| The Ductile-to-Brittle Transition (DBT) is a phenomenon that is widely observed in Body Centered Cubic (BCC) metals and in covalently-bonded materials. Below a critical temperature (DBTT), the material suddenly loses ductility. The controlling mechanism of this transition still remains unclear despite numerous experimental and theoretical investigations. However, all previous theoretical investigations are based on two-dimensional dislocation theory. In this work, the Parametric Dislocation Dynamics (PDD) method is extended and its key features examined. We establish here a numerical method that shows high accuracy and convergence for strong dislocation interactions. Applications of the PDD are demonstrated to the determination of the flow stress in irradiated materials, and to the investigation of the mechanism of Persistent Slip Band (PSB) formation under fatigue condition.; The interaction of dislocations and cracks is a key part in understanding the shielding effect of dislocations at the crack tips. A three dimensional discrete dislocation representation (DDR) method is proposed to determine the stress field around a 3-D crack of arbitrary geometric complexity. It is found that nucleated dislocation loops at crack tips modify the triaxiality of the stress field in a substantial way, and that regions of shielding and anti-shielding are established close to the crack tip. When many dislocation loops nucleate close to the tip, they coalesce and form long, and nearly linear dislocations that move away from the tip. A transition zone of 3D to 2D is shown to occur, when the dislocation is far away from the crack. At lower temperatures, a low dislocation mobility prohibits the dislocation from leaving the crack tip region, and thus inhibits the instantaneous nucleation of further dislocations. The fracture toughness of the crack is shown to be controlled by temperature effects of dislocation mobility. |
