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From molecular dynamics (MD) simulations of melt-quenching and thermal aging procedures in pure Ag, Cu, Ag-Cu binary alloys, and Cu-Zr binary alloys, we have identified two distinct amorphous phases for a metastable undercooled liquid: the homogeneous L-phase with low shear rigidity and the heterogenous G-phase with much higher shear rigidity and a heterogeneity length scale Λ. Here, we examine two-phase equilibration studies showing that the G-phase melts to form the L-phase above ~1,000 K, which then transforms to form the crystal (X) phase; however, below the melting point of the G-Phase (~990 K), the X-and G-phases do not transform into each other. We suggest the presence of a G-phase is likely responsible for embrittlement often observed in metallic glasses. We also consider how mechanical milling or irradiation-induced defect accumulation in the crystalline metallic alloy triggers a crystal-to-glass transition. We use the Cu2Zr system as a model to investigate random interchange of Zr and Cu atoms at room temperature leading to a transition from a crystalline Laves-phase to an L-glass through formation and growth of amorphous regions and destabilization of the crystal. During relaxation of the nonequilibrium structures by annealing, the configurations either reverted to the crystalline phase or evolve to a heterogeneous G-phase equivalent to the G-phase formed during thermal quenching and aging of the melt. Athermal defect accumulation in the G-phase at low temperature led to a transition back to the L-phase. Our findings show how athermal disordering drives transitions among the X-, L-, and G-phases. [ABSTRACT FROM AUTHOR]

The method of constrained equilibria has already found extensive use for a good number of years in conjunction with the programmers library ChemApp and freely adaptable ASCII based thermodynamic datafiles. In the present paper a method is demonstrated which permits the application of this method in the framework of the Integrated Thermodynamic Databank System (ITDS) FactSage. Both, modules which interact with the databases and modules which carry out thermodynamic calculations are used, thus emphasizing the aspect of integration in the ITDS. In the paper a link will be established between original thoughts by J.W. Gibbs concerning the definition of the components of a system and kinetic inhibitions in the system and the method of constrained equilibria as such. Furthermore, reference is made to Mats Hillerts use of driving forces in complex (non-)equilibrium cases. A number of application cases with different degrees of complexity will be demonstrated. These range from partially or fully constrained complex equilibrium calculations to phase diagrams with new types of axis variables. [ABSTRACT FROM AUTHOR]

This work proposes a new design strategy for overcoming the strength-ductility trade-off of γ′′-strengthened multi-component alloys at intermediate-temperatures. By increasing the V content, the alloy’s ductility is enhanced accordingly (650°C tensile-elongation, 9.85%→15.97%) without compromising strength. This enhancement is attributed to the improved stability of γ′′, which results in a suppression of brittle ε lamellae forming at grain boundaries after deformation and a reduction in γ/γ′′ lattice misfit. Concurrently, the increasing V content increases the γ′′ volume fraction, offsetting potential strength loss due to the lattice misfit reduction.

For Nd2Fe14B/α-Fe nanocomposite magnets, which exhibit great superiority in theoretical energy products, it is a great challenge to obtain controlled nano-scale microstructural features while eliminating the harmful metastable phases due to the general metastable fabrication methods. Here, we report a strategy to control the nano-scale features and simultaneously eliminate metastable phases for Nd2Fe14B/α-Fe nanocomposites through low-temperature pre-deformation of amorphous alloys combined with subsequent thermal annealing. The resultant bulk Nd2Fe14B/α-Fe nanostructure exhibits desired ultrafine grain sizes, ∼20 nm for soft-magnetic α-Fe phase with a high fraction of Vα-Fe ≈ 30 wt% and ∼33 nm for hard-magnetic Nd2Fe14B phase. The desired microstructure results in a larger coercivity (Hci = 4.1 kOe) and higher saturation magnetization (Ms = 1.51 T) compared to those (Hci = 3.3 kOe and Ms = 1.35 T) of the counterpart without pre-deformation, contributing to a high energy density of 26.3 MGOe. This energy density is 200% larger than that of the counterpart without pre-deformation and far beyond that (around 20 MGOe) of previously reported bulk Nd2Fe14B/α-Fe nanocomposites with Vα-Fe ≥ 20 wt%. The superior property stems from the purposely introduced low-temperature pre-deformation that changed the structure and local chemistry of the amorphous alloy, facilitating the decomposition of harmful metastable phases and the formation of Nd2Fe14B phase during annealing processes, which lowers the temperature, from 750 to 700 °C for producing the Nd2Fe14B/α-Fe nanostructure and thus yields ultrafine nanograins. These results demonstrate an effective means to prepare high-performance bulk Nd2Fe14B/α-Fe nanocomposite magnets.

