In a model Ni-base superalloy of Ni-23.4Co-4Al-4.3Ti-4.7Cr(wt.%), singular and rough grain boundaries which exist at a temperature range between 1100℃ and 1150℃ can be identified by observing the 3-dimensional shapes of the coherent γ'-precipitates impinging on them by scanning electron microscopy. Some grain boundaries become curved with γ'-precipitates. But some grain boundaries maintain flat shapes with γ'-precipitates as twin boundaries. Such flat shapes are possible only when there are singularities in the Wulff-plots of the γ-γ' incoherent interphase boundaries and γ-γ' grain boundaries. Therefore, the curved grain boundaries should be rough and flat grain boundaries singular. Many flat segments of general high angle grain boundaries lie on low index planes of one of the grain pairs. The size of precipitates at the grain boundaries depends on the grain boundary structure. The average size of precipitates at the rough grain boundaries is significantly larger than that of precipitates at the singular grain boundaries. At the twin boundaries, the average size of precipitates is very small.
