Phase Transformation Boundary Research Articles

Identifying Rare Events in Quantum Molecular Dynamics of Nanomaterials with Outlier Detection Indices.

Nanoscale and condensed matter systems evolve on multiple length- and time-scales, and rare events such as local phase transformation, ion segregation, defect migration, interface reconstruction, and grain boundary sliding can have a profound influence on material properties. We demonstrate how outlier detection indices can be used to identify rare events in machine-learning based, high-dimensional molecular dynamics (MD) simulations. Designed to order data-points from typical to untypical, the indices enable one to capture atomic events that are hard to detect otherwise. We demonstrate the approach with a nanosecond MD simulation of a grain boundary in a metal halide perovskite that is extensively studied for solar energy and optoelectronic applications. The method captures the initial grain boundary sliding and a spontaneous fluctuation half a nanosecond later, both events giving rise to persistent deep electronic trap states that impact charge carrier lifetime and transport and material performance. The approach offers a generalizable and simple method for identifying outlier events in complex condensed matter, molecular, and nanoscale systems.

Heterogeneous‐Structured Refractory High‐Entropy Alloys: A Review of State‐of‐the‐Art Developments and Trends

AbstractRefractory high‐entropy alloys (RHEAs) inspire the development of novel high‐temperature structural materials due to their outstanding resistance to softening and phase stability at elevated temperatures. However, they struggle to simultaneously achieve high‐temperature strength and room‐temperature ductility, while exhibiting insufficient room‐temperature strain hardening capability. Heterogeneous structure strengthening possesses a unique plastic self‐coordinated ability, which can effectively maintain strain hardening rate to achieve an excellent combination of strength and ductility. Benefiting from slow atomic diffusion, severe lattice distortion, and broad compositional design space, RHEAs with heterogeneous structures can be prepared from both chemical composition and interface structure perspectives. Chemical composition heterogeneity primarily focuses on fluctuations of alloying elements at the nanoscale, along with the formation of heterogeneous precipitates and unique lamellar eutectic structures. While, interface structure heterogeneity manifests in the activation of phase transformation and twin boundaries within grains, along with the formation of grains of vastly different sizes. The trend in RHEAs development is toward structural‐functional integration. Heterogeneous structures can also optimize functional properties, such as irradiation resistance, biomedical properties, and high‐temperature softening resistance of RHEAs. Finally, a brief outlook is provided on the future development direction of heterogeneous structure RHEAs.

Grain boundary diffusion and grain boundary phase transition in tungsten in the temperature range of activated sintering

Abstract Grain boundary self- and solute (cobalt) diffusion in tungsten was found [Lee et al., Scr. Metall, 1988; Lee et al., Col. de Physique, 1990] to exhibit discontinuities in the Arrhenius behavior at the homologous temperatures of 0.36 < T/T m < 0.4 that surprisingly match the activation sintering temperature of W (T m is the melting point). In the present work, this unusual grain boundary diffusion phenomenon is discussed in terms of a fundamental grain boundary phase transition in W. The experimental data are analysed with respect to predicted segregation-induced grain boundary phase transformation. Competing co-segregation of impurity elements (carbon and phosphor) might induce a discontinuous grain boundary segregation and invoke a grain boundary phase transition which modifies the grain boundary mobilities of substitutional atoms. The improved understanding of grain boundary phase transitions is expected to provide a breakthrough in interpreting the exact mechanism of W-activated sintering.

Triggering and tracking grain boundary phase transformation at atomic resolution

Triggering and tracking grain boundary phase transformation at atomic resolution

Framework for a chemo‐mechanical model of the breathing effect in lithium–silicon batteries

AbstractWe present a chemo‐mechanical finite element method model for the breathing effect of the silicon anode during lithiation and delithiation. The model is based on the framework for gradient extended standard dissipative solids. It couples diffusion of lithium ions, phase transformation with a moving phase boundary, and the related volume changes, which lead to mechanical stresses in the material. The driving force for diffusion is given by the gradient of the electrochemical potential. Phase transformation and phase boundary movement are described by the Allen–Cahn equation in combination with a constrained double obstacle potential. To shift the constraint from the nodes to the Gauss points, the model makes use of the micromorphic approach.

