Grain boundary composition in NiAl

The results of an atom probe field ion microscopy investigation of the compositions of the grain boundaries in undoped and B-doped NiAl are presented. Two high purity alloys were used. The undoped alloy had a composition of Ni--49 at.% Al. The B-doped material had a stoichiometric NiAl composition to which an addition of 0.04 at.% B (100 wt.ppm) was made. A distinct narrow Al--depleted region was observed at the boundary in the undoped Ni--49Al alloy. The depletion was not observed in the B-doped alloy. Boron segregation to the grain boundary was revealed both in the field ion images and in the atom probe analyses in the B-doped alloy. The coverage of B at the boundary was estimated to be between 4-22% of a monolayer based on a B segregation width of 0.2 nm. The matrix B concentration was determined to be 0.026 plus/minus 0.003% B. The enrichment factor of B at the boundary over the matrix level was found to vary between 180-850.