Solid Solution Limit Research Articles

3D model of a stable triangle LiF–NaBr–KBr four-component reciprocal system Li+, Na+, K+ || F-, Вr-

A 3D model of the phase equilibrium states of the quasi-three-component system LiF–NaBr–KBr, which is a stable triangle of the four-component reciprocal system Li+, Na+, K+ || F-, Br-, has been constructed. Based on the 3D-model, polythermal, isothermal sections and the polytherm of phase crystallization were constructed for the first time. Two polythermal sections contain wide areas of boundary solid solutions based on sodium and potassium bromide. In an isothermal section at 650 оC, the fields of the liquid phase and the coexisting two and three phases are delimited. The crystallization polytherm is represented by three fields. In the crystallization field of lithium fluoride, the area of separation of two liquids is limited. The direction of the ion exchange reaction 2LiBr + NaF + KF = 2LiF + NaBr + KBr was confirmed by thermodynamic calculations at temperatures of 400, 600, 800, 1000K. The exothermic nature of the exchange reaction is confirmed by taking a DTA heating curve for a mixture of powders from 50% LiBr + 25% NaF + 25% KF, and the phase composition of the reaction products LiF + NaBr(OTR) + KBr(OTR) is confirmed by X-ray phase analysis data, where OTR is limited solid solution.

Mutability of Nucleation Particles in Reactive Salt Hydrate Phase Change Materials.

Nucleation particles, solid phases dispersed throughout a medium to decrease the energy barrier for solidification or other reversible phase transitions, are generally selected on the basis of structural or interfacial energy considerations between the host phase and the solid phase that is crystallizing. However, the existence of chemical reactions between the nucleation particles and the host phase can obscure these underlying relationships, thereby complicating the process of selection of active nucleation particle phases. Here, we reveal the origin of nucleation activity of barium-based nucleation particles in the salt hydrate calcium chloride hexahydrate (CCH), a candidate for near room temperature thermal energy storage. We demonstrate that these compounds undergo a series of cation exchange and secondary precipitation reactions, resulting in an assemblage of solid precipitates with some degree of limited solid solution, which collectively dramatically reduce undercooling in CCH, but which obscure the identification of a single crystalline phase primarily responsible for the nucleation of crystalline CCH from the liquid. Importantly, this result illustrates a pathway to harness in situ chemical reactions to generate stable active nucleation particles in reactive phase change materials, which may not be readily synthesized by alternative methods, or which may not be active or remain stable when added in isolation.

Perovskite-like limited solid solution in the BaO–Y<sub>2</sub>O<sub>3</sub>–CuO–MoO<sub>3</sub> system

A new phase Ba2(Y,Cu,Mo)2O6 with the cubic perovskite structure Fm-3m has been obtained in the BaO–CuO–Y2O3–MoO3 quasiquaternary system, and the possibility of coexistence of two limited solid solutions with cubic structures Fm-3m and F-43m has been established. The samples were synthesized by gel combustion followed by calcination at 1000°C and cooling in the inertial thermal regime. The studies were carried out by X-ray phase analysis, X-ray fluorescence spectrometry, infrared spectroscopy, and diffuse reflectance spectroscopy.

Effect of Tb3+ and Ce3+ Co-doping on the Structure and Photoluminescence Properties of Hexagonal Boron Nitride Phosphors.

In the paper, we have successfully prepared hexagonal boron nitride (h-BN:Tb3+, Ce3+) phosphors with melamine as the nitrogen source. The X-ray powder diffraction patterns confirm that the sample possesses a hexagonal crystal structure within the P m2 space group. It is interesting that the co-doping combination of Tb3+ and Ce3+ can markedly enhance the threshold concentration of doped activators within the limited solid solution of h-BN phosphors. Under 302nm excitation, the h-BN:Ce3+ phosphors exhibit broadband blue light emission at 406nm. In h-BN:Tb3+, Ce3+ phosphors, the co-doping of Ce3+ not only ensures high phase purity but also results in strong green light emission. The energy transfer efficiency from Ce3+ to Tb3+ is about 55%. The fluorescence lifetime increases with the increase of Ce3+ and Tb3+ concentration, and the fluorescence lifetime of h-BN:0.025Tb3+, 0.05Ce3+ phosphor reached 2.087ms. Additionally, the h-BN:0.025Tb3+, 0.05Ce3+ phosphor exhibits excellent thermal performance with an activation energy value of 0.2825eV. Moreover, the photoluminescence quantum yield of the sample exceeds 52%. Therefore, the h-BN:Tb3+, Ce3+ samples can be used as green phosphors for solid state lighting and fluorescent labeling.

