Metastable Medium Research Articles

Deformation mechanism of a metastable medium entropy alloy strengthened by the synergy of heterostructure design and cryo-pre-straining

Deformation mechanism of a metastable medium entropy alloy strengthened by the synergy of heterostructure design and cryo-pre-straining

Achieving ultra-high mechanical properties in metastable Co-free medium entropy alloy via hierarchically heterogeneous microstructure

A new metastable dual-phase Fe59Cr13Ni18Al10 medium entropy alloy (MEA) with hierarchically heterogeneous microstructure from micro- to nano-scale was designed in this work. Partially recrystallized FCC phase and lots of NiAl-rich B2 precipitates are obtained by annealing and aging treatment. The yield strength of the MEA at room temperature (298 K) and liquid nitrogen temperature (77 K) increased from ∼910 MPa and ∼1250 MPa in the annealed state, respectively, to ∼1145 MPa and ∼1520 MPa in the aged state, while the uniform elongation maintained more than 15%. The excellent mechanical properties of the MEA both at 298 and 77 K are attributed to the co-activation of multiple strengthening mechanisms, including fine grain, dislocation, precipitation, transformation-induced plasticity, stacking faults, and nano-twins.

Microstructure evolution and mechanical properties of metastable austenitic medium Mn steel fabricated by warm caliber rolling

Warm caliber rolling at 550, 600, and 650 °C were carried out to refine the austenite grains of medium Mn steel by dynamic recrystallization (DRX). The effect of rolling temperature on the microstructure characteristics and mechanical properties of metastable austenitic steel was investigated in detail. The results show that the hierarchical structure of coarse fiber grains (CFGs) and ultrafine grains (UFGs) is obtained by warm caliber rolling method. With the increasing rolling temperature, more obvious DRX behavior occurs, resulting in a high percentage of UFGs. The preactive TRIP effect shortens the yield stress plateau in tensile stress-strain curves caused by CFGs with low stability. Furthermore, the unique hierarchical structure induces crack deflection and delamination, and significantly improves the impact toughness of medium Mn steel at room temperature. However, at the low temperature of −80 °C, CFGs are easy to induce martensite transformation, which seriously deteriorates the impact toughness.

Unusually high corrosion resistance in Mo CrNiCo medium entropy alloy enhanced by acidity in aqueous solution

• A series of ultra-corrosion-resistant medium entropy alloys were developed. • The corrosion resistance of our alloys unusually increases with acidity in Cl − containing solutions. • The unique anti-corrosion behaviour results from the formation of a protective and compact medium entropy oxide film, which can be attributed to the synergic effect of the chemical complexity on absorption of H 2 O and O 2 . High corrosion resistance of alloys is essential for their structural applications; however, most alloys suffer from degradation of their corrosion resistance with the increasing acidity of their surroundings. Nonetheless, we developed a series of medium-entropy alloys (MEAs) in this work, which exhibit high strength, superior fracture toughness and ultra-high corrosion resistance, outperforming the variety of corrosion resistant alloys hitherto reported. Most interestingly, our MEAs exhibit an unusual anti-corrosion behavior and their corrosion resistance increases with acidity in Cl − containing solutions. Through extensive thermodynamic calculations, density functional theory (DFT) simulations and experiments, we reveal that the unusual anti-corrosion behavior of our MEAs can be attributed to their surface chemical complexity, which facilitates the physio-chemical-absorption of H 2 O and O 2 and thus the rapid formation of metastable medium entropy passive films that contain the lowest amount of defects, as compared to the passive films on conventional alloys reported in the literature.

Evolution of a Polydisperse Ensemble of Spherical Particles in a Metastable Medium with Allowance for Heat and Mass Exchange with the Environment

Motivated by a wide range of applications in various fields of physics and materials science, we consider a generalized approach to the evolution of a polydisperse ensemble of spherical particles in metastable media. An integrodifferential system of governing equations, consisting of a kinetic equation for the particle-size distribution function (Fokker–Planck type equation) and a balance equation for the temperature (concentration) of a metastable medium, is formulated. The kinetic equation takes into account fluctuations in the growth/reduction rates of individual particles, the velocity of particles in a spatial direction, the withdrawal of particles of a given size from the metastable medium, and their source/sink term. The heat (mass) balance equation takes into account the growth/reduction of particles in a metastable system as well as heat (mass) exchange with the environment. A generalized system of equations describes various physical and chemical processes of phase transformations, such as the growth and dissolution of crystals, the evaporation of droplets, the boiling of liquids and the combustion of a polydisperse fuel. The ways of analytical solution of the formulated integrodifferential system of equations based on the saddle-point technique and the separation of variables method are considered. The theory can be applied when describing the evolution of an ensemble of particles at the initial and intermediate stages of phase transformation when the distances between the particles are large enough, and interactions between them can be neglected.

