THE INFLUENCE OF PRECURSOR FLUCTUATIONS ON THE KINETICS OF ALPHA-CO PRECIPITATION IN DILUTE CUCO ALLOYS

Small Angle Neutron Scattering on dilute CuCo alloys containing 0.4, 0.5 and 0.8 at.% Co confirms the existence of extended compositional fluctuations, which precede the formation of stable precipitates of the f.c.c. Co-rich alpha-phase. The precursor fluctuations have considerable influence on the reaction kinetics of alpha-Co precipitation: compared to the prediction of classical nucleation theory, the rate of precipitate formation does not follow the sharp drop-off predicted for supersaturation ratios below 2 but shows a rather sluggish dependence on supersaturation. Further, the sizes of initially forming alpha-Co precipitates are significantly increased compared to the theoretically prediction of the critical nuclei size. For supersaturation ratios between about 2 and 3, the initially arising alpha-Co precipitates exhibit an extremely narrow distribution in size.