The paper presents an analysis of the effect of processing powders with high hydrostatic pressure (HHP) (300, 500, 700 MPa) and stabilized zirconium dioxide (ZrO 2 + 3 mol.% Y 2 O 3 , (YSZ)) doping on the compaction of metastable nanopowders mixture with γ+θ-Al 2 O 3 + n% YSZ (n = 0, 1, 5, 10, 15 wt%) composition during sintering. The behavior of nanopowders at the initial stage of sintering was studied by dilatometry at a constant heating rate of 5 °C/min, in temperature range from 20 °C to 1500 °C. It has been determined that sintering of compacts occurs in two stages: compaction of γ + θ-Al 2 O 3 (Ⅰ-st stage metastable phases), compaction of the stable α-Al 2 O 3 (ⅠⅠ-nd stage), accompanied by inhibition between them. An "effect of mutual protection against crystallization" of powder mixtures was determined using the X-ray diffraction method. This is characteristic of systems obtained by co-precipitation. A connection between the above mentioned inhibition and an increase in the YSZ additive was found. That, also, correlates to the effect of "mutual protection against crystallization" of Al 2 O 3 – YSZ powder mixtures. A study of the mechanisms and activation energies of γ+θ-Al 2 O 3 + n% YSZ systems' (n = 0, 1, 5, 10, 15 wt%) sintering depending on the YSZ additive amount and the HHP value showed that a dopant and pressure increase leads to activation energy decrease of the initial sintering stage, and the prevailing sintering mechanism is volumetric diffusion in corundum grains. Also, the work shows that for γ+θ-Al 2 O 3 + ≥5 wt% YSZ systems, the optimal compaction pressure is HHP ≥500 MPa, which corresponds to the transition of the sintering mechanism from grain-boundary diffusion to volumetric diffusion. [ABSTRACT FROM AUTHOR]

Due to their stable crystal framework, promising energy density, and structural versatility, layered 3d transition metal oxides have emerged as the preferred cathodes for lithium‐ion batteries (LIBs) and sodium‐ion batteries (SIBs). While extensive research has individually addressed the lithium and sodium 3d transition metal layered oxides, the differences and interconnections between the two types of materials have largely been overlooked. Effectively utilizing these summaries is essential for driving innovative structural designs and inspiring new insights into the structure‐property relationships. This review comprehensively bridges this gap by meticulously examining the disparities and links in the behavior of the layered oxides upon Li+ and Na+ storage and transfer. Key aspects, including atomic and electronic structure, phase transition mechanisms, charge compensation mechanisms and electrochemical kinetics, are carefully summarized. The implications of these aspects on the battery cycle life, energy density, and rate capability are thoroughly discussed. Additionally, by leveraging the unique characteristics of each oxide structure, this review explores the interconnection between lithium and sodium layered oxides in depth. Finally, a concise perspective on future targets and direction of 3d layered oxides is deduced and proposed. [ABSTRACT FROM AUTHOR]

This work comments on a recent publication by Hajra et al. (Mater. Design 236 (2023) 112483), which claims to have presented compelling experimental and theoretical evidence in favour of the existence of an equilibrium C14-NbCr2 high-temperature Laves phase in the Cr-Nb system. In the present comment, evidence and conclusions reported in the paper of Hajra et al. are critically put into context of insight from previous works. From this it is concluded here, that the evidence in favour of an equilibrium C14-NbCr2 high-temperature Laves phase is, by far, not that compelling as claimed by Hajra et al.. Instead, the most direct evidence presented in the literature does not support the existence of an equilibrium C14-NbCr2 high-temperature Laves phase. Alternative interpretations of Hajra et al.’s evidence and conclusions are offered, and it is elaborated, which true gaps in knowledge exist concerning the Laves phases in the Cr-Nb system.

The development of materials with special microstructures is a continuing theme of research due to their improved physical and mechanical properties. Rapid solidification can lead to amorphous or partially crystalline microstructures, depending on the imposed processing conditions and compositions allowing the production of composite materials. Al–Ni-Sm alloys can be easily obtained in the amorphous state by rapid solidification processing so that composites could be generated depending on the cooling conditions. In this work, new results are shown for Al-based amorphous matrix composites produced during rapid solidification of Al83.5Ni10Sm6.5, Al81.5Ni10Sm6.5Pb2 and Al83.5Ni8Sm6.5Pb2 (atom percent) alloys. Lead (Pb) was added to the amorphous forming composition alloy due to its immiscibility with the main alloying elements. Ribbon samples were obtained by melt spinning technique. Besides the use of ordinary characterization techniques, such as X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM), advanced transmission electron microscopy (TEM) was employed to reveal the special microstructure features forming these alloys. A strong influence of the Pb addition on crystallization behavior has been identified. Also, the size and morphology of the nanocrystalline Pb-rich minority phase within the amorphous matrix is examined. These effects were explained based on the immiscibility of Pb with the alloy components.

