Coherent Interphase Research Articles

Mind the miscibility gap: Cation mixing and current density driven non-equilibrium phase transformations in spinel cathode materials

Cathode materials that exhibit phase transitions with large structural rearrangements during electrochemical cycling are generally seen as disadvantageous. Large volume changes and lattice mismatches between intermediate phases tend to lead to significant kinetic barriers, as well as strain and particle cracking. In this regard, solid solution reactions are more desirable as they provide lower energy barriers and no miscibility gap between co-existing phases. The high-voltage cathode material LiNi0.5Mn1.5O4 is an interesting candidate for high power and rate capability applications, however little is known on how its phase transitions occur on the particle level. In the presented work operando X-ray diffraction was utilized together with detailed peak profile analysis to elucidate the phase transition mechanism dependency on transition metal cation order and current density. When fully disordered, the material was found to undergo a bulk single-phase solid solution reaction between the intermediate phases LiNi0.44Mn1.56O4 and Li0.5Ni0.44Mn1.56O4 followed by a first order phase transition with a coherent interphase between the intermediates Li0.5Ni0.44Mn1.56O4 and Ni0.44Mn1.6O4. When fully ordered and slightly less ordered, two separate first order phase transitions with a coherent interphase between the same intermediate phases were observed. On discharge, the fast kinetics of the transition between Li0.5Ni0.44Mn1.56O4 and LiNi0.44Mn1.56O4 resulted in less strain on the former phase. For all samples the miscibility gap between the intermediate phases narrowed with increased current density, suggesting that the solid solution domain formed at the coherent interphase can be extended when the rate of (de)lithiation exceeds the movement speed of the interphase at the phase transition. This effect was found to be larger with increasing cation disorder. The influence of transition metal ordering on the ability to form solid solutions is in good agreement with computational phase diagrams of LiNi0.5Mn1.5O4, showing that disorder is important for promoting and stabilizing solid solutions. These results indicate that the degree of transition metal ordering within the material is of importance for obtaining a material with small lattice mismatches between the involved intermediate phases and for rational design of full solid solution materials.

Countering Voltage Decay, Redox Sluggishness, and Calendering Incompatibility by Near‐Zero‐Strain Interphase in Lithium‐Rich, Manganese‐Based Layered Oxide Electrodes

AbstractLithium‐rich, manganese‐based layered oxides are considered one of the most valuable cathode materials for the next generation of high‐energy density lithium‐ion batteries (LIBs) for their high specific capacity and low cost. However, their practical implementation in LIBs is hindered by the rapid voltage/capacity decay on cycling and the long‐standing contradictions between redox kinetics and volumetric energy density due to their poor calendaring compatibility. Herein, a coherent near‐zero‐strain interphase is constructed on the grain boundaries of cathode secondary particles by infusing LiAlO2 material through the reactive infiltration method (RIM). Theoretical calculations, multi‐scale characterizations, and electrochemical tests show that this coherent interphase with near‐zero‐strain feature upon electrochemical (de)lithiation inhibits volume changes of the lattice and structural degradation of cathode primary particles during cycling. More importantly, the ionically conductive LiAlO2 nanolayer infiltrated in the grain boundaries of cathode secondary particles can not only promote the rapid Li+ migration and act as a barrier to protect the material from the corrosion of the electrolyte but also effectively improve the mechanical strength of the cathode secondary particles. Collectedly, the LiAlO2‐infiltrated cathode materials display superior electrochemical cyclability, enhanced rate capability, and industrial calendaring performance, marking a significant step toward commercial implementation.

