Kinetics Of Structural Transformations Research Articles

Development of technology for the production of new materials on traditional wide-strip hot rolling mills using modern methods of numerical and physical modeling. Part 1

A description of the technique for developing the technology for hot-rolled coils manufacturing in the conditions of an existing production, the technology and characteristics of which go beyond the passport characteristics of the equipment, is presented. It is proposed to carry out comprehensive laboratory studies of the developed material (the kinetics of recrystallization processes during heating and rolling, the kinetics of structural transformations, material properties at production temperatures), engineering calculations of the equipment operation and simulation of the strip behavior during the rolling and coiling before the industrial testing of the hot rolled products manufacturing technology. The practical application of the proposed approach is demonstrated by the example of the development of a technology for manufacturing hot-rolled coils from low-carbon boron steel with a minimum yield strength of 1000 MPa. For the developed steel, rational temperature parameters for slabs heating are determined, which exclude the development of collective recrystallization and the formation of an uneven-grained structure, and also limiting deformation modes are defined, which ensure efficient grain refinement and implemented on existing hot strip mills. It has been established that in low-carbon steel a high strength with satisfactory ductility, low-temperature toughness, and the ability to cold forming (bending, profiling, etc.) is achieved due to the formation of a lower bainite structure in rolled products. The required structural state was obtained by applying the technology of interrupted quenching from rolling heating at massaverage cooling rates of about 25 °C/s.The studies were carried out within the program of the Russian Federation of strategic academic leadership "Priority-2030" aimed at supporting the development programs of educational institutions of higher education, the scientific project PRIOR/SN/NU/22/SP5/26 "Development of innovative digital tools for the implementation of applied artificial intelligence and advanced statistical analysis of Big Data in production processes of metallurgical products".

Self-Polymerization Promoting Monomers: In Situ Transformation of Disulfide-Linked Benzoxazines into the Thiazolidine Structure.

Polybenzoxazines obtained especially from green synthons are facing challenges of the requirement of high ring-opening polymerization (ROP) temperature of the monomer, thus affecting their exploration at the industrial front. This demands effective structural changes in the monomer itself, to mediate catalyst-free polymerization at a low energy via one-step synthesis protocol. In this regard, monomers based on disulfide-linked bisbenzoxazine were successfully synthesized using cystamine (biobased) and cardanol (agro-waste)/phenol. Reduction of the disulfide bridge in the monomer using dithiothreitol under mild conditions in situ transformed the oxazine ring in the monomer, via neighboring group participation of the -SH group in a transient intermediate monomer, into a thiazolidine structure, which is otherwise difficult to synthesize. Structural transformation of ring-opening followed by the ring-closing intramolecular reaction led to an interconversion of O-CH2-N containing a six-membered oxazine ring to S-CH2-N containing a five-membered thiazolidine ring and a phenolic-OH. The structure of the monomer with the oxazine ring and its congener with the thiazolidine ring was characterized by spectroscopic methods and X-ray analysis. Kinetics of structural transformation at a molecular level is studied in detail, and it was found that the reaction proceeded via a transient 2-aminoethanethiol-linked benzoxazine intermediate, as supported by nuclear magnetic resonance spectroscopy and density functional theory studies. The thiazolidine-ring-containing monomer promotes ROP at a substantially low temperature than the reported mono-/bisoxazine monomers due to the dual mode of facilitation of the ROP reaction, both by phenolic-OH and by ring strain. Surprisingly, both the monomer structures led to the formation of a similar polymer structure, as supported by thermogravimetric analysis and Fourier transform infrared study. The current work highlights the benefits of inherent functionalities in naturally sourced feedstocks as biosynthons for the new latest generation of benzoxazine monomers.

Modeling of structural transformation kinetics in steels during their heat treatment

Analysis of carbide phase growth kinetics during carburizing of steel has been made in this paper, using experimental and calculated data obtained according to the classical theory of transformations in metals and alloys.

Physicochemical Properties of Zirconium Oxide Nanopowder Synthesized in Low-Pressure Arc Discharge Plasma

The method for synthesis of zirconium oxide nanopowder in low-pressure arc plasma was developed. The physicochemical properties of the nanopowder were studied. The estimation was carried out by means of X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The sequence and kinetics of structural transformations occurring in the specified time-temperature heating regimes have been examined using differential thermal analysis. It was shown that the particles have a nanometer size, a narrow particle size distribution and an amorphous-crystalline structure. The maximum specific surface area detected by the method of sorption of surfactants was 470 m2/g. The issues of crystallization were discussed. From comparison between the results of experimental studies from earlier works with the results obtained for zirconium oxide, it is concluded that the proposed method for producing NPs in a low-pressure arc discharge plasma is universal.

