Non-equilibrium Phase Composition Research Articles

Technology of synthesis and electronic structure of triple barium–strontium–calcium carbonates for cathodes of microwave devices

Objectives. Triple barium–strontium–calcium carbonates of various grades are widely used for depositing oxide coatings on cathodes of electrovacuum devices. Of all types of cathodes used in electrovacuum devices, oxide cathodes are among the most common, due to combining efficiency, durability, operation at relatively low temperatures, and a relatively low cost. The aims of this work were to: create a technology for the synthesis of triple barium–strontium–calcium carbonates with nonequilibrium phase compositions that comprise the triple carbonate proper and a pure barium carbonate phase; develop a quality control procedure for such a carbonate for using it as a component of the cathode material for microwave devices; study how the electronic structure of barium oxide crystallites is affected by doping microimpurities from other phases of the cathode material.Methods. The study used precision X-ray diffraction analysis and electron spectroscopy.Results. A technology was developed for the co-precipitation of triple barium–strontium–calcium carbonates from their nitrate salts. Under optimal precipitation conditions, this produces triple carbonate powders with nonequilibrium phase compositions. Electron spectroscopy showed that the parameters of the electronic structure of the crystallites are significantly affected by doping impurities of calcium, strontium, and nickel in barium oxide crystallites formed by heat treatment of triple carbonates.Conclusions. Calcium and strontium have a synergistic effect on the doping of barium oxide with the two other chemical elements. As well as efficiently controlling the quality of the nonequilibrium phase composition of triple carbonates, which is formed during the synthesis of triple carbonates by the titration method, precision X-ray diffraction analysis can be used to efficiently control the processes of agglomeration of nanoparticles or recrystallization of nanostructured phases formed during the synthesis of triple carbonates.

Enhancing the repassivation ability and localised corrosion resistance of an additively manufactured duplex stainless steel by post-processing heat treatment

Duplex stainless steel (DSS) 2205 produced by laser powder bed fusion (LPBF) additive manufacturing was found to possess poor repassivation ability and weak resistance to localised corrosion. This was due to the nonequilibrium phase composition and elemental distribution in the as-built LPBF DSS affected by the rapid solidification inherent to LPBF processing. A short-time post-processing heat treatment at 1000 °C for 10 min followed by water quenching to room temperature was shown to be able to enhance the repassivation ability and localised corrosion resistance by retrieving the equilibrium dual-phase microstructure. A discussion is given on the mechanism behind such behaviour.

Super-hardening and localized plastic deformation behaviors in ZrB2 –TaВ2 ceramics

A systematic study of the Zr-Ta-B system was performed using thermodynamic and ab initio modeling, nanoindentation, and high-resolution TEM and XRD experiments. The phase diagram (convex hull) of the Zr-Ta-B system was constructed at 0 K, and phase stability, lattice parameters, and mechanical properties of ZrB2-TaB2 solid solutions were ascertained in silico. Dense ceramics were manufactured by the combination of combustion synthesis and hot pressing techniques. As opposed to theoretically predicted complete solubility in the ZrB2-TaB2 solid solution series, the highest experimentally attained Ta content in the ZrB2-based solution was 25–27 at%, suggesting that at higher Ta contents a non-equilibrium phase composition is formed during combustion synthesis and inherited during hot pressing. Single-phase ZrB2-TaB2 solid solutions displayed a super-hardness up to 70 GPa and coefficient of elastic recovery as high as 99.7%, which surpasses wurtzite and cubic boron nitride and rivals polycrystalline diamonds. In the non-equilibrium samples with multiple solid solutions, a “pop-in” (localized plastic deformation) effect was discovered and attributed to the formation of shearing bands, microtwins, and dislocation loops. The composition-dependent switching between super-hardened and locally plastic mechanical behavior allows the creation of a new generation of ultra-high temperature ceramic composites.

Theoretical and technological aspects of production mechani‑cally alloyed powders for the production of coatings and products by additive methods

The established regularities of the formation of powders based on iron and nickel, obtained by the method of mechanical alloying and intended for the deposition of thermal spraying coatings, as well as the manufacture of products by layer‑by‑layer synthesis. The structure, phase composition and properties of materials are investigated. Powders consist of particles with a size of 20–70 microns, differ in the submicrocrystalline type structures, and nonequilibrium phase composition. Thermal spray coatings made of them have a set of properties that significantly exceed the properties of coatings made of commercially available materials. The diameter of the grains of the material obtained by the SLМ method from the synthesized powder is 1.5–2.0 times smaller than that produced from the powder of 316L steel, and the heat resistance is higher.

