Metastable Phase Research Articles

Ferroelasticity in Two-Dimensional Hybrid Ruddlesden-Popper Perovskites Mediated by Cross-Plane Intermolecular Coupling and Metastable Funnel-like Phases.

Ferroelasticity is a phenomenon in which a material exhibits two or more equally stable orientation variants and can be switched from one form to another under an applied stress. Recent works have demonstrated that two-dimensional layered organic-inorganic hybrid Ruddlesden-Popper perovskites can serve as ideal platforms for realizing ferroelasticity, however, the ferroelastic (FE) behavior of structures with a single octahedra layer such as (BA)2PbI4 [BA = CH3(CH2)3NH3+] has remained elusive. Herein, by using a combined first-principles and metadynamics approach, the FE behavior of (BA)2PbI4 under mechanical and thermal stresses is uncovered. FE switching is mediated by cross-plane intermolecular coupling, which could occur through multiple rotational modes, rendering the formation of FE domains and several metastable paraelastic (PE) phases. Such metastable phases are akin to wrinkled structures in other layered materials and can act as a "funnel" of hole carriers. Thermal excitation tends to flatten the kinetic barriers of the transition pathways between orientation variants, suggesting an enhanced concentration of metastable PE states at high temperatures, while halogen mixing with Br raises these barriers and conversely lowers the concentration of PE states. These findings reveal the rich structural diversity of (BA)2PbI4 domains, which can play a vital role in enhancing the optoelectronic properties of the perovskite and raise exciting prospects for mechanical switching, shape memory, and information processing.

Formation of metastable phases during spark plasma sintering of Fe@Au core–shell particles

ABSTRACT The present work reports the formation of Fe-Au metastable alloys by galvanic replacement, pressureless spark plasma sintering (SPS) and acid treatment. The reaction of partial galvanic replacement between micrometer-sized iron particles and HAuCl4 solution allowed synthesising core–shell Fe@Au particles. When the Fe@Au particles were treated in HCl solution, iron was selectively dissolved and hollow Au particles were formed. Pressureless SPS of the Fe@Au core–shell particles led to the formation of a metastable intermetallic phase, FeAu3, and a supersaturated solid solution, Au(Fe). The latter was resistant to HCl solution; at the same time, the FeAu3 phase decomposed during HCl treatment to form a porous gold structure. This work shows that a combination of galvanic replacement, SPS and acid treatment can be used for the preparation of metastable Fe-Au alloys with different phase compositions and particle morphologies.

Effect of homogenization process on micro‐structure and mechanical properties of 4047 aluminum alloy

This study investigated the homogenization heat treatment process and microstructural evolution of 4047 aluminum alloy. The results revealed that the homogenization process induced partial dissolution of metastable phases and modification of the fine needle‐shaped eutectic Si. Within the 4047 aluminum alloy, the fine needle‐shaped eutectic Si underwent granulation, involving two stages: crystal dissolution and granulation of broken crystals. After optimal homogenization heat treatment, the eutectic Si phase transformed from a fine needle‐shaped structure to small, regular spherical structures, reducing stress concentration and improving the interfacial bonding capability and internal structure of the alloy matrix. This significantly reduces the likelihood of microcrack formation within the alloy and effectively hinders crack propagation, enhancing the alloy’s mechanical properties.This article is protected by copyright. All rights reserved.

Symmetry Engineering of Epitaxial Hf0.5Zr0.5O2 Ultrathin Films.

Robust ferroelectricity in HfO2-based ultrathin films has the potential to revolutionize nonvolatile memory applications in nanoscale electronic devices because of their compatibility with the existing Si technology. However, to fully exploit the potential of ferroelectric HfO2-based thin films, it is crucial to develop strategies for the controlled stabilization of various HfO2-based polymorphs in nanoscale heterostructures. This study demonstrates how substrate-orientation-induced anisotropic strain can engineer the crystal symmetry, structural domain morphology, and growth orientation of ultrathin Hf0.5Zr0.5O2 (HZO) films. Epitaxial ultrathin HZO films were grown on the heterostructures of (001)- and (110)-oriented La2/3Sr1/3MnO3/SrTiO3 (LSMO/STO) substrate. Various structural analyses revealed that the (110)-oriented substrate promotes a higher degree of structural order (crystallinity) with improved stability of the (111)-oriented orthorhombic phase (Pca21) of HZO. Conversely, the (001)-oriented substrate not only induces a distorted orthorhombic structure but also facilitates the partial stabilization of nonpolar phases. Electrical measurements revealed robust ferroelectric properties in epitaxial thin films without any wake-up effect, where the well-ordered crystal symmetry stabilized by STO(110) facilitated better ferroelectric characteristics. This study suggests that tuning the epitaxial growth of ferroelectric HZO through substrate orientation can improve the stability of the metastable ferroelectric orthorhombic phase and thereby offer a better understanding of device applications.

