| Nanocrystalline materials such as single-crystal copper,single-crystal silicon,polycrystalline copper,and polycrystalline silicon,are widely used in aerospace,weapon equipment manufacturing,electromechanical system fields.However,due to the nano effect(including surface effect,small size effect and macroscopic quantum tunneling effect)in nanocrystalline materials,the traditional machining theory based on continuum mechanics is no longer suitable for studying nanoscale machining mechanism.Therefore,in order to further develop nanoscale machining technology,it is necessary to explore the mechanism of nano-machining,especially the surface integrity and subsurface damage mechanism to explain and guide the nano-machining,which ultimately improves the nano-machining efficiency and surface quality and reduces the process costs.In this paper,the mechanical properties,machining and deformation mechanism in nanocrystalline materials were studied by molecular dynamics(MD)method and elastic-plastic mechanics model.This study is of great theoretical value in understanding ultra-precision machining and mechanical behavior of nanocrystalline material,and it is of practical significance for the machining of micro/nano devices.The mechanical properties of single-crystal silicon in lithium batteries,and the tensile and indentation properties of high entropy alloy(HEA),as well as the special phenomena(e.g,shear deformation induced crack closure)were investigated.Then,the machining properties of HEAs were studied.In order to realize the high-efficiency and low-damage of nanocrystalline material during high-speed machining,this paper systematically studies the interaction between tool and workpiece.The effects of various machining parameters on surface integrity and subsurface damage was studied.The dislocation effect on the diffusion induced stress(DIS)from Li-ion embedded single-crystal silicon electrode and the crack nucleation in single-crystal silicon electrode was discussed.The machining non-smooth surface workpiece was studied to reveal the roughness effect on surface integrity and subsurface damage.The machining of porous and particle reinforced materials was investigated,and the effects of pore and particle on the machining induced mechanical properties were investigated.In addition,the machining properties of polycrystalline and nanotwinned(NT)polycrystalline were analyzed.It is revealed that grain boundaries(GBs)and twin boundaries(TBs)play a key role in the plastic deformation.The main results and innovative work of this paper are as follows:(1)Based on three-dimensional MD simulations,this paper investigates the mechanisms of subsurface damage and material removal of single-crystal copper under a nanoscale high speed grinding of a diamond tip.The key factors that would influence the deformation of the material were carefully explored by analyzing the chip,dislocation motion,and workpiece deformation,which include grinding speed,depth of cut,grid tip radius,crystal orientation and machining angle of copper.An analytical model was also established to predict the emission of partial dislocations during the nanoscale high speed grinding.The investigation showed that a higher grinding velocity,a larger tip radius or a larger depth of cut would result in a larger chipping volume and a greater temperature rise in the copper workpiece.A lower grinding velocity would produce more intrinsic stacking faults(ISF).It was also found that the transition of deformation mechanisms depends on the competition between the dislocations and deformation twinning.There is a critical machining angle,at which a higher velocity,a smaller tip radius,or a smaller depth of cut will reduce the subsurface damage and improve the smoothness of a ground surface.The established analytical model showed that the dislocation emission is most likely to occur with the crystal orientations of(0 0 1)[1 0 0]at the angle 45°.(2)Three-dimensional MD simulations are performed to investigate the nanoscale grinding process of single crystal silicon using diamond tool.The effect of grinding speed on subsurface damage and grinding surface integrity by analyzing the chip,dislocation movement,and phase transformation are studied.We also establish an analytical model to calculate several important stress fields including hydrostatic stress and von Mises stress for studying subsurface damage mechanism,and obtain the dislocation density on the grinding subsurface.The results show that a higher grinding velocity in machining brittle material silicon causes a larger chip and a higher temperature,and reduces subsurface damage.However,when grinding velocity is above 180 m/s,subsurface damage thickness slightly increases because a higher grinding speed leads to the increase in grinding force and temperature,which accelerate dislocation nucleation and motion.Subsurface damage is studied by the evolution of surface area at first time for more obvious observation on transition from ductile to brittle,that provides valuable reference for machining nanometer devices.The von Mises stress and hydrostatic stress play an important role in the grinding process,and explain the subsurface damage though dislocation mechanism under high stress status.The dislocation nucleation and motion induced plastic deformation during grinding process can better reveal subsurface damage mechanism considering to stress and temperature acting on the dislocations.