Solidification Region Research Articles

Experimental determination and thermodynamic reassessment of the Al–Cr–Ru ternary system

The isothermal sections at 950 °C and 1150 °C of Al–Cr–Ru ternary system were determined experimentally by equilibrium alloy method, combined with X-ray diffraction, scanning electron microscopy and energy-dispersive spectrometric. Nine and eight three-phase equilibria were confirmed at the 950 °C and 1150 °C isothermal sections of the Al–Cr–Ru ternary systems, respectively. The liquids surface projection of the Al–Cr–Ru ternary system was determined by identifying the microstructures of the as-cast alloys, and seven primary solidification regions were determined in the liquids surface projection. Besides, the ternary compound τ2 was identified as end-centered monoclinic structure by transmission electron diffraction analysis. The lattice parameters of the τ2 phase are a = 0.5175 nm, b = 1.7532 nm, c = 0.5150 nm and α = γ = 90°, β = 79.167°. Based on the available data of the Al–Cr–Ru ternary system and related binary systems, thermodynamic reassessment of the Al–Cr–Ru ternary system was performed using the Calculation of Phase Diagram approach. A new thermodynamic database for the Al–Cr–Ru ternary system was obtained, and the experimental data are consistent well with the calculated results.

Numerical analysis of MHD free convection in curvilinear porous enclosure with two active cylinders filled with NE-phase change material

In nuclear power industry, the energy conservation of heat exchangers is a crucial demand. The current study addresses the influence of nano-encapsulated phase change material (NEPCM) on free convection under magnetic force in a porous enclosure involving two active cylinders. The curvilinear geometry of the enclosure is set with undulated top wall. The top, horizontal and the inclined walls are set cold while the two cylinders presented the hot surface. All other walls are assumed to be adiabatically isolated. The considered parameters are Rayleigh number, Ra (103–105), Hartmann number, Ha (0–64), fusion temperature, θf (0.1–0.9), Stefan number, Ste (0.1–0.9), volume concentration, ϕ (0–0.05), Darcy number, Da (10−5–10−2). Finite element method has been used in the numerical analysis. Some of the most important results stated that fusion temperature has a significant influence on heat transfer only with high Ra number and low Ste number. Furthermore, the melting solidification region notably changed with the change of Da and θf with less influec under the magnetic force. Also, the heat transfer is enhanced with the increase of Ra, Da and ϕ while it diminished with the increase of Stephan number and MHD. It found that a promotion in the percentage of Nuavg when the volume concentration is raised from 0 to 0.05 is about 65 %, while the Nuavg dwindles by 14 % when Hartman number is raised from 0 to 64. It can be concluded that using NEPCMs is crucial for enhancing heat transfer and hence the improving the related engineering applications.

High efficiency solar-grade silicon producing by directional solidification through controlling diffusion of impurity

The precise distribution of impurity of the whole silicon ingot is investigated through a theoretical model and the experiments. This model solves the problem of inaccurate prediction of impurity distribution in the final solidification region. The model was used to calculate the distribution of iron impurity in polycrystalline silicon ingots after directional solidification and compared with experiments. The calculation accuracy was significantly improved compared to the traditional Scheil equation. This paper investigates the influence of cooling rate, silicon ingot height and solidification rate on the yield of purified products after directional solidification. For the experimental equipment and raw materials in this paper, the optimal cooling rate of silicon ingots after directional solidification is 0.33 °C/s, the optimal height of silicon ingots is 0.34 m, and the yield of silicon purification increases with the decrease of directional solidification rate. Based on the research results, a standard for removing the top skin of silicon ingots after directional solidification and a method for improving the purification yield by suppressing impurity diffusion during the cooling stage were obtained. This model can provide theoretical guidance for the development of low-cost and high-efficiency methods for removing impurities from polycrystalline silicon, and is of great significance for the development of the photovoltaic industry. In addition, the theoretical model proposed in this paper can also be applied to the directional solidification purification process of other materials, such as the preparation of high-purity aluminum and high-purity titanium, and can predict the impurity migration behavior and distribution pattern throughout the purification process.