Unraveling a Unique Metastable Phase of a Proxyphylline Cocrystal Exclusively Produced by Compressed Carbon Dioxide

Unraveling a Unique Metastable Phase of a Proxyphylline Cocrystal Exclusively Produced by Compressed Carbon Dioxide

Deterministic Orientation Control of Ferroelectric HfO2 Thin Film Growth by a Topotactic Phase Transition of an Oxide Electrode.

The scale-free ferroelectricity with superior Si compatibility of HfO2 has reawakened the feasibility of scaled-down nonvolatile devices and beyond the complementary metal-oxide-semiconductor (CMOS) architecture based on ferroelectric materials. However, despite the rapid development, fundamental understanding, and control of the metastable ferroelectric phase in terms of oxygen ion movement of HfO2 remain ambiguous. In this study, we have deterministically controlled the orientation of a single-crystalline ferroelectric phase HfO2 thin film via oxygen ion movement. We induced a topotactic phase transition of the metal electrode accompanied by the stabilization of the differently oriented ferroelectric phase HfO2 through the migration of oxygen ions between the oxygen-reactive metal electrode and the HfO2 layer. By stabilizing different polarization directions of HfO2 through oxygen ion migration, we can gain a profound understanding of the oxygen ion-relevant unclear phenomena of ferroelectric HfO2.

Electrocatalytic Oxygen Reduction Using Metastable Zirconium Suboxide

Strategies for discovery of high‐performance electrocatalysts are important to advance clean energy technologies. Metastable phases such as low temperature or interfacial structures that are difficult to access in bulk may offer such catalytically active surfaces. We report here that the suboxide Zr3O, which is formed at Zr‐ZrO2 interfaces but does not appear in the experimental Zr‐O phase diagram exhibits outstanding oxygen reduction reaction (ORR) performance surpassing that of benchmark Pt/C and most transition metal‐based catalysts. Addition of Fe3C nanoparticles to give a Zr‐Zr3O‐Fe3C/NC catalyst (NC = nitrogen‐doped carbon) gives a half‐wave potential (E1/2) of 0.914 V, outperforming Pt/C and showing only a 3 mV decrease after 20,000 electrochemical cycles. A zinc‐air battery (ZAB) using this cathode material has a high power density of 241.1 mW cm‐2 and remains stable for over 50 days of continuous cycling, demonstrating potential for practical applications. Zr3O demonstrates that interfacial or other phases that are difficult to stabilize may offer new directions for the discovery of high‐performance electrocatalysts.

Electrocatalytic Oxygen Reduction Using Metastable Zirconium Suboxide.

Strategies for discovery of high-performance electrocatalysts are important to advance clean energy technologies. Metastable phases such as low temperature or interfacial structures that are difficult to access in bulk may offer such catalytically active surfaces. We report here that the suboxide Zr3O, which is formed at Zr-ZrO2 interfaces but does not appear in the experimental Zr-O phase diagram exhibits outstanding oxygen reduction reaction (ORR) performance surpassing that of benchmark Pt/C and most transition metal-based catalysts. Addition of Fe3C nanoparticles to give a Zr-Zr3O-Fe3C/NC catalyst (NC = nitrogen-doped carbon) gives a half-wave potential (E1/2) of 0.914 V, outperforming Pt/C and showing only a 3 mV decrease after 20,000 electrochemical cycles. A zinc-air battery (ZAB) using this cathode material has a high power density of 241.1 mW cm-2 and remains stable for over 50 days of continuous cycling, demonstrating potential for practical applications. Zr3O demonstrates that interfacial or other phases that are difficult to stabilize may offer new directions for the discovery of high-performance electrocatalysts.

