Sigma morphology and precipitation mechanism in Udimet 720Li

Sigma formation and its morphology change in long-term exposed Udimet 720Li were examined by the electrochemical extraction of sigma and quantitative X-ray diffraction analysis. As the aging temperature increased, the formation rate of sigma increased and the incubation time for sigma nucleation decreased. In terms of the activation energy, results showed that sigma formation can be controlled by the diffusion of Co in the early stage and by Mo diffusion in the steady state. Some of the sigma phases exhibited dendritic morphology while some possessed a plate or cellular morphology, depending on the nucleation site.