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
This paper investigates the temperature and loading rate dependencies of the critical stress intensity fac-tor(KIC)for dislocation nucleation at crack tips.We develop a new KIC formula with a generalized form by incor...This paper investigates the temperature and loading rate dependencies of the critical stress intensity fac-tor(KIC)for dislocation nucleation at crack tips.We develop a new KIC formula with a generalized form by incorporating the atomistic reaction pathway analysis into Transition State Theory(TST),which cap-tures the KIC of the first dislocation nucleation event at crack tips and its sensitivity to temperature and loading rates.We use this formula and atomistic modeling information to specifically calculate the KIC for quasi-two-dimensional crack tips located at various slant twin boundaries in nano-twinned TiAl al-loys across a wide range of temperatures and strain rates.Our findings reveal that twinning dislocation nucleation at the crack tip dominates crack propagation when twin boundaries(TBs)are tilted at 15.79°and 29.5°.Conversely,when TBs tilt at 45.29°,54.74°,and 70.53°,dislocation slip becomes the preferred mode.Additionally,at TB tilts of 29.5°and 70.53°,at higher temperatures above 800 K and typical exper-imental loading rates,both dislocation nucleation modes can be activated with nearly equal probability.This observation is particularly significant as it highlights scenarios that molecular dynamics simulations,due to their time scale limitations,cannot adequately explore.This insight underscores the importance of analyzing temperature and loading rate dependencies of the KIC to fully understand the competing mechanisms of dislocation nucleation and their impact on material behavior.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
The unveiling of temperature effects on the deformation behaviors of wrought magnesium(Mg)alloys is beneficial for optimizing the hot forming parameters of these alloys with limited room temperature(RT)formability.In ...The unveiling of temperature effects on the deformation behaviors of wrought magnesium(Mg)alloys is beneficial for optimizing the hot forming parameters of these alloys with limited room temperature(RT)formability.In the present work,we performed nanoindentations on individual grains of textured wrought AZ31 alloy along the normal direction(ND)from RT to 300℃to investigate the intrinsic non-basal dislocation behaviors at various temperatures.Interestingly,we observed abnormally enhanced nanoindentation displacement bursts(pop-ins)at elevated temperatures ranging from 150 to 250℃,which is beyond the general scenario that higher temperatures typically result in smoother plastic flow.The bursts exhibited Gaussian-like statistics,which differ from the well-reported bursts with power-law size distributions resulting from the destruction of jammed dislocation configurations.Through transmission electron microscopy(TEM)examination of the microstructure beneath the indentation just after the burst,we found that the abnormal displacement bursts originated from the heterogeneous nucleation of prismatic screw(a)dislocations due to the exhaustion of dislocation sources within the specified temperature range.展开更多
The nanoindentation pop-in behaviors of 13 grains with diverse crystallographic orientations were analysed using a coarse-grained Mg-2 wt.% Gd alloy.Within nanoscale stressed volumes within all grains,the converted sh...The nanoindentation pop-in behaviors of 13 grains with diverse crystallographic orientations were analysed using a coarse-grained Mg-2 wt.% Gd alloy.Within nanoscale stressed volumes within all grains,the converted shear stresses for the first pop-in,calculated using the indentation Schmid factor,ranged from 1 to 1.3 GPa,consistent with theoretical predictions for dislocation nucleation in Mg.The estimated activation volume of the first pop-in was approximately 27–40 A3(involving about ~2 atoms),aligning with reported atomistic simulations of the surface dislocation semi-loop nucleation.While indented near the -axis,grains exhibit higher first pop-in loads and successive pop-ins,implying the possibility of a cross-slip nucleation mechanism to accommodate -axis deformation.展开更多
The effect of hydrogen on the surface morphology and nanomechanical properties of Ni-based Alloy 725 under solution-annealed(SA)and precipitation-hardened(API)conditions was thoroughly studied.The investigation involv...The effect of hydrogen on the surface morphology and nanomechanical properties of Ni-based Alloy 725 under solution-annealed(SA)and precipitation-hardened(API)conditions was thoroughly studied.The investigation involved in situ nanoindentation testing,microscopy characterization,statistical analy-sis,and numerical simulation approaches.The results showed the distinctive effects of hydrogen on the pop-in and hardness in the SA and API samples.For the SA sample,hydrogen mainly dissolved as solid solute in the matrix,causing enhanced lattice friction on the dislocation motion and increasing the in-ternal stress via lattice expansion.Thus,an enhanced hardness,a reduced pop-in width/load ratio,and numerous surface steps were detected in the presence of hydrogen.For the API sample,the strengtheningγ″phases were the stress concentrators,and the dislocations nucleated heterogeneously,demonstrating indistinctive pop-in phenomena.Furthermore,the precipitates in the API sample affected the trapping be-havior of hydrogen,thereby resulting in the hardness change,which reflected the competition between solution hardening in the matrix and vacancy softening mechanism in precipitates.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
