STACKING-FAULTS OF METASTABLE GAMMA'' PHASE PRECIPITATED IN AN NI-15CR-8FE-6NB ALLOY

The stacking faults of a metastable gamma '' phase precipitated in a n ickel base superalloy, a modified JIS NCF 3 type alloy (X-750M), are i nvestigated by TEM and an X-ray diffraction method. The gamma '' preci pitates ale circular shaped plates at the early stage of aging and the y become elliptic or irregular shaped plates at the latter stage of ag ing al temperatures up to 1033 K. Contrast which suggests the existenc e of stacking faults on {112}(gamma '') planes can be seen in many of the large gamma '' precipitates extracted from the specimens aged at 1 033 K for 36 ks or more. It is clear that the values of gamma-gamma '' lattice mismatch increase with increasing aging time from the measure ment of lattice constants of the gamma and the gamma '' phases. The fo rmation of stacking faults on {112}(gamma '') planes in the large gamm a '' precipitates is due to the movement of an a/6 [11 (1) over bar] p artial dislocation introduced by gamma ''-gamma coherency strain. Sinc e a part of the stacking sequence has a similar crystal structure to t hat of stable delta phase precipitates in a gamma phase, the formation of stacking faults in the gamma '' precipitates is considered to beha viour able for their stabilisation. (C) The Institute of Materials.