Aged microstructures of a low thermal expansion 41Fe-29Ni-22Co-4Nb-2Cr-1Ti-0.5Al-0.5Si superalloy

41Fe-29Ni-22Co-4Nb-2Cr-1Ti-0.5Al-0.5Si alloy (alloy 929C) is the latest low thermal expansion superalloy with a good resistance to SAGBO (Stress Accel erated Grain Boundary Oxidation) embrittlement at elevated temperatures. Th is alloy is strengthen by the precipitation of gamma' (Ni-3(Al,Ti); L1(2)) phase in the alloy matrix. This study was carried out to elucidate the rela tion between the age-hardening and the growth of gamma' precipitates in all oy 929C by micro Vickers hardness test and transmission electron microscopy . The results obtained in this study are as follows: (1) Comparing with the similar superalloys like alloys 909 and 929, the age -hardening rate in alloy 929C is slow its peak hardness is high, and the ov eraging is suppressed. (2) Age-hardening proceeds with growing gamma' precipitates and the hardnes s reaches to the maximum in the size of y' precipitates 11 similar to 13 nm . After reaching to the maximum value the hardness decreases gradually acco rding to the coarsening of gamma' phase. Though the gamma' precipitates in the alloy 929C are relatively stable, they transform gradually to a stable eta phase with increasing aging time but never transform to epsilon phase u nlike alloys 909 and 929. (3) The growth kinetics of the gamma' precipitates are explained by Ostwald ripening theory of volume diffusion controlled growth. The apparent activa tion energy for the growth of gamma' precipitates was estimated to be 259 k J/mol which was nearly equal to those of diffusion of Ti or Fe atoms in gam ma -iron or nickel. (4) The gamma' phase precipitates in sphere in the early stage of aging and shifts to in cubic with increasing aging time. The gamma' precipitates wit h the size of 150 similar to 200 nm change to a star-like form with eight h orns according to the growth of [111](gamma') direction.