Cahn's theory of nucleation on dislocations has been analyzed thoroughly and the imperfection of the theory in some cases has been discussed.A revised theory has been proposed with application to the problem of pr...Cahn's theory of nucleation on dislocations has been analyzed thoroughly and the imperfection of the theory in some cases has been discussed.A revised theory has been proposed with application to the problem of precipitation of M(CN)phase in austenite.展开更多
The plastic deformation and the ultrahigh strength of metals at the nanoscale have been predicted to be controlled by surface dislocation nucleation. In situ quantitative tensile tests on individual 〈111〉 single cry...The plastic deformation and the ultrahigh strength of metals at the nanoscale have been predicted to be controlled by surface dislocation nucleation. In situ quantitative tensile tests on individual 〈111〉 single crystalline ultrathin gold nanowires have been performed and significant load drops observed in stress-strain curves suggest the occurrence of such dislocation nucleation. High-resolution transmission electron microscopy （HRTEM） imaging and molecular dynamics simulations demonstrated that plastic deformation was indeed initiated and dominated by surface dislocation nucleation, mediating ultrahigh yield and fracture strength in sub-lO-nm gold nanowires.展开更多
A large-scale atom simulation of nanoindentation into a thin nickel film using the quasicontinuum method was performed. The initial stages of the plasticity deformation of nickel were studied. Several useful results w...A large-scale atom simulation of nanoindentation into a thin nickel film using the quasicontinuum method was performed. The initial stages of the plasticity deformation of nickel were studied. Several useful results were obtained as follows： （1） The response of the load versus indentation depth on the load versus indentation depth curve, besides the straight parts corresponding to the elastic property of nickel, the sudden drop of the load occurred several times; （2） The phenomena of dislocation nucleation -- the dislocation nucleation took place when the load descended, which makes it clear that dislocation nucleation causes the drop of the load; （3） The mechanism of the dislocation emission - the Peierls-Nabarro dislocation model and a pow- erful criterion were used to analyze the dislocation emission. And the computational value was in good agreement with the predict value; （4） The density of geometrically necessary dislocations. A simple model was used to obtain the density of geometrically necessary dislocations beneath the indenter. Furthermore, the influence of the boundary conditions on the simulation results was discussed.展开更多
The hydrogen effect on the nucleation and motion of dislocations in single-crystal bcc Fe with(110)surface was investigated by both nanoindentation experiments and discrete dislocation dynamics(DDD)simulation.The resu...The hydrogen effect on the nucleation and motion of dislocations in single-crystal bcc Fe with(110)surface was investigated by both nanoindentation experiments and discrete dislocation dynamics(DDD)simulation.The results of nanoindentation experiments showed that the pop-in load decreased evidently for the electrochemical hydrogen charging specimen,indicating that the dislocation nucleation strength might be reduced by hydrogen.In addition,the decrease of hardness due to hydrogen charging was also captured,implying that the dislocation motion might be promoted by hydrogen.By incorporating the effect of hydrogen on dislocation core energy,a DDD model was specifically proposed to investigate the influence of hydrogen on dislocation nucleation and motion.The results of DDD simulation revealed that under the effect of hydrogen,the dislocation nucleation strength is decreased and the motion of dislocation is promoted.展开更多
Simulation of molecular dynamics using Embedded Atom Method(EAM) potentials is performed to investigate the mechanical properties of single crystal Al along various crystallographic orientations under tensile loading....Simulation of molecular dynamics using Embedded Atom Method(EAM) potentials is performed to investigate the mechanical properties of single crystal Al along various crystallographic orientations under tensile loading.The specimens are provided with one or two embedded circular voids to analyze the damage evolution by void growth and coalescence.The simulation result shows that the Young's modulus,yielding stress and ultimate stress decrease with the emergence of the voids.Besides,the simulations show that the single-crystal Al in different crystallographic orientations behaves differently in elongation deformations.The single-crystal Al with 100 crystallographic orientations has greater ductility than other orientated specimens.The incipient plastic deformation and the stress-strain curves are presented and discussed for further understanding of the mechanical properties of single-crystal Al.展开更多
In situ observations of the nucleation and growth of ferrous martensites,including ε-, lenticular,butterfly,thin-plate and composite-plate ones at low temperature stage on optical or transmission electron microscope ...In situ observations of the nucleation and growth of ferrous martensites,including ε-, lenticular,butterfly,thin-plate and composite-plate ones at low temperature stage on optical or transmission electron microscope were carried out.The results confirmed the faulting model for the formation of ε-martensite proposed by Olso and Cohen.The grain boundaries and tri- ple points are the favourable nucleation sites for the thin plates,and their surface relief grown initially like a small needle.The lenticular martensite was observed as two stages of formation.According to the dislocation resolving reaction. (a/6)[111]_b→(a/3)[]_b+(a/2)[111]_b,the emission dislocation(=(a/2)[111]_b)acting as the dislocation source for the transition of lattice invariant shear from twinning to slip was proposed.展开更多
