No abstract

The microhardness of samples of VT1-0 titanium with grain sizes ranging from 35 nm to 10 μm is measured at temperatures of 77-300 K. Nanocrystalline samples produced by rolling at low temperatures are found to be quite homogeneous, and their structure is stable with respect to thermal and mechanical interactions. The interrelationship between microhardness and grain size is well described by the Hall-Petch relationship, the parameters of which depend on temperature. Data on the temperature dependence of the microhardness and the Hall-Petch coefficient indicate that the microplastic deformation is of a thermally activated, dislocation character, regardless of grain size.

The results of long-term studies on the physical-mechanical properties of C60 and C70 molecular crystals (fullerites) were systematized and described. These materials are the new allotropic form of carbon in which fullerenes (stable molecules consisting of 60 and 70 atoms) represent the elementary base units. Molecules are combined into crystalline structures mainly by dispersive (van der Waals) forces with a small contribution from covalent bonds. The anomalies of the fullerite crystal structures and features of the phase transitions occurring in them, which are caused by dispersive forces, orientation ordering, and dynamics of the molecules’ rotational degrees of freedom, were discussed. The most interesting transformations of fullerite lattice structures and orientation states were observed in the temperature range of 77 K ≲ Т ≲ 350 K. The majority of the experimental investigations were carried out at these temperatures. The experiments were concentrated on the effects that lattice-orientation phase transitions had on the mechanical properties of single crystals, polycrystalline solids, and compacts. Acoustical spectroscopy at low and high oscillation frequencies, micro- and nanoindentation and macrodeformation methods were used in the experimental research. The crystallogeometric aspects and dislocation mechanisms of plastic slip in fullerites, as well as the methods of observing dislocations and studying their mobility, are described in detail. Also the influence of different external factors, namely, illumination (photoplastic effect), pulsed magnetic field (magnetoplastic effect), and sample compaction pressure (baropolymerization effect) are discussed. The effects of saturating samples of different morphology with hydrogen, oxygen, and inert gas impurities on the fullerites’ mechanical properties were considered. The discussion of the experimental results is accompanied by a brief description of their theoretical interpretation based on analyzing the interaction of elastic and plastic deformations of the fullerite lattice with the processes of orientational ordering, rotation, and librational vibrations of molecules.

Elastic-plastic transition at nanoindentation of (111) plane of pure C60 fullerite single crystals was studied. The onset of plastic deformation in the contact was noted due to the plateau formation in the initial part of loading curve. The estimated stress of plasticity beginning was found to be on the order of the theoretical shear stress required for homogeneous dislocation nucleation in the ideal crystal lattice of C60. The empirical values of elastic modulus E ∼ 13.5 GPa, hardness of the ideal crystal lattice H ∼ 1.4 GPa, and hardness at different indentation loads were obtained. The hardness vs load dependence was found consistent with the model of geometrically necessary dislocations. The loading diagrams shape and the dependencies of contact pressure vs indentation depth were strongly affected by gaseous interstitial impurities (hydrogen, oxygen, nitrogen) in C60 crystal; transition stress was essentially less and plateaus formation was observed at elevated indentation loads and depths as compared with pure fullerite crystal. For crystals, saturated with hydrogen, the enhanced value of elastic modulus (∼ 20.4 GPa) and hardness (∼ 1.1 GPa) were obtained. The results acquired at room temperature for C60 with face-centered cubic lattice are important for the description of the physical-mechanical properties of simple cubic lattice phase of C60 below 260 K (S. V. Lubenets, L. S. Fomenko, V. D. Natsik, and A. V. Rusakova, Fiz. Nizk. Temp. 45, 3 (2019) [Low Temp. Phys. 45, 1 (2019)]).