Структура и фазовый состав плазменных покрытий гидроксиапатита

Changes in the structure, phase composition of the hydroxyapatite (HA) plasma coating were established during deposition at an initial 20 °C and 550 °C temperature of the titanium substrate and a spraying distance of 95 and 150 mm. The structure of HA coatings was analyzed by SEM and optical microscopy. DSC analysis established the transition temperatures of HA coatings to the equilibrium state. When deposited on a substrate with an initial temperature of 20 °C at a deposition distance of 95 mm, a nanostructure with a crystallite size of 21 nm is fixed in the HA coating. With an increase in the deposition distance to 150 mm, the non-equilibrium phase composition increases, the crystallite size decreases to 12 nm, the HA content decreases from 72 to 61 %, TTCP from 10 to 5 %, and the α-TCP content increases from 17 to 30 %. A non-equilibrium nano-structural state passes into a more equilibrium state with the release of heat at temperatures of 615 – 727 °C in DSC studies. The high-temperature α-TCP phase is not fixed when the coating is deposited onto a substrate with an initial temperature of 550 °C at a spraying distance of 95 mm, the content of TTCP increases by 2 times, the size of the HA phase crystallites reaches 36 nm and their size in the sprayed powder is 75 nm. The HA coating has a dendritic microstructure and does not have a thermal effect during DSC heating at an initial substrate temperature of 550 °C.

Plasma coatings made of mechanically synthesized composite powders based on the «iron-aluminum» system

Theregularitiesof the formation of plasma coatings obtained from powders based on the Fe-Al system synthesized by the reaction mechanical alloying method are established. The influence of the processing modes of the compositions in the mechanoreactor on the properties of coatings is revealed. An effective method for controlling the porosity of coatings from developed powders based on a change in the power consumed by the installation is proposed.Plasma coatings have a nonequilibrium phase composition, which, in contrast to the equilibrium one, contains a wide range of metal oxides of the base and alloying elements formed during the spraying. The coating retains submicra – microcrystalline type of structure of powders with a highly developed grain and subgrain boundary. The basis of the coating is intermetallic FeAl. The role of hardening phases is played by ultrafine compounds of intermetallics and oxides.Powder annealing has a significant effect on the structure and properties of coatings and, first of all, on layers from compositions with complex alloying with oxides. Heat treatment increases the density and hardness of coatings and reduces porosity. The study of coating samples after high temperature indicates high heat resistance and the preservation of the microcrystalline type of structure of their bases.

The phase composition, structure and properties of high-temperature-based powders of the system iron-aluminum obtained by the technology of the reaction-mechanical alloying. Foundry production and metallurgy

The regularities of the formation of powders based on the Fe-Al system, obtained by the method of mechanical alloying and intended for the deposition of thermal spraying coatings were established. The morphology, structure, phase composition and properties of materials are investigated. After mechanical synthesis, the powders have an average particle size of 30–100 µm, submicrocrystalline structures, and nonequilibrium phase composition. Annealing of powders causes to an increase in the hardness of particles up to 80%, the value of which is largely preserved after prolonged heating to 1170 K, which indicates the presence of heat-resistant materials.

Effect of Structural and Phase State on the Creep of an Ultrafine-Grained Ti–Al–V–Mo Titanium Alloy

The effect of the phase composition on the regularities of creep of the ultrafine-grained Ti–Al–V–Mo titanium alloy at the temperature of 723 K and in the range of strain rates from 10–7 to 10–5 s–1 has been studied in this work. It has been found that the formation of the nonequilibrium phase composition in the ultrafine-grained alloy leads to a violation of the power law of creep. It has been shown that this violation is associated with the development of the β → α phase transformation during creep and with the redistribution of alloying elements. The physical causes of a decrease in the effective energy of activation of the creep of the alloy with ultrafine-grained structure and nonequilibrium phase composition have been discussed.

Phase Transformation Induced Self-Healing Behavior of Al-Ag Alloy.