Based on the dislocation theory and Olson's stacking fault model, a model describing the nucleation of an hcp(ε) martensite embryo at low-angle grain boundary is proposed with the influence of external stress fie...Based on the dislocation theory and Olson's stacking fault model, a model describing the nucleation of an hcp(ε) martensite embryo at low-angle grain boundary is proposed with the influence of external stress field taken into account. The dependences of temperature (T), shear stress (τ) and dislocation density at grain boundary on the martensite nucleation in FeMnSi based alloy, as an example, are numerically simulated. It has been shown that there exist the subcritical and critical embryos during the course of ε-phase nucleation. The free energy difference between them is just the energy barrier of embryo growth. Depending on T and τ. the characteristic embryo sizes may vary in wide ranges and decreases with increasing σ and decreasing T. The energy condition of martensitic transformation at M s and critical shear stress (τc) is discussed from the viewpoint of kinetics and thus the TEM observed result that stacking fault energy is not zero at M s temperature is reasonably explained. Besides, it is predicted that the high dislocation density at grain boundary can promote the nucleation of fcc→hcp transformation in Fe-based alloys.展开更多
The influence of crystallization temperature （Tc） on the number of spiral growths on poly（butylene succinate） （PBS） single crystals, obtained by self-seeding method, was systematically studied. The studies show ...The influence of crystallization temperature （Tc） on the number of spiral growths on poly（butylene succinate） （PBS） single crystals, obtained by self-seeding method, was systematically studied. The studies show that the statistical average number of spiral growths formed on the PBS single crystals decays exponentially with respect to the To. Inspired by BCF （Bruton, Cabrera and Frank） theory and L-H （Lauritzen and Hoffman） theory, a thermodynamic model has been proposed, in which the origin of spiral growth was treated as a nucleation process. The model suggests that the nucleation rate of spiral growth depends on the inverse square of super-cooling degree, which predicted the density of spiral growth formed on lamellae, and was consistent with the experiments very well.展开更多
The TEM observation on the single-side thinned specimens was made of the nucleation and growth of boride during boronization for steel 20 and the influence of prior room temperature deformation.The results of X-ray di...The TEM observation on the single-side thinned specimens was made of the nucleation and growth of boride during boronization for steel 20 and the influence of prior room temperature deformation.The results of X-ray diffraction and electron probe microstructure analysis show that only Fe_2B formed in the surface layer of specimens at the beginning of boronization;the atomic concentration of B in the surface layer increased with the increase of deformation monotonously;and the segregation of B atoms in the dislocation cells hindered the recovery and recrystallization of the deformed structure of the matrix and accelerated the nucleation and growth of boride in the surface layer.展开更多
基金The project is supported by National Natural Science Foundation of China
文摘Cahn's theory of nucleation on dislocations has been analyzed thoroughly and the imperfection of the theory in some cases has been discussed.A revised theory has been proposed with application to the problem of precipitation of M(CN)phase in austenite.
文摘The plastic deformation and the ultrahigh strength of metals at the nanoscale have been predicted to be controlled by surface dislocation nucleation. In situ quantitative tensile tests on individual 〈111〉 single crystalline ultrathin gold nanowires have been performed and significant load drops observed in stress-strain curves suggest the occurrence of such dislocation nucleation. High-resolution transmission electron microscopy （HRTEM） imaging and molecular dynamics simulations demonstrated that plastic deformation was indeed initiated and dominated by surface dislocation nucleation, mediating ultrahigh yield and fracture strength in sub-lO-nm gold nanowires.
文摘A large-scale atom simulation of nanoindentation into a thin nickel film using the quasicontinuum method was performed. The initial stages of the plasticity deformation of nickel were studied. Several useful results were obtained as follows： （1） The response of the load versus indentation depth on the load versus indentation depth curve, besides the straight parts corresponding to the elastic property of nickel, the sudden drop of the load occurred several times; （2） The phenomena of dislocation nucleation -- the dislocation nucleation took place when the load descended, which makes it clear that dislocation nucleation causes the drop of the load; （3） The mechanism of the dislocation emission - the Peierls-Nabarro dislocation model and a pow- erful criterion were used to analyze the dislocation emission. And the computational value was in good agreement with the predict value; （4） The density of geometrically necessary dislocations. A simple model was used to obtain the density of geometrically necessary dislocations beneath the indenter. Furthermore, the influence of the boundary conditions on the simulation results was discussed.