Excellent combination of cryogenic strength and ductility of a metastable Fe65Ni15Co8Mn8Ti3Si medium entropy alloy through the exceptional deformation-induced martensitic transformation

Excellent combination of cryogenic strength and ductility of a metastable Fe65Ni15Co8Mn8Ti3Si medium entropy alloy through the exceptional deformation-induced martensitic transformation

Constitutive modeling and finite element analysis of metastable medium entropy alloy

In this study, the constitutive model of metastable ferrous Fe60Co15Ni15Cr10 (at%) medium entropy alloy (FeMEA) was constructed based on deformation-induced martensitic transformation (DIMT) behavior under cryogenic temperature. The crystal structure of the FeMEA gradually transformed from a single face-centered cubic (FCC) phase to a body-centered cubic (BCC) phase during deformation. The critical initiation stress criterion and the volumetric fraction evolution model were employed to describe the DIMT kinetics during uniaxial tensile testing. The flow stress model of the FCC phase as a function of the applied strain was developed using the dislocation-based hardening model, and the BCC phase behavior was evaluated based on the macroscopic behavior of the multiphase material and phase transformation kinetics. The decline in the dislocation mean free path for the FCC phase was associated with the grain refinement of the FCC phase due to the transformed BCC islands. To verify its applicability, the model was compared to the experimental data describing phase transformation kinetics and the mechanical behavior.

Approximate analytical solution of the integro‐differential model of bulk crystallization in a metastable liquid with mass supply (heat dissipation) and crystal withdrawal mechanism

This paper deals with an approximate analytical solution of an integro‐differential model describing nucleation and growth of particles. The model includes a thermal‐mass exchange with the environment and the removal of product crystals from a metastable medium. The method developed for solving model equations (kinetic equation for the particle‐size distribution function and balance equations for temperature/impurity concentration) is based on the saddle point technique for calculating the Laplace‐type integral. We show that the metastability degree decreases with time at a fixed mass (heat) flux. The crystal‐size distribution function is an irregular bell‐shaped curve increasing with the intensification of heat and mass exchange.

Decreasing Zr content to improve tensile properties of non-equiatomic TiZrHfNb medium entropy alloys with transformation-induced plasticity

Compositional tuning of phase stability of the body-centered cubic (BCC) matrix is an effective way to improve mechanical properties of refractory medium/high entropy alloys, which are generally brittle or lack of strain hardening capability. Here, reducing Zr content in Ti(80-x)ZrxHf10Nb10 (at.%) alloys was demonstrated to destabilize the BCC phase and trigger the BCC to hexagonal close-packed (HCP) phase transformation. Two metastable Ti50Zr30Hf10Nb10 and Ti60Zr20Hf10Nb10 medium entropy alloys were designed, which exhibit superior strength-ductility combination and evident strain hardening due to the transformation induced plasticity effect.

Физические основы модели саморазвивающихся процессов и вопросы ее применения для прогнозов землетрясений в Дальневосточном регионе

Seismic activation in the period of foreshocks (prior to the mainshock) described by the model of self-developing processes (SDP) is possibly a manifestation of explosive instability of low frequency straining waves in metastable medium. To highlight so nontrivial relationship of continuous wave motions and discrete seismic events flow is a goal of this narrative. Thus, the rationale of the SDP model (the equation, in reality) has been modified, which is of importance in relevance with the article by the Malyshevs in the current issue (A.I. Malyshev, L.K. Malysheva. Precedent-extrapolation estimate of the seismic hazard in the Sakhalin and South Kurils region) which is to improve the seismic hazard estimates by means of this model. A new way to reveal the very beginning of blow-up regime after quasi-stationary one is proposed.