АНАЛИТИЧЕСКИЕ МОДЕЛИ ПРЕВРАЩЕНИЯ АУСТЕНИТА С УЧЕТОМ ВНУТРЕННИХ НАПРЯЖЕНИЙ

Рассмотрены аналитические модели превращения аустенита в стали, учитывающие влияние напряженного состояния на скорость превращения при термообработке сталей. Предложены простые аналитические выражения для расчета скорости и количества образующегося мартенсита в зависимости от величины напряжения в γ-фазе.

Mathematical modelling of formation process for multi-layer 3D structure produced by additive method using arc heat sources

The results of modelling of temperature fields, kinetics of deposition of layers of dissimilar metals and nature of structural transformations in formation of multi-layer structure of 17G1S and 30KhGS steels are presented. Computer modelling was performed using COMSOL Multiphysics software package. The work takes into account effect of temperature on thermal and physical parameters of steels. To increase productivity of additive process the work has studied simultaneous effect of 3 arcs on process of deposit formation, kinetics of structural transformations and diffusion processes of alloying elements redistribution. The calculations show that preheating of the substrate by arc in the beginning of the process before application of deposited material is necessary in order to decrease a stress level between additive deposit and substrate to 50 MPa. It is shown that the time of passing between neighbor arc heat sources shall be kept in 5- 30 s range. It is determined that low power of arc (1 kW) mainly provokes formation of ferrite-bainite structure in the deposit, portion of bainite in which makes 71 %, ferrite 28% and martensite ~ 1%. Application of larger power arc (5 kW) forms in the deposit bainite-martensite structure, portion of bainite in which makes ~ 50%, portion of martensite rises up to 40% and that of ferrite to 10%. Increase of arc power results in rise of maximum temperature of liquid pool to 1750-1850 °C, growth of cooling rate to 15 – 25 °C/s , and, as a consequence, increase of martensite portion in the structure of deposited layers.

Surface Morphology and Structure of Plasma Coatings

Specific features of the structure formation in rapidly quenched high-cobalt alloy coatings formed by plasma sputtering have been investigated. The structural state of the investigated coatings has been identified by the X-ray diffraction technique. The sequence and kinetics of structural transformations occurring in the specified time-temperature heating regimes have been examined using differential thermal analysis. It has been established that heat generation and crystallization temperature depend on the conditions of formation of bulk amorphous coatings.

Structural Features of Cu–Ag Alloys Obtained via Mechanical Alloying with the Use of Cold and Cryogenic Severe Plastic Deformation

The structural features of Cu–Ag alloys mechanically synthesized by means of severe plastic deformation at a room and cryogenic temperature are revealed using X-ray diffractometry and electron microscopy and via in situ measurement of structure-sensitive characteristics such as shear stress depending on the deformation level. The effect of cryogenic strain temperature consists in a special character of change in the structure and shear stress of the formed alloys, which, in turn, affects the composition of the solid solution and the kinetics of structural transformations, as well as the mechanical properties.

New Insights into the Kinetics of Structural Transformation and Hydrogenation Activity of Nano-crystalline Molybdenum Carbide

Transformation of cubic molybdenum carbide into hexagonal β-Mo2C was found to follow the Avrami–Erofeev kinetic model with an n parameter of four indicating a slow nucleation step with a polyhedral growth of the transformed phase. The transformation not only occurred at 773 K under atmospheric H2 pressure, but also imperceptibly occurred under the employed hydrogenation reaction conditions (413 K and 70 psig of H2 pressure) when using toluene or 1-octene as reactants, not affecting the activity under the steady state. Our computational studies also revealed that both crystal structures share common atomic cluster arrangements with small differences in angles and interatomic distances. The transformation was triggered by local distortions which favored shorter and more homogeneous interatomic distances as present in the hexagonal phase. The prepared molybdenum carbide materials were selective to the hydrogenation of 1-octene under the mild hydrogenation conditions tested. Hydrogenation of 1-octene was preferential in a 50 wt% mixture of toluene and 1-octene with minimal toluene hydrogenation under the tested conditions.