Penta-twinned Ag nanowires（pt-AgNWs） have recently attracted much attention due to their interesting mechanical and physical properties. Here we perform largescale atomistic simulations to investigate the influence ...Penta-twinned Ag nanowires（pt-AgNWs） have recently attracted much attention due to their interesting mechanical and physical properties. Here we perform largescale atomistic simulations to investigate the influence of sample size and strain rate on the tensile strength of pt-AgNWs. The simulation results show an apparent size effect in that the nanowire strength（defined as the critical stress for dislocation nucleation） increases with decreasing wire diameter. To account for such size effect, a theoretical model involving the interaction between an emerging dislocation and the twin boundary has been developed for the surface nucleation of dislocations. It is shown that the model predictions are in quantitative agreement with the results from atomistic simulations and previous experimental studies in the literatures. The simulations also reveal that nanowire strength is strain-rate dependent, which predicts an activation volume for dislocation nucleation in the range of 1–10b^3,where b is the magnitude of the Burgers vector for a full dislocation.展开更多
We present an efficient algorithm for calculating the minimum energy path(MEP)and energy barriers between local minima on a multidimensional potential energy surface(PES).Such paths play a central role in the understa...We present an efficient algorithm for calculating the minimum energy path(MEP)and energy barriers between local minima on a multidimensional potential energy surface(PES).Such paths play a central role in the understanding of transition pathways between metastable states.Our method relies on the original formulation of the string method[Phys.Rev.B,66,052301(2002)],i.e.to evolve a smooth curve along a direction normal to the curve.The algorithm works by performing minimization steps on hyperplanes normal to the curve.Therefore the problem of finding MEP on the PES is remodeled as a set of constrained minimization problems.This provides the flexibility of using minimization algorithms faster than the steepest descent method used in the simplified string method[J.Chem.Phys.,126(16),164103(2007)].At the same time,it provides a more direct analog of the finite temperature string method.The applicability of the algorithm is demonstrated using various examples.展开更多
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 China Scholarship Council(Grant No.202007865002)the National Natural Science Foundation of China(Grant Nos.51865027,52065036,and 52065037)+2 种基金the Educational Unveiling Leadership Project of Gansu Province of China(Grant No.2021jyjbgs01)the support by JSPS KAKENHI(Grant No.JP23K20037)MEXT Programs(Grant Nos.JPMXP1122684766,JPMXP1020230325,and JPMXP1020230327).
文摘This paper investigates the temperature and loading rate dependencies of the critical stress intensity fac-tor(KIC)for dislocation nucleation at crack tips.We develop a new KIC formula with a generalized form by incorporating the atomistic reaction pathway analysis into Transition State Theory(TST),which cap-tures the KIC of the first dislocation nucleation event at crack tips and its sensitivity to temperature and loading rates.We use this formula and atomistic modeling information to specifically calculate the KIC for quasi-two-dimensional crack tips located at various slant twin boundaries in nano-twinned TiAl al-loys across a wide range of temperatures and strain rates.Our findings reveal that twinning dislocation nucleation at the crack tip dominates crack propagation when twin boundaries(TBs)are tilted at 15.79°and 29.5°.Conversely,when TBs tilt at 45.29°,54.74°,and 70.53°,dislocation slip becomes the preferred mode.Additionally,at TB tilts of 29.5°and 70.53°,at higher temperatures above 800 K and typical exper-imental loading rates,both dislocation nucleation modes can be activated with nearly equal probability.This observation is particularly significant as it highlights scenarios that molecular dynamics simulations,due to their time scale limitations,cannot adequately explore.This insight underscores the importance of analyzing temperature and loading rate dependencies of the KIC to fully understand the competing mechanisms of dislocation nucleation and their impact on material behavior.
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.
基金supported by the Natural Science Foundation of China(Nos.52031011 and 51971167).
文摘The unveiling of temperature effects on the deformation behaviors of wrought magnesium(Mg)alloys is beneficial for optimizing the hot forming parameters of these alloys with limited room temperature(RT)formability.In the present work,we performed nanoindentations on individual grains of textured wrought AZ31 alloy along the normal direction(ND)from RT to 300℃to investigate the intrinsic non-basal dislocation behaviors at various temperatures.Interestingly,we observed abnormally enhanced nanoindentation displacement bursts(pop-ins)at elevated temperatures ranging from 150 to 250℃,which is beyond the general scenario that higher temperatures typically result in smoother plastic flow.The bursts exhibited Gaussian-like statistics,which differ from the well-reported bursts with power-law size distributions resulting from the destruction of jammed dislocation configurations.Through transmission electron microscopy(TEM)examination of the microstructure beneath the indentation just after the burst,we found that the abnormal displacement bursts originated from the heterogeneous nucleation of prismatic screw(a)dislocations due to the exhaustion of dislocation sources within the specified temperature range.