Self-healing alloys are promising materials that can decrease the consequences of accidents. To detect crack formation in a material is simple task that can be performed by e.g., sonic or ultrasound detection, but it is not always possible to immediately replace the damaged parts. In this situation, it is very advantageous to have the chance to heal the crack during operation, which can be done e.g., by annealing. In this paper, self-healing behavior was proven by TEM (Transmission electron microscope) observation of crack healing after annealing. The crack was observed in the rapidly solidified Al-30Ag alloy with non-equilibrium phase composition formed by a minor amount of Ag2Al and a supersaturated solid solution of Ag in an fcc-Al matrix (fcc = face centered cubic). After annealing at 450 °C, equilibrium phase composition was obtained by forming a higher amount of Ag2Al. This phase transformation did not allow the crack to be healed. Subsequent annealing at 550 °C caused recrystallization to a supersaturated solid solution of Ag in fcc-Al, followed by a return to the mixture of fcc-Al and Ag2Al by cooling, and this process was accompanied by the closing of the crack. This observation proved the self-healing possibilities of the Ag2Al phase. Practical application of this self-healing behavior could be achieved through the dispersion of fine Ag2Al particles in a structural material, which will enrich the material with self-healing properties.

Dependence of the specific features of two PAPVD methods: Impulse Plasma Deposition (IPD) and Pulsed Magnetron Sputtering (PMS) on the structure of Fe–Cu alloy layers

This paper describes the study of the structural properties of the alloy layers prepared by two different, impulsively working PAPVD methods: the Pulsed Magnetron Sputtering (PMS) and the Impulse Plasma Deposition (IPD). The Fe–Cu alloy layers were synthesized. The results of our investigation revealed a nanocrystalline structure of the layers. The differences in the phase composition of the Fe–Cu alloy layers produced by these two methods were observed. The synthesis of the Fe–Cu layers by using the Pulsed Magnetron Sputtering method resulted in obtaining the two-phase, polycrystalline structures (fcc-Cu and bcc-Fe). In this case the clear evidence of mixing between the iron and copper atoms was not observed. The Fe–Cu layers deposited by the Impulse Plasma Deposition method were characterized by the non-equilibrium phase composition – the presence of one-phase supersaturated solid solution (fcc-Cu(Fe) or bcc-Fe(Cu)) was formed in immiscible systems. These results suggest a short-distance diffusion between the neighboring nanoparticles of the two metals (Cu and Fe) occurring during the IPD layers growth.

Effect of Heating Schedule on Fabrication of Ti3SiC2‐based Composites from Ti–SiC Powder Mixture

We herein describe a set of experiments used to study the effect of heating schedule on phase formation during the fabrication of Ti3SiC2‐based materials from Ti–SiC powder mixtures by means of pressureless sintering. We found that combustion was initiated at a temperature of about 1330°C, which resulted in the formation of Ti‐rich eutectic melt followed by crystallization of Ti3SiC2. It was shown that when preannealing was performed at 1150°C, no further combustive process could occur, and an intermediate, nonequilibrium phase composition, consisting of Ti5Si3Cx, TiC, and SiC remained until the end of the heat treatment. We herein discuss the mechanistic interpretation of this behavior.

Effect of Structure on the Electrochemical Oxidation Rate of Copper and Nickel Sulfides

Synthesized samples were crystallized at 10 - 1000 K/sec (νcooling) to study the effect of sulfides structure on the electrochemical oxidation rates. The methods of X-ray, optical and electronic microscopy are used to analyze the phase composition, and the methods of voltamperometry and chronoamperometry are used to study the laws of electrochemical oxidation. Slow cooling of the samples with sulfide phase (Cu1,96S, Ni3S2) leads to crystallization of the metal (Ni, Cu). High cooling rate leads to crystallization of non-equilibrium phases, increasing of proportion of the sulfide phase and decreasing of metallic component up to complete disappearance. It is shown that electrochemical oxidation of copper and nickel sulfides proceeds stepwise: Me2S Me2-XS Me1+XS MeS Me2+ + S. Experiments have revealed the rates of electrochemical oxidation of granular sulfides that exceed the rates established for the slowly cooled samples. The rates of the electrochemical oxidation of Ni3S2 at potential of 1500 mV are 8.810-8 g/sec·mm2 (at vcooling = 10 К/sec) and 1.310-7 g/sec·mm2 (at νcooling = 1000 К/sec). The rates of the electrochemical oxidation of Cu2S at potential of 1000 mV are 2.810-8 g/sec·mm2 (at νcooling = 10 К/sec) and 4.310-8 g/sec·mm2 (at νcooling = 1000 К/sec). It is found that the forming of a passivation layer effects the rate of the electrochemical oxidation of copper sulfide and nickel sulfide. Non-equilibrium phase composition and refinement provide greater reactivity of sulfides in the conditions of anodic polarization. The oxidation and passivation of metallic phase along with sulfide phases proceed from the surface of Me2S Me alloys. As for the samples crystallized at 1000 K/sec only anodic oxidation of sulfides occurs due to a lack of metallic phase. The compositions of passivation films and potentials providing the extraction of sulfur in the elemental state from the sulfides crystallized at high rates were determined.

