IN-SITU ELECTRICAL-RESISTIVITY MEASUREMENTS FOR DETERMINING FORMATIONKINETICS OF GAMMA' PHASE IN NICKEL-BASED WROUGHT SUPERALLOYS

Formation kinetics of gamma (Ni3Al) phase in Ni-based wrought superall oys were investigated using the indirect measurement method of electri cal resistivity change during isothermal holding. It was found that fo rmation of gamma' phase occurs in a very short time (as a function of temperature and alloying level) by a diffusion controlled nucleation m echanism. Al alone enhances the formation of gamma' nuclei in the allo y and enlarges their quantity. On the other hand, addition of Ti or Ta causes delay in nucleation. Any other alloying addition (e.g. Co or W ) delays the primary stage of nucleation. Ta was found to be more subs tantial than Ti in increasing nucleation rate and volume fraction of t he gamma' phase.