Modeling the crystallization kinetics of polymers displaying high levels of secondary crystallization

The isothermal crystallization of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) was evaluated using a range of models, namely, Avrami, simplified Hillier, Tobin, Malkin, Urbanovici–Segal, Velisaris–Seferis, and Hay. Two methods of model evaluation were used: determination of the parameters through traditional double log plots and curve fitting via nonlinear, multivariable regression. Visual inspection of the cumulative crystallization curves, calculation of the R2 value and standard error of the regression, and evaluation of the returned parameters were used to assess which model best describes the experimental data. The Hay model was found to generate the best fit, closely followed by the Velisaris–Seferis parallel model, suggesting that primary and secondary crystallization occur concurrently. The Avrami, Malkin, and Tobin models were found to perform well when the data is restricted to the region where primary crystallization dominates; however, they could not be used to successfully model the entire crystallization process. This work highlights the importance of selecting the most appropriate model for analyzing kinetics, especially when high levels of lamellar thickening and infilling occur during crystallization.

Bifilm Inclusions in High Alloyed Cast Iron

Continuous improvement in the quality of castings is especially important since a cast without defects is a more competitive product due to its longer lifecycle and cheaper operation. Producing quality castings requires comprehensive knowledge of their production, crystallization process, and chemical composition. The crystallization of alloyed ductile iron (without the addition of magnesium) with oxide bifilm inclusions is discussed. These inclusions reduce the quality of the castings, but they are a catalyst for the growth of spheroidal graphite that crystallizes in their vicinity. The research was carried out for cast iron with a highly hyper-eutectic composition. Scanning electron microscopy and EDS analysis were used in the research. A detailed analysis of the chemical composition was also carried out based on the spectrometric method, weight method, etc. Based on the obtained results, a model of spheroidal graphite crystallization near bifilm inclusions was proposed. The surface of the analyzed graphite particles was smooth, which suggests a primary crystallization process. The phenomenon of simple graphite and bifilm segregation towards the heat center of the castings was also documented.

Розробка технології дисперсного модифікування жароміцних нікелевих сплавів для лопаток газотурбінних двигунів

The paper proposes means of improving the properties of nickel alloys for aircraft gas turbine engines by improving the existing alloys. The efficiency of an aircraft engine depends on the quality of the turbine rotor blades, which are made of heat-resistant alloys. According to the studies, the obtained results of pilot tests of a complex of mechanical properties, high-temperature corrosion of a multicomponent high-temperature nickel alloy ХН59МВТКЮЛ (Ni-Cr-W-Co-Al-Ti-Mo systems). The paper proposes the use of modifiers. An effective means of dispersing the structural components of alloys at the macro and micro levels is the modification of multicomponent alloys with dispersed and nano dispersed compositions. Based on titanium nitride powder, a technological regime for modifying nickel melt with dispersed compositions of plasma chemical synthesis has been developed. The modifier was used in tablet form. Investigated different dosages of the modifier: 0.1…0.03 % by weight. The proposed method allows, with minimal losses at the melting stage, to introduce the required amount of modifier. The temperature and time parameters of the modification are 16500С, the modifier action time is 5…7 minutes. Investigations of the complex mechanical properties and the peculiarities of the formation of the structure of the modified alloy have been carried out. According to the research results, significant refinement of the alloy grain was obtained in comparison with the initial state. The modification led to an increase in mechanical properties: strength by 10 %, impact strength by 35...40 %. Comparison of the nano-modified alloy with the obtained one made it possible to increase the corrosion resistance of the ХН59МВТКЮЛ alloy in an oxidizing environment. At a temperature of 1000 0С, the corrosion depth decreased by 15 %. An increase in resistance to high-temperature corrosion is associated with a more uniform distribution of excess phases in the structure of the modified alloy. The mechanism of action of refractory particles of a modifier in a nickel melt, which are the nuclei of primary crystallization, has been established. The results of the work are of practical value in the manufacture of rotor blades from high-quality heat-resistant nickel alloys with increased parameters of heat resistance and heat resistance.

ABOUT THE CLUSTER MECHANISM OF WELDS CRYSTALLIZATION IN ARC WELDING OF METALS AND ALLOYS

In this work, the task was set to analyze the literature data on the cluster mechanism of crystallization of alloys in the field of foundry and metallurgical production and the possibility of its application for the process of crystallization of metal in a weld pool during arc welding and surfacing of metals and alloys. Literature data have shown that the cluster mechanism of their crystallization is widely used to explain many properties of castings and ingots in metallurgical production. It is shown that the previously used diffusion mechanism of crystallization of melts of steels and alloys, that is, the mechanism of crystal growth as the attachment of atoms of a substance from the melt to the solid phase (substrate), does not stand up to criticism. However, this old approach is used in all work on weld solidification in electric arc welding and surfacing. The same approach has always been used in works devoted to the study of the effect of magnetic fields (MF) during arc welding on the refinement of the weld structure. In modern research in the field of foundry and metallurgical industries, it is assumed that there are already clusters in the molten metal. Clusters are centers of crystallization. The process of crystallization of melts is represented as the attachment to larger crystals (clusters) of smaller ones. Earlier it was found that the refinement of the structural components of the weld metal during welding under the influence of MF occurs at the stage of their primary crystallization. Surfacing was performed without and with the influence of a control alternating transverse magnetic field with a frequency of 6 Hz. At the same time, in the structure of the bead deposited with the action of the field, a 2-fold grain refinement was observed. This refinement of the structure of the deposited metal occurred during its primary crystallization. It was assumed that this happened by the cluster crystallization mechanism.

