Reconstructive Phase Transformation Research Articles

Layer-by-layer phase transformation in Ti3O5 revealed by machine-learning molecular dynamics simulations

Reconstructive phase transitions involving breaking and reconstruction of primary chemical bonds are ubiquitous and important for many technological applications. In contrast to displacive phase transitions, the dynamics of reconstructive phase transitions are usually slow due to the large energy barrier. Nevertheless, the reconstructive phase transformation from β- to λ-Ti3O5 exhibits an ultrafast and reversible behavior. Despite extensive studies, the underlying microscopic mechanism remains unclear. Here, we discover a kinetically favorable in-plane nucleated layer-by-layer transformation mechanism through metadynamics and large-scale molecular dynamics simulations. This is enabled by developing an efficient machine learning potential with near first-principles accuracy through an on-the-fly active learning method and an advanced sampling technique. Our results reveal that the β−λ phase transformation initiates with the formation of two-dimensional nuclei in the ab-plane and then proceeds layer-by-layer through a multistep barrier-lowering kinetic process via intermediate metastable phases. Our work not only provides important insight into the ultrafast and reversible nature of the β−λ transition, but also presents useful strategies and methods for tackling other complex structural phase transitions.

Mechanical and thermal properties of Si3N4 ceramics prepared by gelcasting using high-solid-loading slurries

In this study, according to the Dinger model, aqueous Si3N4 slurries with a solid loading of 63 vol% and a viscosity lower than 1.0 Pa s were successfully prepared using two-grade Si3N4 powders with D50 = 7.2 μm and 9.4 μm as starting materials. These resulting slurries exhibited good rheological properties and suspension stability. Notably, the presence of β-Si3N4 seeds existed in the starting materials effectively reduced the nucleation barrier by transforming homogeneous nucleation into heterogeneous nucleation. Consequently, it facilitated the reconstructive phase transformation of α-Si3N4 to β-Si3N4, promoted the anisotropic growth of β-Si3N4 nucleation, and resulted in the final formation of the bimodal microstructure. This unique microstructure significantly enhanced the mechanical properties of Si3N4 ceramics, resulting in a high flexural strength of 1011.4 MPa and a fracture toughness of 14.8 MPa m1/2. These excellent properties were achieved through the combination of gelcasting and gas pressure sintering. Simultaneously, the Si3N4 ceramics exhibited a low thermal conductivity of 47.7 W m−1 K−1, mainly owing to the low density of 95.3 %, the presence of low-thermal-conductivity silicate-based noncrystalline substances, and the possible existence of lattice oxygen in Si3N4 crystals.

K Improves Cu(In,Ga)Se2 Surface Band Alignment through Reconstruction

K, Rb, and Cs improve Cu(In,Ga)Se2 (CIGS) solar cell performance, but the mechanism remains unclear. Here we use air-free transfer of multiple samples to study KF post-deposition treatments (PDTs) by X-ray photoelectron spectroscopy. The KF PDTs do not change the majority carrier concentration or Cd in-diffusion, but they boost efficiency by 6.1% absolute, improve minority carrier lifetime, and shift the surface valence band further from the Fermi energy. Unlike former reports, the valence-band shift is not a result of lower Cu/(Ga+In) or higher K/(K+Cu) composition ratios. We propose that instead, KF PDTs alter the surface valence-band alignment through a reconstructive phase transformation from chalcopyrite CIGS to K2CuIn3Se6 or KInSe2, which have layered structures. These compounds can leave Cu-free cation planes after K is rinsed away, unlike the structure of chalcopyrite CIGS or CuIn3Se5.

Microstructural evolution of a niobium-microalloyed steel during hot shear deformation and subsequent cooling