Назначение рациональных режимов электроэрозионной обработки деталей из титанового сплава ВТ5 на основе решения тепловой задачи о перемещении границы фазового превращения материала

The paper presents results of theoretical studies of the VT5 titanium alloy machinability by the electrodischarge machining method based on solving the thermal problem of the material phase transformation boundary displacement (Stefan problem). It proposes a method for determining parameters of electrical pulses for the VT5 alloy electrodischarge machining in order to increase the process productivity; recommendations are provided for that purpose. Density of the heat flux and its duration were determined necessary for implementation of the VT5 alloy electrodischarge machining process. Dependences were established of the minimum value of the heat flux pulse duration, when the VT5 alloy electrodischarge machining process was possible, and of maximum value of the heat flux pulse duration ensuring maximum removal of the VT5 alloy from the workpiece in one pulse, on the heat flux density. It is shown that for the maximum productivity of the VT5 alloy electrodischarge machining at the heat flux density used, it is necessary to assign effective duration of such a flux. Dependences of the heat flux effective duration on its density were established. Besides, to assign rational modes of VT5 ally electrodischarge machining, relationships were established and provided between the VT5 alloy machinability curves (dependences of the material penetration depth on the pulse duration) and other materials, including those for which rational modes are currently defined

Anomalous effect of nitrogen flow ratio on mechanical properties of NbMoTaW:(N,O) medium entropy alloy films

In this work, nanocrystalline NbMoTaW:(N,O) thin films were prepared by magnetron sputtering. The effect of nitrogen flow ratio [RNN2/(N2 + Ar)] ranging from 0 to 0.5 on the microstructure and mechanical properties of as-deposited films is investigated. It is found that the microstructure and the mechanical properties are both dependent on RN. The structures of NbMoTaW:O films and NbMoTaW:(N,O) films are body-centered cubic and face-centered cubic, respectively. With increasing RN, the hardness of the film reduces first and then increases, showing an anomalous effect of RN on the mechanical properties compared with previous results. The anomalous effect origins from the phase transformation, dislocation density and grain boundary hardening. The surface roughness and residual stress havelittle influence on the mechanical property.

Stress-induced phase transformation and phase boundary sliding in Ti: An atomically resolved in-situ analysis

In-situ tensile experiments on pure Ti were performed in a transmission electron microscope at room temperature. The dynamic process of stress-induced hexagonal closed-packed (hcp) to face-centered cubic (fcc) structural transformation ahead of a crack tip was captured at the atomic level. Intriguingly, a sliding behavior of the ensuing (0001)hcp/(11¯1)fcc phase boundary was observed to further accommodate the plastic deformation until crack initiation. The sliding was accomplished via the successive conservative glide of extended dislocations along the (0001)hcp/(11¯1)fcc phase boundary. A molecular dynamics simulation was carried out to corroborate the experiments and the results confirm the new dislocation-mediated sliding mechanism.

Mathematical model and methods for solving heat-transfer problem during underground coal gasification

Purpose. A mathematical model development for heat transfer during underground coal gasification based on the transcendental equation solution by the Newton-Raphson method. Methods. The heat-transfer model development is based on the research into a temperature field with a variable size of the gasification zone when passing through the phase transformation boundary, which changes abruptly. The research on the coal seam T(x, t) temperature field and the displacement length of the phase transition boundary S(t) is based on the integration of the differential heat-transfer equation with the fulfillment of one-phase Stefan problem conditions. The proportionality factor (β), characterizing the ratio of the displacement length of the “generator gas – coal” phase transition boundary to the time of coal seam gasification, is determined by substituting the Boltzmann equation and using the Newton-Raphson method based on solving the obtained transcendental equation. Findings. The main problems related to laboratory research on the coal gasification process have been identified. A mathematical model of heat transfer during underground coal gasification for a closed georeactor system has been developed, taking into account the effective change in its active zones. Originality. A mathematical model of heat transfer during underground coal gasification at the phase transition boundary has been developed, under which the one-phase Stefan problem conditions are fulfilled. Dependences of the change in the underground gas generator temperature, taking into account the change in the active zones of chemical reactions along the length of the combustion face and the gasification column, have been revealed. In addition, the dependences of the change in the phase transition boundary of a “generator gas – coal” heterogeneous system have been determined, which characterize the displacement length of the phase transition boundary on time and reveal the relationship between the thermal conductivity coefficient, specific heat capacity, as well as bulk density of coal and its calorific value. Practical implications. A method has been developed to determine the displacement length of the phase transition boundary of a “generator gas – coal” heterogeneous system and its relationship between the time and temperature of gasification process. This makes it possible to predict in the future the change in the active zones of the underground gas generator along the length of the gasification column.

