Effect Of Nanoscale Twin And Dislocation Pile-up At Twin Boundary On Crack Blunting In Nanocrystalline Materials

Nanocrystalline materials have attracted much attention due to their unique mechanical characteristics.In most cases,nanocrystalline materials exhibit superior strength,strong hardness and good wear resistance but at the expense of low tensile ductility and fracture toughness,which considerably limit their practical application.However,some nanocrystalline materials such as Cu with good ductility or enhanced toughness have still been studied and reported.The outstanding combination of good ductility and superior strength has created high interest in understanding the toughening mechanisms specific for nanocrystalline materials.Recently,various models have been developed to explain this phenomenon.In most of them,local migration of grain boundaries,rotational deformation,grain boundary sliding and deformation twinning have been theoretically described as specific deformation models in nanocrystalline materials.Research on the effect of twin and dislocation pile-up at the twin boundary on crack blunting in nanocrystalline materials,not only can help us to understand the relationship between the evolution of microstucture caused by twin and dislocation pile-up at the twin boundary and the fracture toughness of materials,but also can lay a theoretical foundation for microstructure design and fracture prevent of nanocrystalline materials.This thesis sets the microstructure changes of nanocrystalline materials as background and researches on the effect of twin and dislocation pile-up at the twin boundary on crack blunting by establishing correlative mechanical models and utlizing elastic complex potential method.The main achievements are summarized as follows:(1)A theoretical model is developed to investigate interaction between nanoscale twin,the dislocation pile-up at the twin boundary and the finite striate crack in infinite plane.Meanwhile,the stress intensity factor expressions from the crack tip are derived,the results show that:(a)Both the twin and the dislocation pile-up at the twin boundary would suppress the dislocation emission from the crack tip,thus weaken the toughness of the nanocrystalline materials.The suppressive effect induced by the dislocation pile-up at the twin boundary is much stronger than that by the twin.(b)The crack length has slight influence on the mode I shielding/anti-shielding effect on the crack tip,but the emission angle has significant effect on that.The location of twin has significant influence on the dislocation emission from the crack tip.Under a certain circumstance,there is a best twin position which can make the dislocation emission easiest.(2)A theoretical model is developed to describe the effect of the nanoscale twin and the dislocation pile-up at the twin boundary on dislocation emission from an elliptical blunt crack tip in nanocrystalline materials.In the model,the nanoscale twin as a stress source approximately equivalent to a quadrupole of wedge dislocation.The expressions of the critical stress intensity factor for the dislocation emission from the elliptical blunt crack tip are derived.The influence of the disclination strength,twin size,orientation of twin,crack length,radius of curvature,as well as the dislocation emission angle on the critical sress intensity factor for dislocation emission from an elliptically blunted crack tip is revealed.