Effects of (KMg)3+ co-doping and sintering temperature on the negative thermal expansion property of Sc2W3O12 ceramics

(KMg)3+ co-doped Sc2W3O12 ceramics have been prepared via co-precipitation method. Effects of (KMg)3+ co-doping amount and sintering temperature on the phase constitution, microstructure, morphology, and negative thermal expansion (NTE) performance of KxMgxSc2-xW3O12 (0 ≤ x ≤ 1.25) ceramics were studied. Results indicate that: Sc3+ cations can be substituted by (KMg)3+ dual-cations and a composition-induced structural phase transition was observed in KxMgxSc2-xW3O12. For x ≤ 0.25, orthorhombic Sc2W3O12 gradually changed into hexagonal KxMgxSc2-xW3O12 and coexisted. Single phase KxMgxSc2-xW3O12 with hexagonal symmetry can be prepared for 0.25 < x ≤ 1. When x ≥ 1.25, (KMg)3+ substitution reaches its solid solution limit in Sc2W3O12, and impurity MgWO4 formed in the samples. As co-doping amount x increased, the NTE performance of the KxMgxSc2-xW3O12 (0 ≤ x ≤ 1) ceramics gradually enhanced. The coefficient of thermal expansion (CTE) of KxMgxSc2-xW3O12 (0 ≤ x ≤ 1) ceramics enhanced from −2.14 × 10−6 °C−1 to −19.68 × 10−6 °C−1 in 30–700 °C. With the increase of sintering temperature from 800 to 1100 °C, the grains grew up and the density of the K0.75Mg0.75Sc1.25W3O12 ceramics increased as well, and the NTE performance was also improved. The linear CTE of K0.75Mg0.75Sc1.25W3O12 ceramics enhances from −2.15 × 10−6 °C−1 to −14.70 × 10−6 °C−1 in 30–700 °C.

Phase evolution, defect chemistry, and electrical properties of Na0.5Bi0.5TiO3-BiNi0.5Ti0.5O3 solid solutions

Solid solutions of Na0.5Bi0.5TiO3 (NBT) and BiNi0.5Ti0.5O3 (BNiT) were prepared by a solid-state reaction route, and their electrical properties investigated by a combination of impedance spectroscopy and electromotive force measurements to explore the possibility of developing mixed ionic-electronic conductors based on NBT. Phase analysis showed that BNiT has a large solid solution limit in NBT (60 mol% based on X-ray diffraction), and the room temperature crystal structure changes from rhombohedral to pseudo-cubic with increasing BNiT content. Neutron diffraction revealed the coexistence of rhombohedral and tetragonal phases when the BNiT content ≥ 40 mol%. Electrically, incorporation of BNiT induces p-type electronic conduction into NBT by hopping of holes between Ni2+ (NiNix) and Ni3+ (NiNi·), and therefore changes the electrical conduction mechanism systematically from predominant oxide-ion conduction to mixed ionic-electronic conduction and then to predominant p-type electronic conduction. The total conductivity of the solid solutions showed a “V-shape” variation with increasing BNiT content. Possible mechanisms for the phase evolution and the conductivity-composition relationships are discussed. Achieving high levels of ionic and electronic conductivity simultaneously in NBT by introducing elements with variable oxidation states remains challenging due to the competition between an enhanced electronic component and a suppressed ionic component. Low levels of BNiT incorporation are, however, beneficial to reducing the dielectric loss of NBT for dielectric applications.