Effect of Heating Schedule on Fabrication of Ti3SiC2-based Composites from Ti-SiC Powder Mixture

We herein describe a set of experiments used to study the effect of heating schedule on phase formation during the fabrication of Ti3SiC2-based materials from Ti–SiC powder mixtures by means of pressureless sintering. We found that combustion was initiated at a temperature of about 1330°C, which resulted in the formation of Ti-rich eutectic melt followed by crystallization of Ti3SiC2. It was shown that when preannealing was performed at 1150°C, no further combustive process could occur, and an intermediate, nonequilibrium phase composition, consisting of Ti5Si3Cx, TiC, and SiC remained until the end of the heat treatment. We herein discuss the mechanistic interpretation of this behavior.

Modeling of phase transitions in three-phase polymorphic systems: Part II. Effects of material characteristics on transition rates

Abstract

Atomistic Processes of Phase Transformation and Dynamic Recrystallization during Hot-Working of Intermetallic Titanium Aluminides

Intermetallic titanium aluminide alloys are multiphase assemblies with complex microstructure and constitution, involving the phases γ(TiAl), α2(Ti3Al), β, and B2. The earlier stages of phase transformation and dynamic recrystallization occurring upon hot-working of such an alloy were investigated at the atomic scale by high-resolution electron microscopy. Accordingly, the conversion of the microstructure is triggered by heterogeneities in the deformation state and non-equilibrium phase composition. The β/B2 phase is apparently unstable under tetragonal distortion, which gives rise to the formation of the B19 phase via distinct shuffle displacements. These processes lead to a modulated microstructure, which is comprised of several stable and metastable phases. The phase transformations are accomplished by the propagation and coalescence of ledges. Large and broad ledges can apparently easily be rearranged into intermediate metastable structures, which serve as precursor for the nucleation of new grains.

Mechanistic understanding of creep in gamma-base titanium aluminide alloys

The paper describes an experimental study of creep processes in two-phase titanium aluminide alloys. The investigations involve long-term creep test at relatively low stresses and temperatures. These test conditions lead to very low strain rates, which are characteristic of the intended service conditions. The creep tests were coupled with detailed electron microscope observations involving high-resolution imaging techniques and in situ heating studies. Long-term creep leads to spheroidization and coarsening of the lamellar morphology, which involve phase transformations and recrystallization. Climb velocities were analyzed in terms of the critical vacancy supersaturation necessary for the operation of diffusion assisted dislocation climb sources. The mechanisms are closely related to the atomic structure of the interfaces and are probably driven by a non-equilibrium phase composition.

Fracture toughness, strength and thermal shock behaviour of bulk plasma sprayed alumina — effects of heat treatment

Bulk plasma spraying Al 2O 3 produces a material with a defect rich splat-internal microstructure and a non-equilibrium phase composition which are both subject to extensive change by heat treatment. In the present work heat treatment induced changes in the strength, fracture and thermoshock characteristics of this material have been investigated and related to microstructural development. It is found that the as-sprayed intrasplat defect structure provides low energy paths for crack propagation. This results in low toughness and strength but excellent thermoshock resistance. Heat treatment increases overall porosity, but heals the defect structure and increases splat integrity, leading to a large increase in strength, accentuated R-curve behaviour and a disproportionate increase in toughness, but also a reduction in thermoshock resistance.

Metastable states in R 2Fe 14B-based alloys processed by severe plastic deformation

The influence of severe plastic torsion straining (SPTS) under high pressure on microstructure, phase composition and coercivity of cast sub-, super- and stoichiometric R 2Fe 14B (R = Nd, Pr)-based alloys was investigated. Deformation results in the formation of a metastable state characterised by ultrafine-grained (UFG) structure and non-equilibrium phase composition. Annealing at 600°C leads to complete or partial restoration of the initial phase composition depending on the alloy composition. Due to SPTS and subsequent annealing of the Pr 20Fe 73.5B 5Cu 1.5 alloy, the formation of high coercivity UFG state with jH c up to 2240 kA/m is observed.

Structure of the heat-affected zone in weldments of steel 24Kh2NMFA

1. With multipass welding of rotor steel 24Kh2NMFA granular bainite is formed in the HAZ. The matrix of the bainite is the α solid solution formed by martensitic kinetics; small areas of granular or elongated form evenly distributed in the matrix consist of martensite and retained austenite. 2. The nonequilibrium phase composition of bainite is responsible for the high hardness of the HAZ in steel 24Kh2NMFA heated during welding.