At high pressures (the pressure is comparable with the bulk modulus) the crystalline lattice may become unstable relative to the uniform shear deformations, and as a result the low symmetric crystalline structures will appear (the so called “elastic phase transitions”). The order parameters at these transitions are the components of the finite deformations tensor. The stability of the high-pressure phases is defined by the nonlinear elasticity of the lattice (the third, fourth etc. order elastic constants). Here the different cases of stability loss at hydrostatic pressure of the cubic and hexagonal structures are considered. The relation between the second, third and fourth order elastic constants is given, which defines the possibility of the first order deformation phase transition. The jump of the order parameter and the height of the potential barrier are defined by the third and fourth order elastic constants. As an example, the experimentally observed elastic phase transition in vanadium at P≈ 69 GPa from bcc to the rhombohedral phase is analyzed, and the possible structural transitions in bcc Mo and W at P≥700 GPa are considered. The stability of the Ru in the pressure interval 0÷600 GPa is also analyzed. The necessary values of the different order elastic constants for Mo, W and Ru are calculated in the framework of the density functional theory using VASP (Vienna Ab initio Simulation Package).
Materials Science. Non-Equilibrium Phase Transformations.
Vol. 3 (2017), Issue 3
Table of Contents
INFLUENCE OF THERMAL-CYCLIC DEFORMATION AND HEAT TREATMENT ON THE STRUCTURE AND PHYSICAL PROPERTIES OF STEEL 10pg(s) 108-110
The results of the influence of preliminary thermal-cyclic deformation and subsequent annealing in the range of 100÷900 ºС with a step of 100 ºС on the microstructure, coercive force and linear expansion of hot-rolled sheet steel 10 are presented. It was found that the use of preliminary thermal-cyclic deformation results in increase in coercive force no more than by 8% in comparison with the steel sheet produced by an industrial technology. Decreasing feasibility in coercive force of the sheet steel produced with the use of thermal-cyclic forging almost by 3 times in comparison with the initial raw condition due to the subsequent annealing at 900ºС during 10 h. is shown. In addition, annealing in accordance with this mode reduces a temperature coefficient of linear expansion of a sheet steel on average by 6 % within the range of the test temperatures 50-450 ºС.
PRESSURE DERIVATIVES OF ELASTIC CONSTANTS OF ALUMINUM ALLOY AMg6 AND NANOSTRUCTURED ALLOY n-AMg6. COMPARISON OF EXPERIMENTAL AND CALCULATED FROM THIRD-ORDER ELASTIC CONSTANTS DATApg(s) 111-114
Research results for the nonlinear elastic characteristics of the polycrystalline aluminum alloy AMg6 and AMg6/C60 nanocomposite (n-AMg6) have been described. The ultrasonic study of the elastic characteristics at pressures up to 1.6 GPa has been carried out using a high-pressure ultrasonic piezometer based on the piston-cylinder device. Pressure dependencies of the longitudinal and shear elastic waves velocities and densities of AMg6 and n-AMg6 were experimentally measured and the pressure derivatives of the secondorder elastic constants were calculated. The results derived by this method have been compared with the results of studies of the nonlinear elastic properties of the test alloys using the Thurston–Brugger quasi-static method, where third-order elastic constants have been measured. The relationships between the pressure derivatives of the second-order elastic constants of these alloys and the Mg-content and nanostructuring were established.
X-ray diffraction analysis studies the formation of a stress-strain state of single-crystal composites LaB6-TiB2 obtained under identical conditions by crucible-free float zone melting (FZM). The composites were obtained using monocrystalline LaB6 substrate seeds with <100>, <110> and <111> orientations. It is shown that indentation-induced deformation in the composite materials obtained by FZM is distinct from the deformation expected in their equilibrium state. This difference arises in part from residual thermal strains in both phases of the composites in a FZM -grown state. Interplay between residual thermal deformations and external mechanical deformation results in a complex distribution of dilatational strain in the LaB6 matrix and TiB2 fibers and differs in composites of different orientations. Reversal sign of the stress-strain state (e.g., alternating tensile/compressive/tensile in central part of the cross-section area) is observed predominantly in the matrix LAB6 and the TiB2 fibers of the composite with the orientation <111>. In the composite with the orientation <100>, this change in the deformation sign was not observed. The size and spread of cracks after indentation-induced deformation shows a decrease in microhardness in FZM -grown composites of <111> significantly more (40%) than in composites with a orientation of <100> (10%) compared with an equilibrium state.
The miniature and low-power devices with long service life in hard operating conditions like the 14C ß-decay energyconverters indeed with eternal resource for integrated MEMS and NEMS are considered. Authors will discuss how to create a power source for MEMS devices, based on SiC/Si porous structure, which are tested to be used as the β-decay energyconverter of radioactive carbon-14 into electrical energy. This is based on the silicon carbide obtaining by self-organizing mono 3C-SiC endotaxi on the Si substrate.
