Simulation Studies On The Mechanical Properties Of Nanometallic Copper Containing Planar/Volume Defects

The plastic deformation and microscopic fracture behavior of nanometallic crystals are closely related to the dynamic evolution and movement of defects within the crystal.An indepth understanding of the evolution of internal defects and the influence of inter-defect interactions on the deformation behavior of nanometallic materials is of great academic significance and application value to enhance the strength,ductility,fracture and radiation resistance and service life of the materials.In recent years,much attention has been paid to enhancement strategies to improve the comprehensive performance of nanometallic materials through reasonable modulation of planar and volume defects.It is necessary to conduct a more in-depth investigation on the plastic deformation mechanism regulated by planar defects and the interaction mechanism between dislocations and volume defects.It is difficult to investigate the dynamic evolution of specific nano-defects in bulk metals based on existing experimental techniques,which severely limits the knowledge of defect structure-mechanical property correlations and their intrinsic deformation mechanisms.Computational simulation methods can help to improve the understanding of the kinetic mechanisms of collective dislocationdefect and defect-defect interactions as well as the quantitative analysis of the temporal and spatial characteristics associated with plastic instabilities.On this basis,this dissertation adopts molecular dynamics,dislocation dynamics methods and multi-scale computational methods with the aim of revealing the microstructure evolution mechanism of nanometallic materials containing planar defects/volume defects,and the following research work is carried out:Firstly,the effects of grain size gradient and twin thickness gradient on uniaxial tensile deformation behaviors of gradient-structured polycrystalline copper by using molecular dynamics simulation are investigated.Simulation results show that the flow stress of gradient nanograined(GNG)or gradielt nanotwinned(GNT)model is linearly dependent on d-1/2(d,the mean grain size)or ?-1/2(?,the mean twin boundary spacing)with different slopes.i.e.,the relationship follows the normal Hall-Petch relation(with d>dc or ?>?c)or the inverse Hall-Petch relation(with d<dc or ?<?c).The transformation of boundary-mediated mechanism to dislocation-based counterpart for accommodating deformation with increasing the applied strain is observed in the GNG model.In the grains with small twin boundary(TB)spacings.the TB migration and annihilation dominate the plastic deformation;while in the grains with large TB spacings,the deformation is accommodated by the dislocation multiplication in the GNT model.Furthermore.the microstructural evolution demonstrates the gradient distribution of strain and the compatibility of deformation induced by the spatial gradient distribution of grain boundary or twin boundary.Secondly,a bulk nano-twinned copper(nt-Cu)model containing spherical/ellipsoidal voids is established,and the relationship between the microstructure of the model and the plastic deformation under uniaxial tensile load is analyzed.By calculating the stress-strain curve and analyzing the dislocation activity during the deformation process,the effects of TB spacing,void size,porosity,void shape and orientation.and loading direction on the mechanical properties of the material are systematically investigated.The results show that the coherent TB can significantly improve the strength of the material.including the yield strength and flow stress.The critical stress of dislocation emission is affected by the size of the voids,which in turn affects the yield stress of nt-Cu.The quantitative results are consistent with the Lubarda theoretical prediction model.Compared with small voids,the critical stress of dislocation emission on the surface of large voids is lower.Larger porosity will increase the area of the stress concentration region.resulting in a decrease in elastic modulus and yield stress.In addition,the transition of the dislocation slip patterns under different loading directions causes anisotropy of the mechanical properties of the structure.When the load is applied perpendicular to the TB,the dislocation moves in the direction approaching the TB.and then is hindered by the TB.When the applied load is parallel to the twin interface,the dislocation slips in the inner region between the two adjacent TBs in the direction parallel to the twin interface,until it is hindered by the grain boundary or other dislocations.Thirdly,a polycrystalline nt-Cu model implanted by helium bubbles is constructed,which comprehensively considers the effects of TB spacing,initial bubble pressure,temperature,and bubble number density on the tensile deformation mechanism of the structure.The research results show that the existence of TBs can significantly enhance the yield strength and flow stress of nanocrystalline copper,and the enhancement effect follows the classic Hall-Petch relationship.The movement of dislocations is hindered by the TBs,and the dislocations are confined between the two layers of TBs,and dislocations are concentrated near the TBs,forming a "dislocation wall".Compared with the average flow stress,the initial bubble pressure has a more significant effect on the yield strength.The critical stress of dislocation emission on the bubble surface decreases with the increase of the initial internal pressure of the bubble.The increase in temperature will weaken the mechanical properties of helium bubbles implanted into nt-Cu.In addition.the increase in temperature will cause the internal pressure of the helium bubble to increase.thereby affecting the yield strength of the material,and this effect will become more significant as the temperature increases.At the nanometer scale,bubbles mainly exist as dislocation emission sources,and their hindering effect on dislocations is not obvious.Therefore,an increase in the number density of bubbles will reduce the mechanical properties of the material including elastic modulus,yield strength and average flow stress.Fourthly,a three-dimensional hierarchical multiscale computational method to quantitatively predict the strength of alloys with shearable or impenetrable precipitates at the microscale is developed.In this method,the behavior of a single dislocation at the nanoscale is quantified in terms of dislocation drag coefficient using molecular dynamics simulations.A physics-based model of precipitate stress field is then constructed and parameterized with molecular dynamics simulation results,which is finally incorporated into dislocation dynamics simulations at the microscale.The model is calibrated by measuring the critical resolved shear stress of a single dislocation bypassing a periodic array of shearable or impenetrable precipitates and the corresponding critical breakaway angle of dislocation configuration.The proposed multiscale approach is applied to model the uniaxial tensile behavior of three-dimensional single crystal copper with constant precipitate volume fraction and number density.The BaconKocks-Scattergood strengthening model is employed to validate the approach and a good agreement between the theoretical predictions and simulation results is obtained,which demonstrates the good capabilities and wide applications of the methodology for the prediction of precipitation strengthening of alloys.