  We have investigated the coarsening mechanism of intergranular Cr-rich M2B
borides after creep and annealing at 850C for approximately 3000 hours in a
polycrystalline nickel-based superalloy. Borides were found to be coarser after
creep, with measured thicknesses in the range of 800-1100nm, compared to
borides annealed in the absence of external applied load (400-600nm). The
borides had a thickness of 100-200nm before exposure at 850C. Transmission
electron microscopy revealed that coarsened borides have either the tetragonal
I4/mcm structure, or the orthorhombic Fddd, with those two structures
coexisting in a single particle. The presence of a very high density of planar
faults is systematically observed within the coarsened borides. The faults were
correlated with chemical fluctuations of B and Cr, revealed by atom probe
tomography. Our results allow us to suggest that borides coarsen by an
epitaxy-like mechanism. In addition, partitioning of Ni and Co was observed at
dislocations within the borides after creep providing insights into the
deformation of borides. Consequences of coarsened intergranular borides on the
creep performance of polycrystalline superalloys are discussed.
