A small-angle neutron scattering study of fine-scale NbC precipitation kinetics in the -Fe-Nb-C system

The fine-scale precipitation of NbC in ferrite has been quantitatively characterized in the temperature range 873-1073 K for two alloy compositions, containing respectively 800 p.p.m. Nb and 400 p.p.m. Nb (by weight). Transmission electron microscopy (TEM) has revealed that the precipitates are located on dislocations, and have a plate-like morphology with an average aspect ratio between 2 and 3. Small-angle neutron scattering (SANS) has been systematically used to determine the precipitation kinetics. The validity of the quantitative SANS measurements of size and volume fraction has been assessed by TEM image analysis and chemical dissolution experiments. The precipitation kinetics is observed to depend strongly on temperature but to be similar for the two alloy compositions. From the measurements, it is inferred that precipitate nucleation is extremely rapid, in relation to the nature of the nucleation sites. A time-temperature transformation diagram is built from the kinetic data, showing a maximum reaction rate between 973 and 1073 K.