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,de...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.展开更多
The strength of polycrystalline metals increases with decreasing grain size,following the classical HallPetch relationship.However,this relationship fails when softening occurs at very small grain sizes(typically less...The strength of polycrystalline metals increases with decreasing grain size,following the classical HallPetch relationship.However,this relationship fails when softening occurs at very small grain sizes(typically less than 10 to 20 nm),which limits the development of ultrahigh-strength materials.In this work,using columnar-grained nanocrystalline Cu-Ag‘samples’,molecular dynamics simulations were performed to investigate the softening mechanism and explore the strengthening strategies(e.g.,formation of solid solution or grain boundary(GB)segregation)in extremely fine nanograined metals.Accordingly,the softening of pure metals is induced by atomic sliding in the GB layer,rather than dislocation activities in the grain interior,although both occur during deformation.The solid solution lowers the stacking fault energy and increases the GB energy,which leads to the softening of NC metals.GB segregation stabilizes GB structures,which causes a notable improvement in strength,and this improvement can be further enhanced by optimizing the solute concentration and GB excess.This work deepens the understanding of the softening mechanism due to atomic sliding in the GB layer and the strengthening mechanism arising from tailoring the GB stability of immiscible alloys and provides insights into the design of ultrahighstrength materials.展开更多
基金financial support from the National Key R&D Program of China(Nos.2017YFB0703001,2017YFB0305100)the National Natural Science Foundation of China(Nos.51134011,51431008)+1 种基金the Research Fund of the State Key Laboratory of Solidification Processing(Nos.117-TZ-2015,159QP-2016)the Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University(No.CX201826)。
文摘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.
基金financially supported by the National Key R&D Program of China(No.2017YFB0703001)the Natural Science Foundation of China(Nos.51971166,51790481 and 52130110)+2 种基金the Fundamental Research Funds for the Central Universities(No.3102017jc01002)the Natural Science Foundation of Shaanxi Province(No.2021JQ-651)supported by High Performance Computation Center of Northwestern Polytechnical University.
文摘The strength of polycrystalline metals increases with decreasing grain size,following the classical HallPetch relationship.However,this relationship fails when softening occurs at very small grain sizes(typically less than 10 to 20 nm),which limits the development of ultrahigh-strength materials.In this work,using columnar-grained nanocrystalline Cu-Ag‘samples’,molecular dynamics simulations were performed to investigate the softening mechanism and explore the strengthening strategies(e.g.,formation of solid solution or grain boundary(GB)segregation)in extremely fine nanograined metals.Accordingly,the softening of pure metals is induced by atomic sliding in the GB layer,rather than dislocation activities in the grain interior,although both occur during deformation.The solid solution lowers the stacking fault energy and increases the GB energy,which leads to the softening of NC metals.GB segregation stabilizes GB structures,which causes a notable improvement in strength,and this improvement can be further enhanced by optimizing the solute concentration and GB excess.This work deepens the understanding of the softening mechanism due to atomic sliding in the GB layer and the strengthening mechanism arising from tailoring the GB stability of immiscible alloys and provides insights into the design of ultrahighstrength materials.
