First-principles Calculations On Stabilities Of Grain Structure And Phase Of Nanocrystalline Alloys

Compared with the conventional coarse-grained polycrystalline material,the nanocrystalline alloys with the same compositions exhibit dramatically enhanced mechanical or functional properties.However,the limited thermal stability and phase stability of the nanocrystalline alloys hinder their large-scale preparations and engineering applications.Thus,the influence factors and mechanism of nanocrystalline alloys stability has been becoming one of the focused researches for nanometer materials.In this study,our research was focused on the key issues in thermal stability and phase stability of nanocrystalline alloys.Multiscale coupling thermodynamic and kinetic models were established based on first principles calculation.According to systemically calculations,four types of representative nanocrystalline alloys were investigated for the functional materials of Lithium-ion batteries?LIBs?and high strength structural materials:nanocrystalline Li₂C₂?Li-Si?Cu-Zn and W-Sc.The intrinsic property,thermal stability,phase stability and the stability strengthening mechanism at different conditions were predicted theoretically.The nanograin alloys with optimized performance were explored using the modeling results,which can provide theoretical basis and a practical way for nanocrystalline alloys fabrication.The main research contents and results are presented as follows:A coupling model of thermodynamics and first-principles calculation was developed for nanocrystalline alloy compounds.The Gibbs free energy of the nanograin boundaries in the nanocrystalline Li₂C₂ system was calculated as a function of temperature and grain size,and the electronic structures at the surface and in the interior of the nanograin were analyzed.The modeling results show that when the grain size is smaller than a critical value,the nanocrystalline Li₂C₂ has higher thermodynamic stability than the conventional polycrystalline counterpart.The structure of the delithiated phase and the optimal theoretical capacity were further predicted by the model.On the other hand,the conventional polycrystalline and nanocrystalline Li₂C₂ were prepared and the electrochemical tests were performed.As compared with the conventional polycrystalline Li₂C₂,the nanocrystalline Li₂C₂ has a larger discharge capacity and much higher cycling stability as the cathode material,which confirms the model predictions.To quantify the phase stability and Li diffusion behavior in nanocrystalline anode materials for LIBs,a hybrid model of the first principles calculation and diffusion kinetics was further developed.The activation barrier for Li diffusion in the Li-Si solid solution decreases with the increase of solute concentration below the critical concentration for amorphization,which is due to the dependence of the formation energy and phase stability of solid solution on Li concentration.The dependence of the Li diffusion on electronic structure,solute concentration,grain size and temperature was described for the nanocrystalline Li-Si system.There exists a coordination effect of solute concentration and grain size on Li diffusion in nanocrystalline materials.A maximum diffusion coefficient can be obtained in the nanocrystalline Li-Si by combination of concentration and grain size,which is increased by two orders of magnitude from that in the coarse-grained counterpart to enhance the rate capability of anode.To describe the thermal stability of the nanocrystalline solid solution with weak segregation,a segregation model was developed based on the coupling model of the first principles calculation and thermodynamics.The dependence of the solute segregation behavior on the electronic structure,solute concentration,grain size and temperature were demonstrated for the nanocrystalline Cu-Zn system.The modeling results show that the segregation energy changes with the solute concentration in a form of nonmonotonic function.The change of the total Gibbs free energy reveals that at a constant solute concentration and a given temperature,a nanocrystalline structure can remain stable when the initial grain size is controlled in a critical range.The experimental measurements confirmed the model predictions that with a certain solute concentration,a state of steady nanograin growth can be achieved at high temperatures when the initial grain size is controlled in a critical range.The present work proposes that in the weak solute segregation system,the nanograin structure can be kept thermally stable by adjusting solute concentration and initial grain size.Based on the coupling model of the first principles calculation and thermodynamics in nanocrystalline solid solution with weak segregation,a real atomic model considering the element characteristic of strong segregation system was developed for nanocrystalline solid solution with strong segregation.The thermal stability mechanism of the systems with weak and strong segregation can be integrated using our current model.In the W-Sc system with strong segregation,the modeling results reveal three types of rules for the relationship between the change of the total Gibbs free energy and grain size,which can provide the universal method and an adjusting approach to enhance the thermal stability of nanocrystalline systems with solute segregation.The experimental measurements in the W-Sc system confirmed the model predictions.By combining model calculation with experimental identification,the nanocrystalline W-10at.%Sc can be designed and prepared with extreme thermal stability.This system possesses great application potential due to the high critical temperature of 1600K for thermal stability of nanograin structure.