On the role of interfacial elasticity in morphological instability of a heteroepitaxial interface

In this paper, we discuss a theoretical approach to morphological instability analysis of the coherent interphase boundaries in strained heterostructures. Taking into account the fact that, under certain conditions, the atomic arrangement of solid–solid interfaces is thermodynamically unstable, the evolution equation describing the kinetics of the relief formation is obtained. The considered process is controlled by interface diffusion activated by the nonuniform stress field which occurs due to mismatch between materials and the initial interface perturbation. To define the stress distribution along the curved interface, we use constitutive equations of bulk and surface/interface elasticity modeling the interphase domain as a negligibly thin layer adhering to the bulk phases. This allows us to take into account the surface energy variation related to the interface relief evolution and analyze, in addition to other parameters, the effect of elastic constants that characterize the mechanical behavior of the interphase layer. With the use of the first-order approximation of the boundary perturbation method, the solution of the linearized evolution equation leads to an estimation of equilibrium surface shape.

High-Performance and Industrially Feasible Ni-Rich Layered Cathodematerials By Integrating Coherent Interphase

For developing the industrially feasible Ni-rich layered oxide cathode with extended cycle life, it is necessary to mitigate both the mechanical degradation due to intergranular cracking between primary particles and gasgeneration from the reaction between the electrolyte and residual Li in the cathode. To simultaneously resolve these two issues, we herein propose a simple but novel method to reinforce the primary particles in LiNi0.91Co0.06Mn0.03O2 by providing a Li-reactive material, whose spinel interphase is coherent with the surface of the cathode. The modified structure significantly outperforms analogous bare samples: they conserve more than90% of the initial capacity after 50 cycles and also exhibit a greater rate capability. By tracking the same particle location during cycling, we confirmed that the current method significantly reduces crack generation because theprovided coating material can penetrate inside the grain boundary of the secondary particle and help maintain the volume of the primary particle. Finally, first-principles calculations were implemented to determine the role of this spinel material, i.e., having intrinsically isotropic lattice parameters, superior mechanical properties, and only a small volume change during delithiation. We believe that the proposed method is straightforward and cost-effective; hence, it is directly applicable for the mass production of Ni-rich cathode material to enable its commercialization. Figure 1

Nanoscaffold WO3 by Kinetically Controlled Polymorphism

Several metastable polymorphs, diverse crystal structures with the same chemical formula, could be accessed through epitaxial interfaces with low energy between epitaxial film and symmetry-matched substrates. Here, we fabricated tungsten oxide (WO3) nanoscaffolds composed of hexagonal WO3 (h-WO3)/monoclinic WO3 (m-WO3) polymorphs, and modulated the proportions of these phases by tuning the W arrival rate during epitaxial growth. The WO3 nanoscaffold is vertically aligned with coherent interphase boundaries between h-WO3 and m-WO3; this structure leads to the persistence of h-WO3 despite its metastable characteristic. The coherent interphase boundaries give rise to the unexpected six-fold m-WO3 phases and a distinct lattice response with intercalated hydrogen defects. This WO3 nanoscaffold with different fractions of two polymorphs would be a good model system to systematically study intercalation chemistry and its application with WO3 polymorphs.

High-Performance and Industrially Feasible Ni-Rich Layered Cathode Materials by Integrating Coherent Interphase.

For developing the industrially feasible Ni-rich layered oxide cathode with extended cycle life, it is necessary to mitigate both the mechanical degradation due to intergranular cracking between primary particles and gas generation from the reaction between the electrolyte and residual Li in the cathode. To simultaneously resolve these two issues, we herein propose a simple but novel method to reinforce the primary particles in LiNi0.91Co0.06Mn0.03O2 by providing a Li-reactive material, whose spinel interphase is coherent with the surface of the cathode. The modified structure significantly outperforms analogous bare samples: they conserve more than 90% of the initial capacity after 50 cycles and also exhibit a greater rate capability. By tracking the same particle location during cycling, we confirmed that the current method significantly reduces crack generation because the provided coating material can penetrate inside the grain boundary of the secondary particle and help maintain the volume of the primary particle. Finally, first-principles calculations were implemented to determine the role of this spinel material, i.e., having intrinsically isotropic lattice parameters, superior mechanical properties, and only a small volume change during delithiation. We believe that the proposed method is straightforward and cost-effective; hence, it is directly applicable for the mass production of Ni-rich cathode material to enable its commercialization.