For the purpose of obtaining a niobium-microalloyed steel with a preferable ultrafine-grained ferrite microstructure, a thermomechanical controlled processing (TMCP) strategy that included a hot shear deformation conducted near the local phase transformation temperature and a subsequent cooling process is proposed. As a large plastic strain was enforced by the simple hot shear deformation, severe plastic deformation (SPD) of the niobium-microalloyed steel was realized. The proposed TMCP strategy was simulated physically on a Thermecmastor-Z compression machine combined with a multi-type cooling system. Various cooling rates of different cooling methods resulted in reconstructive and displacive phase transformations from γ-Fe to α-Fe and led to different microstructural morphologies. In addition to phase transformations, the precipitation of carbides, grain growth, plastic deformation, discontinuous dynamic recrystallization (dDRX) of retained austenite grains, and continuous dynamic recrystallization (cDRX) of ferrite grains occurred during hot shear and subsequent cooling. The effects of the strain rate and forming temperature on the microstructural and textural evolution of niobium-microalloyed steel during hot shear and subsequent cooling were investigated and discussed. A niobium-microalloyed steel with a homogenous ultrafine-grained ferrite microstructure and intense γ-fiber texture was fabricated when the forming temperature, strain rate of hot shear deformation, and cooling rate of subsequent mist cooling were 1073 K, 20 s−1, and 10 K·s-1, respectively.

Recrystallization in commercial grade interstitial-free steel, discussing criticality of martensite and massive ferrite nucleation along with mechanical property

Interstitial-free steel is very soft and ductile, with a fully ferritic microstructure at room temperature. In order to address this issue in the present work, the cold-rolling increases tensile strength vividly by strain hardening. The heat-treatment thereafter optimizes strength-ductility, with an excellent combination, through fine recrystallized grains. The warm-rolled sample provides relatively an inferior property by partial recrystallization of the sample. Irrespective of unannihilated dislocations, both the samples after the recrystallization treatment in the austenitic domain at 925 °C, were water quenched in a laboratory scale to explore phase transformations additionally in the system. The microstructural characterization reveals that the nucleation of massive ferrite under rapid cooling requires austenite conditioning, similar to martensite in the alloy, constrained by strained lattice. The outcome fundamentally argues against the notion that the accumulation of crystal defects/dislocations promotes an extra driving force for a reconstructive phase transformation leading to massive ferrite.

Effect of Copper (II) Oxide on the Microstructure and Phase Transformations of Bismuth Orthoniobate

The microstructure and phase transformations of copper atom doped ceramic BiNbO4 synthesized at 950 and 1100°C were studied. According to XPA the samples in compact, pressed form crystallize in the α-BiNbO4 structure irrespective of the synthesis temperature and in spite of the phase transformation α → β at 1040°C. The substances BiNb1–xO4–δ∙xCuO (x ≤ 0.04) are graphite-colored composites with visually expressed grain microstructure. Thermograms of the samples show near 900°C an endo effect associated with the decomposition of the copper (II) oxide and thermal effects due to reconstructive phase transformations of the type α → γ → β in BiNbO4. It was determined that copper oxide impurity in bismuth orthoniobate ceramic acts as a heat sink, which increases the temperature of the phase transition α → γ on heating of compact samples.

Influence of Interfacial Stress on Microstructural Evolution in NiAl Alloys

A phase-field model for the phase transition between austenite and martensite and twinning between two martensitic variants is presented from our previous theory [1] with the main focus on the influence of interfacial stress that is consistent with the sharp interface limit. Each variant-variant transformation can be represented by only one order parameter. Thus, it allows us to get the analytical solution of interface energy and width. Coupled phase-field and elasticity equations are solved for cubic-to-tetragonal phase transformation in NiAl shape memory alloy. The effects of interfacial stress are studied for martensite-martensite interfaces in detail, which was absent in [1]. Additionally, stress and temperature-induced growth of the martensitic phase inside austenite and twining are simulated. Some of the nontrivial experimentally observed microstructures reproduced in the simulations [1] are analyzed in detail. It includes tip splitting and bending, and twins crossing. This theory can be extended for electric, reconstructive, and magnetic phase transformations.

Effects of interfacial stress in phase field approach for martensitic phase transformation in NiAl shape memory alloys

A phase field approach for phase transition between austenite to martensitic variants and twinning between martensitic variants is presented with the main focus on the effects of interfacial stress on phase transformations. In this theory, each variant-variant phase transformations and twinnings within each martensitic variant can be represented by only one-order parameter. Thus, it allows us to get the analytical solution of the martensite-martensite interface profile, energy, and width. Moreover, this model allows us to include interface stress which is consistent with the sharp interface limit. The finite element method is utilized to solve the coupled phase field and elasticity equations for a cubic to tetragonal phase transformation in NiAl shape memory alloy. The stress fields are obtained and the effects of interfacial stress on the stress field at the interfaces are studied for both austenite–martensite and martensite–martensite interfaces in detail. Additionally, the temperature-induced growth of the martensitic phase inside austenite, martensitic phase transformation, and twining are solved. The evolution of the microstructure and stress fields are obtained and the effects of interfacial stress on the morphological evolution of martensitic nanostructures are examined. It is found that the interfacial stress is an important factor, influencing the stress distribution at interfaces and the phase field solution significantly. This theory can be extended for electric, reconstructive, and magnetic phase transformations.

