Precipitation from supersaturated Nb-C solid solutions

Precipitation of carbides from supersaturated niobium-carbon solid solutions has been measured using electrical resistance and transmission electron microscopy. The process is shown to follow a rate law of the form 1 -w = exp (-t/τ) ⁿ with values ofn around 0.6. The activation energy is consistent with diffusion of carbon as the major rate controlling factor. Transmission electron micrographs of samples aged in the time and temperature range of resistance measurements (days at 180°C to seconds at 400°C) fail to show any visible precipitate. However, foils aged at 400°C for much longer times show very distinct platelike precipitates. On the basis of this then value is taken to signify carbon segregation to solute-rich regions on certain planes which later develop into well defined plates.