TO THE PROBLEM OF APPLICABILITY OF THE TAMMAN TEMPERATURE CONCEPT TO NANOSIZED OBJECTS: TO THE 160TH ANNIVERSARY OF GUSTAV TAMMAN

Во введении представлен краткий критический обзор имеющихся интерпретаций температуры Таммана, обычно определяемой как T = 0,5T, и температуры Хюттига T = 0,3T, где T - макроскопическое значение температуры плавления материала. Для наночастиц предложено в указанных выше определяющих соотношениях заменить T на температуру плавления малого объекта T, т.е. определить T как 0,5T, а T как 0,3T. В молекулярно-динамических экспериментах на наночастицах Au, осуществленных с помощью LAMMPS, было установлено, что при температуре T=T как в центральной части ГЦК-наночастицы, так и в её поверхностном слое возникают локальные очаги квазикристаллической структуры, которые попеременно идентифицируются программой OVITO то как имеющие кристаллическую структуру, то как не имеющие кристаллической упорядоченности. Однако, вопреки мнению Э. Рукенштейна (1984), при T=T жидкий слой на поверхности кристаллической наночастицы еще не образуется. Вместе с тем в наших молекулярно-динамических экспериментах не обнаружено какое-либо проявление температуры Хюттига T в структуре кристаллических наночастиц Au. The introduction provides a brief critical review of the available definitions and interpretations of the Tamman temperature, usually defined as T = 0,5T, and of the Hüttig temperature T = 0,3T where T is the macroscopic value of the melting point of the material. For a nanoparticle we propose to replace in the above relations T by the melting temperature of the small object T , i.e. to define T as 0,5T and T as 0,3T . In our molecular dynamics experiments on Au nanoparticles, carried out using the LAMMPS program, we found that at the temperature T = T , in both the central part of the fcc nanoparticle (the core) and in its surface layer (the shell), some local species of a quasicrystalline structure appear which are alternately identified either as crystalline or as non-crystalline by the OVITO program. However, contrary to opinion of E. Rukenstein (1984), at T = T , a liquid layer on the surface of the crystalline nanoparticle is not formed yet. However, a liquid-like layer was gradually developed in the course of the further temperature elevation. At the same time, in our molecular dynamics experiments we did not reveal any manifestation of the Huttig temperature T in the structure of crystalline Au nanoparticles reproduced in our molecular dynamics experiments. It is also of interest that in our molecular dynamics experiments the nanoparticle sintering took place not only above the Tammann temperature but below it as well.

Eshelbian mechanics of novel materials: Quasicrystals

In this work, the so-called Eshelbian or configurational mechanics of quasicrystals is presented. Quasicrystals are considered as a prototype of novel materials. Material balance laws for quasicrystalline materials with dislocations are derived in the framework of generalized incompatible elasticity theory of quasicrystals. Translations, scaling transformations as well as rotations are examined; the latter presents particular interest due to the quasicrystalline structure. This derivation provides important quantities of the Eshelbian mechanics, as the Eshelby stress tensor, the scaling flux vector, the angular momentum tensor, the configurational forces (Peach–Koehler force, Cherepanov force, inhomogeneity force or Eshelby force), the configurational work, and the configurational vector moments for dislocations in quasicrystals. The corresponding [Formula: see text]-, [Formula: see text]-, and [Formula: see text]-integrals for dislocation loops and straight dislocations in quasicrystals are derived and discussed. Moreover, the explicit formulas of the [Formula: see text]-, [Formula: see text]-, and [Formula: see text]-integrals for parallel screw dislocations in one-dimensional hexagonal quasicrystals are obtained. Through this derivation, the physical interpretation of the [Formula: see text]-, [Formula: see text]-, and [Formula: see text]-integrals for dislocations in quasicrystals is revealed and their connection to the Peach–Koehler force, the interaction energy and the rotational vector moment (torque) of dislocations in quasicrystals is established.

The Phenomenon of Improving of Turbostrate Structure of Carbon Fibers during Dynamic Contact of Surfaces of Solid States

The influence of chemo-mechanical activation of dispersed carbon fibers in their turbostrate and quasicrystalline structure in terms of modeling frictional contact has been researched. It has been shown that change of parameters the structure of activated carbon fibers depends from the temperature of the final heat treatment and technology of obtaining of fibers and from the time of mechanical impact.

The Effect of Heat Treatment on Morphology and Phase Composition of Grain Boundary Phases in Mg-Zn-Y-Nd-Zr

The Mg-Zn-Y system attracts interest of researchers due to the formation of quasicrystalline particles which are believed to improve mechanical properties of Mg-based alloys. In the Mg-Zn-Y-Nd-Zr alloy (WE43 modified by addition of Zn) studied here the formation of icosahedral phase (I-phase) with quasicrystalline structure competes with cubic W-phase. Grain boundary phases in Mg-Zn-Y-Nd-Zr alloys subjected to various heat treatments were characterized. It was found that the portion of the I-phase in Mg-Zn-Y-Nd-Zr alloy can be increased by a suitable heat treatment. Moreover, the solidification process is influenced by the cooling rate resulting in a striking difference in the morphology of grain boundary phases.