage behavior of the martensitic transformation.

Influence of the deformation type and medium on the mechanodynamic penetration of nitrogen molecules into surface layers of armco iron

The extraction of nitrogen molecules from deformed samples of armco iron with different initial structures (annealed and subjected to equal-channel angular pressing) and different deformation prehistories (deformation in liquid nitrogen at 77 K, rolling in air at room temperature, and their combination) has been studied. It has been shown that the preliminary deformation in liquid nitrogen increases its concentration in the surface layer of the material and shifts the principal peak of its release toward low temperatures during heating. The results are associated with the existence of different types of nitrogen traps in annealed and nanostructured armco iron and with their changes during subsequent deformation.

Deformation characteristics of nanocrystalline copper and nickel at low temperatures

Measurements were made of the deformation and fracture characteristics of nanocrystalline copper and nickel at temperatures between 4.2 and 300 K. It was observed that the flow stresses are sensitive to the sign of the load while deformation instability was observed at temperatures close to liquid-helium temperature. The temperature dependence of the yield stress was obtained. It was found that there is a range of a thermal deformation at low temperatures which extends to 60 K for nickel and 200 K for copper. Possible reasons for these characteristics in the deformation behavior of nanocrystalline materials are discussed, especially the role of quantum effects in the low-temperature deformation.

Structural and physicomechanical properties of directionally crystallized aluminum-silicon alloys

Aluminum-silicon alloys (from 8 to 25 wt % Si) have been prepared by directional crystallization of shaped samples by the Stepanov growth at a solidification rate of 103 μm s−1. The dependences of the microhardness, Young’s modulus, internal friction, yield stress, and ultimate tensile stress of the alloys on the silicon content have been studied. It has been shown that the ultimate tensile stress has a maximum, and the yield stress has a kink at 15 wt % Si; the composition corresponds to the eutectic composition at the solidification rate used. The silicon content in the eutectics increases with an increase in the solidification rate. The increase in the ultimate tensile stress is explained by an increase in the volume fraction of the more strength fine-crystalline structure of the eutectics as a result of the decrease in the volume fraction of more plastic dendrites of the primary crystals of the α-Al solid solution. The decrease in the ultimate tensile stress of the hypereutectic alloy is determined by the increase in the volume fraction of brittle primary silicon crystals of various shapes.

Multilevel character of deformation of polymers

The inhomogeneity in the creep rate of polymers on different scales of deformation has been studied by laser interferometry. The main results have been obtained for the amorphous-crystalline polymer polytetrafluoroethylene. The deformation characteristics are the oscillation periods of the rate (jumps of deformation), oscillation amplitudes of the rate, and the scatter of these quantities. Application of computer methods for processing of the results has made it possible to determine the difference and similarity between jumpwise deformations on different structural levels, including the nanolevel. For a more distinct separation of deformation levels, the measurements have been made in a magnetic field and outside the magnetic field. Deformation jumps have been found on five levels: from 4 nm to more than 10 μm. Introduction of a sample into a magnetic field changes the characteristics of jumps; in this case, the scatter in the values of jumps always increases, whereas their average value varies differently on different scale levels. The measurement of the parameters of deformation jumps on different scales allows one to study the laws of the development of the deformation process and the evolution of structural inhomogeneities.

Effect of magnetic field on the microplastic strain rate for C60 single crystals

Microplastic deformation in a magnetic field and in a zero field, as well as after preliminary action of a magnetic field on C60 crystals, is studied with the help of a laser interferometer, which makes it possible to measure the strain rate on the basis of linear displacements of 0.15 µm. It is shown that the introduction of a sample into the field and its removal from a field of 0.2 T directly during sample deformation lead to a change in the strain rate, the decrease in the rate being accompanied by a brief interruption of deformation. The sign of the effect depends on temperature: the magnetic field accelerates deformation at room temperature and slows it down at 100 K. Preliminary holding of a sample in a field of 0.2 or 2 T produces a similar effect on the strain rate. Possible reasons for the observed manifestations of the magnetoplastic effect in C60 and the relation between the sign of the effect and the phase transition at 260 K are considered.

Effect of carbonization temperature on the microplasticity of wood-derived biocarbon

The uniaxial compression strength under stepped loading and the 325-nm-stepped deformation rate of biocarbon samples obtained by carbonization of beech wood at different temperatures in the 600–1600°C range have been measured using high-precision interferometry. It has been shown that the strength depends on the content of nanocrystalline phase in biocarbon. The magnitude of deformation jumps at micro- and nanometer levels and their variation with a change in the structure of the material and loading time have been determined. For micro- and nanometer-scale jumps, standard deviations of the differences between the experimentally measured deformation rate at loading steps and its magnitude at the smoothed fitting curve have been calculated, and the correlation of the error with the deformation prior to destruction has been shown. The results obtained have been compared with the previously published data on measurements of the elastic properties and internal friction of these materials.

Mechanodynamic diffusion of nitrogen molecules into armco-iron under its deformation in liquid nitrogen medium

Data on mechanodynamic penetration of nitrogen molecules are obtained under deformation of armco-iron samples. It is shown that molecular nitrogen diffuses into the surface layer of samples under their deformation in a liquid nitrogen medium, and the nitrogen concentration compares well with the helium concentration in samples deformed in a liquid helium medium and in some cases exceeds the latter.

Micrometer-Scale Deformation Jumps at Different Stages of Creep in Solids

Laser interferometry is used to study micrometer-scale creep-strain nonuniformities (jumps) that occur during compression of metals (Ag, Al, Bi, Cu, Pb, Sn, Zn) and LiF: Mg crystals. The strain rate is found to vary periodically. The average magnitude of deformation L over one period and the variation of L with the total strain are determined. Correlations are found to exist between L and the Mg content in the LiF crystals, between L and the grain size in the metals, and between the magnitude of small jumps and the Burgers vector in the metals.