The monomer method was developed. This was a molecular statics technique for searching for transition states in relatively complex atomistic environments. Like its counterpart, the dimer method, the present technique was based upon force evaluation alone and was able to find the saddle configuration without previous knowledge of the equilibrium state across the saddle; at variance with it, the needed local curvature was determined with just one force evaluation per iteration step. The method was here applied to the location of saddle configurations relevant to the migration of vacancies and self-interstitials in the (11▪1) and (11▪2) twin boundaries of α-Zr modeled with an embedded atom type interatomic potential. As well as the fundamental interest of studying migration in environments of reduced symmetry and dimensionality, the aim was to obtain a better understanding of grain boundaries as agents which contributed to mechanical deformation under irradiation conditions. Whereas vacancies had already been studied in the same boundaries employing a different methodology, self-interstitial results were new. For comparison purposes and completeness however, some results relating to vacancies were also included. The main findings related to the prediction of very low interstitial migration energies that radically changed the anisotropy of bulk migration. This behavior could be associated, though not necessarily, with spatially extended configurations. The results suggested that the picture of grain boundaries as essentially perfect sinks, common to models of irradiation creep and growth, might need revision.

Search of Point Defect Transition States in HCP Twin Boundaries - the Monomer Method. V.P.Ramunni, M.A.Alurralde, R.C.Pasianot: Physical Review B, 2006, 74[5], 054113 (7pp)