АНАЛИТИЧЕСКИЕ МОДЕЛИ ПРЕВРАЩЕНИЯ АУСТЕНИТА В СТАЛИ С УЧЕТОМ ВНУТРЕННИХ НАПРЯЖЕНИЙ

The aim of the work is to review the analytical models of austenite decay, taking into account the internal stresses obtained by various authors. It is shown that a common drawback of the known works is the neglect of the effect of the stress-strain state arising during the heat treatment of parts on the kinetics of structural transformations, while the available experimental data indicate a significant effect of stresses and strains on structural transformations. It has been established that the experimental data accumulated to date do not allow reliably describing the effect of the stress state on structural transitions during the heat treatment of steels for the hot deformation tool. A simple theoretical expression is proposed for calculating the amount of martensite formed as a function of the voltage in the γ phase. It is theoretically shown that taking into account the voltage acting in the γ phase leads to an increase in the temperature of the end of the formation of martensite. Experimental study of this effect is especially convenient to carry out in isothermal conditions under uniaxial tension. It is shown that in order to reliably predict the structural state of the hot deformation tool during heat treatment, it is necessary to conduct a comprehensive study related to the experimental study of the effect of stresses on all types of transformations and the creation of an adequate mathematical model of austenite decomposition based on the obtained experimental data.

Dynamics simulation of disperse systems in the presence of a structure-related mechanical barrier resulting from surfactant adsorption

A computer model has been developed to refine the notions of the kinetics of structural transformations in surfactant-modified disperse systems, the mechanism of the influence of adsorption layers on viscosity, conditions of aggregation, and evolution of nonuniformities in microstructures. Classical concepts of the structure-related mechanical barrier providing disperse systems with stability to aggregation, as well as the criterion of adsorption-layer breakdown upon interparticle collisions under dynamic conditions, have been used. It has been shown that adsorption-layer breakdown under dynamic conditions may lead to the appearance of an extreme in the viscosity curve of a disperse system. The combined effect of additives of surfactants with different molecular masses successively added to the systems has been studied via taking into account the influence of surfactant adsorption layers on the dynamics of contact interactions. The simulation results make it possible to optimize the regulation of structure-related rheological properties of dispersions so as to decrease their apparent viscosity with a simultaneous increase in the uniformity of the structure.

Structural Transformation of Amyloid Peptides Interacting with Lipid Membranes

The inherent cytotoxicity of protein aggregates implies a common mechanism for amyloid diseases (Bucciantini et al). However, accumulated evidence suggests that the insoluble fibrils or aggregates are not the culprit. On the other hand, amyloid peptides in soluble form do interact with lipid bilayers, suggesting that the cell plasma membranes are a target of amyloid pathogeneses. In particular, Keyed et al. have demonstrated that amyloid peptides all increase membrane ion conductance without any evidence of discrete channel or pore formation. In this study we try to find the common molecular process of soluble amyloid peptides interacting with lipid membranes that might induce membrane conductivity. It is difficult to study this molecular process for most amyloid peptides because of its propensity to fibrillize at relatively low solution concentration. PrP 106-126 is a random coil in its soluble form. We study its kinetics of structural transformation in the presence of lipid vesicles. The time dependence of the structural changes was analyzed as a function of the lipid concentration. We demonstrate that the soluble peptides transform from random coils to alpha-helices upon binding to lipid bilayers. The helical state is stable, as long as the bound peptide to lipid ratio P/L on the lipid vesicle is below a critical value P/L∗. But as P/L exceeds P/L∗, the peptides transform from the helical state to beta-aggregates. This is consistent with previous studies of penetration and hIAPP in multiple lipid bilayers. Thus we found the common fibrillization process of amyloid peptides interacting with lipid bilayers. Our proposal is that the process of peptides’ transformation from random coils to helices and then to beta-aggregation creates defects in the membranes that allow ion permeation to occur as observed by Keyed et al.

Mechanisms for surface energy changes observed in plasma immersion ion implanted polyethylene: The roles of free radicals and oxygen-containing groups

Low density polyethylene (LDPE) was modified by plasma immersion ion implantation (PIII) with nitrogen ions of 20 keV. Surface energy and structural transformations were observed during storage of the modified LDPE in air after PIII, by wettability measurements and FTIR–ATR spectra respectively. The appearance of oxygen-containing groups has some kinetic stages with characteristic times from hours to days. The surface energy values attained and comparison with the kinetics of oxidation reveal that the initial changes in the surface energy of LDPE are caused mainly by free radicals and to a lesser extent by oxygen-containing groups. The final surface energies observed after the process known as hydrophobic recovery and the surface energies stabilize are attributable to oxygen-containing groups. The modified surface is “living” and an investigation of the wettability, surface energy, unsaturated and oxygen-containing groups in the surface layer of ion beam modified polymers is incomplete if the kinetics of structural transformations after modification is not taken into account.

Adhesive properties of ethylene and vinyl acetate copolymers

Surface properties of ethylene and vinyl acetate copolymers (EVAs) are considered depending on their composition and history of surface formation. The kinetics of structural transformations in the near-surface layers of copolymers is investigated. Relaxation times and the equilibrium state of the EVA surface are determined. The orrelation dependence between the surface energy of copolymers and their adhesive properties for substrates of different natures is constructed.

