PRECIPITATION AND GROWTH OF GAMMA'- PHASE IN FE-38NI-13CO-4.7NB SUPERALLOY

Fe-38Ni-13Co-4.7Nb base superalloy (alloy 909) is the latest low therm al expansion chromium-free superalloy with a good resistance to SAGBO (Stress Accelerated Grain Boundary Oxidation) embrittlement at elevate d temperatures. This investigation was carried out to elucidate the re lation between the age-hardening and the nucleation and growth behavio r of gamma' precipitates in alloy 909 by micro Vickers hardness test a nd transmission electron microscopy. The results obtained in this stud y are as follows : (1) The hardness of alloy 909 measured at aging tem peratures was ca. HV100 lower than that measured at room temperature. (2) The hardness of specimens aged at 893-1033K for durations up to 72 0ks closely related to the mean size of gamma' precipitates. (3) The g rowth kinetics of the gamma' precipitates in epsilon phase free region was explained by Lifshitz-Slyozov-Wagner's theory of volume diffusion controlled growth at 943-1033K. The activation energy for the growth of gamma' precipitates was estimated to be 254kJ/mol which was nearly equal to those of diffusion of Ti or Fe atoms in gamma-iron or nickel.