ABSTRACTSolute-atom segregation effects to individual stacking faults (SFs) in Co-0.96 at.% Nb and Co-0.98 at.% Fe alloys were studied employing the atom-probe field-ion microscope (FIM) and transmission electron diffraction techniques. The mean composition of individual SFs was measured for bulk specimens which had been equilibrated in the range 450-575°C. In addition, the composition was measured with a spatial resolution, within the plane of the SFs, of ~1 Å and with a spatial resolution of <4 Å perpendicular to the SFs. These measurements demonstrated the following: (a) The meancompostion of the SFs increased with decreasing temperature according to an Arrhenius-like expression; (b) The Nb or Fe concentrations fall off very quickly with distance--within <4 Å from the plane of the SF the bulk concentation is achieved; (c) The SFs equilibrated above 4500 C contained solute-atom fluctuations (≈5 to 20 \ diameter) which correspond to compositions with stoichiometries of ≈Co2Nb or ≈Co3Fe. In the case of the Co(Nb) alloy, a transmission electron diffraction pattern was obtained ofa SF--with the electron beam normal to the SF--which exhibited superlattice reflections with six-fold symmetry around the hcp matrix reflections. This latter result is consistent with a two-dimensional phase in the SF plane with the composition Co2 Nb. Both the atom-probe FIM and the electron diffraction studies are consistent with two dimensional (2D) ordered phases in the SFs.
SOLUTE-ATOM SEGREGATION AND TWO-DIMENSIONAL PHASE TRANSITIONS IN STACKING FAULTS : AN ATOM-PROBE FIELD-ION MICROSCOPE STUDY