Composition and automated crystal orientation mapping of rapid solidification products in hypoeutectic Al-4 at.%Cu alloys

Rapid solidification can produce metastable phases and unusual microstructure modifications in multi-component alloys during additive manufacturing or laser beam welding. Composition and phase mapping by transmission electron microscopy have been used here to characterize the morphologically distinct zones developing in hypoeutectic Al-4 at.% Cu alloy after pulsed laser melting for different crystal growth rate regimes. Deviations of the compositions of the alloy phases from equilibrium predictions and unique orientation relationships between the solidification transformation products have been determined. Specifically, for the columnar growth zone at solidification rates of 0.8ms−1< v <va=1.8ms−1, two distinct orientation relationships were established between the concomitantly forming non-equilibrium phases, supersaturated α-Al solid solution and the discontinuously distributed α-Al2Cu-based θ′-phase, which can be described as {110}θ∥ {001}α, [001]θ∥ [110]α and {001}θ∥ {001}α, [100]θ∥ [100]α. These orientation relationships permit formation of coherent interphase interfaces with low interfacial free energy. This endows a kinetic advantage to the thermodynamically less stable θ′-Al2Cu phase relative to the more stable equilibrium θ-Al2Cu phase during formation of the morphologically modified eutectic of the columnar growth zone grains, since repeated nucleation is required to establish the discontinuous distribution of θ′-Al2Cu phase.

Effect of Deep Cryogenic Treatment on the Microstructure and Mechanical Properties of HY-TUF Steel

This paper analyzes how a deep cryogenic process changes the microstructure and mechanical properties of a medium-carbon low-alloy steel. The light microscopy, microhardness testing, and transmission electron microscopy reveal that η-carbide and martensite are constituent phases of deep cryogenic treated (DCT) steel, while the microstructure of conventional heat-treated (CHT) steel consists of cementite, martensite, and retained austenite. Transmission electron microscopy also shows that the particles of η-carbide have the shape of ultra-fine globules in DCT martensite. The η-carbides grow in a Hirotsu and Nagakura orientation relationship to the martensitic matrix that enables highly coherent interphase boundaries. The results of the mechanical tests, including tensile and Charpy impact tests, show that the deep cryogenic process can improve toughness in terms of elongation (~12.81%), tensile fracture energy (~266 MPa), and the ductile–brittle transition temperature (~−17.2 °C). The results of fractography are also consistent with the improvement in toughness. It is also found that the strength and macrohardness values are increased. Unlike CHT steel, discontinuous yielding is observed in DCT steel. Moreover, there is no change in Young’s modulus due to the deep cryogenic treatment.

Generalized Ginzburg-Landau functional calculations of the structure and energy of the coherent interphase boundaries between austenite and martensite in iron

The earlier-developed methods and experimental data on the phonon spectra and thermodynamics of fcc and bcc iron are used to construct a generalized Ginzburg-Landau functional for microscopic investigations of the austenite-martensite phase transformations in iron. This functional is used to calculate the structure and properties of a plane austenite-martensite interface for arbitrary orientations of this interface at various temperatures. The transformation parameter profiles in the interface region are calculated. The interface width is found to substantially exceed the interatomic distances. The interfacial surface energy is 500–800 erg/cm2, depends strongly on the orientation, and has a sharp maximum when the interface is parallel to a close-packed plane.