(3)This paper is theoretically suggested to describe the combined effects of DIS and dislocation induced stress in a cylinder lithium ion battery electrode on the nucleation and propagation of cracks under galvanostatic or potentiostatic solute insertion and extraction.By the conventional assumption,we develop this model accounting for dislocation mechanics in a cylindrical electrode under axisymmetric DIS,focusing on the dislocation and size effects on the magnitude and distribution of the combined DIS during galvanostatic or potentiostatic condition.The results show that dislocation induced stress can decrease tensile stress,and converts the state of stress from tensile to compressive.The trend of the crack nucleation and propagation can be prevented as the cylindrical particle radius drops down to nanoscale range.Dislocation induced stress suppressing the crack nucleation,however,provides a novel way of mitigating internal damage in a cylindrical lithium ion battery during cycling.It may be used in conjunction with the methods of nano-engineering to create microstructures tailored to maximize suppressing the crack nucleation,yielding new strategy to improve battery life and avoid failure.(4)The process of material removal of single crystal copper with rough surfaces subjected to nanoscale scratching is studied in the present paper.We explore the key material removal mechanism by means of the observed variation of material removal under different surface roughnesses,tool speeds,scratching directions,tip shapes,feeds,double tip and single tip.The investigation reveals that a higher peak on the surface reduces the local area roughness,and a higher valley enhances the stability of surface structure.The plastic deformation by means of dislocation loop transfers from the surface of substrate to the interior of workpiece with the rough or smooth surface during scratching process.A higher scratching velocity results in the increasing surface smoothness and reducing the impact on the rough surface atoms.The scratching along the critical angle 45° between scratching direction and surface texture orientation makes the surrounding atoms produce the minimal variation structure,helps to improve the structural stability,and plays an important role in protecting the scratching surface.The double tip and single tip scratching under different scratching feeds makes the rough surface on the perpendicular to the scratching direction substantially cover by chips or side flow.For different tip shapes,a cone diamond tip causes less plastic deformation in the subsurface than a prismatic diamond tip due to using different diamond tips with a contact area unequal.(5)The subsurface damage and surface integrity of a spherical diamond indenter sliding against a face-centred cubic copper(100)surface considering the pore and second-phase particle effects is investigated by means of MD simulations of nanoindentation followed by nanomachining.In this investigation,we establish an analytical model for pore healing,and provide a criteria to determine whether or not pore can be healed.The results show that with increase of machining distance pore becomes smaller and then closes due to machining-induced compressive stress,resulting in low material damage and strong structure stability.Compared to free pore workpiece,machining force slightly relies upon the existence of pore and second-phase particle while friction coefficient strongly depends on the existence of that.In addition,particle induces work hardening due to Lomer-Cottrel lock and dislocation slip during machining metal matrix composites.It is helpful to understand the relation of machining performance and material parameter for obtaining higher surface integrity and lower subsurface damage during machining porous metals and particle reinforced metal matrix composites.(6)Three dimensional MD simulations are systematically carried out to reveal the mechanism of the crack healing at room temperature,in terms of the dislocation shielding and the atomic diffusion to control the crack closure,in a copper(Cu)plate suffering from a shear loading.The results show that the process of the crack healing is actualized through the dislocation emission at a crack tip accompanied with ISF ribbon forming in the crack tip wake,the dislocation slipping in the matrix and the dislocation annihilation in the free surface.Dislocation included stress compressing the crack tip is examined from the MD simulations and the mathematical models,and then the crack closes rapidly due to the assistance of the atomic diffusion induced by the thermal activation when the crack opening displacement is less than a threshold value.This phenomenon is very different from the previous results for the crack propagation under the external load applied because of the crack healing(advancing)largely dependent on the crystallographic orientation of crack and the direction of external loading.Furthermore,based on the energy characteristic and considering the crack size effect,a theoretical model is established to predict the relationships between the crack size and the shear stress which qualitatively agree well with that obtained in the MD simulations.