Experimental investigation and thermodynamic modeling of the Al–Dy and Ni–Al−Dy systems

The partial phase equilibria of the Ni–Al−Dy ternary system have been systematically investigated via experimental analyses and thermodynamic modeling. Using the equilibrated alloy method, the 1150 °C and partial 800 °C isothermal sections of the Ni–Al−Dy ternary system and related Al–Dy binary system were constructed based on scanning electron microscopy, energy-dispersive spectroscopy and X-ray diffraction. Twelve three-phase regions were confirmed and four three-phase equilibria regions were speculated at 1150 °C, eight three-phase regions were determined at 800 °C, and five kinds of primary solidification regions, DyAl2, τ7, τ5, NiAl and Ni2Al3 were observed. A new ternary compound τ12 was discovered, which was stable at 800 °C and disappeared at 1150 °C, the τ1 phase was not stable at 800 and 1150 °C isothermal sections; the ternary compound τ11 was confirmed to be stable at 800 °C. In addition, the primary solidification phases were also identified, and five different primary solidification phases were found. Based on the experimental results available in this study and the literature, the thermodynamic modeling of the Ni–Al–Dy ternary system was obtained using the CALPHAD method. A set of self-consistent thermodynamic parameters for the Ni–Al–Dy ternary system was first obtained with a satisfactory agreement between the experimental and calculated results.

A New Approach of Solidification Analysis in Modular Latent Thermal Energy Storage Unit Based on Image Processing

The solidification process of RT18HC in a cylindrical shell and tube storage unit has been studied using a new methodology based on image processing. The main idea of the algorithm is to label the region of solidification and use statistical functions to calculate the dimensions of the solidification front over time. Said analysis includes two methods. The first method is to measure the solid fraction changes during solidification. The novelty of this method, as compared to other literature findings, is that pre-processing and calculation process occurs automatically via a calculation algorithm. This method is used to calculate the solid fraction of RT18HC which is reported to be a bit fast at the beginning that 40 % of its volume solidified in 1000 s while the rest of the process is completed in almost 6500 s. The second method is used to measure and calculate the thickness of the solid front by using image processing. This method’s error is calculated to be less than 7% throughout the entire process. The second method also acts as an experimental database of front thickness to use in a novel, simplified, semi-theoretical model proposed to calculate the solid front thickness as a function of time in this paper. It is also worth presenting solution extended by a general definition of thermal resistance for a cylindrical partition. The above study will enable the development of an enhanced and optimized model for complex geometries based on image processing techniques in the future. It will also allow the investigation of both processes i.e. solidification and melting alongside other influencing parameters such as the geometry of the storage unit in future.

Mechanism of columnar to equiaxed to lamellar grain transition during wire-laser directed energy deposition 205 C aluminum alloy utilizing a coaxial head: Numerical simulation and experiment

The microstructure under various conditions shows noticeable differences in the wire-laser directed energy deposition (DED) method using a coaxial head due to the intrinsic properties of aluminum alloy wire materials and the intricate heating process involved. The impacts of varied heat inputs and substrate preheating temperatures on solidification and microstructure evolution were explored in this study. A coupled model, including macroscopic finite element and microscopic cellular automata with interpolation, was employed to investigate the temperature field, thermal cycles, solidification parameters, and microstructure evolution under varied process parameters. This study revealed the transformation of four grain forms in the deposition layer, namely, lamellar grain, equiaxed grain, equiaxed fine grain, and columnar grain, through a comprehensive dynamic simulation of the microstructure. Quantitative research was conducted to determine the solidification region of equiaxed grains and the average length of columnar grain amid solidification under varied crystallization parameters. The findings demonstrated that under a lower heat input and preheating temperature, equiaxed fine grains are generated at the lower region of the deposition layer, while lamellar grains are formed above the equiaxed grains at the upper region of the deposition layer, which is similar to the morphology of a cast microstructure. The typical primary dendritic distance of columnar grain diminishes with the enlargement of the cooling rate and temperature gradient. The findings of this study offer a theoretical framework for regulating the microstructure of aluminum alloy deposition layers manufactured via wire-laser DED technology using a coaxial head.

