The effects of γ′ precipitate coarsening during isothermal aging and creep of the nickel-base superalloy IN-738

The effects of isothermal aging in the temperature range 1023 – 1123 K on the γ′ phase in the nickel-base superalloy IN-738 have been studied both with and without an applied creep stress. The initial heat treatment produces a bimodal distribution of small spheroidal precipitates (about 0.05 μm radius) and larger cuboidal precipitates (about 0.5 μm radius). During aging both types of precipitate obey diffusion-controlled coarsening kinetics (Ostwald ripening) but the cuboids coarsen at the expense of the spheroids until the latter eventually disappear. These microstructural changes are reflected by a decrease in room temperature hardness and a loss of creep resistance. It is demonstrated that at 1123 K the creep strength falls rapidly during about the first 5 × 106 s of aging. At 1023 K the coarsening of the γ′ precipitates is much slower, but their importance is illustrated by the loss in creep resistance resulting from intermediate overaging heat treatments. However, it is shown that the creep strength at this temperature can subsequently be restored using a regenerative heat treatment. These results are discussed in the light of classical theories of creep of dispersion-hardened alloys.