Abstract The VO2 metastable (B) phase is of interest for applications in temperature sensing, bolometry, and Li‐ion batteries. However, single‐phase growth of thin films of this metastable phase is a challenge because vanadium oxide exhibits many polymorphs and the VO2 stable (M1) phase is usually present as a secondary phase. Additionally, the phase transition at 350 °C in the (B) phase severely narrows the processing window for achieving phase‐pure films. Here, single‐phase growth of 5‐to 50‐nm thick VO2 (B) films on muscovite, mica, by pulsed direct‐current reactive magnetron sputtering at 400 °C is demonstrated. The films are phase‐pure and exhibit a high density of 4.05 g cm−3 and low resistivity of about 50 mΩcm at 30 °C. Increasing the film thickness to 100 nm results in a V2O5‐capped VO2 (B) film with a resistivity of 8000 mΩcm. These results indicate that the stability of the VO2 (B) phase is sensitive to in situ annealing during deposition. These findings should serve as a basis to design processes to exclusively obtain phase‐pure VO2 (B) films.

Topotactic transition is a structural phase change in a matrix crystal lattice mediated by the ordered loss/gain and rearrangement of atoms, leading to unusual coordination environments and metal atoms with rare valent states. As early as in 1990s, low temperature hydride reduction is utilized to realize the topotactic transition. Since then, topological transformations are developed via multiple approaches. Especially, the recent discovery of the Ni‐based superconductivity in infinite‐layer nickelates has greatly boosted the topotactic transition mean to synthesizing new oxides for exploring exotic functional properties. In this review, a detailed and generalized introduction is provided to oxygen‐related topotactic transition. The main body of the review includes four parts: the structure‐facilitated effects, the mechanism of the topotactic transition, some examples of topotactic transition methods adopted in different metal oxides (V, Mn, Fe, Co, Ni) and the related applications. This study is to provide timely and thorough strategies to successfully realize topotactic transitions for researchers who are eager to create new oxide phases or new oxide materials with desired functions. [ABSTRACT FROM AUTHOR]

For the processing of diamond-metal matrix composites, the powder bed fusion using a laser for metals (PBF-LB/M), represents a new promising method for the additive manufacturing of diamond tools for concrete and rock machining, even with more complicated geometries. Previous research activities show a strong tendency for cracking and delamination during the construction process of the samples. This behavior is caused by thermal residual stresses associated with the embedded diamonds. To control these negative effects on the process side, the volume energy density is reduced accordingly, which, however, led to increased pore formation. This publication deals with an approach on the material side to modify a 316L stainless steel base powder with an addition of 20 wt% bronze via a high energy ball milling (HEBM) process in such a way that a homogeneous solid solution phase is created. A significantly increasing of the melting interval and a decreasing of both solidus and liquidus temperature was observed, which can reduce pore formation in the PBF-LB/M-process. In addition, XRD-diffractometry and SEM/EDS-analysis showed that the homogeneous solid solution phase of this alloyed powder segregates again into Fe- and Cu-rich phases when heated up to the melting point. [ABSTRACT FROM AUTHOR]

We have carried out phase polymorphism studies of 4-n-hexadecyloxybenzoic acid (16OB) in temperature range: from 10 to 150 °C with the use of Differrential Scanning Calorimeter, Thermal Microscopic Polarization and Transmitted Light Intensity. 16OB is a liquid crystal which has smectic C phase (SmC) and rich polymorphism in solid phase. Particularly interesting is one of the transitions in solid state possessing characteristic clear exothermic effect (during heating cycle) which is associated with the transition between the metastable and stable phases. Fourier transform infrared spectroscopy as a function of temperature was used to analyze changes in the nature of hydrogen bonds. The exact analysis of effects shows an existence of the metastable phases. The stable and metastable phases differ from each other in the structure made of hydrogen bonds. [ABSTRACT FROM AUTHOR]

Direct synthesis of hydrogen peroxide (H2O2) through electrochemical oxygen reduction has gained close attention yet remains a great challenge due to the slow kinetics. Herein, combining with the virtues of the native high energy state and fascinating surface environment of metastable materials and doping strategy, an efficient phosphorus‐optimized metastable hexagonal‐close‐packed phase nickel catalyst (P‐hcp Ni), belonging to the space group (P63/mmc, 194), with P doping is demonstrated. Significantly, it achieves high selectivity of 97% and a high intrinsic turnover frequency of 2.34 s−1, much better than those of the stable face‐centered‐cubic Ni catalyst. It also displays high stability with remaining in the metastable phase after the stability test. More importantly, P‐hcp Ni also achieves a productivity of 4917.2 mmol gNi−1 h−1 and an accumulated concentration of (H2O2) of 2.38 mol L−1 after 130 h stability test in pure water with a solid electrolyte. Further investigation reveals that the P doping not only greatly enhances the stability of metastable phase, but also weakens the *OOH adsorption on the active site, promoting the high production of H2O2 in the neutral media. [ABSTRACT FROM AUTHOR]