The carbide M 7C 3 in low-temperature-carburized austenitic stainless steel

Prolonged low-temperature gas-phase carburization of AISI 316L-type austenitic stainless steel can cause intragranular precipitation of the carbide M 7C 3 (M: randomly dispersed Fe, Cr, Ni). Transmission electron microscopy revealed that the carbide particles have the shape of needles. They grow by a ledge-migration mechanism and in a crystallographic orientation relationship to the austenite matrix that enables highly coherent interphase interfaces. A small solubility limit of Ni in the carbide and restricted Ni diffusivity at the processing temperature leads to Ni pileup around the particles and may explain the extreme aspect ratio of the particle shape. These characteristics closely resemble what has been observed earlier for precipitates of M 5C 2 under slightly different processing conditions and can be rationalized by considering the particular constraints imposed by carburization at low temperature.

Geometrical and physical models of martensitic transformations in ferrous alloys

The classical theory of the crystallography of martensitic transformations developed in the 1950s is based on the notion that the interface between the parent and product phases is an invariant plane of the shape deformation. Underlying this hypothesis is the expectation that such interfaces do not exhibit long-range strain, and the geometric theory is an algorithm for finding invariant planes, the orientation relationship and transformation displacement. In the context of ferrous alloys, the classical theory has been applied successfully to transformations with {295} habit planes, but is less satisfactory for {575} for example. A new model of martensitic transformations has been presented recently based on dislocation theory, incorporating developments in the understanding of the topological properties of interfacial defects. Topological arguments show that glissile motion of transformation dislocations, or disconnections, can only occur in coherent interphase interfaces. Hence, the interface in the model comprises coherent terraces with a superimposed network of disconnections and crystal dislocations. It is demonstrated explicitly that this defect network accommodates the coherency strains, and that lateral motion of the disconnections across the interface effects transformation in a diffusionless manner. Moreover, it is shown that a broader range of habit planes is predicted on the basis of the semi-coherent interface model than the invariant plane notion. In the case of ferrous alloys, it will be shown that a range of viable solutions arise which include {575}.

Atomic bonding and mechanical properties of Al–Li–Zr alloy

The atomic bonding of Al–Li alloy with minor Zr is calculated according to the “Empirical Electronic Theory in Solids”. The result shows that the stronger interaction between Al and Zr atoms, which leads to form the Al–Zr segregation regions, promotes the precipitation of Al 3Zr particles and produces a remarkable refinement of Al 3Li grains in the alloy. Because there are the strongest covalent Al–Zr bonds in Al 3Zr and Al 3(Zr, Li) particles, these covalent bonds can cause a great resistance for dislocation movement, and is favorable to strengthen the alloy. On the other hand, with precipitating the Al 3(Zr, Li) particles, it causes the coherent interphase boundary energy of Al/Al 3Li to decrease, and atomic bonding is well matched in between the interface of two phases.

Precipitation- and stress-influenced coarsening in Mg-based Mg–Zn–Sn–Y and Mg–Zn–Sn–Sb alloys

Extensive experimental research work has been carried out to investigate precipitation peculiarities in Mg–Zn–Sn-based alloys during aging at different temperatures. This in-depth research was conducted on Mg–4.4wt%Zn–4.0wt%Sn–0.6wt%Y and Mg–4.4wt%Zn–4.4wt%Sn–1.1wt%Sb using x-ray diffraction (XRD), transmission electron microscopy (TEM) including high-resolution TEM, and scanning electron microscopy (SEM) equipped with an energy-dispersive x-ray spectrometer (EDS). It was found that, first, a hexagonal close-packed (hcp)-MgZn2phase nucleates and grows in the form of needles having coherent interphase boundaries with α-Mg matrix. Then the face-centered cubic (fcc)-Mg2Sn-phase nucleates heterogeneously, mainly at the tips of MgZn2needles. A very certain mutual orientation of crystal lattices of MgZn2, Mg2Sn, and α-Mg matrix was revealed. The orientation of Mg2Sn precipitates is perpendicular to that of MgZn2needles. They grow in the form of plates parallel to the basal planes of α-Mg matrix. Two-phase T-like particles are very typical of alloys aged for 1 to 16 days at 175 to 225 °C. The width/length ratio of MgZn2needles inside T-like particles differs substantially from that found in single needles. The elastic/surface energy balance of needles and its influence on the morphology and coarsening behavior has been analyzed.