Phase evolution and enhanced Q×f in BaSi2O5 ceramics with Co2O3 substitution

According to the chemical formula Ba1-xCoxSi2O5, low permittivity ceramics were prepared using BaCO3, Co2O3 and SiO2. Orthorhombic structured BaSi2O5 limited solid solution was obtained at x≤0.1. It coexisted with BaCo2Si2O7 and SiO2 in the range of 0.3≤x≤0.5. While x=0.7, the three-phase equilibrium was destroyed, accompanied by BaSi2O5 vanishing. At x=1, all Si existed in the form of Co2SiO4 and SiO2. Co2O3 melting decreased the sintering temperature from 1200 °C to 1025 °C. Because of the dielectric polarizability decrease, Co introduction brought in low dielectric constant εr. Notably, the quality factor Q×f was boosted to 60,340 GHz, which was almost 4 times than the original (∼16,526 GHz). This occurred because the intrinsic loss and extrinsic loss were significantly reduced by the average bond covalence increase and SiO2 generation (Q×f∼80,400 GHz), respectively. The variation of resonant frequency temperature coefficient τf was explained from the view of the change in the bond valence of oxygen. When x=0.5, the system with a high relative density of 98.63% exhibited superior microwave dielectric properties: εr=6.77, Q×f=60,340 GHz, and τf=-47.11 ppm/°C.

Structural specific features of Mn5Si3-type quasibinary Gd-, Tb- and Dy-based compounds and magnetic structure of quasibinary Tb5Sb2Bi

Owing to the fascinating structural and physical properties of Mn5Si3 type quasibinary compounds, in this work a set of quasibinary Mn5Si3-type Gd5{Si, Ge}3-x{Al, Ga}x, Gd5Sb2{Si, Bi}, Gd5Bi2Ga, Gd5Bi1.8Sn1.2, Tb5Bi2Sn, Tb5Sb2Bi, Dy5Sb1.6-1.7{Si, Ge}1.4-1.3, Dy5{Sb, Bi}2In, Dy5Sb1.5Bi1.5, Dy5Sb1.3Te1.7 compounds (P63/mcm, N 193, hP16) have been synthesized by arc melting under argon atmosphere followed by annealing between 870 and 1070 K. These compounds belong to the continuous or limited solid solutions rows of initial Mn5Si3-type binary compounds with corresponding change/distortion of initial unit cells.Neutron diffraction study in zero applied magnetic field shows sine modulated antiferromagnetic ordening of quasibinary Tb5Sb2Bi with wave vector Kxx = [±Kx, ±Kx, 0] (Kx = 0.0869–0.1356) below ∼90 K and non-collinear antiferromagnetic ordering with additional Kz = [0, 0, 1/2] wave vector below ∼50 K as a representative of magnetic ordering of Tb sublattice in the Mn5Si3-type compounds.

Improving Mechanical Properties of Co-Cr-Fe-Ni High Entropy Alloy via C and Mo Microalloying.

The as-cast [Co40Cr25(FeNi)35-yMoy]100-xCx (x = 0, 0.5, y = 3, 4, 5 at.%) HEAs (high-entropy alloys) were prepared by a vacuum arc melting furnace and were then hot rolled. The effect of C and Mo elements on the microstructure evolution and mechanical properties of HEAs was systematically analyzed. The results showed that when no C atoms were added, the HEAs consisted of FCC + HCP dual-phase structure. In addition, as the Mo content increased, the grain size of the alloy increased from 17 μm to 47 μm. However, only the FCC phase appeared after adding 0.5 at.% carbon in Mo microalloyed HEAs, and the grain size of the Mo4C0.5 HEA decreased significantly. Due to the Mo atom content exceeding the solid solution limit, the carbides of Mo combined with the C element appeared in the Mo5C0.5 HEA. The strength of C and Mo microalloyed HEAs significantly increased compared to HEAs with no C added. However, the Mo4C0.5 HEA exhibited excellent comprehensive mechanical properties, which was superior to a majority of reported HEAs and conventional metal alloys. Its yield strength, tensile strength, and elongation were 757 MPa, 1186 MPa, and 69%, respectively. The strengthening mechanism was a combination of fine grain strengthening, TWIP effect, and solid solution strengthening.

Effect of Ca substitution on crystal structure and band gap of solar cell material BaSi2

To ameliorate the potential of a promising solar cell material BaSi2, the effects of substituting Ba with Ca atoms on the crystal structure and band gap Eg of BaSi2, were investigated both experimentally and computationally. The solid-solution limit of the Ca atoms in BaSi2 was approximately 2.3 at.%. Single-crystal X-ray diffraction analysis of Ba1−xCaxSi2 (0.025 ≤ x ≤ 0.072) revealed that the unit cell volume decreases with Ca content x, and the Ba atoms at the A1 crystallographic site are preferentially substituted by Ca atoms. Diffuse reflectance measurements indicated that Eg decreases with x (1.24 eV at x = 0 and 1.17 eV at x = 0.07). The density functional theory calculations demonstrate that the experimentally observed decrease in Eg by Ca substitution can be explained qualitatively by the combination of the substitution of Ca atoms in the unit cell volume of BaSi2 and the volume reduction.