A volumetric heat source model for the approximation of the melting pool in laser beam welding

AbstractLaser beam welding is a metal fusion technique that, with its high advance speed and low thermal distortion, is used more frequently nowadays in industry. During welding, the solidification front is located in the so‐called mushy zone, which forms the transition region between the completely solidified and mixture zone of solid and fluid material behind the laser beam. In this zone, various process parameters can lead to the formation of included melt areas that may cause cracking when they solidify and shrink. The properties of the solidification region are influenced by several parameters. Factors such as welding speed, temperature gradient, and chemical composition affect the likelihood of solidification cracking. To study the heat affected zone more accurately, different models describing the melting pool in finite element approaches using a heat source are discussed. The goal is to develop a volumetric heat source model based on the conical model, which closely represents the temperature distribution illustrated by the Lamé curves. Boundary value problems are presented to showcase the results.

Numerical simulation of melt pool size and flow evolution for laser powder bed fusion of powder grade Ti6Al4V

The study of molten pool characteristics is a powerful means of determining the quality of laser additive manufacturing forming. In this paper, a three-dimensional transient thermal flow field numerical model of Ti6Al4V powder processed by LPBF is developed based on FLOW-3D software, and the dynamic evolution of the molten pool with fixed process parameters is quantitatively described using dimensionless numbers in computational fluid dynamics. It is shown that the main heat transfer mode of the molten pool is thermal convection; the evaporative back sitting pressure, surface tension and Marangoni shear force are the main driving forces for the evolution of the molten pool. Furthermore, the influence of key process parameters on the heat flow field of the molten pool was analyzed. When the laser power was 300 W, the depression depth and molten pool depth caused by the recoil pressure were significantly larger, and the isothermal density of the solidification region was more intensive; when the scanning speed was increased from 0.4 m/s to 0.8 m/s, the molten pool length was reduced from 524 μm to 410 μm, and the line energy density was reduced making the amount of melted powder decrease. The wettability and fluidity of the molten state metal becomes worse; when the scanning interval increases to 120 μm, a large number of incompletely melted powder tracks cannot lap correctly at the midline of the double track. The results of the above simulations achieve a high degree of agreement with the surface quality of the specimens during the experiments, which provides guidance for quantitative analysis of molten pool evolution and prediction of Ti6Al4V forming quality.

Experimental Investigations of Ni-Ti-Ru System: Liquidus Surface Projection and 1150 °C Isothermal Section.

Ruthenium addition inhibits the formation of the topologically close-packed phases in Ni-based superalloys and improves the solid solution strength of Ni-Ti shape memory alloys. Therefore, the Ni-Ti-Ru phase stability is a very valuable indicator of the effects of Ru in Ni-based superalloys and Ni-Ti shape memory alloys. In this study, the isothermal section at 1150 °C and liquidus surface projection of the Ni-Ti-Ru ternary system were determined experimentally using the equilibrated alloy method and diffusion couple method, respectively. Alloys were prepared through the arc-melting of Ni, Ti, and Ru (all 99.99% purity), and then vacuum encapsulation in quartz tubes, followed by annealing at 1150 °C for 36 to 1080 h depending on the alloy composition. Diffusion couples were fabricated by joining one single-phase block (τ1) with one two-phase block (Ni3Ti + γ(Ni)), and the couples were annealed under vacuum at 1150 °C for 168 h. Reaction temperatures of as-cast alloys were determined by differential scanning calorimetry performed with heating and cooling rates of 10 °C/min. Scanning electron microscopy and X-ray diffraction were used to analyze the microstructure. Seven three-phase regions were found at the 1150 °C isothermal section. Seven primary solidification regions and five ternary invariant reactions were deduced in the liquidus surface projection. A new ternary compound τ1 was discovered in both the isothermal section at 1150 °C and liquidus surface projection. The results aid in thermodynamic modeling of the system and provide guidance for designing Ni-based superalloys and Ni-Ti shape memory alloys.

