High-melting Phase Research Articles

Liquid AlCoCrFeNi and AlCoCrFeNiX (X = Mo, Ta) high-entropy alloys on graphite: Wetting, reactivity and CALPHAD modelling

Wettability and interfacial reactivity with graphite of three equimolar High-Entropy Alloys (HEAs), namely AlCoCrFeNi (HEA-base), AlCoCrFeNiMo (HEA-Mo) and AlCoCrFeNiTa (HEA-Ta), were investigated for the first time, aiming to support industrial sectors involving liquid-phase processing routes. Isothermal high-temperature wettability tests were performed at 1400 °C for HEA-base and HEA-Mo, and at 1580 °C for HEA-Ta. The samples were then analysed by SEM-EDS. Based on the in-house-built GHEA thermodynamic database, CALPHAD calculations were used to simulate and discuss liquid-solid interactions occurring at high temperatures as well as transformations taking place in the samples during cooling. HEA-base wetted the graphite well after 5 min and the wetting behaviour significantly improved for HEA-Mo after the same contact time. HEA-Ta, after initial melting and wetting stage, solidified due to high-melting phases formation. All the graphite/HEA samples formed refractory carbides. For HEA-base and HEA-Mo, such carbides were found homogeneously dispersed in the whole drop. In HEA-Ta, TaC formed a compact layer separating two liquids of different compositions, with only one of these being in contact with the graphite substrate. The here proposed combination of experiments and thermodynamic calculations allowed to understand and discuss both high-temperature reactivity and evolution of the systems over cooling. Overall, a remarkable agreement between equilibrium calculations and experimental findings was observed for these complex dynamic systems.

Preparation and photothermal properties of latent functional thermal fluid based on size-controllable phase change nanocapsules

To improve the solar heat utilization efficiency of working fluid in direct solar collector, a novel latent functional thermal fluid (LFTF) based on self-designed 1-octadecanol@silica phase change nanocapsules (NEPCMs) with excellent heat storage capacity and photothermal conversion efficiency were prepared. The NEPCMs prepared by sol–gel method were dispersed in water containing multi-walled carbon nanoparticles to prepare the LFTF. The prepared NEPCMs have high encapsulation efficiency (85.12 %) and melting phase change enthalpy (235.1 J/g), controllable particle size (27.13–167.30 nm), excellent thermal stability and cycle stability. The LFTF containing 10 wt% NEPCMs was considered the optimal choice for its excellent dispersion stability (zeta potential 35.63 mV), thermal storage capacity (peak specific heat capacity 5.11 J·g−1·K−1) and superior photothermal conversion efficiency (up to 98 %), which is roughly 3.29 times that of water. These results offer valuable insights for the future development of LFTF in the field of solar thermal applications.

Enhanced comprehensive performance of Y2O3-Al2O3-SiO2 glass filler for SiC brazing by Li2O additive

Effects of Li2O additive on the network structure, crystallization behavior, thermal expansion properties and wettability of Y2O3-Al2O3-SiO2 glass were investigated. The results showed that Li2O could depolymerize the network structure, decreasing the characteristic temperatures of glass. The coefficient of thermal expansion of glasses varied from 5.09×10−6 K−1 to 8.03×10−6 K−1 by changing the Li2O addition and heat treatment process. Meanwhile, Li2O inhibited the crystallization of high-melting phases, consequently improving the fluidity of molten glass. The glass with 3 wt% Li2O showed an excellent wettability on SiC ceramic with a contact angle of 18°, and a sound joint mainly consisted of glassy phase was obtained at 1430 ℃ for 30 min. The prepared joints exhibited a shear strength of 119.7±20.3 MPa, which is 5.4 times of that without Li2O. After 3 times of water-quenching from 1200 ℃, the joints remained 68.4% of its original strength, demonstrating graceful reliability.