The results of studying the influence of high hydrostatic pressure (HHP) on the structure formation and properties of composite ceramics of the Al 2 O 3 -ZrO 2 + 3mol% Y 2 O 3 (YSZ) system, based on the metastable Al 2 O 3 phase, are presented in this work. The phase composition of the powders, the structure, physical and mechanical properties of the Al 2 O 3 - YSZ ceramics were studied taking into account the YSZ concentration and the pressing pressure. We have found that various crystallization processes occur during the sintering of ceramics and two types of structure are realized: aggregate-hardened and disperse-hardened, depending on the HHP value. It is shown that the processing of compacts under HHP conditions at 600–700 MPa prevents the formation of agglomerates of YSZ grains in the intergranular space of Al 2 O 3 , and the maximum values of physical and mechanical characteristics are achieved at concentrations of 10 and 15% YSZ and HHP values of 700 MPa. [ABSTRACT FROM AUTHOR]

Conventional alloys show low compressive strength and strain accommodation due to their propensity for limited strengthening mechanisms and rapid compressive instability. High‐entropy alloys (HEA) may overcome this limitation by metastability engineering. Inspired by this, herein, a dual‐phase Fe44Mn20Cr15Ni7.5Co6Si7.5 HEA (M‐HEA) which shows maximum engineering strength of ≈4.4 GPa at true compressive strain of ≈2.2 (engineering strain of 90%) in as‐cast condition due to controlled hardening and sustained softening response during deformation is presented. The controlled hardening phenomenon (up to 60% strain) is attributed to transformation‐induced plasticity in γ‐phase and nanotwinning in untransformed γ‐phase. The sustained softening is a result of extensive strain accommodation by both γ‐ and β‐phases via dynamic recovery in γ‐phase and shearing of β‐phase during deformation. Hence, this superformability in M‐HEA makes it an excellent material for structural applications in comparison with other conventional alloys and HEAs. [ABSTRACT FROM AUTHOR]

Understanding and hence predicting the stability and metastability of multi-principal-element alloys (MPEAs) is crucial for their design and applications, but it remains a complex and time-consuming task. Descriptors based on the configurational entropy alone are often insufficient in determining the relative stability of MPEA solid solutions, since they predict, against experimental evidence, that alloys containing a large number of elements will be eventually stable. Here we introduce two characteristic temperatures, derived from the temperature dependence of the configurational entropy, which effectively act as (meta-)stability indicators. These can be further combined in a dimensionless quantity, T d0 , which enables us to rank alloys according to their compositional and structural (meta-)stability across a broad composition range. In particular, we are able to map equiatomic and non-equiatomic alloys, and even regions covered by conventional alloys. Our proposed descriptors are validated against a large body of experimental results and compared to other (meta-)stability descriptors. Furthermore, they allow us to revise the alloys classification scheme into high-entropy, medium-entropy and low-entropy. Our work sheds new light into the thermodynamic origin of alloying metastability, it provides a potential tool to correlate metastability thermodynamics and kinetics, and ultimately may help in alloys design and discovery. [Display omitted] [ABSTRACT FROM AUTHOR]

Permanent magnets based on neodymium‐iron‐boron (Nd‐Fe‐B) alloys provide the highest performance and energy density, finding usage in many high‐tech applications. Their magnetic performance relies on the intrinsic properties of the hard‐magnetic Nd2Fe14B phase combined with control over the microstructure during production. In this study, a novel magnetic hardening mechanism is described in such materials based on a solid‐state phase transformation. Using modified Nd‐Fe‐B alloys of the type Nd16Febal‐x‐y‐zCoxMoyCuzB7 for the first time it is revealed how the microstructural transformation from the metastable Nd2Fe17Bx phase to the hard‐magnetic Nd2Fe14B phase can be thermally controlled, leading to an astonishing increase in coercivity from ≈200 kAm−1 to almost 700 kAm−1. Furthermore, after thermally treating a quenched sample of Nd16Fe56Co20Mo2Cu2B7, the presence of Mo leads to the formation of fine FeMo2B2 precipitates, in the range from micrometers down to a few nanometers. These precipitates are responsible for the refinement of the Nd2Fe14B grains and so for the high coercivity. This mechanism can be incorporated into existing manufacturing processes and can prove to be applicable to novel fabrication routes for Nd‐Fe‐B magnets, such as additive manufacturing. [ABSTRACT FROM AUTHOR]