Temperature-dependent structural evolution and stability of divalent anion bearing alumosilicate cancrinites

Abstract A comprehensive study of XO42--cancrinites with X = Mn, Cr, Se, Mo and W was carried out, focusing on the effect of incorporated anions on the framework structure. At room temperature an anisotropic unit cell expansion with increasing anion size was observed. The size of the incorporated anions has no influence on the framework angles or the channel radii. In temperature-dependent (TD) studies four different regimes could be identified: first a quasi-linear expansion of the cancrinites containing water molecules in the e -cages; second, the dehydration process; third the quasi-linear expansion regime of the dehydrated phase to a maximum expanded structure, followed by a phase transformation or decomposition (fourth). The temperature at which the maximum expanded structure is reached, decreases with increasing size of the incorporated anion. Further, it is shown, that thermally stable anions can promote a reconstructive phase transformation to the corresponding cubic nosean-type structure. The absence of an amorphous intermediate state as well as the complete ordering of the template ions indicate a fast transformation mechanism based on the translation of six-ring layers from a hexagonal ABAB to a cubic ABCABC staking.

High-pressure phase transitions of TiN: an ab initio constant pressure study

An ab initio constant pressure molecular dynamics technique is carried out to explore the behaviour of rock salt-structured titanium nitride (TiN) under pressure. Two successive phase transformations are successfully observed in the dynamical simulations. The first one is an isostructural phase transition accompanied by an anomalous volume compression without any symmetry breaking. The second one is a reconstructive phase transformation into a CsCl-type structure. For the first time, the previously proposed two-phase transformations for TiN are confirmed through the simulations.

Following a Photoinduced Reconstructive Phase Transformation and its Influence on the Crystal Integrity: Powder Diffraction and Theoretical Study

In the course of solid-state photoreactions, a single crystal (SC) of the reactant can be transformed into an SC of the product or it can lose crystallinity and become amorphous. In-between these two scenarios exist the reconstructive phase transformations, where upon irradiation, the reactant SC becomes a powder or an SC with increased mosaicity. We present a detailed description of reconstructive photodimerization, where the structural changes are directly correlated with the disintegration process. The kinetics of the reaction is explained by two kinetic regimes, forming an autocatalytic autoinhibition photoreaction set with high quantum yield. In addition, the photoreaction pathways were studied theoretically.

Following a Photoinduced Reconstructive Phase Transformation and its Influence on the Crystal Integrity: Powder Diffraction and Theoretical Study

AbstractIn the course of solid‐state photoreactions, a single crystal (SC) of the reactant can be transformed into an SC of the product or it can lose crystallinity and become amorphous. In‐between these two scenarios exist the reconstructive phase transformations, where upon irradiation, the reactant SC becomes a powder or an SC with increased mosaicity. We present a detailed description of reconstructive photodimerization, where the structural changes are directly correlated with the disintegration process. The kinetics of the reaction is explained by two kinetic regimes, forming an autocatalytic autoinhibition photoreaction set with high quantum yield. In addition, the photoreaction pathways were studied theoretically.

New Polymorph of InVO4: A High-Pressure Structure with Six-Coordinated Vanadium

A new wolframite-type polymorph of InVO4 is identified under compression near 7 GPa by in situ high-pressure (HP) X-ray diffraction (XRD) and Raman spectroscopic investigations on the stable orthorhombic InVO4. The structural transition is accompanied by a large volume collapse (ΔV/V = -14%) and a drastic increase in bulk modulus (from 69 to 168 GPa). Both techniques also show the existence of a third phase coexisting with the low- and high-pressure phases in a limited pressure range close to the transition pressure. XRD studies revealed a highly anisotropic compression in orthorhombic InVO4. In addition, the compressibility becomes nonlinear in the HP polymorph. The volume collapse in the lattice is related to an increase of the polyhedral coordination around the vanadium atoms. The transformation is not fully reversible. The drastic change in the polyhedral arrangement observed at the transition is indicative of a reconstructive phase transformation. The HP phase here found is the only modification of InVO4 reported to date with 6-fold coordinated vanadium atoms. Finally, Raman frequencies and pressure coefficients in the low- and high-pressure phases of InVO4 are reported.

