Atomistic simulation of [001] symmetrical tilt grain boundaries in NiAl

Abstract

We study the atomic structure of [001] symmetrical tilt grain boundaries (GBs) with stoichiometric composition in the B2 compound NiAl using molecular statics with an embedded-atom potential. The simulations are performed in the misorientation range 0° ⩽ θ 90° in every 3° in average. The multiplicity of stable GB structures is addressed by using the γ-surface technique combined with full atomic relaxation. The GBs typically have only a few nonidentical stable structures. All other structures either are symmetrically related to those few structures or are their strained variants and reduce to one of them if relative translations of the grains are allowed. All ground-state structures are composed of only four types of structural unit, or their variants obtained by site substitution. Many structural features of the GBs can be understood in terms of the significant atomic size effect peculiar to NiAl. The ground-state Σ = 5, (310), θ = 36.87° and Σ = 5,(210),θ = 53.13° GBs, as well as all GBs with intermediate orientations, show a relative shift of the grains by 1/2[001]. All such GBs follow the structural unit model with the two Σ = 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 θ< 30° and θ> 65°. In some GBs, the primary or secondary GB dislocations are observed to dissociate in partials associated with GB steps. Many GBs contain regions of antiphase boundaries on (100) and (110) planes. The behaviour of GBs under applied shear stress parallel to the GB plane is discussed.