We identified dmisteinbergite, the rare trigonal polymorph of CaAl2Si2O8, for the first time in high-grade metamorphic rocks. Dmisteinbergite occurs as a crystallization product of silicate melt inclusions (nanogranitoids) in garnet from three host rocks with different protoliths and re-equilibration conditions, i.e., from 1.0 to 4.5 GPa. Raman spectra and compositions of the dmisteinbergite here investigated are overall identical to those of previously characterized artificial and natural dmisteinbergite. In nanogranitoids, this phase coexists with other metastable polymorphs of feldspar (kumdykolite, kokchetavite) and SiO2 (quartz, cristobalite), recently interpreted as the result of undercooling, supersaturation and rapid crystallization of a silicate melt confined in a micrometric pore. Dmisteinbergite formation likely results from a similar process, and thus it should be regarded as a kinetically controlled phase. Moreover, the investigation of dmisteinbergite as well as of other metastable feldspar polymorphs offers new insights into the behavior of natural materials under non-equilibrium conditions. [ABSTRACT FROM AUTHOR]

We have studied the effect of heat treatment (isothermal holding in the temperature zone C, initial cooling temperature, and cooling rate) of a melt with the composition 85.7 mol % Bi2O3 + 14.3 mol % SiO2 (6 : 1) on the phase composition and microstructure of the forming crystals and found conditions for crystallization of a δ-Bi2O3-based metastable solid solution (δ*) at high melt cooling rates. The results demonstrate that low melt cooling rates lead to crystallization of the δ*-phase and an α-Bi2O3-based solid solution, followed by partial or complete eutectoid decomposition of dendritic δ*-phase crystals into a mixture of metastable phases, containing β*-Bi2O3 and η-Bi2SiO5 as well. Raising the melt cooling rate limits both the formation of secondary phases and eutectoid decomposition. The results we obtained make it possible to control the formation of polycrystalline Bi12SiO20 and δ-Bi2O3 upon 6Bi2O3⋅SiO2 melt solidification. [ABSTRACT FROM AUTHOR]

Crystallization and transformation of iron (oxyhydr)oxides occur widely in redox-dynamic environments, which are closely linked to iron cycling, (im)mobilization of co-associated elements, and microbial iron metabolisms. Although numerous studies have been conducted to investigate the occurrence and fate of iron (oxyhydr)oxides under various redox conditions, the mechanism controlling these processes is still incompletely understood. Here, we present a theoretical model, based on Classical Nucleation Theory (CNT), that can be used to predict and rationalize phase-selection and crystallization pathways of iron (oxyhydr)oxides under different redox conditions. By calculating the energy landscapes of dissolved or solid-phase iron species in a Fe-H 2 O system as a function of Eh, pH, and particle size, we show how the stability regions of iron (oxyhydr)oxides change with geochemical environments (pH and redox potential) and initial properties of precursors (particle size and crystallinity). Through the calculation of thermodynamic driving forces and energy barriers for the nucleation of different metastable phases, the crystallization pathways of iron (oxyhydr)oxides via hydrolysis of Fe3+ (aq) , oxidation of Fe2+ (aq) , and Fe(II)-catalyzed ferrihydrite transformation can be predicted. The computational results show good agreement with previously reported experimental data. This study provides a unified model to predict and understand relative stability, transformation, and persistence of iron (oxyhydr)oxides under varying pH and redox conditions in natural environments. [ABSTRACT FROM AUTHOR]

Co-Al based superalloys have been considered as the promising alternatives to Ni-Al based superalloys. Appropriate alloy design can stabilize a dual phase structure of face-centered cubic (FCC) Co matrix and L1 2 Co 3 (Al, X) intermetallics and avoid precipitation of B2 CoAl intermetallics for Co-Al based superalloys. It has been shown that stable Co 3 Al single phase compound does not exist. Instead, at such a chemistry, Co-Al binary phase diagram predicts the coexistence of hexagonal close-packed (HCP) Co and B2 CoAl at room temperature. Here we report the synthesis of stoichiometric Co 3 Al via magnetron sputtering. Transmission electron microscopy investigations identified metastable phases of Co 3 Al, dominated by HCP supersaturation with minor L1 2 intermetallics. Furthermore, the phase transformation from HCP-to-FCC Co 3 Al induced by stacking faults in Co/Co 3 Al multilayers is also discussed. This investigation provides new insights on the crystal structures of stoichiometric Co 3 Al and the design of novel Co-Al based alloy systems. [Display omitted] [ABSTRACT FROM AUTHOR]

Tunable magnetic anisotropy can greatly facilitate the integration of perovskite oxides in emerging magnetic memory devices. Tailoring the metastable structures is a powerful and effective method to tune the magnetic anisotropy of perovskite oxides possessing a large spin–lattice correlation. Herein, we stabilized SrRuO 3 (SRO) at two metastable phases by tailoring the interfacial octahedral coupling between SRO and LaCoO 3 (LCO). Two metastable phases, distorted-rhombohedral and tetragonal structures, were stabilized in turn as the SRO thickness increased. Through the stabilization of these metastable phases, we realized a strongly lattice-dependent magnetic anisotropy and magnetic structure symmetry of SRO, employing its large spin–orbit coupling. Detailed octahedral rotation analysis demonstrated that when the SRO film was thin, the rotation phase mismatch between SRO and LCO played a decisive role in stabilizing the distorted-rhombohedral phase, while the competition between the rotation phase mismatch and tilt propagation relaxation induced the tetragonal phase when the SRO film was thick. These results demonstrate the feasibility of tailoring various structural symmetries and exotic functionalities through interface coupling. [ABSTRACT FROM AUTHOR]