Critical assessment of thermal conductivity models for Miscibility Gap Alloy-based composite Phase Change Materials for high temperature Thermal Energy Storage

Miscibility Gap Alloys (MGAs), such as Al-Sn-based systems, provide a viable solution for the development of composite Phase Change Materials (PCMs) for Thermal Energy Storage (TES) purposes. Their successful production depends on the cooling rate imposed to the melt. Finite Element Analyses (FEA), which relies also on thermal conductivity values, represent a powerful tool for the design of the production process. Thermal conductivity, which depends on the arrangement of the phases in the system, also affects the thermal response of the alloy. In the view of evaluating the impact of the phase morphology, the authors adapted some of the models developed for composites and solutions to Al-Sn and Al-Sn-Si-Mg alloys, characterized by broad solidification ranges in terms of composition and temperature and by significantly different phases thermal conductivity. In the fully-liquid range, Filippov and Novoselova model was selected for the description of both alloys. Models that consider sphere-like dispersions give values quite close to the theoretical upper Wiener bound when the high-melting phase is solid. The phase morphology impact is relevant when the solidification range is considered. The resulting arrangement-related thermal conductivity curves are compared to those supplied by CALPHAD-based software and to available literature data.

Utilization of Hydrated Lime as Binder and Fluxing Agent for the Production of High Basicity Magnesium Fluxed Pellets

The present work investigates the manufacture of high basicity (CaO/SiO2 = 1.45) magnesium containing fluxed pellets with hydrated lime used as binder and fluxing agent. The fired pellets and lump ores could constitute a binary burden for blast furnace with lower carbon emission. Both of the green and fired pellets with high basicity possessed good physical and chemical properties, and its metallurgical performances were better than the fired pellets in which bentonite and limestone were used as binder and fluxing agent, respectively. The optical microscopy, SEM/EDS and thermodynamic calculation were employed to clarify the difference between limestone fluxed pellets and hydrated lime fluxed pellets. The effect of basicity on the properties of green and indurated pellets was also studied. The results found that basicity and decomposition characteristics of the fluxing agents had great influence on the mineral composition of the fired pellets. Larger amounts of high-melting phases like MgFe2O4, strong bonding phases such as CaFe2O4 and good reducibility phases like SFCA could be generated in the hydrated lime fluxed pellets with high basicity during induration process, thus its metallurgical performances being improved.

THE PECULIARITIES OF THE FRACTURE OF HETEROPHASE INCLUSIONS OF THE TYPE "HIGH- MELTING PHASE SURROUNDED BY A LIGHT-MELTING SHELL" IN THE DEFORMATION OF STEELS

Purpose. The aim of the work was to study of the peculiarities of crack initiation in heterophase inclusions of the "high-melting phase surrounded by a low-melting shell" type during deformation of steels. Methods. The destruction of heterophase inclusions of various types was investigated during deformation of specimens from steels 08T, 08Yu, 12GS, 08kp, 09G2S, NB-57, 08GSYuTF in the temperature range 20...1 200 °C [1; 5]. Samples of steels were subjected to tension, compression and bending in vacuum at temperatures of 20…1 200 °C on Instron-1195 and IMASH-5C with special grippers, the movement speed of which was 20 mm / min. Methods of investigation –petrography and optical microscopy (Neophot-21) were used. Results. It was found that in the process of deformation of the steels in different temperature regimes, microcracks originating near heterophase inclusions of the "high-melting phase surrounded by low-melting shell" type can be brittle and ductile. In this case, the role of the level of plasticity of the phases making up the inclusion, which depends on the deformation temperature, is important. The features of microcrack initiation are shown for different combinations of plastic and non-deformable phases of inclusions. The interaction of heterophase inclusions and a steel matrix during deformation is analyzed. It has been established that the nucleation and growth of microcracks within inclusions of the "high-melting phase surrounded by a low-melting shell" type occurs both in the phases and along the internal interphase boundaries. Scientific novelty. Specific features of the initiation of microcracks associated with heterophase inclusions of the " high-melting phase surrounded by a low-melting shell" type, which have a different combination of brittle and plastic phases during deformation of steels, have been established. It is shown that the critical degrees of deformation of the samples, upon reaching which noticeable microcracks appeared along the internal interphase boundaries, depend on the temperature and the nature of the phases of the inclusions. Practical significance. The use of the results obtained will make it possible to develop technologies for producing steels with regulated types of heterophase nonmetallic inclusions, which will significantly increase their technological and operational characteristics, as well as prevent the formation of various kinds of defects during the processing of steels by pressure and the operation of products.