High-pressure Phase Transition of Olivine-type Mg2GeO4 to a Metastable Forsterite-III type Structure and their Equation of States

Abstract Germanates are often used as structural analogs of planetary silicates. We have explored the high-pressure phase relations in Mg2GeO4 using diamond anvil cell experiments combined with synchrotron X-ray diffraction and computations based on density functional theory. Upon room temperature compression, forsterite-type Mg2GeO4 remains stable up to 30 GPa. At higher pressures, a phase transition to a forsterite-III type (Cmc21) structure was observed, which remained stable to the peak pressure of 105 GPa. Using a 3rd order Birch Murnaghan fit to the experimental data, we obtained V0 = 305.1 (3) Å3, K0 = 124.6 (14) GPa and K0′ = 3.86 (fixed) for forsterite- and V0 = 263.5 (15) Å3, K0 = 175 (7) GPa and K0′ = 4.2 (fixed) for the forsterite-III type phase. The forsterite-III type structure was found to be metastable when compared to the stable assemblage of perovskite/post-perovskite + MgO, as observed during laser-heating experiments. Understanding the phase relations and physical properties of metastable phases is crucial for studying the mineralogy of impact sites, understanding metastable wedges in subducting slabs and interpreting the results of shock compression experiments.

Investigation of the Impacts of Nonpolar Amino Acids on Air Lime Mortars

ABSTRACT Natural protein-based additives have been frequently added to lime-based mortars to enhance their properties. However, the possible mechanism underlying this effect is not well understood. All proteins are built up by the basic units of amino acids, which may be viewed as a useful starting point for the research topic. The present study focused on the impacts of nonpolar amino acids on air lime mortars. The results obtained from the present study highlighted that some nonpolar amino acids (represented by glycine, alanine, valine and proline) decrease mortar water retention, decrease consistency, delay and prolong setting, increase compressive strength, and decrease water absorption. However, other nonpolar amino acids (such as leucine, isoleucine, phenylalanine and tryptophan) showed almost opposite results. The addition of all the nonpolar amino acids slowed mortar drying and carbonation, decreased the carbonation degree, inhibited calcite formation, and tended to form metastable calcium carbonate phases, such as vaterite. Studying the effects of single amino acids on lime mortars could contribute to the design of repair mortars with more specific requirements, and the results of the present study could open up a series of future topics to be investigated.

The water bimodal inherent structure and the liquid-liquid transition as proposed by the experimental density data.

The bulk water density data are studied in a very large temperature-pressure range, from stable liquid to glass in the frame of water polymorphism. Because this thermodynamic variable evidences a crossover T*, above which the hydrogen bond (HB) is unable to arrange tetrahedral networks, the T-dependence of their isobars was considered. Such an analysis also shows pressure, P*, around which their thermal behaviors are completely different: concave below P* (with maxima and minima) and convex above (without extremes). Having ρ's measured values of the bimodal structures of the liquid phase, HDL (ρHDL), made of not-bonded monomers (ρNHB) and partially bonded dimers plus trimers (ρNHB), and LDL tetramers (ρLDL) the isobars of the relative distributions [W(P, T)] of the three species (WLDL, WPHB, and WNHB) have been evaluated. The results were studied by means of a logistic function (LF) that details the evolutions of the relative polpulations of the water LDL and HDL phases by decreasing T (for the isobars, in the range of 0.1-400 MPa). The LFs analysis obtained by proposing a full connection between liquid water from its supercooled metastable phase to the stable up to the boiling temperature identifies the Widom line quite satisfactorily and fully supports the presence of the liquid-liquid critical point in the deep supercooled region, located at about 190K and in the range 200 > P > 180 MPa.

