Funding

surface tension in NSC. The first factor governs regularities of strength dependence on temperature and the feature of the bcc!fcc transition under hydrostatic tension. The second one influences the absolute value of nanocrystal strength and determines the main regularities in manifestation of the size effect in nanospecimens of bcc and fcc metals, as well as the orientation dependence of the size effect in nanospecimens of bcc metals.

3. Dependence of the magnitude of local shear stresses in the surface layer of nanospecimen, induced by the surface tension, on its diameter (cross-sectional size) is the reason for existence of a size effect for the strength of nanospecimens. The "sign" of a size effect (increase or decrease in strength with decreasing diameter) is determined by the kind of non-equilibrium defect that is formed as a result of local instability of the nanocrystal.

4. Strength of nanospecimen (nanowire) increases with decrease in its diameter (crosssectional size) if the value of this strength is controlled by the formation of a stacking fault (fcc lattice, orientations <100>, <110>, <111>) or a twin (bcc lattice, <100>). Formation of non-equilibrium dislocations (bcc lattice, <110>, <111>) gives rise to an anomaly of the scale effect, i.e., to fall in strength with decreasing diameter. This is due to the fact that nonequilibrium dislocations form in a thin surface layer, where the action of surface tension gives rise to an increase in the level of local shear stresses, i.e., promotes realisation of the local instability. Size of the region required for the formation of a non-equilibrium stacking fault or twin exceeds thickness of the stretched surface layer, and this region falls within the range of compressive stresses. The latter reduces the magnitude of the resulting local shear stresses in the region where a twin or a stacking fault is formed, which leads to an

5. Physical mechanisms of the thermally activated reduction in strength of nanosized and macrosized crystals are fundamentally different. In first case, the thermal vibration of atoms causes local instability in defect-free structure and formation of non-equilibrium defects. In the second case, the atom vibrations raise mobility of already existent defects. As a result, the laws of temperature dependence differ as well as change in the absolute value of strength. For instance, over the temperature range 77К…300К, the value of yield strength of typical bcc transition metals decreases 3–5 times, while the changes in a

6. Instability in bcc nanocrystals on the Bain path is possible under uniform triaxial tension of nanosized specimens. The peculiarity of the manifestation of "localised instability" under these conditions is that the fluctuations of local tensile stresses not only lead to a decrease in the global (average) stress required for the bcc!fcc transition but also cause deviation from triaxial uniform tension within the local region where this transition realises. It means that at a global uniform triaxial tension of nanospecimen, locally, bcc!fcc transition occurs under non-uniform triaxial tension. This leads to a decrease in the magnitude of stress

The maximum attainable experimental values of the strength of Mo and W nanoneedle specimens in the crystallographic direction [110] are approximately 40% less than the results of MD simulation. This may be caused by stress raising due to rough lateral surface of nanoneedle specimens

required for such a transition. For W at T = 77 K, this decrease is 25–30%.

increase in the strength of nanospecimen.

56 Molecular Dynamics

strength of nanosized crystals do not exceed 15–25%.

This work was supported by the National Academy of Sciences of Ukraine [grants number #0117 U002131; #0117 U006351].
