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

摘要

The stacking faults of a metastable gamma '' 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 gamma '' precipitates ale circular shaped plates at the early stage of aging and they become elliptic or irregular shaped plates at the latter stage of aging al temperatures up to 1033 K. Contrast which suggests the existence of stacking faults on {112}(gamma '') planes can be seen in many of the large gamma '' precipitates extracted from the specimens aged at 1033 K for 36 ks or more. It is clear that the values of gamma-gamma '' lattice mismatch increase with increasing aging time from the measurement of lattice constants of the gamma and the gamma '' phases. The formation of stacking faults on {112}(gamma '') planes in the large gamma '' precipitates is due to the movement of an a/6 [11 (1) over bar] partial dislocation introduced by gamma ''-gamma coherency strain. Since a part of the stacking sequence has a similar crystal structure to that of stable delta phase precipitates in a gamma phase, the formation of stacking faults in the gamma '' precipitates is considered to behaviour able for their stabilisation. (C) The Institute of Materials.