Al-Sn Miscibility Gap Alloy produced by Power Bed Laser Melting for application as Phase Change Material

Alloys based on the Al-Sn system are Miscibility Gap Alloys consisting of two phases: a low-melting Sn phase and a high-melting Al phase. The two phases are almost immiscible in solid state, with minimal miscibility of Al in liquid phase just above the Sn-phase melting temperature. This situation allows the alloy to be used as composite Phase Change Material, where the low-melting (active) phase (Sn) acts as the actual Phase Change Material, storing/releasing latent heat at each thermal cycle across its activation temperature, and the high-melting (passive) phase (Al) plays its main role in fast driving heat within the composite material, thanks to its high thermal conductivity. Thermal response of PCM composites is expected to be faster if the active phase is finely combined to the matrix. Moreover, if the active phase is embedded in the passive phase, leakage phenomena can be prevented and the continuity of the passive phase, acting as matrix, also allows residual structural properties to the composite above the temperature at which the low-melting phase is activated. In these conditions, the composite PCM can be considered as a form-stable composite material. In this paper, the effect of rapid solidification has been investigated as a tool to get suitable microstructure of an Al-Sn alloy in view of its application as a composite PCM. The rapid solidification has been obtained by a Power Bed Laser Melting (also known as Selective Laser Melting) process. In the case of positive results for microstructure, the process will allow to overcome critical points in the manufacturing of composite PCM parts by powder metallurgy, mainly consisting in limitations on size and geometrical features of the parts. Data availabilityThe raw data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. The processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Roles of the semisolid heating on the microstructure of a hypereutectic Al\u2013Fe\u2013Cu alloy

A hypereutectic Al–Fe–Cu alloy with a high-volume fraction ferro-aluminum second phase (AlFe phases for short) was reheated in the solid–liquid region and the microstructure evolution was investigated. During semisolid heating, the high-melting AlFe phases in the Al–Fe–Cu alloy were demonstrated to stunt the grain growth and to block the liquid coalescing and the solid moving. Consequently, the grain sizes in the alloy increased rapidly and then slowly with increasing holding time, and the grains increased gradually with increasing temperature. Smaller grain grew into the large grain but it did not continually grow into the larger grain with increasing temperature or holding time. The shape factor (SF) of the alloy increased gradually and then decreased quickly with increasing temperature or holding time. The major alloying elements in addition to magnesium in the hypereutectic Al–Fe–Cu alloy were finally enriched at the grain boundaries or around the AlFe phases. Besides dissolving in the grains or AlFe phases, copper also diffused between the grains or around AlFe phases, resulting in the formation of diverse Cu-enriched zones. Cu constituents in the inter-grains are outnumbered in the intra-grains. The coarsening kinetics of the alloy was controlled by grain boundary diffusion. The coarsening rate constants K in the initial stage of heating (5–20 min) were several times larger than that in the later stage of heating (20–60 min), indicating the blocking effect of AlFe phases on coarsened grain being obvious.

Effect of Different Production Processes on Metallic Composite Phase Change Materials for Thermal Energy Storage

Phase Change Materials (PCMs) can be applied in Thermal Energy Storage and Thermal Management systems, exploiting the storage and release of latent heat associated to a phase transition. Among them, metallic PCMs can be used at medium and high temperatures (i.e. above 150°C), storing higher heat per unit volume at higher temperatures with respect to the most widely investigated polymeric and salt-based PCMs. Miscibility Gap Alloys (MGAs) can be used to obtain multiple-phase mixtures in which the active phase (the actual PCM) is mixed to a second, high-melting temperature phase, with negligible interaction between them. These can actually be considered as fully metallic composite materials specifically developed for thermal management. Suitable microstructures can prevent leakage of active phase when the solid-liquid transition occurs, resulting in a form-stable PCM (FS-PCM). However, obtaining these microstructures it is not trivial. The present study focuses on a solid-liquid FS-PCM consisting of a ‘classical’ fully metallic FS-PCM, an Al-Sn based MGAs produced by powder metallurgy. The goal was to evaluate the effect of different production processes on thermal and mechanical behaviour of the PCM. Particularly, powder metallurgy routes including both simple mixing and ball milling were compared and further combined. Moreover, several compression and sintering conditions were considered, also substituting Al powders with Al-alloy powders, in order to optimize the material microstructures in view of suitable thermal and mechanical properties. Finally, the casting route with a rapid solidification approach was investigated for the same alloy.