In the present review, new approaches for the stabilization of metastable phases of zinc oxide and the growth of ZnO single crystals under high pressures and high temperatures are considered. The problems of the stabilization of the cubic modification of ZnO as well as solid solutions on its basis are discussed. A thermodynamic approach to the description of zinc oxide melting at high pressures is described which opens up new possibilities for the growth of both undoped and doped (for example, with elements of group V) single crystals of zinc oxide. The possibilities of using high pressure to vary phase and elemental composition in order to create ZnO-based materials are demonstrated. [ABSTRACT FROM AUTHOR]

The influence of heat treatment of the melt (composition 40 mol % Bi2O3 and 60 mol % GeO2), temperature of the beginning of its cooling and cooling rate on the phase composition, macro- and microstructure of the solid phases formed during crystallization were studied. The role of platinum in the formation of a metastable state of the melt and, as a consequence, the formation of metastable phases during crystallization is also considered. Using X-ray phase analysis, optical and scanning electron microscopy, it was found that the main factors determining the formation of the phase composition of crystallization products on the composition of 40 : 60 mol % (Bi2O3 : GeO2) are the preliminary heat treatment of the melt, which provides the most complete implementation of the metastable equilibrium in the melt (thermodynamic factor), and its interaction with solid platinum oxides on the walls and bottom of the crucible (kinetic factor). It is shown that at slow cooling rates (with a furnace) the melt crystallizes with the formation of a mixture of phases consisting of metastable Bi2GeO5 and eutectoid (Bi2GeO5 + α-GeO2). In this case, the decomposition of this metastable mixture is possible with a partial or complete transition to a stable state with the formation of Bi4Ge3O12. It was found that high-speed cooling of the melt is accompanied by the formation of either glass (quenching with a crucible in water) or a mixture of metastable Bi2GeO5 and an amorphous component. [ABSTRACT FROM AUTHOR]

Metastable β titanium alloys are widely used in the biomedical, automotive, and aerospace industry, due to their excellent corrosion resistance, fatigue strength, biocompatibility, and easy formability. Besides all these use areas, the suitable microstructure of the alloys for heat treatment increases the efficient usability day by day. In literature research, it has been found that heat treatment types such as cryogenic treatment and precipitation hardening can be applied efficiently to the alloys. Optimum strength/ductility, wear resistance, creep strength, and fatigue strength can be obtained with these heat treatments. For this reason, it has become important to understand the effect of heat treatments on the microstructural and mechanical properties of the alloys and parameters affecting this heat treatment efficiency. Precipitation hardening includes solution and aging treatment steps. The solution treatment can be applied at temperatures below and above the β transition temperature. While the aging treatment can be applied in four different ways, in the review article, the effects of single step and duplex aging treatment, which are applied with high efficiency, are emphasized. Precipitation hardening efficiency affects the chemical composition of the alloys, heat treatment steps, treatment temperature and times, and heating/cooling rate. The cryogenic treatment provides the formation of martensite α phases in metastable β titanium alloys cooled below the martensitic transformation temperature. Higher-strength and hardness have been obtained in the studies where aging treatment was applied after cryogenic treatment. Cryogenic treatment efficiency determines chemical composition of the alloys, treatment temperature and time, the heating/cooling rates, and heat treatments applied after cryogenic treatment.

• We report the in-situ formation of nonstoichiometry and metastable Nb 2 O 5-x phases within the range of 0.04 < x < 0.21. • Metastable phases T, TT-Nb 2 O 5 are formed at the splat core and nonstoichiometry phases NbO 2 , Nb 12 O 29 at the splat boundaries. • Ar/H2 thermal plasma-Nb 2 O 5 interaction is based on Nb 2 O 5 incongruent vaporization and rapid solidification. • We report interesting thermal, electrical and thermomechanical properties of Nb 2 O 5-x deposits. Nb 2 O 5 polymorphism and defect chemistry depend on the temperature, pressure, atmosphere composition and the initial crystallography. Plasma spray of Nb 2 O 5 is a pathway to form coatings with in-situ metastable and nonstoichiometric phases, however so far unexplored. This study aimed to understand the phase evolution of plasma sprayed Nb 2 O 5 coatings, and its effect on their morphology and properties. Phase evolution from H-Nb 2 O 5 in the feedstock, to T-Nb 2 O 5 , TT-Nb 2 O 5 , N-Nb 2 O 5 , H-Nb 2 O 5 , Nb 12 O 29 and NbO 2 in the coatings depends on the plasma Ar/H 2 ratio and its related enthalpy. The microstructure shows a layered distribution of nonstoichiometric phases at the splat boundaries and splat cores composed of T-Nb 2 O 5 or TT-Nb 2 O 5. The presence and distribution of these phases are related to the thermomechanical and electrical properties. The mechanisms driving the formation of these coatings are based on the Nb 2 O 5 incongruent vaporization which promote retention of nonstoichiometric phases and the rapid solidification of metastable phases. [ABSTRACT FROM AUTHOR]