Effect of pulse electron beam characteristics on internal friction and structural alterations in epoxy

Temperature dependence of internal friction is experimentally studied in epoxy irradiated by 2.5 MeV pulse electron beam to different doses. Time dependence of internal friction characteristics associated with radiation-induced processes of polymer scission and cross-linking is analyzed and discussed. Experimental data on kinetics of structural transformations in epoxy are interpreted on the base of analytical solutions of differential equations for free radical accumulation during and after irradiation subject to the pulse irradiation mode and an arbitrary effective order of radical recombination.

The Influence of Multiple-Solution Annealing on Kinetics of Structural Transformation of Maraging Steels

The paper presents effects of multiple solution annealing on kinetics of structural transformation of maraging steels 18 9 5. Multiple solution annealing testing was carried out in dilatometer by simultaneous registration of transformation temperatures austenite start As at heating and martensite start Ms at cooling and registration of linearly dilatatic changes. Analysis of dilatometric curves has quantified coefficients of dilatation during heating and coefficient of contraction during cooling, and has established mathematical model for determination of quantity of retained austenite in microstructure of maraging steels.

Controlling light with light via structural transformations in metallic nanoparticles

Reversible light-induced structural transitions in metallic nanoparticles, taking the form of surface-driven dynamic phase coexistences, provide an optical nonlinearity suitable for controlling light with light in nanophotonic and plasmonic switching devices. Using low-power diode lasers (giving excitation powers of a few tens of nanowatts per particle) operating at telecom wavelengths, we studied this nonlinearity in gallium nanoparticles grown from atomic beam at the tip of silica optical fiber. The nonlinear response characteristics indicate the occurrence of both solid-solid and solid-liquid transitions and are consistent with an effective medium model for the optical properties of closely packed nanoparticle films. The megahertz dynamics of light-by-light control in the particle film provide an insight into the kinetics of structural transformations in nanoparticles. Transitions between two solid phases of gallium in the nanoparticles, occurring on application and withdrawal of optical excitation, take considerably less than a microsecond. The transient characteristics of the nonlinearity associated with the solid-liquid transition are dominated by the recrystallization time, which is typically of the order of a microsecond.

Preparation, infrared spectroscopy, and phase stability of nanosized iron silicide powders

Nanosized powders of iron silicides were prepared by the arc plasma method. These silicide powders were characterized by X-ray diffraction and transmission electron microscopy. The lattice vibrations of nanosized ε-FeSi were investigated by Fourier-transformation infrared spectroscopy and compared with those of bulk FeSi. To determine the stability of different iron silicides, powder mixtures of ε-FeSi and α-FeSi 2 were treated thermally in different atmospheres, and at high pressure as well. The different behaviors of α-FeSi 2 could be understood by means of the thermodynamics and kinetics of structural transformation in nanosized α-FeSi 2 grains.

Kinetics of structural transformations in silica gel and physicochemical processes involved in the firing of dinas refractories containing dry mineralizers

The laws of phase formation in silica gel products obtained using a limestone residue are established. Results of x-ray diffraction, petrographic, and other analyses show that the process of transformation of quartz into high-temperature modifications of silica gel proceeds at a markedly higher rate if the limestone residue is used in a mixture with pyrite cinder rather than with a conventional lime-iron additive. This is related to certain special features of the mineralizer composition and the physicochemical processes involved in the firing. The temperature dependence and kinetics of the conversion of silica gel into its high-temperature structural modifications during the firing of dinas with dry mineralizers were studied in the course of heating in the temperature range from 1200 to 1420°C and during a hold at the maximum heat-treatment temperature. Equations describing the structural transformation processes are derived. A relationship between the physico-chemical properties of the samples and the features of the phase formation processes is demonstrated.

Microraman Spectroscopy and X-Ray Diffraction Studies of Ti-W-O Thin Films

AbstractThin films of the Ti-W-O system grown by r.f. reactive sputtering from a Ti-W (10%–90% weight) target have been studied by Raman and microraman spectroscopy, X-ray diffraction and scanning electron microscopy with the aim to investigate their microstructural and morphological properties. To this purpose, the kinetics of structural transformations at different temperatures (600 °C, and 800 °C) have been studied, and the effect of Ti on the WO3 lattice has been singled out. The results show that annealing at different temperatures induces a microstructural evolution from the amorphous phase of the as-deposited thin film to WO3 crystalline phases via an intermediate cubic disordered phase of WO3. The effect of Ti on this cubic phase and on the thin film morphology is also investigated with the aid of microraman and scanning electron microscopy analysis. The results show that two distinct phases arise upon long annealing treatments; namely, small crystallites belonging to the WO3 monoclinic phase are dispersed on a layer composed of a disordered cubic WO3 phase with a high Ti content.