PROSPECTS FOR IMPROVING THE PROPERTIES OF SECONDARY FOUNDRY ALLOYS OF THE AL-SI SYSTEM USING THE MODIFICATION PROCESS

The results of analytical studies of the use of modern modifiers for secondary aluminum alloys, which affect the structure of the metal of castings and allow to obtain the necessary physical and mechanical characteristics. It is shown that modifiers influencing the size of the primary grain and the shape of eutectic silicon inclusions are of the greatest interest for the production of castings from secondary silumins. It is shown that according to modern ideas the structure of the metal melt is not homogeneous. In some temperature range, complete mixing of atoms does not occur, and microregions with a short-range structure characteristic of the crystalline phase appear. These formations are called differently: atomic groups, clusters, clots, islands, complexes of atoms, clusters, etc. In the last decade, ultrafine powders of chemical compounds (nanopowders), which act as additional crystallization centers during primary crystallization, have become increasingly used as modifiers of cast alloys.

Phase relations in the Tl2 Te–TlBiТe2 –TlTbTe2 system

The phase equilibria in the Tl2Te–TlBiТe2–TlTbTe2 concentration area of the Tl–Bi–Tb-Te quaternary system were investigated by using the differential thermal analysis and powder X-ray diffraction techniques. The diagram of the solid-phase equilibria of this system at room temperature was constructed. It was established that the Tl9BiTe6–Tl9TbTe6 section divides the Tl2Te–TlBiТe2–TlTbTe2 system into two independent subsystems. It was found that the Tl2Te–Tl9BiTe6–Tl9TbTe6 subsystem is characterized by the formation of a wide field of solid solutions with a Tl5Te3 structure (δ-phase) that occupy more than 90% of the area of the concentration triangle. The results of X-ray phase analysis of alloys of the Tl9BiTe6–Tl9TbTe6–TlTbTe2–TlBiТe2 subsystem showed the formation of wide regions of solid solutions based on TlTbTe2 and TlBiTe2 along the section of TlTbTe2–TlBiTe2 ((β1- and β2-phases) and made it possible to determine the location of the heterogeneous phase regions in this subsystem. The parameters of crystal lattices of mutually saturated compositions of the β1-, β2-, and δ-phases are calculated from powder diffraction patterns.The paper also presents some polythermal sections, isothermal sections at 740 and 780 K of the phase diagram, as well as projections of the liquidus and solidus surfaces of the Tl2Te–Tl9BiТe6–Tl9TbTe6 subsystem. The liquidus surface consists of three fields of the primary crystallization of α (Tl2Te)-, δ- and β1-phase. The constructed isothermal sections clearly demonstrate that the directions of the tie lines do not coincide with the T–x planes of the studied internal sections, which is characteristic of non-quasi-binary polythermal sections. The obtained new phases are of interest as potential thermoelectric and magnetic materials.

Phase Equilibria of the MnTe-Sb2Te3 System and Synthesis of Novel Ternary Layered Compound – MnSb4Te7

By using Differential Thermal Analysis (DTA) and Powder X-ray Diffraction (PXRD) techniques, the phase diagram of the MnTe-Sb2Te3 system has been constructed for the first time in the entire composition range. The system features two ternary layered van der Waals (vdW) compounds. Apart from known MnSb2Te4, novel MnSb4Te7 which a structural analogous of the known MnBi4Te7 was found in the system. Crystal structure parameters of both compounds were determined by Rietveld refinement using the fundamental parameter approach. Both compounds were found to decompose via peritectic reactions and possess significant homogeneity ranges. The title system is also characterized by the existence of the wide solid solution field based on the starting Sb2Te3. The present results would be useful for the bulk single crystal growth of both compounds from the liquid phase via the determination of primary crystallization areas.

Nanocrystallization and phase formation in Fe73.5Nb3Cu1Si15.5B7 amorphous ribbon under laser heating

Laser-induced local crystallization in Finemet-type alloy was studied using X-ray diffraction, SEM and EDX methods. For investigated conditions of irradiation (wavelength λ = 1.06 µm, laser power density 50 W/cm2), it was found that primary crystallization starts with the formation of the nanocrystalline α-Fe(Si) solid solution at shorter exposure time and the second step crystallization with the nanocrystalline hexagonal H-phase formation occurs in longer exposure time. Changes in the local element concentration were observed at the surface of the irradiated zone and at the ribbon fracture. It was shown that the nonlinear temperature field due to the laser irradiation resulted in changes of the local elements concentration and this feature changed crystallization mechanism of the Finemet-type alloy.