Effect of misorientation on microstructures and properties of a bicrystal alloy

Effect of mathematical misorientation on microstructures and properties of bicrystal superalloy RR2086 have been investigated. Prior to thermal exposure, precipitates of modified RR2086 bicrystals (BXs) at grain boundaries were mainly MCs. After the exposure, a reasonable amount of discontinuous and blocky M₂₃C₆ precipitates which were embedded in a γ′ layer along the grain boundary were found at grain boundaries of the modified RR2086 BXs. This type of precipitates is known to be ductile and creep-resistant. No cellular M₂₃C₆ appeared during exposure. This structure results in improved creep performance of bicrystals of high-angle boundaries in the modified BXs compared to their base counterparts. The modified RR2086 BXs exhibited higher grain boundary creep resistance and higher grain boundary ductility of creep fracture. This is attributed to the grain boundary strengthening of the MC and M₂₃C₆ precipitates, the mechanical keying effect due to their coherence with one neighbouring grain, which effectively suppresses grain boundary sliding. The γ′ layer also played an important role in improving creep resistance of the grain boundaries. The formation of a γ′ layer along grain boundaries was initiated by the conversion of M₂₃C₆ in modified BXs. (paper)