On grain boundary phases in B-doped Ni3Al

Polycrystalline Ni3Al normally fractures intergranularly with little plastic deformation. Its ductility can be greatly increased by adding small amounts of boron. Plastic tensile strains as high as 50% have been reported by C.T. Liu and coworkers. A peculiar aspect of this ductilization phenomenon is that boron is only effective in Ni-rich alloys. Much effort, both experimental and theoretical, has been expended to understand the boron effect in the hope of producing ductile intermetallics that might be even more suitable for high-temperature structural applications than Ni3Al. It is generally agreed that the boron must be enriched at the grain boundaries in order to impart ductility to the aluminide. But this cannot be the only criterion since boron also segregates to the boundaries in Al-rich alloys which remain brittle. To find additional criteria for ductility, attention has been focused on the composition and state of ordering of the grain boundary regions. Earlier FIM/AP studies concluded that the matrix is ordered up to the boundary, at least on one side of the interface (4,8). In this paper the authors report the results of more recent work in which the authors examined the state of order on both sides of boundaries in ductile Ni3Al and estimated the maximum width of any disordered boundary phase that would escape detection. We also discuss the occurrence of other types of grain boundary phases observed in the FIM/AP