Experimental study on phase relations in the Al-Mo-Nb ternary system

Aluminium alloying can improve the anti-oxidation property of the Nb/Mo-Si-based superalloy. Therefore, the Al-Mo-Nb ternary system is one of the important subsystems of the Al-alloyed multi-component system. The isothermal sections at 1200 and 1000 ℃ and the liquidus projection of the Al-Mo-Nb system were studied experimentally. The phase structures and the phase constituents of the as-cast alloys and the annealed alloys were analyzed and measured using X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) as well as electron probe micro-analysis (EPMA) techniques. Five three-phase regions and eleven two-phase regions in isothermal section at 1000 °C and six three-phase regions and twelve two-phase regions in isothermal section at 1200 °C were determined, and no ternary compound was observed. Fourteen primary solidification regions and eleven invariant reactions were deduced based on the study on the solidification processes of the as-cast alloys. The present experimental results can be used for the future thermodynamic assessment of the Al-Mo-Nb ternary system, and be helpful for the database development of the multi-components Nb/Mo-Si-based superalloy system.

Experimental investigation and thermodynamic optimization of the Co–Ta–Zr system

This work is focused on a deeper understanding of the solidification microstructure of the as-cast alloys, as well as the phase constituents and phase fractions of annealed samples in the Co–Ta–Zr system. Nine primary solidification regions were experimentally determined in the liquidus surface projection, and three primary solidification regions were deduced based on binary phase diagrams. Six three-phase equilibria were observed in the isothermal sections at 1373 and 1273 K. The maximum solubilities of Zr in μ and CoTa2 were determined to be ∼ 6.1 and ∼ 6.7 at% at 1373 K, and ∼ 3.9 and ∼ 5.9 at% at 1273 K, respectively. The maximum solubilities of Ta in Co23Zr6 and CoZr were determined to be ∼ 3.3 and ∼ 14.1 at% at 1373 K, and in Co23Zr6, CoZr and CoZr2 were determined to be ∼ 2.6, ∼ 13.5 and ∼ 11.9 at% at 1273 K, respectively. The Co–Ta–Zr system was optimized by CALculation of PHAse Diagram (CALPHAD) method based on the experimental data. Four solution phases, liquid, bcc, fcc, and hcp, were described as substitutional solution. The thermodynamic models of intermetallic phases Co11Zr2, Co23Zr6, CoZr2, Co3Ta, CoTa2, λ1, λ2 and λ3 were treated as (Co)11(Ta,Zr)2, (Co)23(Ta,Zr)6, (Co,Zr)(Ta,Zr)2, Co3(Co,Ta), (Co,Ta,Zr)(Co,Ta,Zr)2, (Co,Ta)2(Co,Ta), (Co,Ta,Zr)2(Co,Ta,Zr) and (Co,Ta)2(Co,Ta) using a two-sublattice model, respectively. CoZr3 was described as (Co,Zr)(Co,Zr)Zr2 by a three-sublattice model. μ phase was modeled as (Co,Ta,Zr)1(Ta,Zr)4(Co,Ta,Zr)2(Co,Ta,Zr)6 using a four-sublattice model. And CoZr with a B2 structure was described as an ordered phase of bcc_A2, and the model was treated as (Co,Ta,Zr,Va)0.5(Co,Ta,Zr,Va)0.5(Va)3. A set of self-consistent thermodynamic parameters of the Co–Ta–Zr system was obtained using CALPHAD method. The calculated solidification paths and phase fractions can provide theoretical guidance for material design and thermal processing.

A dielectric investigation of structural-phase transitions in oils

The article deals with issues related to the mechanism of solidification of oils. The authors present the results of a dielectric investigation of structural-phase transitions in oils from the fields of Tyumen region in the temperature range -110 ÷ +20 °С. The dielectric relaxation of oils has been established, the values of the activation energy and dielectric relaxation time have been calculated. The phase transition determined by dielectric relaxation is interpreted as a transition from the glassy state to the associated state. The glass transition of oils, accompanied by the cessation of internal rotation in hydrocarbon molecules, is a sign of their true (or viscous) solidification. The glass transition temperature and the temperature region of the structural solidification of oils were determined. The glass transition temperature is considered to be the true pour point. The relationships between the physicochemical characteristics of oils and the parameters characterizing their dielectric properties were established, which were studied by the methods of correlation and regression analysis. The obtained regression equations can be used to predict the physico-chemical characteristics of oils in the technological processes of their extraction, field preparation and transportation.

