Molecular Dynamics Simulation of Mechanical Behaviors of a System Including Both a Crack and Grain Boundaries under Cyclic Loading

摘要

The mechanical behaviors around a crack tip for a system including both a crack and two tilt grain boundaries under cyclic loading are examined using a molecular dynamics simulation. The grain boundary, whose direction of the axis of misorientation angle is <110> and whose plane is {112}, is considered in this simulation. This grain boundary has the lowest grain boundary energy among all tilt grain boundaries. The Johnson potential for alpha-Fe is used in the analysis to describe interaction between atoms. Not only a structural transition from bcc to hcp but also ductile deformation occurs around the crack tip during the first loading in order to relax stress concentration. Edge dislocations emitted from the crack tip are observed and they move to the <111> direction on the {112} plane, which is a slip system of alpha-Fe. Then, two dislocation pile-ups near the grain boundaries are formed after the edge dislocations reach the grain boundaries, because they cannot move beyond the grain boundaries. Alternating slipping-off occurs at the crack tip because of the emission of the edge dislocations. During the first unloading, the edge dislocations emitted from the crack tip return to the crack tip and disappear in the system. Then, twin deformation occurs from the crack tip and expands along the slip direction until it reaches the grain boundary. We observe that not only the crack does not propagate in the cleavage plane of alpha-Fe but also several vacancies are generated along the slip direction from the crack tip during cyclic loading. Conclusively, we suggest the fatigue crack growth mechanism for the initial phase of the fatigue fracture. That is, the fatigue crack propagates along the slip direction due to coalescence between the crack and vacancies which are caused by the emission and absorption of the dislocations and the twin deformation around the crack tip.