Becoming and Individuation on the Encounter between Technical Apparatus and Natural System

Becoming and Individuation on the Encounter between Technical Apparatus and Natural System

Metastability alloy design

Abstract

Simulation of Self-sustained Relaxation of a Two-dimensional Metastable Medium by Means of a Traveling Wave

The traveling wave mode of self-sustained relaxation of a metastable medium, such as combustion and annealing of radiation-induced defects in solids, is studied. The mechanism of self-sustained relaxation and propagation of the traveling wave is a positive thermal-concentration feedback and heat transfer. The thermal-concentration feedback develops because the relaxation rate depends strongly on temperature, and the energy of the metastable state transforms into thermal energy in the process of relaxation. Hence, the relaxation leads to an increase in the medium temperature and, consequently, to further support of the relaxation. In this work, traveling waves are considered, and their features are investigated by computer simulation. The shape and speed of the traveling wave are investigated as a function of the medium properties, and the way of its initiation. We examined the shapes of wavefronts as they propagate on a plane, and observed two different regimes of traveling wave propagation. Sometimes the front of the traveling wave spreads as a whole and velocity of propagation is perpendicular to the wavefront. Sometimes small perturbations arise on the wavefront, and then grow by virtue of relaxation along the front, thus, the new area of the relaxed medium has the form of a narrow strip that propagates along the line of the front and maintains a constant width in the direction perpendicular to the front. We observe the appearance of one or more small perturbations, which arise in a symmetric way. The influence of boundaries on the traveling waves was also investigated.

On the theory of nucleation and nonstationary evolution of a polydisperse ensemble of crystals

The process of nucleation and unsteady-state growth of spherical crystals in a supersaturated solution is considered with allowance for the Weber-Volmer-Frenkel-Zel’dovich and Meirs kinetic mechanisms. The first two corrections to the steady-state growth rate of spherical crystals are found analytically as the solution of the moving boundary problem. On the basis of this solution, we formulate and solve the integro-differential model consisting of the Fokker-Planck type equation for the particle-size distribution function and of the balance equation for the system supersaturation. The distribution function dependent on the nucleation kinetics is found as a functional of the supersaturation. The integro-differential equation for the system supersaturation is solved by means of the saddle-point method. As a result, a complete analytical solution of the problem of nucleation and nonstationary evolution of a polydisperse ensemble of crystals in a metastable medium is constructed in a parametric form. How to use the obtained solutions for supercooled liquids is discussed.

The Effect of Ion Irradiation on Nanocrystallization and Surface Relief of a Ribbon from Fe72.5Cu1Nb2Mo1.5Si14B9 Alloy

Using the methods of X-ray diffraction and atomic force microscopy, the process of crystallization of an amorphous Fe72.5Cu1Nb2Mo1.5Si14B9 alloy irradiated with accelerated Ar+ ions is investigated. It is found out that an irradiation by the Ar+ ions with the energy 30 keV at the ion current density 300 μA/cm2 (fluence 3.75·1015 cm–2, irradiation time ~2 s, ion-beam short-duration heating up to 350°С, which is 150°С lower than the thermal crystallization threshold) results in a complete crystallization of this amorphous alloy (throughout the bulk of a 25 μm ribbon) followed by precipitation of solid solution crystals of α-Fe(Si), close in its composition to Fe80Si20, stable phase of Fe3Si, and metastable hexagonal phases. By the methods of atomic force and scanning tunneling microscopy it is shown that nanocrystallization caused by ion irradiation is accompanied by surface relief changes both on the irradiated and unirradiated sides of the Fe72.5Cu1Nb2Mo1.5Si14B9 alloy ribbon at the depth exceeding by a factor of 103 that of the physical ion penetration for this material. The data obtained, taking into account a significant temperature decrease and multiple acceleration of the crystallization process, serve an evidence of the radiation-dynamic influence of accelerated ions on the metastable amorphous medium.

Loess as a collapsible soil: Some basic particle packing aspects

Loess is the most important collapsible soil; possibly the only engineering soil in which real collapse occurs. A real collapse involves a diminution in volume – it would be an open metastable packing being reduced to a more closely packed, more stable structure. Metastability is at the heart of the collapsible soils problem. To envisage and to model the collapse process in a metastable medium, knowledge is required about the nature and shape of the particles, the types of packings they assume (real and ideal), and the nature of the collapse process – a packing transition upon a change to the effective stress in a media of double porosity. Particle packing science has made little progress in geoscience discipline – since the initial packing paradigms set by Graton and Fraser (1935) – nevertheless is relatively well-established in the soft matter physics discipline. The collapse process can be represented by mathematical modelling of packing – including the Monte Carlo simulations – but relating representation to process remains difficult. This paper revisits the problem of sudden packing transition from a micro-physico-mechanical viewpoint (i.e. collapse imetan terms of structure-based effective stress). This cross-disciplinary approach helps in generalization on collapsible soils to be made that suggests loess is the only truly collapsible soil, because it is only loess which is so totally influenced by the packing essence of the formation process.