Synthesis of SrHfO3 Co-doped with Gd2O3 and Yb2O3 for thermal barrier coating applications

In thermal barrier coatings (TBCs), ceramic material (YSZ) acts as a topcoat layer. Recently, perovskites (ABO3) materials are consider as a substitute for YSZ, which are used in gas turbines, aircrafts and jet engines etc. The SrHfO3 perovskite has high melting point but phase transformation occurs at 1130°C. Due to this phase transformation, its shows significant changes in its volume. To overcome this problem, SrHfO3 is co-doping with the rare earth oxide materials (Gd2O3 and Yb2O3). The main aim of this work is to co-dope 5 mol% Gd2O3 and 5 mol% Yb2O3 with SrHfO3 by using a mechanical solid-state method. The obtained mixture is annealed at 1450°C at a rate of increase of 3°C/min for 24 h. The phase and structural information are characterized by X-Ray Diffraction (XRD). The attained powder is sintered at 1700°C for 15 min with a spark plasma sintering (SPS), to form a pellet. The pellet density and the relative density is measured by using Archimede’s technique. Thermal gravimetric and differential thermal analysis (TG-DTA) are used to examine the phase purity. The morphology, and composition analysis are examined by using Field emission scanning electron microscopy (FESEM) followed by Energy dispersive spectroscopy (EDS) respectively. The refined powder is in an orthorhombic structure and observed by using XRD. The relative density of pellet is 95.1% and fractional porosity is 4.9%. TG-DTA graph shows no phase transformation and weight loss is <3%. Morphology of the grain boundaries and elemental analysis are observed by FESEM & EDS respectively.

Effect of basicity and chromium oxide on the viscosity of boron-containing slags

The main diluent for chromium-containing slags is CaF2. However, fluorspar disadvantages make it necessary to search for a replacement. As a substitute, boron oxide allows to improve slag physical properties and environmental situation. The paper presents the experimental study results of the chemical composition and temperature influence on viscosity of slags of CaO–SiO2–Cr2O3 system containing 8% MgO, 3% Al2O3 and 6% B2O3. It was found that a slag free of chromium oxide with basicity of 1.0 has a sufficiently high fluidity of 0.2–0.6 Pa·s in a wide temperature range of 1200–1350°C, due to high concentration of low-melting phases, reaching 22%, and only 11 % high-melting. A slag with 18% Cr2O3 and the same basicity retains a low viscosity of 0.1–1.0 Pa·s but at higher temperature of 1450–1570 °C due to increase in high-melting compound content to 27%. The viscosity of a slag with basicity of 2.5 without chromium oxide is 0.07–1.0 Pa·s in a narrow temperature range of 1650–1700°C and 0.14–1.0 Pa·s for a ‘shorter’ slag with 18% Cr2O3 and same basicity in even narrower range of 1650–1670°C. These slags have the highest high-melting phase content of about 50%.

The behaviour of heterophase inclusions “high-melting phase surrounding with low-melting cover” under steels pressure forming

Purpose. It is necessary to study of the effect of heterophase inclusions on the technological ductility of steels for various purposes. The goal of the work was to study the plasticity level of heterophase inclusions “high-melting phase surrounded with low-melting cover” during the processing of steels by pressure.Methods. Comprehensive methods for the study of heterophase non-metallic inclusions (metallographic, petrographic, X-ray microanalysis methods) were used. Results.It was found that heterophase inclusions “high-melting phase surrounded with low-melting cover” have different structures and phase compositions. The features of the plastic behavior of various phases in inclusions “high-melting phase surrounded with low-melting cover” with various combinations of phases were shown. The interaction of phases in the inclusions during the processing of steel by pressure was analyzed.Scientific novelty.The special features of the plastic behavior of heterophasic ones were identified, including the “high-melting phase, surrounded with low-melting cover” with steel sampling with a vice. It was shown that the process development of interphase rubbing at the boundaries of the included-matrix and at the internal boundaries of the inclusions was shown. The inverse phase inclusion at the third stage of deformation was presented. The possibility of transforming the inclusion of a “high-melting phase, surrounded with low-melting cover” in the process of hot and cold deformation when machining steel with a vice is shown. Practical significance.The results obtained will allow developing technologies for producing steels with regulated content and types of heterophase non-metallic inclusions, which will significantly increase their technological plasticity, as well as prevent the formation of various kinds of defects during the processing of steels by pressure.

