Molecular dynamics (MD) simulation study of low angle grain boundary (LAGB) mobility in pure Al and Al-Mg alloys

Abstract

<p>Low angle grain boundary (LAGB) mobility is an essential parameter for developing the analytical models that describe the kinetics of recovery and predict the nucleation of recrystallized grains. The thesis is aimed at the molecular dynamics (MD) simulations study of LAGB mobility determination in pure Al and Al-Mg alloys. All the previous experimental studies reported that the presence of several defects, such as solutes and dislocations, retard the boundary motion and provide lower mobility. However, very few studies have been conducted in MD simulation to capture the interactions of those defects with the migrating grain boundary. This thesis is focused on providing complete understanding of LAGB determination along with a comprehensive explanation of solute and dislocation retarding effects on boundary motion.</p> <p>The LAGB mobility in pure Al was computed from two different MD techniques as a function of temperature and misorientation. Within numerical uncertainties, both techniques provide the same magnitude of mobility at 300K for 7.785^o boundary and at 700K for 23.07^o boundary. It was observed that ADF method is not applicable to determine LAGB mobility at high temperature due to failure of order parameter computation. The MD derived activation energy is found to be approximately ten times lower than the experimental observations.</p> <p>A strong solute pinning effect on boundary motion was observed at all misorientations and solute concentrations studied in Al-Mg alloys. An approximate linear relationship is found between the restraining force and the solute concentration in a distributed solute approach. In addition, the extrinsic dislocations are found to completely pin both 7.785^o and 23.07^o boundary motion at low driving forces in pure Al at 300K. The MD results do not reveal significant qualitative differences of the pinned boundary structure for the low and high angle boundaries and will be discussed in terms of the previous experimental observations.</p>