Studying the features of complex beryllium-containing silicate phases co-crystallization in zonal samples using the x-ray electron probe microanalysis method

Research subject. Silicate ingots containing four species-forming components Be, Mg, Al, and Si and belonging to the crystallization region of beryllium indialite (with the formula of Mg2BeAl2Si6O18 and a beryl-type structure). Aim. To investigate the fundamental problem of identifying the patterns of matter differentiation and the stable and metastable phase formation in silicate matrices. Methods. The evolution of the phase composition of silicate melts was registered using a temperature gradient method. Results. New data on the features of phase transformations in silicate melts belonging to the region of beryllium indialite were obtained by electron probe microanalysis (EPMA). Co-existing metastable and stable mineral phases were identified, and the similarity of their compositions with different structures was shown. The nature of impurity phases at each stage of crystallization was established. Conclusions. The evolutionary sequence of phase associations ensuring the crystallization of beryllium indialite and metastable phases of a similar composition, the nature of which is determined by the initial ratio of components, was experimentally recorded. The range of possible phase associations that co-crystallize or replace a stable phase with a beryl structure in melts from the region of existence of beryllium indialite in the BeO–MgO–Al2O3–SiO2 system was extended. The selectivity of the coloring element chromium entry into various phases of the studied system is shown depending on the capabilities of their structure. The addition of a chromophore is a reliable criterion for visualizing successive layers, zones, and areas of changing phase associations in the final ingot.

THERMOPHYSICAL PROPERTIES OF HYPOEUTECTIC, EUTECTIC AND HYPEREUTECTIC Al-Si AUTOMOTIVE ALLOYS UNDER AGEING TREATMENT

Influence of different levels of silicon under a variety of thermal treatment is investigated on the thermophysical behaviour of Al-Si-Cu-Mg automotive alloys. Conventional cast alloys are subjected to T6 heat treatment for age hardening. Hardness values, thermal conductivity along with microstructural observation under different heat-treated conditions are well thought-out to comprehend the ageing performance and the precipitation behaviour of the alloys. From these investigational results it is indicated that two successive hardening peaks take place in the agedalloys. Ageing sequence consists of different phases like a supersaturated solid solution, GP zones, intermediate βʹʹ, intermetallic βʹ, equilibrium β and the Q phase but GP zones and the metastable phases are most likely responsible for these ageing peaks. Thermal conductivity decreases for these precipitates formation during ageing and increases for relieving the internal stress, dissolution of metastable phase and coarsening the precipitation of the alloys. Addingup of Si to the alloy showed earlier ageing peaks with higher intensities intended for its increasing properties of heterogeneous nucleation and diffusion kinetics. After ageing around at 200°C for four hours the alloys offer the height hardness. Microstructural study reveals that eutectic silicon makes the grain boundary coarsen and beyond the eutectic composition the star-shaped blocky primary Si is formed. Subsequent to ageing at 350oC for one hour the alloys reach completely recrystallized state.

Identification of an affordable and printable metastable β Ti alloy with outstanding deformation behaviour for use in laser powder bed fusion

Research on metastable β Ti alloys has recently shown the possibility to achieve outstanding ductility and work hardening behaviours by engineering the materials’ unique strain transformable attributes and responses to heat treatment. As quenching is central to the development of the metastable phase in these alloys, it is of interest to understand how these materials could be designed and developed for use in laser additive manufacturing where feedstocks experience numerous rapid thermal cycles. In this context, we first propose a material design framework to identify printable metastable Ti alloys for laser powder bed fusion (L-PBF). We focus on the ternary Ti-Cr-Sn system for both its affordable character and interest in the biomedical field. The design framework combines criteria based on well-established empirical parameters that we use to compare stability of β phase and resistance to cracking during L-PBF, i.e. its printability. The design work is supported by series of microstructural investigations on arc-melted buttons of selected compositions. Using such methodology we identify Ti-7.5Cr-4.5Sn as strain transformable alloy of good printability and we then characterise this alloy in depth in a series of laser powder bed fusion experiments. Printability of this alloy is further demonstrated via printing experiments using a variety of processing conditions while the deformation character is studied using compressive testing. It was found that the identified Ti-7.5Cr-4.5Sn alloy can achieve a high compressive strain exceeding 70 % thanks to deformation mechanisms that combine dislocation slip and {3 3 2}<1 1 3> twinning. This research paves the way towards the identification of novel Ti alloys for L-PBF beyond those used in the aerospace sector, opening the way for a broader field of applications.