On the phase separation in nanostructured solid solutions under the effect of a flux of nonequilibrium vacancies

The effect of a directional flux of nonequilibrium vacancies in a nanograin of a binary substitutional solid solution is considered for the case of a limited solubility of the components. On the assumption that the initial state is thermodynamically equilibrium and single-phase, an analytical solution for a two-phase problem is given in the regime of quasi-stationary diffusion. An analysis has been performed for solid solutions based on the more mobile and less mobile components for the cases of coherent and incoherent interphase boundaries. The main features of the phase separation have been established, the fractions of the new phases have been estimated, and the distributions of the components and vacancies in the arising phases have been found. The possibility of decreasing nanograin sizes upon the decomposition of the solid solution with a specific distribution of the precipitated phases is demonstrated.

Precipitation Sequence in Mg-Zn-Sn Based Alloys

AbstractExtensive experimental research work has been carried out in order to develop an Mg-Zn-Sn based creep resistant alloy. It was found that the addition of Y improves the structural stability of this alloy. The main goal of this work was to investigate precipitation sequence in Mg-Zn-Sn-Y alloys during aging at different temperatures. In order to elucidate the sequence of phase formation, XRD, TEM, SEM and EDS analyses were applied. It was found that, firstly, an hcp-MgZn2 phase nucleates homogeneously, and grows in the form of needles having coherent interphase boundaries with Mg-matrix. High-resolution TEM images in combination with Fourier transformed images reveal a topotaxial orientation relationship which is given by {110}MgZn2 &lt;100&gt;MgZn2∥(002)α-Mg&lt;110&gt;α-Mg. The cubic fcc Mg2Sn-phase nucleates heterogeneously, mainly at the tips of MgZn2-needles when the needle's width reaches a certain critical value. The orientation of Mg2Sn-precipitates is usually perpendicular to the one of MgZn2-needles. Two-phase T-like particles are very typical of alloys aged for 1-16 days at 175-225C. The width/length ratio of MgZn2-needles inside T-like particles differs substantially from the one found in single needles. The elastic/surface energy balance of needles and its influence on the morphology and coarsening behaviour has been analyzed.

Application of the cluster/site approximation to the calculation of coherent interphase boundary energies

The cluster/site approximation (CSA) is computationally less demanding than, whilst maintaining comparable accuracy to, the cluster variation method (CVM) in the calculation of configurational entropies in alloys. This computational advantage would seem particularly attractive in calculations involving multicomponent phase equilibria, phase transformation kinetics or in the calculation of coherent interphase boundary (IPB) energies. In this communication, we report some results from IPB energy calculations as a function of temperature for some prototype binary face-centered cubic-based alloy systems. The CSA calculated results are compared with those obtained from the tetrahedron and the tetrahedron–octahedron approximation of the CVM and it is demonstrated that the CSA offers comparable accuracy to the CVM in calculating IPB energies. Moreover, CSA-calculated IPB energies for (Al)/Al 3Li in Al–Li and γ/γ′ in Ni–Al are also in accord with experimental data. We conclude that the CSA offers the possibility of accurately calculating IPB energies for real binary and higher order alloys, i.e., for practical applications.

Coherent interfacial energy and composition profiles in ternary FCC systems by a discrete lattice plane analysis

A discrete lattice plane, nearest neighbor, broken bond model which had been previously used to calculate the energy of coherent interphase boundaries in binary substitutional and ternary substitutional-interstitial systems was extended to ternary substitutional alloys to study the chemical interfacial energy and composition profiles across the interphase boundary in fcc solid solution. Compared with a binary system, the segregation of the third component atoms at the interphase boundary is demonstrated both with symmetric and asymmetrical composition profiles. Accordingly the interfacial energy is reduced by the presence of the third component. Increasing temperature has an effect of decreasing the gradient of composition profiles across the interface, as well as absolute value and anisotropy of the interfacial energy in ternary system.

