PNNL

Abstract

Tens-nanometer sized defect substructures such as dislocation network, nanotwin and new grain or phase may form during solid phase processing (SPP), which affect the energy landscape, hence, the stability and evolution of phase and structure. Developing non-equilibrium thermodynamic models needs the correlation among the energy, defect substructure and deformation. In the current work, we use molecular dynamics (MD) method to simulate defect substructure evolutions in polycrystalline Al under compress and shear stresses. The effect of local stresses on the formation and transformation of typical defect substructures were analyzed. It was found that transitions from FCC, nanotwin, HCP, BCC, HCP to FCC lead to the formation of subgrains facilitated with large grain rotation. Our results demonstrate that point defect concentrations (e.g., HCP, dislocation core atoms) can be used as internal variables to describe defect substructures, such as dislocations, nanotwins and sub-grain boundaries. An energy landscape of defect substructures in polycrystalline Al under compress and shear stresses was established which shows unstable and metastable defect substructures. Approaches to developing more accurate energy landscapes for understanding and predicting microstructure evolution under SPP was discussed.