Experimental investigation and thermodynamic description of the Fe–Hf–Nb system

According to solidification microstructure of as-cast alloys and phase constituents of annealed samples, the liquidus surface projection and phase relationships at 1373 and 1273 K in the Fe–Hf–Nb system were constructed. A ternary phase τ with AlCuHf-structure was found, and the composition ranges of Nb in τ at 1373 and 1273 K were ∼ 20.5–30.8 at.% and ∼ 20.8–30.2 at.%, respectively. Moreover, the maximum solubilities of Nb in FeHf2 was determined to be ∼ 16.9 at.% and ∼ 16.0 at.% at 1373 and 1273 K, and the maximum solubilities of Hf in μ was measured to be ∼ 4.6 at% and ∼ 3.3 at.% at 1373 and 1273 K. Fe2Hf and Fe2Nb with the same C14 structure formed a homogeneous solid solution from Fe2Hf to Fe2Nb at 1373 and 1273 K. Seven primary solidification regions in the liquidus surface projection and two three-phase regions and seven two-phase regions in the isothermal sections at 1373 and 1273 K were experimentally determined. On the basis of experimental data, the thermodynamic parameters of individual phases in the Fe–Hf–Nb system were optimized by CALculation of PHAse Diagram (CALPHAD) method. Four solution phases (liquid, bcc, fcc, and hcp) were described as the substitutional solution. The intermetallic phases FeHf2, λ1, λ2, λ3 and τ were modeled as (Fe)1(Hf,Nb)2, (Fe,Hf,Nb)2(Fe,Hf,Nb)1, (Fe)2(Hf)1, (Fe)2(Hf,Nb)1, and (Fe,Hf,Nb)1(Fe,Hf,Nb)1 by a two-sublattice model, respectively. μ was described as (Fe,Hf,Nb)1(Hf,Nb)4(Fe,Hf,Nb)2(Fe,Hf,Nb)6 by a four-sublattice model. A set of self-consistent thermodynamic parameters of the Fe–Hf–Nb system was obtained.

Effect of nitrogen content on solidification behaviors and morphological characteristics of the precipitates in 55Cr17Mo1VN plastic die steel

The effect of nitrogen content on as-cast microstructure, particularly the precipitates (M7C3, M2(C, N), Fe3C) of 55Cr17Mo1VN plastic die steel was investigated. It's found that higher nitrogen content would enlarge the solidification region, and increase the area ratio of precipitates. The eutectic M7C3 carbides and M2(C, N) carbonitride are mainly distributed in the interdendritic region. The eutectic M7C3 carbides mainly possess coarse (M7C3-C) and lamellar (M7C3-L) morphologies. With increasing nitrogen content from 0.16% to 0.31%, the M7C3 carbide transforms from M7C3-C to M7C3-L and promotes M2(C, N) carbonitride precipitates. Based on Bramfitt's two-dimensional lattice misfit theory, Al2O3 inclusion is the very potent heterogeneous nucleation site for the formation of M2(C, N). In contrast, M2(C, N) could be a poor heterogeneous nucleation site for M7C3. Eutectoid transformation γ-Fe → α-Fe + Fe3C occurred in 0.23% and 0.31% nitrogen content samples when solidifying, which caused the outer layer carbides in lamellar morphology. The excessive precipitates weaken the austenite stability and eventually promote martensite transformation.

Mechanism of Shrinkage in Compacted Graphite Iron and Prediction of Shrinkage Tendency

Shrinkage greatly influences the mechanical and fatigue properties of compacted graphite iron and it is necessary in order to study the causes of shrinkage in compacted graphite iron and to predict it effectively. In this paper, a kind of cylindrical necking test sample was designed to evaluate the shrinkage in compacted graphite iron, and a method to calculate the size of shrinkage was proposed. By observing the microstructure around the shrinkage zone, it is concluded that concentrated shrinkage mainly appears in the solidification region where the dendritic gap is closed, and the isolated shrinkage mainly occurs in the final solidification region, and the supersaturated carbon elements are gathered on the surface of the shrinkage. The cause of shrinkage in compacted graphite iron is caused by its solidification method, where the austenite dendrites and the eutectic clusters are generated close to the melt zone during the solidification process, leading to the inability to feed the shrinkage. Based on the thermodynamic analysis, the equations between the volume change of each phase, solid phase rate, and time during solidification of compacted graphite iron were established to theoretically explain the formation mechanism of the shrinkage. Taking nine parameters such as the chemical elements and characteristic values of thermal analysis as the input nods, a four-layer BP neural network model for predicting the size of shrinkage in compacted graphite iron was constructed, and the R-squared of the model reached 97%, which indicates it could be used to predict the shrinkage tendency.

