GRAIN BOUNDARY CHEMISTRY IN Ni₃Al AND Ni₃Si

Abstract
Measurements of the Ni/Al ratio at grain boundary regions in Ni₃Al have been performed, using x-ray microanalysis of thin foils in a scanning transmission electron microscope, as a function of both alloy stoichiometry and the addition of boron. It is shown that whilst grain boundaries of Al-rich and stoichiometric Ni₃Al have a composition similar to the matrix composition, boundaries in Ni-rich Ni₃Al can be highly Ni-enriched. The addition of B to Ni₃Al produces Ni enrichment in grain boundaries, the extent of which increases with increasing Ni content of the alloy. Also Auger electron spectrometry has been used to show that B segregates to the grain boundaries irrespective of the Ni : Al ratio. The chemistries of grain boundaries in Ni-rich Ni₃Si (Ni₃Si only exists on the Ni-rich side of stoichiometry), with and without B, have also been measured by AES. The results are similar to those from Ni₃Al, that is, the grain boundaries are enriched in Ni and the addition of B further increases this Ni-enrichment. The implications of these results on the degree of grain boundary atomic order and on the ductility of polycrystals are discussed.