X-ray and neutron diffraction study of the defect crystal structure of the as-grown nonstoichiometric phase Y0.715Ca0.285F2.715

This work begins a series of papers aimed at studying the defect structure of nonstoichiometric phases R1 − yCayF3 − y with a tysonite-type (LaF3) structure. In the single-crystal structure of Y0.715Ca0.285F2.715 with a tysonite-type small unit cell (sp. gr. P63/mmc, a = 3.9095(2) A, c = 6.9829(2) A; Z = 2; Rw = 2.16%), the displacements of Y3+ cations and F2− anions from 63 symmetry axes were observed for the first time. The X-ray diffraction pattern shows weak satellites insufficient for structural calculations. The LaF3 structure type is stabilized up and down on the temperature scale due to anion vacancies and the symmetrizing effect of Ca2+ cations lying on 63 symmetry axes. At 120°C the fluoride-ion conductivity in the nonstoichiometric phase Y0.715Ca0.285F2.715 is five orders of magnitude higher than that in the stoichiometric phase β-YF3. The transition to a superionic state is caused by a deviation from stoichiometry and is not associated with reconstructive phase transformation.

Displacive Phase Transitions at Large Strains: Phase-Field Theory and Simulations

The Landau potential for multivariant displacive phase transformations (PTs) is derived for the most general case of large rotations, elastic and transformational strains, as well as nonlinear and different elastic properties of phases. The method of repetitive superposition of large strains is extended for PTs and is utilized in the finite-element method approach for solution of corresponding coupled phase-field and mechanical problems. Problems of martensitic variants nucleation and evolution in a nanosize sample, including a sample with two nanovoids, are solved. A similar approach can be applied for twinning, dislocations, and reconstructive PTs.

Nucleation mechanism for reconstructive solid-solid phase transitions via melt mediated nanocluster transformation

A general nucleation mechanism is proposed and justified thermodynamically and kinetically. The authors apply it to the β-δ phase transformation (PT) in the HMX crystal. It explains the observation of a reconstructive PT very close (0.6K) to the phase equilibrium temperature, despite the large volume change and interface energy. Nanosize clusters of β phase dissolve in a liquid and transform into δ phase clusters. The liquid completely removes the elastic energy generated by a large volume change. Cluster to cluster PT also drastically reduces the change in interfacial energy. Suggested kinetics for the β-δ PT is in good agreement with experiments.

Multicenter multidomainB1−B2pressure-induced reconstructive phase transition in potassium fluoride

The reconstructive pressure-induced phase transformation of potassium fluoride from $B1$ to the high-pressure $B2$ structure type is investigated by means of molecular dynamics simulations. Application of the transition path sampling approach allows investigation the phase transition without driving the kinetics by overcritical pressure or artificial forces along a predefined reaction coordinate. This offers a unique perspective for studying the atomistic mechanisms involved in the nucleation and growth of phases. From our simulations, we demonstrate the ability of investigating coexisting nucleation centers, resulting in the formation of local phase domains. Depending on the respective inset of ionic translocations, local domains may be formed in different manners. In this case, phase growth is observed to lead to domain-domain interfaces, which result in grain boundaries once the phase transition is completed.