基金financial support of NSERC Discovery Grant (RGPIN-2019–05882) and Canada Research Chair program (CRC-2021–00512)。
文摘The nanoindentation pop-in behaviors of 13 grains with diverse crystallographic orientations were analysed using a coarse-grained Mg-2 wt.% Gd alloy.Within nanoscale stressed volumes within all grains,the converted shear stresses for the first pop-in,calculated using the indentation Schmid factor,ranged from 1 to 1.3 GPa,consistent with theoretical predictions for dislocation nucleation in Mg.The estimated activation volume of the first pop-in was approximately 27–40 A3(involving about ~2 atoms),aligning with reported atomistic simulations of the surface dislocation semi-loop nucleation.While indented near the -axis,grains exhibit higher first pop-in loads and successive pop-ins,implying the possibility of a cross-slip nucleation mechanism to accommodate -axis deformation.
基金The authors acknowledge voestalpine BOHLER Edelstahl GmbH&Co KG for providing laboratory-scale Alloy 725 samples.The authors acknowledge the support provided by Research Council of Norway through the HyLINE(294739)and M-HEAT(294689)projectsD.P.acknowledges SFI Manufacturing,funded by the Research Council of Norway(237900)the financial support from the Research Council of Norway to the NORTEM project(197405).The authors are greteful for the Steel Institute of the RWTH Aachen Univeristy for providing the atom probe facility.
文摘The effect of hydrogen on the surface morphology and nanomechanical properties of Ni-based Alloy 725 under solution-annealed(SA)and precipitation-hardened(API)conditions was thoroughly studied.The investigation involved in situ nanoindentation testing,microscopy characterization,statistical analy-sis,and numerical simulation approaches.The results showed the distinctive effects of hydrogen on the pop-in and hardness in the SA and API samples.For the SA sample,hydrogen mainly dissolved as solid solute in the matrix,causing enhanced lattice friction on the dislocation motion and increasing the in-ternal stress via lattice expansion.Thus,an enhanced hardness,a reduced pop-in width/load ratio,and numerous surface steps were detected in the presence of hydrogen.For the API sample,the strengtheningγ″phases were the stress concentrators,and the dislocations nucleated heterogeneously,demonstrating indistinctive pop-in phenomena.Furthermore,the precipitates in the API sample affected the trapping be-havior of hydrogen,thereby resulting in the hardness change,which reflected the competition between solution hardening in the matrix and vacancy softening mechanism in precipitates.
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.
基金supported by the National Natural Science Foundation of China (Grants 11372152 and 51420105001)the National Natural Science Foundation of United States (Grant CMMI-1161749)
文摘Penta-twinned Ag nanowires（pt-AgNWs） have recently attracted much attention due to their interesting mechanical and physical properties. Here we perform largescale atomistic simulations to investigate the influence of sample size and strain rate on the tensile strength of pt-AgNWs. The simulation results show an apparent size effect in that the nanowire strength（defined as the critical stress for dislocation nucleation） increases with decreasing wire diameter. To account for such size effect, a theoretical model involving the interaction between an emerging dislocation and the twin boundary has been developed for the surface nucleation of dislocations. It is shown that the model predictions are in quantitative agreement with the results from atomistic simulations and previous experimental studies in the literatures. The simulations also reveal that nanowire strength is strain-rate dependent, which predicts an activation volume for dislocation nucleation in the range of 1–10b^3,where b is the magnitude of the Burgers vector for a full dislocation.
基金support by the Department of Energy under Grant No.DE-SC0002623.
文摘We present an efficient algorithm for calculating the minimum energy path(MEP)and energy barriers between local minima on a multidimensional potential energy surface(PES).Such paths play a central role in the understanding of transition pathways between metastable states.Our method relies on the original formulation of the string method[Phys.Rev.B,66,052301(2002)],i.e.to evolve a smooth curve along a direction normal to the curve.The algorithm works by performing minimization steps on hyperplanes normal to the curve.Therefore the problem of finding MEP on the PES is remodeled as a set of constrained minimization problems.This provides the flexibility of using minimization algorithms faster than the steepest descent method used in the simplified string method[J.Chem.Phys.,126(16),164103(2007)].At the same time,it provides a more direct analog of the finite temperature string method.The applicability of the algorithm is demonstrated using various examples.