DOUBLE-DOPED CALCIUM PHOSPHATE COATINGS WITH ANTIMICROBIAL EFFICACY FOR BIODEGRADABLE METAL BIOMEDICAL IMPLANTS

Over the last decades, biodegradable metals emerged as promising materials for various biomedical implant applications, aiming to reduce the use of permanent metallic implants and, therefore, to avoid additional surgeries for implant removal. However, among the important issue to be solved is their fast corrosion - too high to match the healing rate of the bone tissue. The most effective way to improve this characteristic is to coat biodegradable metals with substituted calcium phosphates. Tricalcium phosphate (β-TCP) is a resorbable bioceramic widely used as synthetic bone graft. In order to modulate and enhance its biological performance, the substitution of Ca2+ by various metal ions, such as strontium (Sr2+), magnesium (Mg2+), iron (Fe2+) etc., can be carried out. Among them, copper (Cu2+), manganese (Mn2+), zinc (Zn2+) etc. could add antimicrobial properties against implant-related infections. Double substitutions of TCP containing couples of Cu2+/Sr2+ or Mn2+/Sr2+ ions are considered to be the most perspective based on the results of our study. We established that single phase Ca3−2x(MˊMˊˊ)x(PO4)2 solid solutions are formed only at x ≤ 0.286, where Mˊ and Mˊˊ—divalent metal ions, such as Zn2+, Mg2+, Cu2+, Mn2+, and that in case of double substitutions, the incorporation of Sr2+ ions allows one to extend the limit of solid solution due to the enlargement of the unit cell structure. We also reported that antimicrobial properties depend on the substitution ion occupation of Ca2+ crystal sites in the β-TCP structure. The combination of two different ions in the Ca5 position, on one side, and in the Ca1, Ca2, Ca3, and Ca4 positions, on another side, significantly boosts antimicrobial properties. In the present work, zinc-lithium (Zn-Li) biodegradable alloys were coated with double substituted Mn2+/Sr2+ β-TCP and double substituted Cu2+/ Sr2+ β-TCP, with the scope to promote osteoinductive effect (due to the Sr2+ presence) and to impart antimicrobial properties (thanks to Cu2+ or Mn2+ ions). The Pulsed Laser Deposition (PLD) method was applied as the coating's preparation technique. It was shown that films deposited using PLD present good adhesion strength and hardness and are characterized by a nanostructured background with random microparticles on the surface. For coatings characterization, Fourier Transform Infrared Spectroscopy, X-ray Diffraction, and Scanning Electron Microscopy coupled with Energy Dispersive X-ray and X-ray Photoelectron Spectroscopy were applied. The microbiology tests on the prepared coated Zn-Li alloys were performed with the Gram-positive (Staphylococcus aureus, Enterococcus faecalis) and Gram-negative (Salmonella typhimurium, Escherichia coli) bacteria strains and Candida albicans fungus. The antimicrobial activity tests showed that Mn2+/Sr2+ β-TCP -coated and Cu2+/Sr2+ β-TCP coated Zn-Li alloys were able to inhibit the growth of all five microorganisms. The prepared coatings are promising in improving the degradation behavior and biological properties of Zn-Li alloys, and further studies are necessary before a possible clinical translation.

Engineering Er-doped ZnO nanocrystals for As-removal: Targeting water remediation