Samples obtained by heating a CeCo3-based intermetallic hydride with a low hydrogen content (1.0 H/FU) to a high temperature in an inert atmosphere have been characterized by X-ray diffraction. X-ray diffraction data for the samples have been used to determine relationships between phases forming at different temperatures and structural parameters of their lattice. In the temperature range 200–400°C, partially amorphous samples have been obtained. Raising the temperature to 600°C has led to the formation of essentially crystalline samples. The samples have been found to contain the high-temperature, cubic phase of cobalt, stable at room temperature. [ABSTRACT FROM AUTHOR]

Nitrides are commonly studied for their properties relevant to numerous coating applications, and have been recently used to synthesize newly predicted materials in thin‐film form. However, the thermodynamic stability of such nitride materials is difficult to experimentally evaluate, due to the limited thermal budget of thin films. Herein, it is shown that the thermal stability of nitride films can be extended using protection layers. Specifically, it is found that zirconium nitride (ZrN) thin films are stable up to at least 1200 °C annealing temperature in N2 atmosphere, if aluminum nitride (AlN) diffusion barriers and capping layers are used. X‐ray diffraction (XRD) measurements show the expected thermodynamically stable rocksalt structure of ZrN, and scanning electron microscopy (SEM) confirms the compact microstructure of these ZrN films. Without such protection layers, the transient bixbyite Zr2ON2 phase and high‐temperature ZrSi2 phase are observed after annealing by XRD, SEM, and Auger electron spectroscopy (AES), due to side reaction with the native surface oxide (ZrOx) and the substrate (Si), respectively. These results demonstrate that protection layers are beneficial to increase the thermal budget of nitride thin films, for evaluating rgw thermodynamic stability of new nitride materials, for capturing transient metastable phases during crystallization, and for using functional nitride coatings in extreme environments. [ABSTRACT FROM AUTHOR]

In situ observation of a phase transformation during annealing of Si containing diamond-cubic (dc) and metastable phases has been performed using synchrotron X-ray diffraction. Metastable body-centred-cubic (bc8) and rhombohedral (r8) phases were formed by high-pressure torsion. These metastable phases gradually disappeared when increasing the annealing temperature to ∼ 180°C while another metastable phase having a hexagonal diamond (hd) structure appeared at ∼ 190°C. High-resolution transmission microscopy analysis revealed that hd and dc grains were present after annealing at 200°C. The results indicate that a phase transformation, mainly from bc8 to hd, occurred during annealing. [ABSTRACT FROM AUTHOR]

Ionic liquid syntheses have proven proficiency in the production of metal oxides with diverse functionalities. In this work we use the potential of these solvents to form multicationic solutions to directly synthesize five strontium niobate metastable crystal compositions. Such metastable crystals are characterized via UV–Vis spectroscopy, inductively coupled plasma optical emission spectrometer, powder X-Ray diffraction, high-resolution transmission electron microscopy, and crystalline refinements. After characterization, all metastable crystalline structures were found to be strontium rich with Sr/Nb molar ratios Sr/Nb = 3.54, 3.23, 3.09, 2.66, and 2.22, and are found to be consistent with triclinic with space group P-1 (2). [ABSTRACT FROM AUTHOR]

In the present review, new approaches for the stabilization of metastable phases of zinc oxide and the growth of ZnO single crystals under high pressures and high temperatures are considered. The problems of the stabilization of the cubic modification of ZnO as well as solid solutions on its basis are discussed. A thermodynamic approach to the description of zinc oxide melting at high pressures is described which opens up new possibilities for the growth of both undoped and doped (for example, with elements of group V) single crystals of zinc oxide. The possibilities of using high pressure to vary phase and elemental composition in order to create ZnO-based materials are demonstrated.

