The currently known main mechanisms of martensitic transformation of austenite in steel during cooling, physical models, and schemes of shear rearrangement of the iron crystal lattice during gamma ->alpha transformation under the action of internal stresses are considered. An analysis of the available kinetic model of the shear transformation of austenite in steel is carried out considering the influence of stresses. As shown, for the transformation of retained austenite into martensite, two main conditions have to be fulfilled. The first one is thermodynamic condition due to decreasing the temperature of the steel down to the temperature of the end of the transformation of retained austenite. The second condition is kinetic one due to increasing the level of internal stresses in austenite, the cooling rate at the final treatment stage or mechanical impact. To calculate the effect of stresses on the transformation of retained austenite in steel, a new equation is proposed that takes into account the minimum stress required for a crystallogeometrically ordered displacement of Fe atoms during the formation of martensite. As shown, the effect of alloying elements and cooling rate on the temperature of the beginning and end of the austenite transformation can be calculated using the relations given in the article. To calculate the quantity of martensite formed depending on the transformation temperature, an improved Koistinen-Marburger equation is suggested. This equation takes into account not only the temperature of the beginning, but also the temperature of the end of the austenite transformation, i.e., increases the accuracy of the obtained result.
The influence of deposited Al, Pd, Co, Au, Ni atoms on a single-layer graphene substrate is investigated using computer simulations. The computer modelling of spraying nanoparticles on the basis of molecular dynamics method is implemented using the NVIDIA®CUDATM technique. According to the results of model calculations, the general behaviour of the system is investigated. The experiments are performed to study the sputtering of atoms of different metals under different initial conditions of the system. Based on these sprays, the behaviour at the beginning of the interaction of the deposited atoms with the substrate is analysed. The time dependences of the lateral position of nanoparticles’ centre of mass and the substrate force throughout the experiment for different sprayed materials are compared. The behaviour of total and potential energies, temperature and momentum of the system is studied. As shown, there is a jump in total energy and temperature as well as a change in the behaviour of the momentum and the substrate force when carbon atoms of graphene are reached for all depositions.
The melting temperature, the jumps of volume, enthalpy and entropy at the melting point, the isothermal compressibility, the thermal expansion coefficient, the heat capacity at constant volume, the Grüneisen parameter, the Debye temperature, the electrical resistivity, the thermal conductivity, and the thermal diffusivity for defective and perfect f.c.c. metals are studied by combining the statistical moment method (SMM), the limiting condition of the absolute stability of the crystalline state, the Clapeyron–Clausius equation, the Debye model, the Grüneisen equation, the Wiedemann–Franz law, and the Mott equation. Numerical calculations are carried out for Au under high temperature and pressure. The calculated melting curve of Au is in good agreement with experiments and other calculations. Obtained results are predictive and orient towards new experiments.
The main advantages of hydrogen as an energy carrier in comparison with currently used hydrocarbons are determined. By comparing the advantages and disadvantages of available methods of hydrogen storage, it was proved that storage of it in a bound state (hydrides) provides the largest amount of stored hydrogen per unit weight of the container and is the safest method. As shown, the alloys based on AB(2)-type intermetallics are the most promising materials for safe storage and transportation of hydrogen in the bound state.
As known, the surface phenomena play a crucial role in the formation of strong interatomic bonds while joining dissimilar materials and the deposition of metal films. Thus, the presence of various contaminants, including oxides, on the metal surface reduces drastically the metal surface energy, thereby, preventing the diffusion processes in the contact zone and wetting them with liquid solder and adhesion of condensed films on the substrate surface. As a result, the processes of cleaning (activating) of metal surfaces before welding or coatings’ deposition begin to play a significant role. In some cases, metal surfaces have to be modified in order to give them the desired properties. Recently, for activation and modification of surfaces before welding and coatings’ deposition, gas-discharge plasma of abnormal glow discharge is widely used. The latter allows treating the surfaces of different configurations, including internal cavities, and various areas from units to tens of thousands of square centimetres. This review contains the results of research on the activation and modification of metal surfaces with low-energy ions (< 10 keV) initiated in the plasma of an abnormal glow discharge for welding, brazing, and coatings’ deposition. Particularly, we present results of studies of ion treatment with the glow discharge surface of samples, which are made of steels С45 and DC04, a number of active metals and alloys as well as chromium-containing steels 41Cr4, X20Cr13, and X6CrNiTi18-10, which possess the chemically and thermally stable Cr2O3 oxides on their surfaces. The decisive influence on the efficiency of purification and modification of metal surfaces with glow discharge by means of such regime parameters as electrode voltage, discharge current density, working chamber pressure, and ion exposure time is indicated. The optimal values of these parameters, in most cases, are determined by the technological conditions of the process and vary in the following ranges: U = 1500–3500 V, J = 0.4–1 mA/cm2, P = 3.99–7.98 Pa, t = 120–300 s, respectively.