A Variational Model for Reconstructive Phase Transformations in Crystals, and their Relation to Dislocations and Plasticity

1We study the reconstructive martensitic transformations in crystalline solids (i.e., martensitic transformations in which the parent and product lattices have arithmetic symmetry groups admitting no finite supergroup), the best known example of which is the bcc–fcc transformation in iron. We first describe the maximal Ericksen-Pitteri neighborhoods in the space of lattice metrics, thereby obtaining a quantitative characterization of the “weak” transformations, which occur within these domains. Then, focusing for simplicity on a two-dimensional setting, we construct a class of strain-energy functions admitting large strains in their domain, and which are invariant under the full symmetry group of the lattice. In particular, we exhibit an explicit energy suitable for the square-to-hexagonal reconstructive transformation in planar lattices. We present a numerical scheme based on atomic-scale finite elements and, by means of our constitutive function, we use it to analyze the effects of transformation cycling on a planar crystal. This example illustrates the main phenomena related to the reconstructive martensitic phase changes in crystals: in particular, the formation of dislocations, vacancies and interstitials in the lattice.

Geological and anthropogenic factors influencing mercury speciation in mine wastes: an EXAFS spectroscopy study

The speciation of Hg is a critical determinant of its mobility, reactivity, and potential bioavailability in mine-impacted regions. Furthermore, Hg speciation in these complex natural systems is influenced by a number of physical, geological, and anthropogenic variables. In order to investigate the degree to which several of these variables may affect Hg speciation, extended X-ray absorption fine structure (EXAFS) spectroscopy was used to determine the Hg phases and relative proportions of these phases present in Hg-bearing wastes from selected mine-impacted regions in California and Nevada. The geological origin of Hg ore has a significant effect on Hg speciation in mine wastes. Specifically, samples collected from hot-spring Hg deposits were found to contain soluble Hg-chloride phases, while such phases were largely absent in samples from silica-carbonate Hg deposits; in both deposit types, however, Hg-sulfides in the form of cinnabar (HgS, hex.) and metacinnabar (HgS, cub.) dominate. Calcined wastes in which Hg ore was crushed and roasted in excess of 600 °C, contain high proportions of metacinnabar while the main Hg-containing phase in unroasted waste rock samples from the same mines is cinnabar. The calcining process is thought to promote the reconstructive phase transformation of cinnabar to metacinnabar, which typically occurs at 345 °C. The total Hg concentration in calcines is strongly correlated with particle size, with increases of nearly an order of magnitude in total Hg concentration between the 500–2000 μm and <45 μm size fractions (e.g., from 97–810 mg/kg Hg in calcines from the Sulphur Bank Mine, CA). The proportion of Hg-sulfides present also increased by 8–18% as particle size decreased over the same size range. This finding suggests that insoluble yet soft Hg-sulfides are subject to preferential mechanical weathering and become enriched in the fine-grained fraction, while soluble Hg phases are leached out more readily as particle size decreases. The speciation of Hg in mine wastes is similar to that in distributed sediments located downstream from the same waste piles, indicating that the transport of Hg from mine waste piles does not significantly impact Hg speciation. Hg LIII-EXAFS analysis of samples from Au mining regions, where elemental Hg(0) was introduced to aid in the Au recovery process, identified the presence of Hg-sulfides and schuetteite (Hg3O2SO4), which may have formed as a result of long-term Hg(0) burial in reducing high-sulfide sediments.

Synthesis of Monolithic ZrO2 in a New Polymorph of Orthorhombic Crystal Structure with a Mesoporous Precursor

A high energy amorphous ZrO(OH)2.xH2O precursor (porous) is obtained by a controlled hydrolysis of dispersed Zr4+ cations in an aqueous solution in reaction with NH4OH at 5°C followed by aging and drying of the recovered gel at room temperature. At 20° to 40°C in air, the excess H20 evaporates leaving behind a porous ZrO(OH)2.xH2O powder with φ ∼ 37% porosity. On heating, a reconstructive ZrO2 phase transformation occurs from a refined amorphous state at 500°C to form a new orthorhombic structure in a mesoporous structure. Average crystallite size is D ∼ 15 nm. X-ray diffractogram determines the lattice parameters of a = 0.3340, b = 0.5535 and c = 0.6364 nm, with V0 = 0.1177 nm3 lattice volume or ρ = 6.96 g.cm−3. A high-pressure phase of Pnma o-ZrO2 is known of a similar value of ρ = 6.78 g.cm−3. Other known c-, t- or m-ZrO2 polymorphs of the cubic, tetragonal, or mono-clinic structures have somewhat lower ρ-values of 6.21 g.cm−3or still smaller. The o-ZrO2 irreversibly transforms to m-ZrO2, D = 23 nm, at as early a temperature as 800°C.