Erbium-doped zinc oxide nanocrystals (Zn1−xErxO) produced by the polymeric precursor method are the subject of a comprehensive study in this report. Our results indicate that only the wurtzite ZnO phase is produced up to x = 0.050. After that (x > 0.050), a mixed system with a secondary phase (Er2O3) is found. The lower solubility limit of erbium in the ZnO crystal lattice is correlated with the higher ionic radii of Er3+-ions compared to Zn2+-ions, impacting remarkably the solid solution limit. Magnetic measurements show the coexistence of the two phases: paramagnetism (PM) related to Zn1−xErxO nanocrystals (free and short-range coupled Er3+-ions) and a weak ferromagnetic (FM) contribution assigned to the presence of bound magnetic polarons. UV–vis spectra show a transition from one to two excitonic peaks as the Er-content increases; the second peak related to the nucleation process of the secondary Er2O3 phase, which was successfully acknowledged using the Elliott’s model. Fourier transform infrared (FTIR) spectroscopy was used to characterize vibrational properties and the results are in agreement with the x-ray diffraction data analysis. To assess the efficiency of using the Zn1−xErxO samples to remove As(V) from contaminated water, a reference water solution containing 500 ppb As(V), at a neutral (pH = 7), was prepared to simulate drinking water. Our results show the effectiveness of As-removal. It was determined that the effectiveness is improved with the Er-content, reaching the limit imposed by World Health Organization. According to the linearity achieved while applying the pseudo-second-order-adsorption model, our findings strongly suggest that a chemisorption mechanism takes place between the Zn1−xErxO nanoparticle surface and As(V) in solution.

Phase formation and hydration of alumina cements on the basis of cobalt‐molybdenum‐containing waste

AbstractThe subject of the research is features of synthesis of alumina cements on the basis of compositions of system CaO – CoO – Al2O3 – MoO3 with use of cobalt‐molybdenum‐containing waste for reception of refractory materials with a complex of the set properties. The processes of phase formation in raw mixtures have been investigated. X‐ray phase studies of clinkers fired at different temperatures and holding times proved that as a result of the interaction of the raw materials of the mixture in the material synthesizes a mixture of hydraulically active calcium mono‐ and di‐aluminate and refractory cobalt spinel, which provides the resulting complex high strength, accelerated curing time, fire resistance. The absence of phases corresponding to molybdenum compounds is explained by the fact that they are part of hydraulically active aluminates as limited solid solutions, deforming the crystal lattice and increasing hydraulic activity. Thus, it was found that the high strength of cement is due to the presence of calcium hydroaluminates such as C2AH8, aluminum hydroxide, as well as unhydrated grains of calcium aluminates, which will contribute to further long‐term strength. It is this coexistence of phases in both the crystalline and colloidal states provide high strength cement stone.

Hydration mechanism of special‐purpose cements

AbstractDuring baking of the raw mixture, a weak crystallization of hydraulically inert compounds is observed for these materials, providing a substrate for the growth of hydraulic compound crystals with limited solid solutions. As a result of grinding, cleavage of crystals will occur along the growth steps and intercrystalline boundaries, creating additional mechanical stress concentrators, which provides an increase in hydraulic activity. It appears that the full destruction of solid solutions does not occur, which results in strength enhancement of resulting hardened cement stone. The electron defects can also occur, having the form of deviations from distribution of electric charge corresponding to the regular structure of crystals. Upon hydration of the cements obtained from clinkers, highly basic hydrated newgrowths and active hydroxides are formed. At the same time, some leaching occurs on the surface of hydraulically inert compound resulting in a gelatinous hydroxide. With the lapse of time, gelatinous hydrate newgrowths transform to a crystalline state with decrease of basicity and lattice rearrangement into a more closely packed one. Amorphous hydroxides undergo crystalline aging and form a compactive crystallization frame around the grains of alkaline‐earth hydroaluminates.

Structural evolution and chemical durability of uranium-doped high-entropy pyrochlore

The high-entropy ceramic solidified body ((LaPrEuGdYb)yUx)2Zr2O7+δ (y = (1−x)/5, x = 0.05, 0.10, 0.15, 0.20, 0.25) was prepared by immobilizing uranium (U) on the A-site of the pyrochlore in this study. The phase composition and evolution of solidified body were determined by XRD, Raman and SEM analysis. The phase of (LaPrEuGdYb)yUx)2Zr2O7+δ ceramic was varied with U doping amounts: a dual-phase of pyrochlore and fluorite with no U doping. A single-phase pyrochlore with U doping amounts from x = 0.15 to x = 0.20, and the U3O8 phase was precipitated at doping levels above x = 0.25. The solid solution limit for high entropy pyrochlore in relation to uranium is roughly x = 0.2. The results should be linked to the increase in configurational entropy and the variation of U valence in the ceramic bodies, as evidenced by XPS. The normalized leaching rate of U from high-entropy ceramic waste forms (x = 0.20) was about 1.65 × 10−5 g/m2⋅d after 42 d leaching. It performed well in the leaching tests. Our study is expected to provide candidate materials for solidifying complex high-level radioactive waste (HLW).