Magnetic properties evolution with grain boundary phase transformation and their growth in Nd-Fe-Cu-Ga-B sintered magnet during post-sinter annealing process

The formation of Nd6Fe13Ga phase promoted the continuity and non-ferromagnetism of grain boundary (GB) phases of Nd-Fe-Cu-Ga-B magnets during proper post-sintering annealing process. The coercivity increased dramatically with a decrease of remanence. After further annealing at the sintering temperature, the remanence recovered with a drop of coercivity. It indicated that Nd2Fe14B phase decomposed with the formation of Nd6Fe13Ga phase. Overextending the annealing time led to the gradual decrease of coercivity as the growth of Nd6Fe13Ga phase weakened the continuity of GB phases. Increasing the annealing temperature to the optimum temperature for Nd6Fe13Ga phase can aggravate the decrease of coercivity and remanence. Owing to the intensifying segregation of Ga and Fe in GB phases with the formation of Nd6Fe13Ga phase by dehydrogenation (HD) at 560 °C, high coercivity can be obtained by annealing at a lower temperature or for shorter time compared with HD at 480 °C. The enrichment of Cu in Nd-rich phase ensured its wettability and non-ferromagnetism when the diffusion of Fe and Ga became more local with the formation of Nd6Fe13Ga phase. Therefore, the combined effect of Cu doping and reasonable high temperature HD was conducive to obtain high coercivity in a larger range of annealing time and temperature for Nd-Fe-Ga-B magnet, which was a potential method for its mass production.

Segregation-assisted phase transformation and anti-phase boundary formation during creep of a γ′-strengthened Co-based superalloy at high temperatures

In this study, the creep defects and segregation-assisted local phase transformation processes in a γ′-strengthened Co-based superalloy crept at 982 °C/248 MPa-1% and 1000 °C/137 MPa-1% were analyzed. A non-coplanar deformation configuration, i.e. a repetition of superlattice intrinsic stacking faults and antiphase boundaries (…SISF-APB…), was discovered in a Ni-free Co-based alloy, and found to be assisted by the local elemental segregation and favored more at higher stress conditions. A SISF with a/6<11¯2> type displacement in the (1¯11) plane and APB with a/2<110> type displacement nucleating in the (11¯1) plane were observed to be connected. The γ former segregation-assisted local γ′→γ phase transformation process near LPD was estimated to evolve in the order: γ′→metastable γ→γ+γ′1. The formation of metastable γ generates a new γ/γ′ interface with misfit strain/stress, which is responsible for the nucleation of dislocations, accompanied by the creep stress, thus generating APBs in the γ′ phases. The separation of γ and γ′1 seems to occur by a directional redistribution of γ and γ′ formers along the near-(1¯11) plane and toward the SISF side. The W segregation-assisted γ′→χ phase transformation along the planar fault is estimated to decrease the deformation resistance by forming SISFs with single-layer a/6<112> type displacement, while the Co segregation-assisted γ′→γ phase transformation at the LPD is believed to improve the deformation resistance by trapping the moving dislocation at the local γ/γ′ interface.

Grain boundary phase transformation in a CrCoNi complex concentrated alloy

The phase-transformation-like behavior of grain boundaries, where their chemistry, structure and properties change discontinuously, is emerging as a fundamental interest in the context of grain boundary engineering. The cases studied so far pertain primarily to elemental metals and dilute alloy systems. The next step of complexity would be a system still of a single phase structure but involving multiple and concentrated elements. The recently emerging high/medium entropy alloys (H/MEAs), alternatively known as complex concentration alloys (CCAs), fit the bill in this regard. Here we use CoCrNi as a model CCA to highlight that intricate interatomic interactions influence the grain boundary element distribution and consequently phase transitions. By combining classical molecular dynamics simulations and first-principles density functional theory calculations, we reveal that grain boundary phase transition temperature is sensitive to the grain boundary atomic configuration. The CCA grain boundary with random atomic distribution is more apt to transform due to a larger thermodynamic driving force and faster diffusion kinetics, compared to a hypothetical reference that bears the same bulk properties but no distinguishable constituent components. In contrast, when the three elements redistribute with local Ni-clustering and Co–Cr ordering in the grain boundary structure, diffusion is rendered more sluggish, delaying grain boundary phase transformation. Our work is a step forward in understanding critical phenomena in grain boundaries and CCAs, and enriches the knowledge base for materials design via grain boundary engineering.

