THE GROWTH OF GAMMA' PRECIPITATES IN A 53NI-20CO-15CR SUPERALLOY

The age hardening behavior and microstructural characteristics of a 53Ni-20Co-15Cr-5Mo-4.9Al-1.3Ti alloy has been investigated by micro-Vickers hardness test and transmission electron microscopy. The shape of gamma' precipitates was spherical at the early stage of aging and was cuboidal at the later stage. The aged hardness of the alloy closely related to the size and the quantity of gamma' precipitates. The gamma' precipitation cannot be suppressed by water quenching after solution treatment. It is necessary to cool the specimen at rates of higher than 10(4) K/s for restraining the precipitation of gamma' phase. The growth kinetics of gamma' precipitates in the alloy followed the prediction of Lifshitz-Slyozov-Wagner (LSW) theory for growth under volume diffusion control at all aging temperatures examined. The activation energy for the growth was estimated to be 266 kJ/mol which was close to those of diffusion of solute atoms such as Al, Ti and so on in Ni and Ni-base alloys. The actual particle size distribution of gamma' precipitates was somewhat different from distributions predicted by the LSW and Brailsford-Wynblatt encounter modified (BWEM) theories.