Atomistic simulation of dislocation core structures in B2 NiAl

A systematic study of the core structures of (100), (110), and (111) dislocations in B2 NiAI has been conducted using atomistic simulations with an embedded atom method (EAM) potential. New flexible boundary conditions and a new method of graphic representation of dislocation core structure have been employed. The main findings are the following:

Core structures: There are no planar core structures of the dislocations found in B2 NiAl. The core spreading of (100) dislocations in NiAl can occur along a variety of planes depending on dislocation slip plane and line orientation. Discrete lattice effects reduced the high strain levels from anisotropic elasticity solution at the dislocation core considerably and resulted in asymmetrical core structures. The core structure of the (110) dislocations is mutilayered with spreading on the {110} plane. The extent of the same strain level comparing with (100) and (111) dislocations is much larger. The complete (111) dislocations in NiAl are also highly non-planar and are stable with respect to splitting into exact 1/2(111) partials as well as to alternative splittings that correspond to the stable fault in the vicinity of the antiphase boundary (APB), in both {110} and {112} planes. Peierls stresses: Peierls stresses of the dislocations have been calculated and have been compared for their relative ease of motion. Local disordering effects: The local disordering effects on the core structure are found to be significant only in the immediate vicinity of the point defect. Compositional deviation from stoichiometry: The simulation results of (100), (110), and (111)dislocations in off stoichiometric NiAl show that the core structures became more extended than the ones in the stoichiometric NiAl. The core structures are not only dependent on the overall composition but also on their local atomic arrangement near the core region. When compositional deviation from stoichiometry is introduced, the response to the applied stress is different for the various slip systems. The Peierls stresses for the usually easiest moving (100){110} dislocations increased and for the (100){100} dislocations decreased, and the latter are expected to be more active in the deformation processes.

The practical implications of these results are that it seems very difficult to modify the alloy behaviors through local changes in stoichiometry and ordering state. The best way to improve the ductility of B2 NiAl is to stabilize (111) slip through the addition of alloying elements that can lower the APB energy.