Research On Nonhomogeneous Dislocation Nucleation And Interaction Between Dislocation And Grain Boundary

High strength and hard toughness of crystalline solid are intensively investigated atsub-micron and nano scales. The yield strength of perfect crystal depends on thedislocation nucleation. The interactions between dislocations and grain boundary affectthe plastic properties of crystal material. The research work is mainly focused on thedislocation nucleation and the interactions between dislocations and grain boundary.Based on the atom potential method, the stability theory is developed, and thenonhomogeneous dislocation nucleation of crystal sheet is studied. The Cauchy-Bornrule is introduced to describe the deformation of2D hexagonal crystal sheet. Thenonhomegeneous dislocation nucleation criterion is deduced. The onset of dislocationnucleation and the criterion distribution map are illustrated. Furthermore, the loadingamplitude corresponding to the dislocation nucleation can be determined by comparisonbetween the stress-strain curve and criterion-strain curve easily. The results show thatthe dislocation nucleation stress is related to the crystal sheet orientation angles.By using molecular dynamics method, yield strength of copper with (100) twistgrain boundary effect are investigated under shear loading. The critical twist angle isobtained, which mediates the hardening mechanism of twist grain boundary. When twistangle is lower than critical value, the yield strength increases with the increase of thetwist angle, and the yield form of twist grain boundary is the dislocations nucleation attwist grain boundary and dislocations piling-up in the grain interior. When twist angle isgreater than critical value, the yield strength decreases with increasing the twist angle,and the yield form of twist grain boundary is the grain boundary sliding instead ofdislocation nucleation. Furthermore, the hardening mechanism of copper with (111)twist grain boundary is illustrated also. The yield form of twist grain boundary fortensile stress is the partial dislocation nucleation and propagation. The yield form oftwist grain boundary for compression is emission and propagation of dislocation loops.When two dislocation loops meet, the intersection of dislocation loops occur. The yieldform of twist grain boundary for shear load is proliferation of the twist grain boundary.The relationship between the twin boundary spacing and yield strength is obtained.The critical twin boundary spacing is founded for shear strength and compression strength. When the twin boundary spacing is lower than the critical value about1nm,the compression strength and shear strength increase with increase of the twin boundaryspacing. When the twin boundary spacing is greater than the critical value1nm, thecompression strength and shear strength decreases with increasing the twin boundaryspacing. The tensile and compression yielding forms are the dislocation nucleation andpropagation from the section of free surface and twin boundary. The shear yielding formis the twin boundary motion perpendicular to the direction of shear load.Based on the gradient plastic theory of defect energy, the implementation finiteelement is illustrated. Considering rectangular and hexagonal form grains, themechanical properties of multi-grain thin film have been studied. There are someconclusions obtained. It is illustrated that slip system affects the hardening of thin film.The fact that dislocations escape from free surface more easily, makes the thin filmsoftening. The dislocations, which pile up at grain boundary, lead to the thin filmhardening. With increasing the grain boundary density, the hardening influence of thinfilm becomes stronger. The simulation results reasonably show that the grain boundarydensity is linear to the yield strength and hardening rate, respectively.