The thermoelectric performance in transition metal-doped PbS influenced by formation enthalpy

Transition metals have excellent valence electrical properties and unique electronic state distribution and are regarded as potential materials for improving thermoelectric performance. However, the impact of transition metals on thermoelectric materials is restricted to the solid solution limit and doping efficiency, reinforcing the shortcomings in systematic research. Here, thermoelectric properties of transition metal (Ti, V, Cr, Zr, Nb, Mo)-doped PbS are compared and analyzed systematically based on the formation enthalpy. The DFT calculation indicates that the doping (except Zr) leads to the bandgap expansion and the density of states distortion near the Fermi level, while the localization property of the latter results in an invalid resonance level. The formation enthalpy dominates the carrier concentration due to the opposite trend of carrier concentration and formation enthalpy. The formation enthalpy of Zr, Ti, and Nb doping is more negative than others, leading to the more significant optimization of carrier concentration. The Moss–Burstein effect promotes the bandgap expansion, leading to weaker bipolar effects for Zr, Ti, and Nb doping. Eventually, the thermoelectric performance for Ti, Zr, and Nb doping is superior to others at high temperature. The Hume-Rothery rule of the formation enthalpy supplementation is more suitable for the doping and alloying in thermoelectricity. Thermodynamic stability analysis based on the formation enthalpy contribute the PbS-based thermoelectric devices evaluation. The present finding demonstrates the significant effect of formation enthalpy on the thermoelectric properties of PbS and provides a useful avenue for the doping modification and thermodynamic stability analysis of other thermoelectric alloy materials.

Synthesis, structure and thermal expansion of new complex phosphates of titanium and metals in the +2 oxidation state

The phosphates of the composition Ca0.5+xMxTi2-x(PO4)3 (M − Ni, Mg) were synthesized by a co-precipitating of salts in aqueous solutions followed by thermal treatment stages. The studied series formed limited NZP solid solutions within the regions 0 ≤ x ≤ 0.5 (Ni) and 0 ≤ x ≤ 1 (Mg). The Rietveld refinement of the Ca1.2Mg0.7Ti1.3(PO4)3 structure showed that its basis is framework formed by TiO6 and (Mg/Ti)O6 octahedra (together with PO4 tetrahedra), and the Ca2+ ions are situated in the cavities. Thermal expansion of the phosphates depending on their chemical composition was characterized in the temperature range from 173 to 473 K. There was a gradual decrease in the linear thermal expansion coefficients with x growth, so that average linear coefficients varied from (11.0–2.6)∙10−6 K−1. Low average thermal expansion coefficient and its low anisotropy allow the studied phosphate ceramics to withstand temperature changes.

The ternary system Gd–Ge–Sn at 600 0C

The isothermal section at 6000C of the phase diagram of the ternary system Gd–Ge–Sn was constructed in the whole concentration range, based on X-ray powder diffraction and energy-dispersive X-ray spectroscopy. The formation of two continuous solid solutions, Gd5Ge4–xSnx (x=0–4, structure type Sm5Ge4, Pearson symbol oP36, space group Pnma, a=7.8565(12)–8.040(2), b=14.812(2)–15.552(3), c=7.7781(12)–8.201(2) Å) and Gd5Ge3–xSnx (x=0–3, structure type Mn5Si3, hP16, P63/mcm, a=8.5702(8)–9.0306(13), c=6.4305(5)–6.5941(10) Å), and limited solid solutions based on the binary compounds Gd11Ge10 (6 at.% Sn), GdSn3 (5.5 at.% Ge), Gd3Sn7 (2 at.% Ge), GdSn2 (5 at.% Ge), and Gd11Sn10 (3.5 at.% Ge) was established. Three following ternary compounds were found in the system at 6000C: Gd2Ge3.84Sn0.92, GdGe0.75–0.85Sn1.25–1.15, and Gd2Ge2.91Sn0.80. The detailed crystal structures of two of them were refined on X-ray powder diffraction data: GdGe0.75–0.85Sn1.25–1.15, structure type ScCo0.25Si1.75, oS12, Cmcm, a=4.3206(4)–4.3035(4), b=16.4824(15)–16.4433(14), c=4.1270(4)–4.0961(4) Å and Gd2Ge2.91Sn0.80, structure type Gd2Ge3.38Bi0.42, oS32, Cmcm, a=4.0445(6), b=30.473(5), c=4.1694(6) Å. The third compound, Gd2Ge3.84Sn0.92, adopts the structure type Nd2Ge3.55Sn1.24. The crystal structures are closely related and are built from layers of Gd6 trigonal prisms centered by Ge atoms (or in part by Sn atoms) and square-mesh nets of Sn atoms (or in part Ge atoms), which alternate along the crystallographic direction [010]. Partial disorder of Ge and Sn atoms is observed.

