Phase-Field Crystal Investigation On Grain Boundary Evolution Behaviors Of Nanocrystalline Grains

As a new category of materials,nanocrystalline materials have attracted widespread attention by scientists of condensed matter physics,chemistry and materials science because of the unique and excellent mechanical,electromagnetic and chemical properties.The most prominent microstructural feature of nanocrystalline materials is that a large number of grain boundaries(GBs)and triple junctions(TJs)exist in nanocrystalline materials,so the evolution of GBs and TJs driven by capillary or external forces can largely determine the properties of nanocrystalline materials.However,evolution of nanocrystalline boundaries is a complex multi-scale physical process that involves not only atomic rearrangement but also interfacial diffusion.In order to understand essentially the effects of boundary movement on the evolution of nanograins under different conditions,it is necessary to have an in-depth knowledge of the evolution of GBs and TJs at multi-spatial-temporal scales.The phase field crystal model,based on the classical density functional theory,is a newly atomic scale methodology of modeling materials microstructure evolution over diffusive time scales.The greatest advantage of the PFC model is its ability to describe complex defect evolution phenomena on atomistic length scales and over diffusive time scales.In this dissertation,on the basis of accurate description of atomic characteristics of GBs,we investigate the evolutionary behaviors of BCC nanocrystalline [001] tilt GBs driven by capillary forces or external shear strains as well as the impact of TJs by using both the standard and minimized bulk dissipation PFC models,which could provide a theoretical foundation for the microstructure controlling of nanocrystalline materials.The main conclusions are as follows:(1)Through high performance computing of parallel programs for PFC models,we have successfully conducted the numerical simulation of the whole processes with respect to the grain boundary evolution driven by capillary forces and external shear strains.Furthermore,we have obtained the accurate description of atomic characteristics for BCC [001] tilt GBs,including the equilibrium structures,thermodynamic energy and the nearby elastic strain field.It is found that the simulated GB structure is a mixture of two types of edge dislocations(a<100> and a<110>/2),which is in agreement with dislocation theory.In addition,the strain field around a simulated dislocation is quantitatively consistent with anisotropic elastic theory of dislocations.(2)We have obtained the scope of applicability of different GB motion mechanisms,including purely capillary driven,purely coupled and mixed ones,with respect to the misorientation in a BCC bicrystal system with an embedded grain.When misorientation is higher than 27.5°,GB migrates under the pure capillary forces,resulting in a linear and rapid decrease in the embedded grain area while the orientation remains constant;when misorientation is less than 20°,GBs migrates in accordance with the pure coupled theory,leading to a linear but slow decrease in the grain area as the orientation gradually increases;in the middle range,GBs move in a partially capillary and partially coupled mechanism,i.e,mixed mechanism,which results in a nonlinear and very slow decrease in the grain area as the orientation slightly increases.(3)We have revealed the mechanism of grain rotation induced by curved GB movements with a very small misorientation.Generally,a curved GB consists of multiple dislocations.When the misorientation is small,there are significant elastic distortions in the coherent region between adjacent dislocations,which causes the radial motion of GB dislocations and rotational torque for the grain.When the misorientation is large,the dislocation spacing is very small as well as the elastic distortion.Meanwhile,dislocation reactions,such as annihilation and decomposition,become dominant,resulting in the pure capillary driven GB motion.In view of atomic structures of GBs,different evolutionary behaviors of GBs with different misorientations can be attributed to the asynchronous spacing changes of multiple GB dislocations with respect to the misorientation.(4)Planar symmetrical tilt GBs migrates in a shear-coupled way under external shear strains,which can be interpreted from the microscopic point of view as nucleation and propagation of disconnections.It is found that a preexisting disconnection dipole plays an important role in the process of shear-coupled GB migration,where the PeierlsNabarro(PN)barrier is determined by the nucleation barrier of disconnections and the coupled factor depends on the dual characteristics(partly partial dislocation and partly step)of disconnections.(5)The effect of inclination angles on the shear-coupled migration of asymmetrical tilt GBs relies on the relative number of the two types of GB dislocations and their responses to external shear strains.When the inclination angle is small,one type of dislocation suffers the largest amounts of deformation and the other type of dislocation moves to follow the former one in an asynchronous way.When the inclination changes to the angle where the amounts of the two types of dislocations are approximately equal to each other,both the onset and proceeding of GB migration are difficult to occur due to the requirement of cooperative movement of dislocations,resulting in the increase of PN barriers and coupled factors.(6)Mechanism concerning the effect of TJs on the evolution of nanocrystalline GBs is obtained and a discontinuous migration model of TJs is constructed.It is found that the movement of GB dislocations is altered by the drag effect of TJs,which also slows the evolution dynamics of grains.Under the external shear strains,dislocations emission near TJs is caused by the uncoordinated movement of GBs.Besides,the grains with the same TJs can exhibit different stabilities to pure capillary driven condition and shear strains.