STRUCTURES AND ENERGIES OF COMPRESSION TWIN BOUNDARIES IN HCP TI AND ZR

Using a combination of first-principles and embedded-atom techniques, we have calculated the structure and energy of compression twin bounda ries in hcp Zr and Ti, the most ductile of the hcp elements. These cal culations are important tests of classical nucleation theory, which pr edicts that the energy of these structures controls the nucleation rat e. Our results contradict this: the {11 (2) over bar 2} twin boundary, which is prevalent at low temperatures, is significantly higher in en ergy than the competing {10 (1) over bar 1} twin which is only formed at higher temperatures.