(7)The plastic deformation mechanisms of nanoscratching process are investigated through the study of a rigid diamond tip sliding against nanocrystalline Cu using MD simulation.Special attentions are paid to the scratching rate effects,as well as the crystal structural effects from single crystalline,polycrystalline and NT polycrystalline.With the increase of scratching rate,scratching force and workpiece temperature increase continuously due to severe plastic deformation and large chip volume,resulting in dislocation slip,GB slip,and twining/detwining.Scratching rate also governs the distributions of potential energy and kinetic energy of all the atoms,revealing the rate-dependent plastic deformation.Specifically,the plastic deformation for different scratching rates depends on the competition of scratching force,workpiece temperature and tool-workpiece contacting time that affect dislocation evolution.In addition,the results show that the plastic deformation due to scratching of single crystalline Cu is dominated by the dislocation-dislocation interactions.And the scratching induced plastic deformation of polycrystalline Cu is determined by the dislocation-GB interactions.As for NT polycrystalline Cu under scratching,it is the dislocation-GB-TB interactions accompanied with the twining/detwining process.While the presented MD simulations and the associated conclusions are based on nanocrystalline Cu,it is believed that the current deformation mechanism could also be applied to other face-centered-cubic nanocrystalline metals.(8)Although HEA has exhibited promising mechanical properties,little attention has been given to dynamics deformation mechanism during uniaxial tension to limit its widely practical utility.According to the experiment,AlCrFeCuNi HEA of atomic model is built using a melting and quick quenching method.In this work,the mechanical behaviors of AlCrFeCuNi HEA under uniaxial tensile loading are studied using atomistic simulation to investigate the evolution of dislocation and SF as well as deformation twin.The results show that calculations for the elastic properties and stress-strain relations are in excellent agreement with recent experimental results.Above all,AlCrFeCuNi 1.4 HEA not only has high strength,but also exhibits good plasticity which is qualitatively consistent with the experiment.Similar to mechanical properties of single-crystal metals,stress fluctuation during the plastic deformation of HEA is always accompanied with the generation and motion of dislocation and SF with the increase of strain.In addition,the dislocation-dislocation interaction,dislocation-solid solution interaction,deformation twinning and detwinning occur after the yield point.Furthermore,the dislocation gliding,dislocation pinning due to the severe lattice-distortion and solid solution,and twinning are still the main mechanism of plastic deformation in AlCrFeCuNi 1.4 HEA.This atomistic mechanism provides a fundamental understanding of plastic deformation in HEA.(9)Using MD simulations we study the elastic and plastic deformations of indentation in AlCrFeCuNi HEA.The indentation tests are carried out using spherical rigid indenter to investigate the effects of high-entropy and severe lattice distortion in terms of shear strain,indentation force,surface morphology,defect structure,dislocation evolution and radial distribution function on the deformation processes.It can be found that when the indentation depth increases,the shear stress requires for the occurrence of the contact area between the indenter and the substrate increased,which is attributable to a higher probability to observer the dislocation evolution under a large indentation depth.The indentation test also shows that the equal element addition can significantly improve the mechanical properties of HEA compared with the conventional alloy.Based on the Hertzian fitting,the AlCrFeCuNi HEA has the Young's modulus of 161 GPa and hardness of 15.4 GPa,respectively.These values are higher than that of traditional metal materials,due to the low stacking fault energy(SFE)and the dense atomic arrangement in the slip plane of HEA.In the plastic region,the Fe element causes the more stable crystal structure,much stronger than the Cu element,presumably resulted from a variety of crystal structures for Fe in the multicomponent AlCrFeCuNi alloy.Further,this effective strategy is used to accelerate the discovery of excellent mechanical properties of HEAs.(9)The mechanical behaviors and deformation mechanisms of scratched AlCrCuFeNi HEAs have been studied by MD simulations,in terms of the scratching forces,atomic strain,atomic displacement,microstructural evolution and dislocation density.The results show that the larger tangential and normal forces and higher friction coefficient take place in AlCrCuFeNi HEA due to its outstanding strength and hardness,and high adhesion and fracture toughness over the pure metal materials.Moreover,SFE in HEA increases the probability to initiate dislocation and twinning,which is conducive to the formation of complex deformation modes.Compared to the single element metal workpieces,the segregation potency of solutes into TB is raised due to the decreasing segregation energy of TB,resulting in the stronger solute effects on improving twinning properties for HEA workpiece.The higher dislocation density and the more activated slipping planes lead to the outstanding plasticity of AlCrCuFeNi HEA.The solute atoms as barriers to hinder the motion of dislocation and the severe lattice distortion to suppress the free slipping of dislocation are significantly stronger obstacles to strengthen HEA.The excellent comprehensive scratching properties of the bulk AlCrCuFeNi HEAs are associated with the combined effects of multiple strengthening mechanisms,such as dislocation strengthening,deformation twinning strengthening as well as solute strengthening.This work provides a basis for further understanding and tailoring SFE in mechanical properties and deformation mechanism of HEAs,which maybe facilitate the design and preparation of new HEAs with high performance. |
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