A strategy for designing stable nanocrystalline alloys by thermo-kinetic synergy

Aiming to design stable nanocrystalline(NC)materials,so far,it has been proposed to construct nanostructure stability maps in terms of thermodynamic parameters,while kinetic stabilization has seldom been considered,despite the synergy of thermodynamics and kinetics.Consequently,the thermodynamically stabilized NC materials may be easily subjected to grain growth at high temperatures due to the weakly kinetic stabilization.Starting from the thermo-kinetic synergy,a stabilization criterion is proposed as a function of intrinsic solute parameters(e.g.the activation energy for bulk diffusion and the segregation enthalpy),intrinsic solvent parameters(e.g.the intrinsic activation energy for GB migration and the GB energy)and processing parameters(e.g.the grain size,the temperature and the solute concentration).Using first-principles calculations for a series of combinations between fifty-one substitutional alloying atoms as solute atoms and Fe atom as fixed solvent atom,it is shown that the thermal stability neither simply increases with increasing the segregation enthalpy as expected by thermodynamic stabilization,nor monotonically increases with increasing the activation energy for bulk diffusion as described by kinetic stabilization.By combination of thermodynamic and kinetic contributions,the current stabilization criterion evaluates quantitatively the thermal stability,thus permitting convenient comparisons among NC materials involved by various combinations of the solute atoms,the solvent atoms,or the processing conditions.Validity of this thermo-kinetic stabilization criterion has been tested by current experiment results of Fe-Y alloy and previously published data of Fe-Ni,Fe-Cr,Fe-Zr and Fe-Ag alloys,etc.,which opens a new window for designing NC materials with sufficiently high thermal stability and sufficiently small grain size.

Thermo-kinetic connectivity by integrating thermo-kinetic correlation and generalized stability

Designing structured materials with optimized mechanical properties generally focuses on engineering microstructures,which are closely determined by the processing routes,such as phase transformations(PTs)and plastic deformations(PDs).Both PTs and PDs follow inherent trade-off relation between thermodynamic driving force ΔG and kinetic energy barrier Q,i.e.,so-called thermo-kinetic correlation.By analyzing nucleation and growth and proposing a conception of negative driving force integrating strain energy,interface energy and any kind of energy that equivalently inhibits the PT itself,ΔG^(S),unified expressions for the thermo-kinetic correlation and generalized stability(GS)were derived for three kinds of PTs,i.e.,diffusive PTs with simultaneously decreasedΔG and increased Q,diffusive PTs with simultaneously increasedΔG and decreased Q,and displacive PTs with simultaneously increased ΔG and decreased Q.This leads to so-called thermo-kinetic connectivity by integrating the thermo-kinetic correlation and the GS,where,by application in typical PTs,it was clearly shown,a criterion of high ΔG-high GS can be predicted by modulating chemical driving force,negative driving force and kinetic energy barrier for diffusion or nucleation.Following thermo-kinetic connectivity,analogous procedure for dislocation evolution upon PDs was performed,and materials design in terms of the highΔG-high GS criterion was discussed and prospected.

Application of non-equilibrium dendrite growth model considering thermo-kinetic correlation in twin-roll casting

Upon non-equilibrium solidifications, dendrite growth, generally as precursor of as-solidified structures,has severe effects on subsequent phase transformations. Considering synergy of thermodynamics and kinetics controlling interface migration and following conservation of heat flux in solid temperature field, a more flexible modeling for the dendrite growth is herein developed for multi-component alloys,where, two inherent problems, i.e. correlation between thermodynamics and kinetics(i.e. the thermokinetic correlation), and theoretical connection between dendrite growth model and practical processing,have been successfully solved. Accordingly, both the thermodynamic driving force G and the effective kinetic energy barrier Qeffhave been found to control quantitatively the dendrite growth(i.e. especially the growth velocity, V), as reflected by the thermo-kinetic trade-off. Compared with previous models, it is the thermo-kinetic correlation that guarantees quantitative connection between the practical processing parameters and the current theoretical framework, as well as more reasonable description for kinetic behaviors involved. Applied to the vertical twin-roll casting(VTC), the present model, realizes a good prediction for kissing points, which influences significantly alloy design and processing optimization.This work deduces quantitatively the thermo-kinetic correlation controlling the dendrite growth, and by proposing the parameter-triplets(i.e. G-Qeff-V), further opens a new beginning for connecting solidification theories with industrial applications, such as the VTC.

