Microstructure Evolution of a Multimodal Gamma-Prime Ni-Based Superalloy Characterized by In Situ Diffraction

The relationship between the evolution of microstructure, deformation micromechanisms and mechanical properties is difficult to establish in multimodal size distribution gamma ' superalloys, as the microstructure evolves with both temperature and time, and multiple strengthening mechanisms across each size distribution contribute to mechanical performance. In situ X-ray scattering can offer unparalleled insight regarding microstructure evolution at the temperatures and stresses of importance to gas-turbine applications; however, in situ X-ray diffraction has not been applied to the study of multimodal gamma ' distribution superalloys. Herein, lattice parameter evolution of secondary and tertiary gamma ' precipitates in a representative superalloy, Nimonic 115, is determined between 750 degrees C and 950 degrees C and correlated to room-temperature SEM and microhardness values. A large positive lattice parameter misfit of secondary gamma ' induces precipitate splitting, and the tertiary gamma ' goes into dissolution at similar to 800 degrees C, but with little apparent change in hardness values. The volume fraction of gamma ' decreases above 900 degrees C and precipitate-matrix coherency is lost, and there is a corresponding decrease in microhardness values. The diffraction analysis demonstrates the capability to determine critical microstructural parameters of both precipitate size distributions in situ, representing an additional tool for determining microstructure-mechanical property relationships of multimodal superalloys.