Search of point defect transition states in hcp twin boundaries: The monomer method

RV.P. RAMUNNI, M. A. ALURRALDE, R.C. PASIANOT

PHYSICAL REVIEW B - CONDENSED MATTER AND MATERIALS PHYSICS

APS

Lugar: Maryland; Año: 2006 vol. 74 p. 541131 - 541137

0163-1829

We develop the monomer, a molecular statics technique for the search of transition states under conditions of relatively complex atomistic environments. As its counterpart from the literature, the dimer, our method is based on forces evaluation alone and is able to find the saddle configuration without previous knowledge of the equilibrium state across the saddle; at variance with it, the needed local curvature is determined with just one force evaluation per iteration step. The method is here applied to the location of saddle configurations relevant to the migration of vacancies and self-interstitials in the (11-21) and (11-21) twin boundaries of hcp-Zr modeled with an embedded atom type interatomic potential. Besides the fundamental interest of studying migration in these environments of reduced symmetry and dimensionality, we aim at a better understanding of grain boundaries as agents contributing to the mechanical deformation under irradiation conditions. Whereas vacancies have already been studied in the same boundaries employing a different methodology, self-interstitial results are new. For comparison purposes and completeness however, some results relating to vacancies are also included. Our main findings relate to the prediction of very low interstitial migration energies that radically change the anisotropy of bulk migration. This behavior may be associated, though not necessarily, with spatially extended configurations. The results suggest that the picture of grain boundaries as essentially perfect sinks, common to models of irradiation creep and growth, may need revision.