Experimental investigation and thermodynamic description of the Co–Nb–Ti system

The solidification microstructures of as-cast alloys and the equilibria microstructure of annealed alloys at 1273 K in the Co–Nb–Ti system were determined by scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD), and the liquidus surface projection and partial isothermal section at 1273 K were constructed. Nine primary solidification regions, bcc(Nb,Ti), fcc(Co), Co3Ti, CoTi, CoTi2, λ1, λ2, λ3, and μ and five ternary invariant reactions, liq. → λ1 + λ2 + μ, liq. → CoTi + λ2 + μ, liq. → bcc(Nb,Ti) + CoTi + μ, liq. + fcc(Co) + λ3 → Co3Ti, and liq. + CoTi → bcc(Nb,Ti) + CoTi2 were experimentally determined or deduced in the liquidus surface projection. Three three-phase regions, CoTi + λ2 + μ, bcc(Nb,Ti) + CoTi + μ, and bcc(Nb,Ti) + CoTi + CoTi2 and six two-phase regions, Co3Ti + λ3, λ2 + μ, CoTi + λ2, CoTi + μ, bcc(Nb,Ti) + μ, and bcc(Nb,Ti) + CoTi at 1273 K were experimentally determined. The continuous compound λ2 with a C15 structure from the Co–Nb side to Co–Ti side was formed at 1273 K. The solubilities of Ti in μ and Nb in CoTi were determined to be ~11.1 at% and ~18.4 at% at 1273 K, respectively. And no new phases were found. According to the available experimental data from the present work and literatures, the Co–Nb–Ti system was optimized by CALPHAD (CALculatiuon of PHAse Diagram) method. Four solution phases (liquid, bcc, fcc, and hcp) were described using the substitutional solution model. A two-sublattice model (Co,Nb,Ti)2(Co,Nb,Ti)1 was used to described three compounds λ1, λ2, and λ3 according to their crystal structures and homogeneity ranges. Co7Nb2 and CoTi2 were treated as Co7(Nb,Ti)2 and Co(Nb,Ti)2 by a two-sublattice model, respectively. μ was treated as (Co,Nb,Ti)1(Nb,Ti)4(Co,Nb,Ti)2(Co,Nb,Ti)6 by a four-sublattice model. Co3Ti with L12 structure and CoTi with B2 structure were treated as the ordered phases of fcc and bcc solutions using the thermodynamic models (Co,Nb,Ti)0.75(Co,Nb,Ti)0.25Va1 and (Co,Nb,Ti,Va)0.5(Co,Nb,Ti,Va)0.5Va3, respectively. A set of self-consistent thermodynamic parameters of the Co–Nb–Ti system was obtained.

Experimental determination of the isothermal sections and the liquidus surface projection of the Mo-Si-Cr ternary system

The isothermal sections at 1200 and 1000 ℃ and the liquidus projection of the Mo-Cr-Si system have been experimentally determined based on the microstructures, the phase structure analyses and the phase constitutions of the as-cast and the isothermally heat treated alloys in whole composition range, using scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) techniques. In the isothermal section at 1000 and 1200 °C, five three-phase regions and ten two-phase regions were derived, and no ternary compounds were observed. Nine primary solidification regions and six univariant reaction lines were deduced based on the study on the solidification processes of the as-cast alloys in the liquidus surface projection. In addition, the stoichiometry compounds Mo5Si3 and αCr5Si3 are the phases of the same structure (W5Si3-type) but different compositions, forming two intermetallic compounds Mo-rich Mo(Cr)5Si3 and Cr-rich Cr(Mo)5Si3. The present experimental results could be used for the further thermodynamic assessment of the Mo-Si-Cr ternary system, and be helpful for the database development of the multi-components Nb/Mo-Si-based superalloy system.

