Segregation and structural transformations at (sigma) = 3 grain boundaries in NiAl: A Monte-Carlo study

Grain boundary structure and grain boundary segregation in stoichiometric and Ni-rich compositions of NiAl are studied by molecular statics and grand-canonical Monte-Carlo simulations in conjunction with an embedded-atom potential. Significant Ni segregation is found at the (sigma) = 3(1 1 1) and (sigma) = 3(2 1 1) grain boundaries, with segregation isotherms showing a saturation effect. The amount of Ni segregation increases and the excess free energy of the grain boundaries decreases with increased Ni bulk concentration. In strongly Ni-rich bulk compositions, both boundaries undergo transformations to new structures capable of accommodating more excess Ni atoms than the initial structures. The structural transformation is clearly manifested by a non-monotonic behavior of the segregation isotherm. In the (sigma) = 3(2 1 1) grain boundary, the transformation is accompanied by a relative tangential translation of the grains.