ATOMIC RESOLUTION OBSERVATIONS OF SOLUTE-ATOM SEGREGATION AND TWO-DIMENSIONAL PHASE TRANSITIONS AT INTERNAL INTERFACES

Abstract
Solute-atom segregation effects to individual stacking faults (SFs) in Co-0.96 at.% Nb and Co-0.98 at.% Fe alloys have been studied employing the atom-probe field-ion microscope (APFIM) and transmission electron diffraction techniques. /1-7/ 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 of the SFs was measured--employing the APFIM technique-- with a spatial resolution, within the plane of the SFs, of ≈ 0.1 nm and with a spatial resolution of < 0.4 nm perpendicular to the plane of the SFs. These measurements demonstrated the following : (a) The mean composition 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 < 0.4 nm form the plane of the SF the bulk concentration is achieved ; (c) The SFs equilibrated above 450°C contained solute-atom fluctuations (≈ 0.5 to 2.0 nm diameter) which correspond to compositions with stoichiometries of ≈ Co₂Nb or ≈ Co₃Fe. The temperature dependence of these fluctuations suggests that in addition to solute-atom segregation we have observed a two-dimensional phase transition in the stacking fault. In the case of the Co(Nb) alloy a transmission electron diffraction pattern was obtained of a SF-- with the electron beam normal to the SF--which exhibited superlattice reflections with six-fold symmetry around the Bragg reflections from the hexagonal close-packed reflections. The latter result is consistent with a two-dimensional phase in the SF plane with the composition Co₂Nb. Both the APFIM and the transmission electron diffraction studies are consistent with the existence of two-dimensional ordered phases within the SFs. The APFIM and transmission electron diffraction results demonstrate that it is possible to study both the chemistry and the spatial arrangement of solute atoms at a well-defined internal interface--the stacking fault. The APFIM measurements yield unique information about the chemistry of this interface--which, at present, is unobtainable by any other technique. Whereas the transmission electron diffraction patterns yield information about the ordering of solute and solvent atoms at the interface. The combination of these two techniques is a particularly powerful approach for the study of internal interfaces on an atomic scale.