Effect of titanium on the skull formation of the blast furnace hearth

The erosion of the blast furnace hearth has become one of the key factors affecting the life of the blast furnace, because of the limited on-line monitoring and control means in the blast furnace hearth area, the burn-through accidents of the blast furnace hearth and bottom occur occasionally. In this work, based on the self-built platform of heat flow regulation, the control mechanism of the high melting phases of TiC is studied. The on-line control mechanism of the hearth erosion by adding titanium-containing materials was discussed. There are three steps for the on-line control of iromaking using titanium-containing materials in blast furnace. Firstly, Ti(C, N) precipitated from the molten iron due to the decrease of temperature in the erosion zone of the hearth. Secondly, the viscosity of the molten iron increased with the precipitation of Ti(C, N) in the erosional cryogenic zone and temperature further decreasing. Finally, the protective layer, included pig iron matrix and Ti(C, N) solid fulcrum, formed because of the solidification of the molten iron. The protective layer can replace the corroded lining of the blast furnace to prevent its hearth from being eroded.

Types and structure of heterophase inclusions in steels

Purpose. The problem of the influence of heterophase nonmetallic inclusions on the properties of steels is actual. The goal of the work was to study the main types and structures of heterophase nonmetallic inclusions in steels. Methods. Comprehensive methods for the study of heterophase non-metallic inclusions (metallographic, petrographic, X-ray microanalysis methods) were used. Results. It has been established that heterophase inclusions have different structure and phase composition. The features of various combinations of phases in microcomposite inclusions are shown. The main types of hetero-phase inclusions in steels are established. Scientific novelty. The structural features of heterophase inclusions in steels are established. A classification of microcomposite heterophasic inclusions is proposed: inclusions of the first type “high-melting phase surrounding with low-melting cover”; the inclusion of the second type of “phases are beside”; the inclusion of the third type of “eutectics”; Inclusions of the fourth type “dispersed phases in a non-metallic matrix”; inclusions of the fifth type "eutectics inclusion-matrix". Practical significance. The use of the obtained results will make it possible to develop technologies for the production of steels with regulated content and types of heterophase non-metallic inclusions, which will significantly improve their technological, mechanical and operational properties.

Effect of titanium content on the precipitation behavior of carbon-saturated molten pig iron

The use of iron ores bearing titanium as a raw material is an effective measure to prevent hearth erosion and prolong the life of a blast furnace. In this research, the effect of titanium content on the precipitation behaviors of high-melting phases of carbon-saturated molten pig iron were studied by confocal scanning laser microscopy. The results showed that, when the titanium content was less than 0.25wt%, Fe3C was precipitated as a single phase from the molten carbon-saturated iron. The growth rate of the precipitated Fe3C crystals was very high, reaching 7387 μm2/s. When the titanium content in the molten pig iron was greater than 0.47wt%, TiC crystals precipitated first. The shape and size of the precipitated TiC crystals did not obviously change. After TiC was precipitated, the fluidity of the molten pig iron worsened. With a decrease in temperature, Fe3C was also precipitated but the growth rate of Fe3C was limited by the presence of the first precipitated TiC phase. The crystal size of the precipitated Fe3C was much smaller than that of pure Fe3C.