Topology-transport correspondence in silicene under the modulation of electric and antiferromagnetic fields

The low-buckled material silicene undergoes abundant topological phase transitions under external fields. The applications in transport related to the topological properties remain the focus of attention. Utilizing the Berry curvature formula and Boltzmann transport equation, we investigate the spin- and valley-dependent anomalous Nernst effect in silicene, which is subjected to the antiferromagnetic field and perpendicular electric field. The results reveal that the antiferromagnetic field can be used to modulate the pure spin Nernst current; the pure valley Nernst current appears only when electric field exists; the charge Nernst current needs both fields. It is found that the inflection points on the Nernst conductivity correspond exactly to the phase transformation boundary. In the quantum spin Hall (QSH) phase, the spin beam splitting can be realized, in the quantum valley Hall (QVH) phase, the valley beam is split, and in the spin-polarized quantum anomalous Hall (SQAH) phase, the single spin and valley beam can be collected. These findings related to topology are expected to facilitate some applications in spin and valley caloritronics and thermal logical devices.

Modeling of Heat and Mass Transfer in Heat-Shielding Composite Materials Based on the Universal Law of Binder Decomposition

A physical and mathematical model of heat and mass transfer under phase transformations during the high-temperature, aerodynamic heating of high-speed aircraft is constructed in this work based on the identified law of the decomposition of binders of most heat-shielding composite materials. The mathematical model includes a description of the occurrence and advancement of the binder-decomposition zone (pyrolysis) limited by two moving boundaries of phase transformations, the heat transfer and filtration of pyrolysis gases in the porous coke residue, their injection into the gasdynamic boundary layer, and the distribution of the temperature and density of the composite material in the zone pyrolysis. The mathematical model and analytical method for the solution of the nonlinear heat and mass transfer problem allow the Stefan-type problem to be reduced to the solution of the transcendental equation with respect to the mass velocity of the pyrolysis zone. The results were obtained for temperature fields with allowance for the filtration in the porous residue and the pressure distribution of pyrolysis gases and for temperature fields in the pyrolysis zone that were not affected by binder decomposition, as well as the distribution of pyrolysis gas density in the pyrolysis zone.

Calculation of heat transfer during the manufacture of railway wheels of increased hardness

The article is devoted to the study of temperature conditions in the tread of a railway wheel during the production of wheels with increased hardness of the rolling surface. It is shown that obtaining a metal structure on the wheel surface without martensite and with hardness of 360-390 HB required by technical conditions at a depth of 30 mm from the rolling surface is difficult to realize by varying technological parameters: the number of sprayers, changing the water supply to the rolling surface due to the switch on/off the sprayers. To achieve the highest cooling speeds in the mass of the wheel tread, including at a depth of 30 mm, it is necessary to use the largest possible rolling surface in the heat removal process. It is advisable to remove heat from the rolling surface according to a predetermined dependence of the specific heat flux on time. It was established by experimental studies that the required hardness at a depth of 30 mm was achieved with an increased carbon content in steel and with an increase in the temperature of heating of the wheel before cooling with water spayers of more than 900 °C. To optimize the cooling process, it is advisable to carry out additional theoretical studies with more correctly specified boundary conditions of heat transfer on the tread surface and with refinement of the boundaries of phase transformations. It is advisable to carry out experimental studies on cooling the wheel surface with the possibility of varying the heat removal when using a gas-liquid flow as a cooling agent.

Observations of grain-boundary phase transformations in an elemental metal.

While the theory of grain boundary (GB) structure has a long history1 and the proposition that grain boundaries can undergo phase transformations was made already 50 years ago2,3, the search for these GB transitions has been in vain. The underlying assumption was that multiple stable and metastable states exist for different grain boundary orientations4,5,6. The terminology complexion was recently proposed to distinguish between those interfacial states that differ in any equilibrium thermodynamic property7. Different types of complexions and transitions between them have been characterized mostly in binary or multicomponent systems 8,9,10,11,12,13,14,15,16,17,18,19. Novel simulation schemes have provided a new perspective on the phase behavior of interfaces and showed that grain boundary transitions can occur in a multitude of material systems20,21,22,23,24. However, their direct experimental observation and transformation kinetics in an elemental metal remained elusive.Here, we show atomic scale grain boundary phase coexistence and transformations at symmetric and asymmetric [11-1] tilt grain boundaries in elemental copper (Cu). We found the coexistence of two different grain boundary structures at Σ19b grain boundaries by atomic resolution imaging. Evolutionary grain boundary structure search and clustering analysis21,25,26 finds the same structures and predicts the properties of these GB phases. Furthermore, finite temperature molecular dynamics simulations explore their coexistence and transformation kinetics. Our results demonstrate how grain boundary phases can be kinetically trapped enabling atomic scale room temperature observations.Our work paves the way for atomic scale in situ studies of grain boundary phase transformations in metallic grain boundaries. In the past, only indirect measurements have indicated the existence of interfacial transitions9,15,27,28,29. Now, their atomic scale role on the influence of abnormal grain growth, non-Arrhenius type diffusion or liquid metal embrittlement can be explored.