Effect of refractory element addition on high temperature oxidation process of complex concentrated alloy coatings

The sluggish diffusion effect of complex concentrated alloys (CCAs) affords them considerable application potential in the field of high temperatures. In this study, the role of refractory elements in the high temperature oxidation process of denary complex concentrated alloy coatings (CCACs, namely NiCoCrFeAl0.8Si0.1(MoNbTaW)x, where x is 0.025, 0.05, 0.1 and 0.15) prepared via laser cladding was investigated. Four groups of the CCACs with different refractory contents were subjected to 100 h of high-temperature oxidation experiments, and the oxidation products and microscopic characteristics of the samples were observed via XRD and SEM, respectively. Atomic-scale characterisation and analysis of the oxidised samples were performed using TEM and XPS technology to explore the diffusion behaviour of elements in the high-temperature process. Results indicate that the solid solution limit of the CCACs is obtained by regulating the topological instability of the lattice matrix via the addition of refractory elements. Consequently, when 0.1 > x > 0.025, the coating is a body-centred cubic (BCC) + face-centred cubic (FCC); when x = 0.1, the coating maintains a single-phase FCC structure; and when x > 0.1, the coating changes to an FCC + BCC duplex structure. The x0.1 group exhibits the smallest relative strain among all coatings, which is conducive to maintaining the structural stability of the face-centred cubic at high temperatures and delaying atomic migration within the lattice. Furthermore, the oxidation kinetics curve of the CCACs with a single- phase FCC exhibits low Kp values. In a single- FCC lattice, the atom diffusion rate through volume slows down and grain boundary diffusion decreases. Considering the Weibull modulus for the hardness statistical value after coating oxidation, the modulus of single-FCC phase CCACs is also higher than others. This study has the potential to contribute to the application of high-temperature thermal protection for CCACs.

An experimental investigation of phase transformations in the Al-Fe-V system

This paper presents a comprehensive study of phase equilibria in the Al-Fe-V system by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential thermal analysis (DTA) and electron probe microanalysis (EPMA). Liquidus and solidus projections, a number of isothermal and vertical sections, as well as a reaction scheme (Scheil diagram) are constructed. On the solidus surface, there is a continuous solid solution Al8(Fe,V)5 (Cu5Zn8-type structure, cI52-I-43 m), which decomposes with decreasing temperature. The limited solid solution formed in this case has the widest homogeneity region. At 1000 and 700 °C, the solubility of iron in it is 25 and 18 at.% Fe, respectively. The solidus surface is characterized by six three-phase fields: (Al) + Al13Fe4 + Al45V7, (Al) + Al21V2 + Al45V7, Al3V + Al23V4 + Al45V7, Al3V + Al45V7 + Al13Fe4, Al8(Fe,V)5 + Al3V + Al13Fe4 and Al8(Fe,V)5 + Al13Fe4 + Al5Fe2, which result from invariant four-phase equilibria. One of them is of the peritectic type, the others are of transition type. Equilibria take place at 653, ∼660, 735, 755, 1086, and 1121 °C, respectively. In the two-phase area AlFe + Al8(Fe,V)5 the solidus surface has a temperature minimum at 1188 °C, resulting from the invariant eutectic three-phase equilibrium L ⇄ AlFe + Al8(Fe,V)5. At ∼1080 and 730 °C, solid-state invariant four-phase equilibria Al8V5 + Al2Fe ⇄ AlFe + Al5Fe2 and Al8V5 + Al5Fe2⇄ AlFe + Al13Fe4, respectively, occur.