ATOMISTIC SIMULATION OF [001] SYMMETRICAL TILT GRAIN-BOUNDARIES IN NIAL

We study the atomic structure of [001] symmetrical tilt grain boundari es (GBs) with stoichiometric composition in the B2 compound NiAl using molecular statics with an embedded-atom potential. The simulations ar e performed in the misorientation range 0 degrees less than or equal t o theta less than or equal to 90 degrees in every 3 degrees in average . The multiplicity of stable GB structures is addressed by using the g amma-surface technique combined with full atomic relaxation. The GBs t ypically have only a few nonidentical stable structures. All other str uctures either are symmetrically related to those few structures or ar e their strained variants and reduce to one of them if relative transl ations of the grains are allowed. Ail ground-state structures are comp osed of only four types of structural unit, or their variants obtained by site substitution. Many structural features of the GBs can be unde rstood in terms of the significant atomic size effect peculiar to NiAl . The ground-state Sigma=5, (310), theta=36.87 degrees and Sigma=5,(21 0), theta=53.13 degrees GBs, as well as all GBs with intermediate orie ntations, show a relative shift of the grains by 1/2[001]. All such GB s follow the structural unit model with the two Sigma=5 GBs being the delimiting boundaries. For all other orientations, the ground-state GB structures have no shift along the tilt axis and are consistent with the structural unit model only at theta < 30 degrees and theta > 65 de grees. In some GBs, the primary or secondary GB dislocations are obser ved to dissociate in partials associated with GB steps. Many GBs conta in regions of antiphase boundaries on (100) and (110) planes. The beha viour of GBs under applied shear stress parallel to the GB plane is di scussed.