EFFECT OF OXYGEN AND NITROGEN CONTENTS ON THE STRUCTURE OF THE Ti-6Al-4V ALLOY MANUFACTURED BY SELECTIVE LASER MELTINGpg(s) 2-103
TiAl6V (ELI) has the low density, low elastic modulus and high strength. Biocompatibility of this material allows one to adapt it to human implant production and successfully use in the manufacture of surgical implants. Spherical argon-atomized Ti6Al4V (ELI) (45µm) powder from TLS Technik was used for study. The chemical composition complies with the ASTM F-136 (grade 5), ASTM B348 (grade 23) standard for surgical implant applications. Two machines from two scientific centers (Russia and South Africa) were used for the manufacturing of the alloys. Analysis of the oxygen and nitrogen contamination in SLM alloys was done with Van de Graaff accelerator with 2 Mega Volts. It is found that the oxygen concentration in both samples is about 0.2 wt. % and decreases with the increasing of the sample depth; the nitrogen concentration is about 0,02 wt%. X-Ray results show an absence of beta (BCC) phase in both samples. TEM studies found the metastable martensitic structure and silicon nitride Si3N4.
STRUCTURAL-PHASE TRANSFORMATIONS DURING CONTINUOUS COOLING OF MEDIUM-CARBON HIGH-STRENGTH STEELS WITH DIFFERENT CONTENT OF MOLYBDENUM AND NIOBIUMpg(s) 78-81
In the article are considered structural phase transformations during continuous cooling of medium-carbon steel with different contents of molybdenum and niobium. The features of the phase transformations course during decomposition of austenite depending on the cooling rate are revealed. The structure and microhardness were studied. Continuous cooling transformation (CCT) diagrams are constructed for all studied chemical compositions. The effect of molybdenum and niobium on the kinetics of phase transformations and the hardness of the steels studied are shown.
TRANSFORMATION OF HETEROPHASE NON-METALLIC INCLUSIONS “HIGHMELTING PHASE SURROUNDING WITH LOW-MELTING COVER” IN STEELS UNDER LASER ACTIONpg(s) 82-86
Melting and crystallization of heterophase non-metallic inclusions “high-melting phase surrounding with low-melting cover” was investigated. It was shown that under laser action the initial structure of inclusion-steel matrix boundaries transits into unstable equilibrium high-energy condition that cause development of the dissipation processes connecting with aspiration of system inclusionmatrix to the state with minimum of free energy. In the result of the system heterophase inclusion-matrix transits to the state of unstable equilibrium which determines structure and properties of laser-quenched interphase boundary. Processes of melting, fusion and dissolution of non-metallic inclusions “high-melting phase surrounding with low-melting cover” and also of the melting of steel matrix play the great role in transformation of interphase inclusion-matrix boundaries under laser action.
SCHEME OF TREATMENT AND ITS EFFECT ON THE STRAIN HETEROGENEITY AND STRUCTURAL CHANGES IN BILLETS OF LOW-CARBON STEELpg(s) 87-91
The paper presents the results of numerical modeling of the conventional drawing and free torsion processes of rods made of low-carbon steel using the "DEFORM-3D" software package. The heterogeneity of the strain intensity distribution in the investigated processes revealed in the model basically corresponds to real physical experiments with similar parameters of the structural heterogeneity. A mutual correspondence of the ductility diagram to the character of the structural changes in the samples deformed by free torsion is established, and the limiting values of the ductility margin, the excess of which leads to the development of fracture processes.
NANOCOMPOSITE MATERIALS BASED ON THERMOPLASTIC BLENDS FOR THE TECHNOLOGICAL EQUIPMENT WITH A LONG SERVICE LIFEpg(s) 92-94
The composite materials based on polymer and oligomer blends with different thermodynamic compatibility have been developed. It is shown that for polymer and oligomer blends of products formed as result of the thermogasdynamic synthesis of fluorine-containing compounds, the ability of oligomeric matrix to multiply deformation and alternating transfer is the most important. In polymer-polymer systems formed in a melt by various technologies the most important factor is the structure of the boundary layers determining the parameters of the stress-strain and tribological characteristics of the composites. The effect of physical compatibilization during the introduction of nanoscale metal-containing and carbon-containing particles into the blend compositions has been established. This effect promotes the thermodynamic compatibility and resistance to the action of thermal-oxidative medium on composites.
EFFECT OF HOT PLASTIC DEFORMATION ON DECOMPOSITION OF MAGNESIUM SOLID SOLUTION CONTAINING RARE-EARTH METALSpg(s) 95-97
In the work effect of the rare-earth metals (REM) on structure of the decomposed Mg supersaturated solid solution after hot deformation was investigated. Investigation indicated Mg solid solution decomposition during hot deformation with precipitation of the RErich particles preferably on the Mg grain boundaries and boundaries between originated blocks. Precipitations on the grain and block boundaries prevent recovery, recrystallization, the grain growth, and increase plasticity of alloys.
The results of the development of a process for the production of powder porous materials (PPM) based on a granular mechanically activated aluminum oxide powder have been presented. The dynamics of the particle size change of alumina powder depending on the time of mechanical activation by means of a high-energy mill of planetary type has been studied. The development results of permeable materials based on granular mechanically activated oxide ceramic powder have been presented. The developed PPMs are applied for liquid and gas filtration, production of calibration elements for control and measuring devices and equipment.