Stacking faults of metastable γ″ phase precipitated in an Ni-15Cr-8Fe-6Nb alloy

The stacking faults of a metastable γ″ phase precipitated in a nickel base superalloy, a modified JIS NCF 3 type alloy (X–750M), are investigated by TEM and an X-ray diffraction method. The γ″ precipitates are circular shaped plates at the early stage of aging and they become elliptic or irregular shaped plates at the latter stage of aging at temperatures up to 1033 K. Contrast which suggests the existence of stacking faults on {112}_γ″ planes can be seen in many of the large γ″ precipitates extracted from the specimens aged at 1033 K for 36 ks or more. It is clear that the values of γ″ −γ lattice mismatch increase with increasing aging time from the measurement of lattice constants of the γ and the γ″ phases. The formation of stacking faults on {112}_γ″ planes in the large γ″ precipitates is due to the movement of an a/6 [111] partial dislocation introduced by γ″ −γ coherency strain. Since a part of the stacking sequence has a similar crystal structure to that of stable δ phase precipitates in a γ″ phase, the formation of stacking faults in the γ″ precipitates is considered to be favourable for their stabilisation.