文摘The hydrogen effect on the nucleation and motion of dislocations in single-crystal bcc Fe with(110)surface was investigated by both nanoindentation experiments and discrete dislocation dynamics(DDD)simulation.The results of nanoindentation experiments showed that the pop-in load decreased evidently for the electrochemical hydrogen charging specimen,indicating that the dislocation nucleation strength might be reduced by hydrogen.In addition,the decrease of hardness due to hydrogen charging was also captured,implying that the dislocation motion might be promoted by hydrogen.By incorporating the effect of hydrogen on dislocation core energy,a DDD model was specifically proposed to investigate the influence of hydrogen on dislocation nucleation and motion.The results of DDD simulation revealed that under the effect of hydrogen,the dislocation nucleation strength is decreased and the motion of dislocation is promoted.
基金Supported by the National Science Foundation of China under Grant No.11572259
文摘Simulation of molecular dynamics using Embedded Atom Method(EAM) potentials is performed to investigate the mechanical properties of single crystal Al along various crystallographic orientations under tensile loading.The specimens are provided with one or two embedded circular voids to analyze the damage evolution by void growth and coalescence.The simulation result shows that the Young's modulus,yielding stress and ultimate stress decrease with the emergence of the voids.Besides,the simulations show that the single-crystal Al in different crystallographic orientations behaves differently in elongation deformations.The single-crystal Al with 100 crystallographic orientations has greater ductility than other orientated specimens.The incipient plastic deformation and the stress-strain curves are presented and discussed for further understanding of the mechanical properties of single-crystal Al.
文摘In situ observations of the nucleation and growth of ferrous martensites,including ε-, lenticular,butterfly,thin-plate and composite-plate ones at low temperature stage on optical or transmission electron microscope were carried out.The results confirmed the faulting model for the formation of ε-martensite proposed by Olso and Cohen.The grain boundaries and tri- ple points are the favourable nucleation sites for the thin plates,and their surface relief grown initially like a small needle.The lenticular martensite was observed as two stages of formation.According to the dislocation resolving reaction. (a/6)[111]_b→(a/3)[]_b+(a/2)[111]_b,the emission dislocation(=(a/2)[111]_b)acting as the dislocation source for the transition of lattice invariant shear from twinning to slip was proposed.
文摘Based on the dislocation theory and Olson's stacking fault model, a model describing the nucleation of an hcp(ε) martensite embryo at low-angle grain boundary is proposed with the influence of external stress field taken into account. The dependences of temperature (T), shear stress (τ) and dislocation density at grain boundary on the martensite nucleation in FeMnSi based alloy, as an example, are numerically simulated. It has been shown that there exist the subcritical and critical embryos during the course of ε-phase nucleation. The free energy difference between them is just the energy barrier of embryo growth. Depending on T and τ. the characteristic embryo sizes may vary in wide ranges and decreases with increasing σ and decreasing T. The energy condition of martensitic transformation at M s and critical shear stress (τc) is discussed from the viewpoint of kinetics and thus the TEM observed result that stacking fault energy is not zero at M s temperature is reasonably explained. Besides, it is predicted that the high dislocation density at grain boundary can promote the nucleation of fcc→hcp transformation in Fe-based alloys.
基金financially supported by the National Natural Science Foundation of China(No.21434003),the National Natural Science Foundation of China(No.21374054)the National High Technology Research and Development Program of China(No.2015AA033801)+3 种基金the National Science and Technology Major Project of the Ministry of Science and Technology of China(No.2013ZX09J13110-11B)the Fundamental Research Funds for the Central Universitiesthe Program for Changjiang Scholars and Innovative Research Team in Universitythe Sino-German Center for Research Promotion
文摘The influence of crystallization temperature （Tc） on the number of spiral growths on poly（butylene succinate） （PBS） single crystals, obtained by self-seeding method, was systematically studied. The studies show that the statistical average number of spiral growths formed on the PBS single crystals decays exponentially with respect to the To. Inspired by BCF （Bruton, Cabrera and Frank） theory and L-H （Lauritzen and Hoffman） theory, a thermodynamic model has been proposed, in which the origin of spiral growth was treated as a nucleation process. The model suggests that the nucleation rate of spiral growth depends on the inverse square of super-cooling degree, which predicted the density of spiral growth formed on lamellae, and was consistent with the experiments very well.
文摘The TEM observation on the single-side thinned specimens was made of the nucleation and growth of boride during boronization for steel 20 and the influence of prior room temperature deformation.The results of X-ray diffraction and electron probe microstructure analysis show that only Fe_2B formed in the surface layer of specimens at the beginning of boronization;the atomic concentration of B in the surface layer increased with the increase of deformation monotonously;and the segregation of B atoms in the dislocation cells hindered the recovery and recrystallization of the deformed structure of the matrix and accelerated the nucleation and growth of boride in the surface layer.