Evolution of Coherent Interphase Boundaries during Annealing of Multilayers: A Monte Carlo Study

Some properties of nanoscale multilayers, as for example the giant magnetoresistance, are expected to depend sensitively on the structure of the interphase boundaries. Recent experimental work aimed to elucidate the effect of annealing on the multilayer structure and properties. In the present study, thermally induced changes of coherent phase boundaries in multilayers with largely immiscible components were investigated by means of the Monte Carlo method based on a vacancy migration algorithm. Two limiting cases of the as-deposited state were considered: chemically sharp, ideally planar phase boundaries as well as strongly mixed, diffuse interfaces. Initially, a rapid demixing and sharpening of concentration profiles were observed. The morphological roughness of chemically nearly sharp interfaces was found to increase in the course of annealing at higher temperatures. The analysis of the evolving phase boundary topography suggests that the observed instability of thin layers of a few monolayers thickness is due to the development of a long-wavelength roughness. The Monte Carlo studies are compared with predictions of an analytical theory on surface roughening.

Thermoelastic Martensitic Transformation and Shape Memory Effect in Sr2(Si, Ge)O4.

Crystals of Sr2(Si1-xGex)O4 with 0≤x≤1 were prepared and examined by powder XRD, optical microscopy, and AFM. The crystals with 0.3≤x≤0.6 were composed of both the α′ (orthorhombic)- and twinned β (monoclinic)-phases. On the basis of the lattice correspondence between the two phases and their cell parameters, the phenomenological crystallographic theory has been applied to determine the habit planes and shape deformations upon α′-to-β martensitic transformation. The habit planes, which define the coherent interphase boundaries between α′ and β, were nearly parallel to either (100) or (001). Because the transformation was accompanied by a small volumetric shrinkage of -0.3%, the parent α′-phase would elastically accommodate the strains upon the polysynthetic twin formation of the β-phase. At 293K, the crystals with x=0.6 were composed of -92.5mass%α′ and -7.5mass%β. During further cooling in liquid nitrogen, the α′-to-β transformation proceeded which increases the phase composition of β up to -56.3%. The transformation is accompanied by the formation of plate-like surface reliefs. The surface relief angles have been determined from both observations (7.4±0.2°) and calculations based on a phenomenological analysis (7.55°). The fair agreement of these values indicates that the transformation is martensitic and mainly governed by a shear mechanism. The shape memory effect has been demonstrated by the reproducibility of the surface reliefs. The coherency at the interface boundaries between the α′ and β-phases as well as the effective strain accommodation substantially account for the thermoelasticity of the solid solutions.

Thermodynamic properties of coherent interfaces in f.c.c.-based Ag–Al alloys: a first-principles study

The thermodynamic properties of coherent interphase boundaries (IPBs) between the Al-rich-matrix and Guinier–Preston-zone (GP-zone) precipitate phases in Ag–Al are studied from first principles. The cluster-variation-method (CVM), with effective-cluster-interaction (ECI) parameters derived from the results of ab initio total energy calculations, is used to compute the interfacial free energies ( γ) and composition profiles of flat {111} and {100} IPBs as a function of temperature ( T). The calculated values of γ increase monotonically from zero to 35 (37) mJ/m 2 for {111} ({100}) IPBs as T is lowered from the critical temperature (calculated to be 760 K) to 450 K. Monte-Carlo simulations, based on the same set of ECIs used in the CVM work, have been performed to compute GP-zone morphologies at 450 K. Simulated precipitate shapes are found to be anisotropic, consistent with experimental observations. The CVM is used also to compute the gradient coefficient ( κ) in the Cahn–Hilliard coarse-grained free energy. Calculated values of κ are found to display non-negligible concentration and temperature dependencies, in contrast to the predictions of regular-solution theory.