Shear-Induced Homogeneous Deformation Twinning in FCC

The {111}<11 2 > shear stress-displacement behavior for face-centered cubic (fcc) metals, aluminum and copper, is calculated using empirical potentials proposed by Mishin and by Ercolessi, based on the embedded atom method (EAM), and compared with published ab initio calculations. In copper close agreement is observed in the results given by the Mishin potential for both the ideal shear strength and local atomic relaxation during shear, although the extent of plastic deformation before failure is over-predicted. In aluminum, both the Mishin and Ercolessi potentials are used, with only the former able to capture the majority of the behavior exhibited in first principle calculations. Both potentials are shown to have difficulties modeling the effects of directional bonding. Calculations of the multiplane generalized stacking fault energy in both materials reveal that aluminum has a longer range of atomic interaction than copper. Using molecular dynamics and static energy calculations, deformation twins are shown to form by homogeneous nucleation, slip and subsequent coalescence of partial dislocations in both copper and aluminum. The minimum energy path for formation of a two-layer microtwin, and the energy barriers to its further growth are analyzed for the two metals. The mechanism observed is interpreted with reference to existing work on the nucleation of microtwins in bodycentered cubic metals. Thesis Supervisor: Sidney Yip Title: Professor of Nuclear Engineering