Nucleation/growth design by thermo-kinetic partition

Departing from nucleation and growth involved in phase transformations (PTs) and/or plastic deformations (PDs), thermodynamics, kinetics, and thermo-kinetic partition are described. It has been shown that the thermo-kinetic partition reflects the scale of the so-called thermo-kinetic correlation, and by combining with the reference state of PT or PD, corresponds to so-called generalized stability. Regarding the universality of nucleation/growth and thermo-kinetic partition, the principle of high thermodynamic driving force-high generalized stability has been reinterpreted by integrating nucleation and growth, separating nucleation and growth, and designing so-called negative driving forces, respectively. As such, the current materials design is classified, summarized, and prospected. This work is helpful to realize high strength and high plasticity by designing nucleation and growth.

Thermo-kinetic investigation of heavy metal ions adsorption onto lignin considering coupled adsorption–desorption mechanisms:Modeling and experimental validation

A coupled adsorption–desorption thermo-kinetic model is developed incorporating both adsorption and desorption reactions.A local pseudo-equilibrium condition at the interface of adsorbent and adsorbate bulk phases was used as isotherm equation which can even be applied for multi-pollutants scenarios.The developed model is then validated using collected experimental data of heavy metal ions(Pb,Cu,Cd,Zn,and Ni).Comparisons were made for a number of isotherm and kinetic models to examine the performance of the proposed model.The developed model revealed desirable accuracy and superiority over other models in predicting the adsorption behavior and can be used for other systems of concern.The model correlates the adsorption kinetic with an R2 value of 0.9391 and desorption kinetic with an R2 value of 0.9383.By application of the proposed model to any available adsorption datasets,the individual characteristics of adsorption and desorption can be determined.

Generalized stability criterion for controlling solidification segregation upon twin-roll casting

Macro-and micro-segregation formed upon twin-roll casting(TRC)can be inherited from sub-rapid solid-ification to solid-state transformation,even to plastic deformation,thus deteriorating drastically mechan-ical properties of as-produced thin sheets.Although many works focusing mainly on controlling fields of thermal,concentration and convection have been reported,how to control artificially and quantitatively the segregation using a theoretical connection between processing parameters and solidification models,has not been realized,yet.Regarding it,a systematical framework integrating non-equilibrium dendritic growth and overall solidification kinetics with the TRC parameters,was constructed applying a general-ized stability(GS)conception deduced from transient thermodynamic driving force△G^(t)and transient ki-netic energy barrier Q_(eff)^(t)evolving upon solidification.Departing from this framework considering synergy of thermodynamics and kinetics(i.e.,thermo-kinetic synergy),a criterion of high△G^(t)-high GS guaranteed that the macro(i.e.,the centerline)and the micro(i.e.,the edge)segregation can be suppressed by in-creasing△G^(t)and GS at the beginning and the ending stage of sub-rapid solidification,respectively.This typical thermo-kinetic combination producing the microstructure can be inherited into the plastic de-formation,as reflected by corresponding strength-ductility combinations.This work realized quantitative controlling of TRC by a theoretical connection between processing parameters and solidification models,where,an optimization for sub-rapid solidification segregation using the GS conception including△G^(t)and Q_(eff)^(t)has been performed.

Mechanism and kinetics of the BaMgAl_(10)O_(17):Eu^(2+) alkaline fusion reaction

Knowledge of the kinetics and mechanism of BaMgAl10017：Eu2＋ （BAM） fusion with sodium hydroxide will benefit recy- cling rare earth elements （REEs） from the waste phosphors. The reaction temperature range of 290-375 ~C and the reaction mecha- nism were determined using X-ray diffraction, scanning electron microscopy and differential scanning calorimetry. Activation energy was determined by the four model-free methods, and calculated results showed that the Kissinger method value of 579.5 KJ/mol was close to the average value of the Kissinger-Akahira-Sunose （KAS） and the Flynn-Wall-Ozawa （FWO） methods of 563.5 kJ/mol. The calculated activation energy variation tendency versus conversion factor agreed with the proposed mechanism.