Direct observation of the compositional disordering of Ni3Al in the vicinity of grain boundaries using high resolution electron microscopy techniques

The ordered compound Ni/sub 3/Al is potentially useful as a high temperature structural material, since its yield stress increases with increasing temperature up to --650/sup 0/. The major obstacle to the structural use of polycrystalline Ni/sub 3/Al is its tendency to fail intergranularly. The ductility of Ni/sub 3/Al can be improved by the addition of small amounts of boron to a slightly Ni-rich compound. The boron is found to segregate to the grain boundaries, and changes the fracture mode from intergranular to transgranular by a mechanism which is not yet understood. Possible explanations for the presence of a/2 -type dislocations are: (i) the dislocations are imperfect, with the associated antiphase boundary contrast not visible, or (ii) the Ni/sub 3/Al is disordered in the vicinity of the interface, resulting in a thin layer of f.c.c. material at the boundary, and the dislocations then have the expected f.c.c. Burgers vector. In support of the possibility of disordering are atom-probe field ion microscopy observations which showed that the boundary region is Ni-rich, with deviations from the average alloy composition of up to 5 at % Ni. It is reasonable to predict that the thickness of the disordered layer will be a function of temperature,more » and so the contribution of the disordered layer to the mechanical properties may also be temperature dependent. It can be speculated that the solubility of boron, and any other impurities that are present, will be different in disordered and ordered Ni/sub 3/Al.« less