Solute atom segregation effects to individual stacking faults in Co-0.96 at.% Nb and Co-0.98 at.% Fe alloys have been investigated using the atom-probe field-ion microscope. The composition of individual stacking faults has been directly measured in the range 450-575°C. The diameter of the analyzed cylinder of alloy was ~ 30-50 Å. It is shown that the solute concentration profile falls off very quickly with distance-within less than 4 Å-from the plane of the fault in both alloys. And it is suggested that the Nb and Fe segregation effects are most likely confined to the plane of the fault. Very strong solute atom segregation effects to stacking faults have been observed in both alloys. After correction for the matrix contribution to the chemical analyses, the Nb concentration in the stacking faults in a Co-0.96 at.% Nb alloy is greater than the bulk composition by factors of 30, 12 and 7 at annealing temperatures of 450, 525 and 575°C. The corrected Fe concentration in the stacking faults in a Co-0.98 at.% Fe alloy is greater than the bulk composition by factors of 10, 6 and 4 at the annealing temperatures 450, 550 and 575°C. Small solute-rich fluctuations, (~ 5-20 Å dia), whose compositions differ significantly from the average fault composition, have been observed in the plane of the stacking faults at 525 and 575°C in the Co-0.96 at.% Nb alloy. This, in combination with the experimental observation that at 450°C the stacking fault composition is close to that of Co2Nb, suggests that in addition to solute atom segregation we have evidence for a disorder-order transformation within the plane of the fault with decreasing temperature. Small solute-rich fluctuations (~ 20 Å dia) have also been observed in the Co-0.98 at.% alloy at 550 and 575°C.
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