Phase equilibria and liquidus projection determination of the Hf–Si–V ternary system

The phase equilibria including the isothermal sections at 1100 and 1300 °C as well as the liquidus surface projection of the Hf–Si–V ternary system were experimentally determined. The microstructures, crystal structures and phase constitutions of the annealed and the as-cast alloys in whole composition range were investigated by means of scanning electron microscopy equipped with energy dispersive spectroscopy, X-ray diffraction and electron probe microanalysis, respectively. The isothermal sections of the Hf–Si–V system at 1100 and 1300 °C were accordingly constructed. Fifteen three-phase regions were confirmed, the ternary compound HfVSi was identified, and the solubilities of the third element in the binary compounds were determined. In the meanwhile, based on the XRD data obtained in the present work and by means of the JADE software package, the lattice parameters of each compound are calculated. Furthermore, the solidification paths of the Hf–Si–V as-cast alloys were analyzed to verify the primarily solidified phases from the liquid phase. Fifteen primary solidification regions were finally confirmed in the liquidus surface projection. Particularly, transmission electron microscopy results revealed that the binary stoichiometric compound V6Si5 extended into the Hf–Si–V ternary system, forming the line compound (V,Hf)6Si5. With the decrease of temperature, the line compound (V,Hf)6Si5 can decompose into two phases, one of which V(Hf)6Si5 has the composition near V6Si5, another near Hf2V4Si5, but with the same structure type. The Hf2V4Si5 phase is proposed to form due to the miscibility gap of the line compound (V,Hf)6Si5. The present experimental results are useful for the thermodynamic assessment of the Hf–Si–V ternary system, and the database development for the Nb/Mo–Si–V–Hf containing multi-component superalloy materials.

Investigating the low-temperature properties of oil by the method of dielectric spectroscopy

The article is devoted to the possibility of using the dielectric spectroscopy method for assessing the low-temperature properties of oil. For 25 samples of oil in the fields of Tyumen region the physical and chemical characteristics have been determined, in the temperature range dielectric permittivity ℇ and the tangent angle of dielectric losses tg δ were obtained. The dielectric relaxation of oil was established. The calculated values of dielectric relaxation parameters made it possible to interpret this process as a glass transition process. The glass transition temperature tС and the region of structural solidification ∆t of oil have been determined. The dependences of tС and ∆t of oil on its physico-chemical characteristics have been established and investigated by methods of correlation and regression analysis. The glass transition temperature tС and the temperature range of structural solidification ∆t have been proposed to estimate the lowtemperature properties of oil. In the future, on the basis of data obtained, recommendations for application of dielectric spectroscopy method for forecasting and operational control of some (individual) characteristics of oil in the processes of its production, collection, preparation and transportation can be developed.

Effect of Cu on the Microstructure of Al-8Zn-2.5Mg-xCu Alloys Fabricated by Twin roll casting

The effect of Cu content on the microstructure and mechanical properties of Al-8.0Zn-2.5Mg-xCu (x: 0, 1, 2, 3) aluminum alloys manufactured by the twin-roll casting process was investigated. The Al-8.0Zn-2.5Mg-xCu alloy showed an increase in surface defects with increasing Cu content. This is because the amount of residual liquid in the final solidification region increased from 9.6 wt.% to 18.3 wt.% as the Cu content increased from 0Cu to 3 Cu alloy. For the 3Cu alloy, as the amount of residual liquid in the final solidification region exceeded the critical point, a large number of surface defects and internal shrinkage defect were observed. The main secondary phases of the four alloys were the T(Mg32(Al, Zn)49) and η(MgZn2) phases, and their fraction increased with Cu content. These secondary phases mainly existed in the center segregation band, and a fine η(MgZn2) phase was additionally observed. In terms of mechanical properties, as the Cu content increased, the hardness of the center matrix, secondary phase, and overall hardness increased respectively. Although the yield strength increased, the tensile strength and elongation decreased because the center segregation band was widened from 684 μm to 790 μm with increasing Cu content.