Orientation-dependent deformation mechanisms of alpha-uranium single crystals under shock compression

Large-scale non-equilibrium molecular dynamics (NEMD) simulations were employed to investigate the dynamic deformations of alpha-uranium (α-U) single crystals subjected to varying shock strengths along low-index crystallographic orientations. The pronounced anisotropy of α-U gives rise to a complex microstructural evolution under shock loading. In-depth microstructural analysis of post-shock specimens reveals the identification of multiple dynamic deformation mechanisms. Notably, when the shock loading direction aligns with the a-axis, dynamic deformation of the α-U single crystals is primarily dominated by lattice instability, which attributes to a crystalline-to-amorphous transition serving as the dominant shear stress relaxation pathway. On the other hand, shock loading along the b-axis results in an abundance of deformation twins, with twinning planes identified as (130) and (13¯0). During the twinning event, the α-U matrix undergoes a transition to a metastable intermediate phase, subsequently decomposing into a composite structure comprising α-U twins and matrix. This unconventional twinning mechanism significantly deviates from classical theories. Furthermore, upon loading along the c-axis, twinning and a phase transition from α-U to body-centered tetragonal phase (bct-U) occur in α-U single crystal samples. Given that the pressure threshold of this phase transition predicted by ab initio calculations is as high as ∼270 GPa, the phase transition from α-U to bct-U might be implausible. An alternative interatomic potential of uranium with the higher pressure threshold was employed to reinvestigate the shock response of α-U single crystals along the c-axis. The phase transition of α-U to bct-U disappears, and twinning dominates the plastic deformation, with the twinning orientation conforming to the {112} twinning. The strong anisotropy of the α-U lattice triggers a wealth of orientation-dependent dynamic deformation mechanisms. The activation of the twinning system is evidently associated with the loading direction, constituting the potential cause for the discovery of multiple twinning variants during the deformation in polycrystalline uranium.

Dilaton in a multicritical 3+epsilon-D parity violating field theory

The multi-critical behavior of an approximately scale and conformal invariant quantum field theory, which can be regarded as the deformation of the critical Gross-Neveu model in 3+epsilon dimensions by a nearly marginal parity violating operator, is studied using a large N expansion. When epsilon is greater than a number of order 1/N, the deformation is marginally relevant and it is found to exhibit spontaneous breaking of the approximate scale symmetry accompanied by the appearance of a light scalar in its spectrum. The scalar mass is parametrically small, of order epsilon times the fermion mass and it can be identified with a light dilaton. When the dimension is reduced to 3 the deformation of the Gross-Neveu model becomes marginally irrelevant, what was a minimum of the potential becomes a maximum and the theory has a non-perturbative global instability. There is a metastable perturbative phase where the scalar does not condense and the fermions are massless separated by an energy barrier with height of order one (rather than N) from an energetically favored phase with a runaway condensate.

Experimental and thermodynamic assessment of the Cu–In system

Owing to the low melting temperature and excellent mechanical properties, In is considered a potential low-temperature solder. However, the interfacial reaction between In and Cu substrates is still unclear, specifically regarding the stability of CuIn2. Although CuIn2 was often observed at the Cu–In thin film after long-term aging below 100 °C, it is considered as a metastable phase and has not been added into the Cu–In phase diagram yet. In this study, the stability of CuIn2 and the peritectoid reaction Cu11In9 + In → CuIn2 were established using the Cu/In diffusion couple, Cu–In alloy phase equilibria, and decomposition investigation of CuIn2. The peritectoid temperature was determined to be in the range of 110–105 °C. Finally, thermodynamic assessment was conducted based on the experimental data and the revised Cu–In phase diagram.

