1995 Volume 81 Issue 9 Pages 924-929
The stacking faults of a metastable γ″ phase precipitated in a nickel-base superalloy, a modified NCF 3 type alloy (X-750M), were investigated by transmission electron microscopy and 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 up to 1033K. Contrast which suggests the existence of stacking faults on {112}γ″ planes can be seen in many of large γ″ precipitates extracted from the specimens aged at 1033K. It is clear that the values of γ″/γ lattice mismatch increase with increasing the aging time from the measurement of lattice constants of the γ and the γ″ phase. The formation of stacking faults on {112}γ″ 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 stacking sequence has a similar crystal structure to that of a stable δ phase precipitates in γ phase, the formation of stacking faults in the γ″ precipitates is considered to be favorable for the stabilization of them.
