Interphase boundary fracture and grain boundary precipitation of Ni3Al(γ') phase in β-NiAl bicrystals

The effect of the crystallography of film-like Ni3Al(γ') precipitates along grain boundaries of NiAl(β) on the fracture stress at room temperature was examined using β bicrystals with controlled orientations. The selected variant of γ'-film satisfied the Kurdjumov-Sachs (K-S) relation with a neighbouring β crystal, and deviated from the relation with an adjacent β crystal. In the course of tensile deformation at ambient temperature, fracture occurred preferentially at the (β/γ') interphase boundary deviating from the K-S relation, which showed no plastic flow, and the fracture stress decreased with increasing deviation angle. In contrast, slip transfer from γ'-film to β crystal across coherent (β/γ') interface was observed, when the variant of γ'-film satisfied the K-S relation with both neighbouring β crystals. To clarify the relation between the interphase boundary fracture and the deviation angle from the K-S relation, the boundary structure of incoherent (β/γ') interfaces was discussed using O-lattice theory and transmission electron microscopic observations