Investigation on the Interface Segregation and Thermal Stability of Nanocrystalline Alloys/Ceramics

Author:Gong Ming Ming

Supervisor:liu feng


Degree Year:2017





Nanocrystalline metals(alloys)and ceramics,due to their excellent mechanical and physical properties induced by extremely tiny grain sizes,have great potential of application in industry.However,enlarged grain boundary(GB)area in these systems compared with their bulk counterpart renders them thermodynamically unstable against grain growth.Significant grain coarsening often occurs upon their preparation process or the service condition when exposed at moderate to high temperatures,decreasing their original excellent properties.Given this,considerable efforts have been addressed to the issue of how to retard the grain growth of nanocrystalline materials and improve their thermal stability.Solute addition or doping has been demonstrated as an effective way to stabilize nanostructures.Nevertheless,the mechanism of its action has not yet been fully elucidated,deserving to be further explored.In this paper,systematic theoretical study of the grain growth and thermal stability of nanocrystalline alloys and ceramics were carried out.Firstly,for nanocrystalline alloys,the models regarding solute segregation to GBs and the relevant thermal stability,under the influence of GB excess volume,were established.Then,the thermo-kinetic model of grain growth of nanocrystalline alloys was deduced by coupling the GB-segregation-dependent thermodynamic and kinetic stabilization effects.For nanocrystalline ceramics,considering the coexistence of GBs and free surfaces,a model of segregation of dopant cations at GBs and free surfaces was established,and then a method for obtaining GB and surface segregation enthalpies was proposed.On this basis,the grain-growth and densification models under the action of dopant-cation segregation were derived by coupling the thermodynamic and kinetic stabilization effects of GB and surface concurrent segregation.The main conclusions are as follows.(1)Based on the assumption that the GB is treated as an expanded lattice,the model of GB segregation affected by GB excess volume was established by applying Embedded-atom-method(EAM)to standard thermodynamic relations and then employing the Hillert‘s parallel tangent rule,exhibiting the solute enrichment ratio that increases with the increase of GB excess volume while decreases with the increase of temperature or bulk(i.e.grain interior)solute content;accordingly,good agreement between theoretical and experimental/simulated results for Cu-Ag and W-Nb were achieved.Regarding that the excess volume in GBs is released as the vacancies which are accommodated by the crystal bulk during grain growth,a free-energy function for binary nanocrystalline solid solution was proposed within the framework of statistical thermodynamics,predicting an equilibrium grain size subjected to a mixed effect due to solute segregation and due to excess vacancies for a given composition and temperature;the excess-vacancy-inhibited grain coarsening can be attained,which plays a minor role in holding the thermal stability of nanocrystalline alloys,as compared to the effect of solute segregation.(2)The grain growth model of binary substitutional alloys was established by applying the thermodynamic extremal principle(TEP)within the framework of statistical thermodynamics,exhibiting the self-consistent GB energy and GB mobility expressions subjected to GB segregation;a criterion used for evaluating the thermodynamic and kinetic stabilization effects was then proposed by comparing the normalized GB energy and GB mobility;this model describes well the experimental results for Fe-P alloys,nanocrystalline Ni-P and Fe-Zr alloys.(3)The concurrent segregation model at GBs and free surfaces was explicitly demonstrated,based on the concept that the solute distribution at equilibrium corresponds to the minimum Gibbs energy of a system;a method to evaluate the enthalpies of segregation for GB and surface was then proposed,by combining the specified-interface energy formula and this concurrent segregation model with experimentally measured GB energy and surface energy;employing this method,the segregation enthalpies for GB and surface for La3+in YSZ were obtained,and further applied to evaluate the energy change of nanocrystalline La2O3 doped YSZ during the final sintering stage,which is consistent with the corresponding microstructural evolution,as a result of the decrease in total interface energy caused by concurrent segregation of La3+at GBs and free surfaces.(4)A grain growth model to describe dopant effects on nanocrystalline ceramics was proposed by incorporating the dopant-segregation-dependent GB energy and the GB mobility subjected to intrinsic drag and pore drag(both affected by dopant segregation)into the curve-driven grain-growth equation and then combining it with the GB-diffusion-mediated densification equation under the influence of dopant segregation;applying this model,the substantially suppressed grain growth in La doped YSZ as compared to La-free YSZ should be attributed to the combined effect of thermodynamically reduced GB energy and kinetically reduced GB mobility.(5)Employing the TEP,a model for grain growth and densification in the final sintering stage of doped ceramics was derived,with segregation-dependent interface energy and mobility(or diffusivity);as compared to previous works,where the GB(surface)energy is the only constituent of the driving force for grain-growth(densification),it was shown that the surface energy contributes positively to the driving force for grain growth while the GB energy negatively to the driving force for densification;model calculations demonstrated that the dopant with more negative GB(or surface)segregation enthalpy or lower GB diffusion coefficient can achieve higher relative density at the same grain size,which was further supported by applying this model to the sintering data of alumina doped individually with yttria and lanthana,due to the enhanced thermo-kinetic effect caused by the larger dopant cationic radius.