Structures and energies of compression twin boundaries inhcpTi and Zr

Using a combination of first-principles and embedded-atom techniques, we have calculated the structure and energy of compression twin boundaries in hcp Zr and Ti, the most ductile of the hcp elements. These calculations are important tests of classical nucleation theory, which predicts that the energy of these structures controls the nucleation rate. Our results contradict this: the {1122} twin boundary, which is prevalent at low temperatures, is significantly higher in energy than the competing {1011} twin which is only formed at higher temperatures.