Experimental Study on Performance Test of Serpentine Flat Plate Collector with Different Pipe Parameters and A New Phase Change Collector

Abstract In this paper, the thermal performance of serpentine pipe flat plate collectors with different pipe diameters and pipe spacings was tested by experiments. On this basis, two phase change materials with high melting point (70 °C) and low melting point (15 °C) were laid on the upper and lower parts of the pipeline to enhance the frost resistance and high temperature resistance of the collector. The experimental results show that under strong solar radiation conditions, high-melting phase change materials can absorb a lot of heat when the water temperature of the collector is high, thereby slowing down the water temperature rise and avoiding overheating, and when the collector is cooled, phase change can be effectively utilized to slows down the rate of temperature drop of the collector, so as to achieve reasonable utilization of the overheating of the collector; in the low temperature environment, the low melting point phase change material can significantly reduce the cooling of the collector and improve the frost resistance of the collector. The placement of the two kinds of different melting point phase change materials was investigated, and the results show that the low-melting phase change material under the pipe is more conducive to the collector antifreezing. However, for the hot regions, the high-melting phase change material placed at the top of the pipe is more conducive to preventing overheating.

Corrosion and penetration behaviors of slag/steel on the corroded interfaces of Al2O3-C refractories: Role of Ti3AlC2

In this study, the degradation behavior of Al2O3-Ti3AlC2-Si-C refractories and a complete degradation process of Ti3AlC2 under a temperature gradient along the penetration direction were investigated by a dynamic corrosion at approximately 1630 °C. The degradation process of Ti3AlC2 affected the local liquid components by assisting to form CaAl12O19 and restrained CaO penetration via trapping into high-melting phase. On refractories-steel/slag interfaces affected by Marangoni convection, the molten steel penetration was suppressed in the samples with Ti3AlC2. A refined texture and increased viscosity, accompanying Ti3AlC2 oxidation and dissolution in molten steel, blocked the pathways and restrained the penetration of molten steel.

Preparation and thermophysical properties of plasma sprayed lanthanum zirconate

Lanthanum Zirconate (LZ) has been found as prosperous thermal barrier coating (TBCs) material for its high melting point, phase stability and lower thermal conductivity. LZ has been synthesized by planetary ball milling method using a mixture of La2O3 and ZrO2 powders in a 1:2 mol ratio for 24 h. The synthesized LZ was deposited on SS substrate by a DC non-transferred atmospheric plasma spray (APS) system. X-ray diffraction and field emission scanning electron microscopic investigations revealed the formation of a single phase formation with the occurrence of pores and the formation of lamella particles. Thermal conductivity of LZ has been found in the range 0.46–0.6 W m−1 K−1 at 24 kW which was much lower than that shown by YSZ (yttria stabilized zirconia).

Effect of Cr2O3 Pickup on Dissolution of Lime in Converter Slag

AbstractApplication of low-nickel laterite ore containing chromium as charging material for ironmaking can reduce raw material costs, but result in an increase of chromium content in the hot metal and hence, Cr2O3 content in the steelmaking slag, which subsequently causes many problems related to lime dissolution for the steelmaking operation. In this work, a rotating cylinder method was employed to study the effect of Cr2O3 on lime dissolution in steelmaking slag. The lime dissolution mechanism, rate control step and affecting factors, including slag basicity, FeOx and B2O3 content, and the formation of phases at reacted layer, were discussed. It was found that mass transfer was the rate control step in slag phase, increase of Cr2O3 and slag basicity delayed lime dissolution due to the formation of high-melting temperature phases of FeO · Cr2O3 spinel and 2CaO · SiO2 at the slag/lime reacted interface. Addition of B2O3 promoted lime dissolution and suppressed formation of FeO · Cr2O3 spinel.

Ablation Resistance Property of TiC/ZrO 2 Coating Deposited by Plasma-spraying

In the high temperature ablation process, the TiC coating will be oxidized to TiO 2 . The thickness of molten TiO 2 increases when the temperature is higher than 2000 °C, which decreases the bonding strength of the coating. In the present study, the TiC/ZrO 2 coatings with different contents of high melting phase ZrO 2 were deposited by plasma spraying, in order to improve the anti-scouring property of TiC coatings. The phase structure, microstructure and composition of the coatings, before and after ablation, were characterized by XRD, SEM and EDS, respectively. The ablation resistant mechanism of the coatings with high melting phase ZrO 2 was discussed. Results indicate that the TiC/ZrO 2 coating shows good ability of ablation resistance, and its mass loss is only 1.5×10 −4 g-cm −2 -s −1 at 2000 °C. The ZrO 2 layer formed during the ablation can restrain the further oxidation of inner TiC and reduce the shearing force of oxyacetylene flame, which improves the anti-scouring property of the coatings.