Crystallographic account of nano-scaled intergrowth of M2B-type borides in nickel-based superalloys

M2B-type borides in nickel-based superalloys containing boron subjected to long-term aging treatments were investigated by means of analytical transmission electron microscopy. Contrast analysis and electron diffraction show that the boride precipitations have dual-phase structures (I4/mcm, C16; Fddd, Cb) with a high density of stacking faults. The faulted planes and displacement vectors are determined as (1 1 0)C16 and [11¯1]C16/4 in C16 structure and (1 0 0)Cb and [0 1 1]Cb/4 in Cb structure. Structural models based on a polyhedron of square anti-prisms are proposed not only to rationalize the stacking faults, 60° rotation twin and the intergrowth of structural variants, but also to interpret the newly observed structure in M2B-type borides.