The stage of nonisothermal nucleation of supercritical particles of a new phase under nonstationary conditions of particle diffusion growth and heat transfer to a medium

An analytical theory has been formulated for the stage of nonisothermal nucleation of supercritical particles in a metastable medium with instantaneously generated initial supersaturation. The theory takes into account the nonuniformities of metastable substance concentration and temperature, which result from the nonstationary diffusion of the substance to growing particles and the nonstationary transfer of the heat of the phase transition from the particles to the medium. The formulated theory extends the approach based on the concept of excluded volume that has recently been used in the theory of the stage of nucleation under isothermal conditions. This approach implies that the nucleation intensity of new particles is suppressed in spherical diffusion regions with certain sizes that surround previously nucleated supercritical particles and remaining unchanged in the rest of the medium. It has been shown that, when self-similar solutions are used for nonstationary equations of substance diffusion to particles and heat transfer from the particles, the ratio between the excluded volume and the particle volume is independent of particle size, thereby enabling one to analytically solve the integral equation for the excluded volume throughout a system as a time function at the stage of nucleation. The main characteristics of the phase transition have been found for the end of the stage of nucleation. Comparison has been carried out with the characteristics obtained in terms of the isothermal and nonisothermal nucleation theory upon uniform vapor consumption and heat dissipation (the mean-field approximation of vapor supersaturation and temperature).

MgH2 thin films deposited by one-step reactive plasma sputtering

The morphology, crystal structure, hydrogen content, and sorption properties of magnesium hydride thin films prepared by reactive plasma-assisted sputter deposition were investigated. Few micrometers-thick films were deposited on Si and SiO2/Si substrates, at low pressure (0.4 Pa) and close to room temperature using (Ar + H2) plasma with H2 fraction in the range 15–70%. The microstructure and hydrogen content of the films are closely related to the surface temperature and hydrogen partial pressure during the deposition process. Operating in pulsed-plasma mode allows the hydrogenation rate of the MgH2 thin film to top up to 98%, thereby producing a nearly fully hydrogenated film in a single-step process. The positive effect of the pulsed process is explained by the significant decrease in the whole energy flux incident on the surface and the favourable impact of the transient process for the rearrangement/relaxation of the materials. As for the hydrogen storage properties, desorption experiments and cycling of the films show the destabilizing effect of Mg2Si formation at the interface between the film and the Si substrate resulting in a drastically increased desorption kinetics compared to less reactive SiO2 substrate. However, the reaction is regrettably not reversible upon hydrogenation and the hydrogen storage capacity is consequently reduced upon cycling. Nevertheless, the deposition process carried out on inert substrates would offer true potential for reversible storage. Finally, our experimental results, which show the possibility to preferentially grow the metastable medium pressure γ-MgH2 phase, open possibilities for the synthesis of more complex metastable phases such as magnesium-based ternary compounds.

Structural evolution from TiO2nanoparticles to nanosheets and their photocatalytic performance in hydrogen generation and environmental pollution removal

The rapid depletion of the fossil fuel reserves, and the increase of green-house gas emissions and other environmental pollutants, bring up an urgent need for the development of clean energy and sustainable environmental solutions. The ability to use sunlight to produce fuels such as H2 from abundant, non-toxic resources, and to decompose environmental pollutants with benign photocatalysts, would revolutionize our civilization. TiO2 has attracted substantial interest due to its activity for generating hydrogen from water under ultraviolet (UV) light irradiation. The discovery of (001) facet control with the fluoride additive in the reactants has triggered intensive studies for model photocatalysis. Here, we found that small TiO2 nanoparticles were formed at the very beginning stage of the reaction, then were transformed into large TiOF2 crystals, and finally turned into (001)-faceted TiO2 nanosheets. TiOF2 acted as a metastable intermediate medium in the transformation from TiO2 nanoparticles into (001)-faceted TiO2 nanosheets. The (001) face showed higher activity for the photocatalytic generation of hydrogen and the photodecomposition of methylene blue and rhodamine B. The synergic effect between TiOF2 and TiO2 nanosheets or nanoparticles further improved the photocatalytic activities. This study provides us with a promising solution for solving the energy and environment problems.

The formation of a condensate film of methane with chlorine at helium temperatures

An amorphous film of a condensate with large excess energy, configuration entropy, and free volume was prepared from the gas phase on a cold substrate at helium temperatures in the form of vacancies and their aggregates. The temperature dependence of the decomposition of this metastable state at various process stages was studied. The medium is active, because an important role in it is played by self-action effects, in particular, during sintering under vibration conditions. Structure-sensitive local sample probing by laser initiation of a photochemical reaction was used. The occurrence of this reaction can be used to draw conclusions about metastable medium activity.