The metastability of a typical non-equiatomic FeMnCoCr high-entropy alloy (HEA) system was demonstrated to be dormant upon co-doping with C and N interstitials, i.e., displacive γ-ε phase transformation disappears during tensile deformation. Interestingly, we found in the present study that the displacive phase transformation prevails again in the C-N co-doped HEA upon severe deformation via cold-rolling. The generation of ε-phase lamellae is found to be motivated by strain localization which facilitates the formation of shear bands. Such highly concentrated strains significantly intensify the local elastic fields to neighboring γ matrix and finally eke out the driving force to overcome the interstitials-enhanced energy barrier of ε-martensite transformation. The results suggest that the suppressed phase metastability is awakened, which inspires efforts to tune the phase metastability via modifying the strain state during alloy processing and gives new insights into the development of HEAs with desirable phase stability and mechanical properties. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Using high‐pressure–high‐temperature treatment at P = 4–7.7 GPa; T = 1373–1473 K with subsequent quenching, a new metastable phase of bismuth selenide (m‐Bi2Se3) is synthesized and its crystal structure, electrical resistivity, and annealing at heating are investigated. Using X‐ray powder diffraction, and analogy with high‐pressure structures of the rare‐earth element sesquichalcogenides, the crystal structure of the new metastable phase of m‐Bi2Se3 is determined as an orthorhombic distorted cation‐deficient structure of the Th3P4 type with the Fdd2 space group and Z = 10.66. The unit‐cell dimensions are: a = 13.4660(7) Å, b = 12.7772(7) Å, c = 9.0896(5) Å. The density of m‐Bi2Se3 (7.47 g cm−3) is slightly less than the initial rhombohedral phase of Bi2Se3, but the coordination number (CN) = 8 is higher than CN = 6 of the initial phase. The period of stability under ambient conditions of the new m‐Bi2Se3 phase is only about 2 months. After 300 °C annealing, m‐Bi2Se3 completely returns to the initial layered structure, and differential scanning calorimetry confirms that the reverse transformation as well as the direct transition occur through an amorphous state. The new phase is a narrow bandgap semiconductor with the energy gap of about 80 meV. [ABSTRACT FROM AUTHOR]

In this work, the effect of polyvinylpyrrolidone (PVP) concentration as a surfactant was studied in the stabilization of the metastable phase γ-Ag 2 WO 4 by the coprecipitation method at room temperature. The phases obtained were characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and spectroscopy in the ultraviolet–visible region (UV–Vis). The photocatalytic activity was estimated by varying the concentration of methylene blue dye. The diffractograms indicate that 0.3 g of PVP is ideal for obtaining the single phase of γ-Ag 2 WO 4 , as larger or smaller amounts favor the appearance of β-Ag 2 WO 4 as a secondary phase while the absence of PVP formed the α-Ag 2 WO 4 phase. UV–Vis spectroscopy showed that both phases have absorption in the ultraviolet region, where the γ-Ag 2 WO 4 phase has a higher absorption at shorter wavelengths. The micrographs indicate that the β-Ag 2 WO 4 morphology is formed by rods, while the γ-Ag 2 WO 4 is formed by octahedrons, both in micrometer scale and with well-defined morphology. The photocatalytic tests indicate that the γ-Ag 2 WO 4 phase has better photocatalytic activity compared to the β-Ag 2 WO 4 phase. The scavenger's methodology indicated that h+ charges are the main mechanism in the photocatalysis of methylene blue dye. [ABSTRACT FROM AUTHOR]

We investigate the reactive flash sintering of Mg 0.2 Ni 0.2 Co 0.2 Cu 0.2 Zn 0.2 O precursor powders prepared by a polymeric synthesis route. The flash experiments were performed at a constant heating rate of 10°C min−1 with electric fields from 15 to 60 V cm−1. The single-phase rock-salt structure was obtained in just a few seconds at furnace temperatures as low as 500°C, under an electric field of 60 V cm−1 and a current density of 150 mA mm−2. Success at processing an entropy-driven phase in a remarkably short time and low furnace temperature demonstrates the potential of reactive flash sintering for producing entropy-stabilized materials. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Martensite that is mechanically induced from reverted austenite can be reverted again to austenite upon annealing. Carrying out mechanical tests, electron backscatter diffraction, electron channeling contrast imaging, and energy-dispersive X-ray spectroscopy analyses, we observe that Mn micro-segregation governs this re-reversion process in a martensite-reverted-austenite steel. The annealing treatment cannot fully revert all strain-induced effects in this multi-phase alloy (i.e., ductile damage, grain shape change) and introduces some new changes (e.g. precipitate size in martensite). However, the resulting microstructure exhibits the original mechanical response even after multiple reversions, demonstrating the governing role of the mechanically induced martensitic transformation on strain hardening. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Reducing the enthalpy of as-deposited amorphous films is desirable as it improves their kinetic stability and enhances the reliability of resulting devices. Here we demonstrate that Ge 15 Te 85 glass films of lower enthalpy are produced by increasing the voltage during magnetron sputter deposition. An increase of ~100 V leads to a drop in effective cooling rate of almost three orders of magnitude, thereby yielding markedly lower enthalpy glasses. The sputtering voltage therefore constitutes a novel parameter for tuning the fictive temperature of glass films, which could help to obtain ultra-stable glasses in combination with substrate temperature control. Image, graphical abstract [ABSTRACT FROM AUTHOR]