Papers by Author: Ya. Matychak

Abstract: The diffusion saturation of (α + β) titanium alloy by nitrogen under rarefied medium taking into account the peculiarities of interaction both at the interface and in the metal core is modeled. The corresponding diffusion problem using the diffusion equations in the medium consisting two families of diffusion paths and non-stationary boundary condition at interface is analytically solved. The effect of the temperature and time parameters, volumetric quantity of β- phase on the kinetics of nitriding is estimated. It is established that with the increase of isothermal exposure temperature and quantity of β-phase the surface saturation by nitrogen decreases. It is caused by more intensive nitrogen penetration into bulk and formation of a deeper diffusion zone.

Abstract: This paper deals with the development of a theoretical model for reactive diffusion during new-phase formation when its volume fraction is negligible. A mathematical analysis of this physical model is carried out and an equation for mass balance at the interface is derived. The latter includes a correlation between the transport flux of nitrogen to the surface (J1) followed by both diffusion dissolution (Jdiff) and segregation at the defects due to chemical interaction with metal atoms (JR.). The analytical-experimental procedure for the determination of the coefficient of mass transfer which controls the density of the outer flux (J1) is proposed. The effect of temperature and time upon the kinetics of nitriding is estimated. The additive dependences of both the nitrogen concentration in a distant band and the depth of the diffusion zone upon time, temperature and pressure during isothermal exposure are shown. Peculiarities of the nitrogen concentration distribution at the interface, as a function of temperature of isothermal exposure, are found. It is established that, for a given duration of exposure, an increase in temperature does not always tend to increase the concentration of the impurity at the interface. This is caused by a more intense diffusion flow into the matrix. In accordance with the concept of time-independence of the surface concentration during saturation, the diffusion fluxes Jdiff → ∞ with decreasing time (τ→ 0). This artefact is eliminated by an equation, derived here, for a diffusion flux defined using new boundary conditions. The equations for calculating mass increase include the effect of segregation to the surface, which causes a deviation from a parabolic dependence of the mass change.

Abstract: Process of high-temperature (650°С) interaction of Cr(Si)-containing steel EP-823 with stagnant Pb melt containing oxygen (CО[Pb] » 10-5 - 10-6 mass.%) was theoretically and experimentally investigated. The structure and composition of oxide layer formed on the steel surface during exposure to Pb melt was examined. It is determined that thin (£ 1000 Å) Cr(Si) - rich oxide layer is formed on the steel surface in the early stages of oxidation. Oxide layer is being formed intensively over the grain boundaries followed by a lateral diffusion of Cr and spreading of Cr2O3 over alloy surface. Iron diffuses through the Cr(Si) - rich continuous oxide layer in the course of time. The formed oxide layer protects the steel against liquid metal penetration. Kinetics of iron diffusion dissolution in the liquid Pb is analytically described taking into account the chemical interaction between iron and oxygen. It is assumed that oxygen ions serves as a “traps” for iron ions and eliminates them from the diffusion flux. Fe−O complexes are considered as separate slowmoving components of the melt. In order to formulate the diffusion problem equations with additional parameter describing the volume reaction between Fe and O in melt and boundary conditions involved the time dependence of oxygen concentration at the interface of both melt and solid metal sides were used. Result is obtained in analytical form using the Laplace transformation. Analysis of obtained relations allow to assert that in the case of dissolution of iron in the lead melt containing “oxygen traps” the diffusion zone are less than that in the conditions of dissolution (without “traps”). However, the total concentration of iron both on surface of oxide layer and in the contact zone of melt is increased.

Abstract: The phenomenological theory for describing high-temperature interaction between metal and diluted gaseous medium has been developed. The theory is based on the assumption of duplex contact layer existence in the vicinity of interface (with relative thickness 2 d), where chemical reactions and processes of gas component migration occur. The non-stationary conditions of mass transfer at the interface are described involving effective average parameters. These conditions allow considering a wide spectrum of boundary diffusion phenomena (in a short and prolonged time ranges), in order to describe the kinetics of accumulation of diffusing component close to the interface. The description of the kinetic of gaseous saturation of metal (nitriding and borating) in the diluted medium becomes a partial proof of the suggested models. In order to approach the diffusion phenomena, boundary conditions, which contain, besides the coordinate derivative of concentration function, also the time derivative, were suggested. The derived equations describe the time dependence of change of surface concentration of gaseous component, the kinetics of its accumulation owing to chemical reaction, the specimen mass change owing to both, the diffusive addition dissolution in metal and its chemical interaction. The role of temperature is also discussed.