| As nanocrystalline(NC)materials exhibit many unique properties,which are often superior to the properties of their coarse-grained counterparts,they offer significant potential for use in a variety of applications.However,due to poor thermal stability,these materials are subjected to significant grain coarsening at relatively low temperatures,even room temperature,which significantly limits the potential application of NC materials under engineering environment.Accordingly,impeding grain coarsening(or improving the thermal stability)of NC materials is becoming one of the most important issues in the field of NC materials.So far,it has been found that GB segregation,solute drag,particle pinning and other intrinsic crystal defects can highly effectively improve the thermal stability of NC materials,attributing to the thermodynamic stabilization and the kinetic stabilization.However,several key scientific issues have not been solved,including that the theoretical description and experiment/simulation evidences for the thermo-kinetic stabilization is lack,and that the pinning mechanism during particle-GB interaction is not revealed,and that existing alloy design frameworks ignores the synergy of thermodynamics and kinetics.These above key issues,if solved,can help us deeply understand the stabilization mechanism of NC materials,and further guide us to design highly stable NC materials.In current work,theoretical modeling,experiments and multiple computer simulation methods(e.g.first-principle calculations and molecular dynamics simulations)are employed to study the thermodynamics and kinetics influenced by GB segregation,solute drag,and particle pinning,etc.The main conclusions are listed as follows:1)For NC materials without solute-riched phases,a GB-excess-dependent activation energy for GB migration is derived.By combining the expression of activation energy for GB migration with the Krill et al.'s GB energy model,a thermokinetic correlation during grain growth is obtained.Furthermore,by determination of the GB energy and the activation energy for GB migration as functions of amount of segregated solute atoms,the above thermo-kinetic correlation is proved.Accordingly,the stabilization of segregated solute atoms should be thermo-kientic,rather than pure thermodynamic or pure kinetic.2)Based on the regular solution model,the mixing Gibbs free energy is constructed as a function of solute/solvent parameters,for two cases: equibrium solute distribution and the non-equibrium solute distribution,with which,the driving force for grain growth is further obtained.It is found that the effective driving force of grain growth can be expressed as the difference between the classical driving force and the solute drag force.By substituting the expression of the effective driving force into the parabolic equation and combining the Borisov's equation,a thermo-kinetic grain growth equation is derived.Applying this equation to describe the grain growth behavior in Fe-4at.% Zr and Pd-19 at.%Zr alloys,the effects of thermodynamic(GB energy and driving froce)and kinetic parameters(solute drag force and GB mobility)on grain growth are discussed.3)On the base of thermo-kinetic correlation,a stabilization criteria is derived as a function of intrinsic solute(activation energy for bulk diffusion and segregation enthalpy)and solvent(intrinsic GB energy and activation energy for GB migration)parameters and processing parameters(temperature,grain size and solute concentration),which,combining both the thermodynamic and kinetic contributions from segregated solute atoms,firstly quantitatively describes the thermal stabilty of NC materials.Calculating the solute and solvent parameters using first-principle calculation and molecular dynamics simulations,and the thermal stability for each solute atom can be theoretically calculated,with which high stable NC materials can be designed.The Y atom is selected according to the model calculation and the thermal stabilty of Fe-1at.%Y alloy is studied.The experimental results show that Fe-1at.%Y alloy has a remarkablely high thermal stability.4)For particle-GB interaction system,the maximum/average pinning forces model as functions of particle parameters(the size,the volume fraction,the shape,the orientation of particle)are derived.Furthermore,the particle-GB interaction is investigated using MD simulation for a serious of particle parameters.A combination of theoretical model and MD simulation reveals the multiple mechanisms during particle-GB interation,shows the thermo-kinetic correlation during particle-GB interaction,and describes the dependence of the maximum/average pinning force on inherent particle parameters(the size,the volume fraction,the shape and the orientation of particle). |
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