Crack Nucleation Related Dislocation Dynamics - A Numerical Study on the FCC Metal

Abstract

The present thesis consists of a number of studies on the fatigue related to dislocation dynamics phenomena. These studies were intended to provide an in-depth understanding of the dynamics at the atomic scale. Until now, our understanding on such atomistic dynamics remains sparse and incomplete. Special attention was paid to two particular dislocation processes, which were the dislocation dipoles disintegration and the triple junction deformation. Both scenarios are closely related to a fatigue crack initiation at the cycle fatigue loading condition. Studies were carried out by means of numerical molecular dynamics simulation. The software used in these studies was developed by the author, and was tailored for superior performance and efficiency.

One of the most important questions addressed in this thesis was the intermediate disintegration pathway of a group of dislocation dipoles. The disintegration of dipoles was realised in experiments for decades. It was known to produce massive amount of point defects as a result. However, the process cannot be observed directly. The steps between dislocation dipole accumulation to point defects production are in the missing puzzle. Molecular dynamics simulation with a precise force description was deployed to investigate the scenario in a pure aluminum sample. The analysis suggests that cross-slip is the major mechanism for the disintegration. The cross-slip leads to the formation of stacking fault tetrahedrons, which then collapse to form vacancies clusters.

Another question addressed in the thesis was the deformation mechanism at the vicinity of a triple junction. It is known that such process leads to the embrittlement of a material and is related to the intergranular crack initiation. Models in the past did not consider the dynamical nature at the molecular scale. Investigation was therefore carried out to address this issue. The study illustrates that triple junction can deform via dynamic recrystallization and amorphization. Such illustration can be useful for the development of a more comprehensive model of crack initiation at a triple junction.