Mechanical and magnetic properties of phase separated Cu50Fe50 (at%) alloys synthesized by mechanical alloying and spark plasma sintering

A bulk Cu50Fe50 (at%) alloy was prepared by mechanical alloying (MA) of elemental Cu and Fe powders for 20h, followed by the spark plasma sintering (SPS). All the powder and bulk samples were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The mechanical and magnetic properties of the bulk samples were also evaluated. From 1h to about 10h of milling, the formation of nanocrystalline Cu-rich face-centered cubic solid solution (FCC γ phase) and Fe-rich body-centered cubic solid solution (BCC α phase) was observed. Above 10h of milling, the γ and α phases merged to form a single-phase face-centered cubic (γS) solid solution till 20h. Subsequently, the 20h MA powder was consolidated using spark plasma sintering (SPS) at three different temperatures: 519 ℃ (0.5Tm), 677 ℃(0.6Tm), and 835 ℃ (0.7Tm), where Tm is the average melting point of the alloy (1310 ℃). In all three samples, the metastable γS phase decomposed into the nanocrystalline Cu-rich γ and Fe-rich α phases during SPS. Notably, the 0.7Tm sample exhibited a better balance between strength and ductility by demonstrating a compressive yield strength of 886MPa and an ultimate compressive strength of 1166MPa with a ductility of 15%. The high strength and ductility are attributed to a bimodal grain size distribution, i.e., a few microns thick and continuous Cu-rich γ grain boundary phase and an intragranular nanocrystalline mixture of γ and α phases. In addition to its excellent mechanical properties, the 0.7Tm sample displayed favorable soft ferromagnetic characteristics exhibiting a saturation magnetization (Ms) of 66emu/g and a coercivity (Hc) of 16Oe.

Enhancing the adiabatic shear resistance of Ti-Zr-Nb-Al high entropy alloy via metastability engineering

Dual-phase Ti-Zr based high entropy alloys are lightweight structural materials with high specific strength, which are promising for the application in warheads and munitions. However, they usually experience adiabatic shear under impact, resulting in early failure. Here, we demonstrate an effective method to enhance the adiabatic shear resistance, benefiting from the transformation-induced plasticity (TRIP) effect. The composition of Ti-Zr-Nb-Al is designed based on a combination of lattice distortion enthalpy, mixing enthalpy, and Bo¯-Md¯ criteria, to enable a metastable solution structure. The alloy is directly water quenched from 900 ℃ to maximally keep the metastable BCC phase (MA), while an annealed alloy with the BCC+HCP dual-phase structure (PA) is also fabricated for comparison. The metastable MA alloy exhibits a doubled fracture strain and 79% enhancement of impact energy compared with the dual-phase PA alloy, while its dynamic ultimate strength remains almost unchanged. Microstructural analyses at different dynamic stain levels reveal a BCC-α" phase transformation in MA alloy, which delays the generation of phase boundaries, avoids the premature stress concentration, and thereby delays the occurrence of adiabatic shear.

Microstructure and Mechanical properties of CMT welded AA6063-T6/B4C/ES metal matrix composite using ER5356 filler wire

Aluminium alloy 6063-T6 is a popular choice among different industrial sectors as it portrays various desirable properties like moderate strength, improved corrosion resistance, better weldability and formability etc. These advantages make AA6063-T6 a common choice for matrix in a metal matrix composite. Mostly conventional fusion welding methods like Metal Inert Gas (MIG) and Tungsten Inert Gas (TIG) seem preferable options for welding aluminium and its alloys but it remains a challenge to use them to weld aluminium metal matrix composite. A modified version of MIG i.e. CMT welding, can be explored to overcome the challenges. In this study, an aluminium metal matrix composite has been fabricated with AA6063-T6 as matrix and two reinforcements −one organic, i.e., Eggshell (ES) and one inorganic, i.e., Boron Carbide (B4C), using the conventional stir casting method. CMT welding has been performed on plates cut out from the composites. Microstructure, hardness, tensile strength and residual stress of weld joint of composite have been observed. From micrographs, it is observed that a finer grain has been formed at the Fusion Zone (FZ) in comparison to the Heat Affected Zone (HAZ) and Parent Material (PM). XRD analysis at the fusion line has identified several intermetallic stoichiometric compounds (Al12Mg17, Mg2Si, CaMgSi, CaO, B2O3) and reinforcement B4C and CaCO3 within the matrix volume. The FZ exhibits higher hardness than the HAZ and the PM. Further, the tensile strength of the FZ exceeds that of the unwelded composite due to the presence of metastable strengthening phases of Mg2Si in the FZ, along with reinforcement (B4C and ES) particles. Residual stress analysis reveals tensile stress in the FZ and more compressive stress in the HAZ than in the PM.