Mechanical Behavior And Deformation Mechanism In Nanocystalline Metal Materials By Molecular Dynamics Simulation

When the grain size of metal material decreases to nano-scale,the mechanical behavior and deformation mechanism of Ni and NiTi shape memory alloys show fundamental changes.The researches about microstructure evolution during deformation in materials are mostly focused on the experiment observation.It is difficult to observe the whole microstructure evolution process by in-situ experiment at atomic level,so the microstructure deformation mechanism is not very clear.To reveal the microstructure deformation mechanism,it is necessary to study the temperature,stress,and grain size effects on the microstructure evolution and deformation mechanism by computational simulation.Molecular dynamics simulation can provide the stress-strain response,detailed microstructure morphology,and energy evolution law of system at atomic level,which is helpful for us to understand the mechanical behavior and deformation mechanism of materials.The main work and conclusion of this paper is as follows:(1)The temperature,stress,and grain size effects on creep mechanism in nanocrystalline Ni,and the evolution law of creep mechanism are studied.The results show that the defects in materials play an important role in creep behavior.Single crystal Ni with few defects do not show obvious creep phenomenon,but nanocrystalline Ni with much more defects show obvious creep phenomenon(including primary creep and steady-state creep).With temperature and the level of stress increasing and grain size decreasing,the creep behavior in nanocrystalline Ni becomes more obvious and the steady-state creep rate increases too.The creep mechanism is obtained by analysing the stress exponent,grain size exponent,and microstructure evolution law.With increasing temperature and the level of stress,decreasing grain size,the creep mechanism is found changes from lattice diffusion and grain boundary sliding to grain boundary diffusion and grain boundary sliding,then to dislocation nucleation.It is the much smaller grain size(2.8-5.6 nm)of nanocrystalline Ni that leads to the observed creep mechanism transition law at lower temperature(0.23-0.58 times melting temperature).(2)The grain size effects on phase transition in nanocrystalline NiTi shape memory alloys(nc NiTi)are studied.The results show that when the grain size is reduced,the atomic percentage of interface especially grain boundary increases rapidly,which brings remarkable effects on the phase transition behavior in nc NiTi.The temperature and stress induced martensitic phase transition are almost suppressed,when the grain size is reduced to a certain value.Phase transition temperature(martensitic phase transition start temperature and austenite phase transition finish temperature)decreases with grain size decreasing,and phase transition cannot induced under overcooling in smaller grain size sample.The phenomena caused by martensitic phase transition of average atomic volume increases rapidly,potential energy changes suddenly,and the shear strain appears in grains inside disappear gradually in smaller grain size sample.During stress induced phase transition process,the formed martensite phase in loading decreases and the incomplete phase transition phenomenon occurs in smaller grain size sample.Combining the stress-strain response and microstructure evolution morphology,the energy of system and the energy of each component the energy evolution law of system,energy partition in each component(austenite,martensite,grain boundary,and phase boundary)are counted.The evolution law of the system energy,energy partition in each component,and energy dissipation are obtained,and the grain size effects on those evolution laws are gained too.It is shown that the potential energy landscape of the system changes significantly with grain size reduction from nonconvex(grain size is 11.97 nm)to convex(grain size is 4.02 nm).This brings the corresponding changes in stress-strain response and microstructural evolution.With grain size decreasing,the main energy dissipation mechanism changes from martensitic phase transition behavior to the plastic behavior of grain boundary sliding.It is the gradual dominance of the interfacial energy terms(especially grain boundary)in the total potential energy of nc NiTi with grain size reduction that leads to the observed fundamental changes of phase transition behavior and mechanical behavior.(3)The effects of cyclic loading on phase transition and the tension-compression asymmetry phenomenon in nc NiTi are studied.The results show that the mechanical behavior and phase transition behavior are influenced by cyclic loading.With the number of cyclic loading increasing,almost no plastic strain is produced in larger grain size sample,but few plastic strain is produced in smaller grain size sample.The plastic strain is almost concentrated in the grain boundary,the grain boundary sliding is the main plastic deformation mechanism.The plastic strain leads to the residual stress in grains inside.So the phase transition stress in nc NiTi decreases,which brings the corresponding changes in the law of microstructure evolution and the energy evolution law of system.On the other hand,nc NiTi with random grain size orientation show obvious tension-compression asymmetry phenomenon.The grain orientation affects the phase transition stress,phase transition strain and the scope of phase transition.The phase transition is less suppressed,more martensite appears in compression.Under the same strain,the external force input more energy in compression,which causes the corresponding changes in energy evolution law of system.When the strain is high,the material compression modulus is higher than tensile modulus,which is the major factor of the tension-compression asymmetry phenomenon of stress-strain response in nc NiTi.