Solute drag and dynamic phase transformations in moving grain boundaries

A discrete model and the regular solution approximation are applied to describe the effect of grain boundary motion on grain boundary phase transformations in a binary alloy. The model predicts all thermodynamic properties of the grain boundary and the solute drag force, and permits a broad exploration of the parameter space and different dynamic regimes. The grain boundary phases continue to exist in the moving grain boundary and show a dynamic hysteresis loop, a dynamic critical line, and other features that are similar to those for equilibrium phases. Grain boundary motion strongly affects the relative stability of the phases and can even stabilize phases that are absolutely unstable under equilibrium conditions. Grain boundary phase transformations are accompanied by drastic changes in the boundary mobility. The results are analyzed in the context of non-equilibrium thermodynamics. Unresolved problems and future work are discussed.

In Situ Measurement of Phase Boundary Kinetics during Initial Lithiation of Crystalline Silicon through Picosecond Ultrasonics

Studying the kinetics of phase transformation and phase boundary propagation during initial lithiation of silicon electrodes in lithium ion batteries is relevant to understanding their performance. Such studies are usually challenging due to the difficulties in measuring the phase boundary velocity in the interior of the sample. Here we introduce a non-invasive, in situ method to measure the progression of the phase boundary in a planar specimen geometry while maintaining well-controlled lithium flux and potential. We developed an apparatus integrating an electrochemical cell with picosecond ultrasonics to probe the propagating phase boundary in real time. Phase propagation during initial lithiation of crystalline silicon, which is an example of a high capacity anode, is investigated. The primary objective of this manuscript is to report on the experimental technique development and some preliminary results. For lithiation normal to the (100) plane, we observe the phase boundary velocity to be approximately 12 pm/s and x to be 3.73 in LixSi under galvanostatic lithiation with a current density of 40 μA/cm2. The growth rate of the lithiated phase and the reaction rate coefficient are examined using a Deal-Grove type model.

Solute Drag and Dynamic Phase Transformations in Moving Grain Boundaries

A discrete model and the regular solution approximation are applied to describe the effect of grain boundary motion on grain boundary phase transformations in a binary alloy. The model predicts all thermodynamic properties of the grain boundary and the solute drag force, and permits a broad exploration of the parameter space and different dynamic regimes. The grain boundary phases continue to exist in the moving grain boundary and show a dynamic hysteresis loop, a dynamic critical line, and other features that are similar to those for equilibrium phases. Grain boundary motion strongly affects the relative stability of the phases and can even stabilize phases that are absolutely unstable under equilibrium conditions. Grain boundary phase transformations are accompanied by drastic changes in the boundary mobility. The results are analyzed in the context of non-equilibrium thermodynamics. Unresolved problems and future work are discussed.

Phase Relations in the Model System SiO2–MgO–Cr2O3: Evidence from the Results of Experiments in Petrologically Significant Sections at 12–24 GPa and 1600°C

The new results of an experimental study of the majorite MgSiO3–magnesiochromite MgCr2O4 model section are discussed, and general topology of the SiO2–MgO–Cr2O3 system is analyzed. Despite the absence of some petrogenic components (CaO, FeO, Al2O3, Na2O, K2O, and others) in this system, our study, performed in wide pressure range (10–24 GPa), allows us to consider all of the most important phase transformations (in this case, magnesium silicates and oxides) in the upper mantle, transition zone, and uppermost lower mantle. Addition of Cr shows the influence of a minor element on the phase transition parameters. New data on the solubility of Cr in deep minerals (garnet, olivine, wadsleyite, ringwoodite, and bridgmanite) were obtained, which allowed us to determine the influence of Cr on the structural patterns of the major mantle phases. It is shown that addition of 1 wt % Cr2O3 shifts the boundaries of phase transformations by 50 km (olivine/wadsleyite) and 10 km (wadsleyite/ringwoodite) to a lower-pressure domain in comparison with Cr-free systems. In a first approximation, the results of experimental study of phase relations in pseudobinary sections of the SiO2–MgO–Cr2O3 system simulate the phase composition of the restitic part of the upper mantle, transition zone, and uppermost lower mantle under partial melting conditions. It is shown that the Cr concentration in mantle phases is significantly controlled by the Cr/Al ratio in the protolith.