ATOMIC RESOLUTION OBSERVATIONS OF SOLUTE-ATOM SEGREGATION EFFECTS AND PHASE-TRANSITIONS IN STACKING-FAULTS IN DILUTE COBALT ALLOYS .1. EXPERIMENTAL RESULTS

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 degree C. The solute concentration profile falls off with distance - within less than 4 A - from the plane of the fault. 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 is Co-0. 96 at. % Nb is greater than the bulk composition by factors of 30, 12 and 7 at annealing temperatures of 450, 525 and 575 degree C. The corrected Fe concentration in the stacking faults in Co-0. 98 at. % Fe is greater than the bulk composition by factors of 10, 6 and four at temperatures 450, 550 and 575 degree C. Small solute-rich fluctuations (5-20 A dia), whose compositions differ from average fault composition have been observed in the plane of the stacking faults at 525 and 575 degree C in Co-0. 96 at. % Nb. This, in combination with the observation that at 450 degree C stacking fault composition is close to that of Co//2Nb, suggests that in addition to solute atom segregation there is evidence for a disorder-order transformation with decreasing temperature. Small solute-rich fluctuations (20 A dia) are also observed in Co-0. 98 at. % at 550 and 575 degree C.