Mechanisms of Precipitation of Different Generations of Gamma-Prime Precipitates During Continuous Cooling of a Nickel Base Superalloy (PREPRINT)

Abstract

Abstract : This paper focuses on developing a mechanistic understanding of the process of multiple nucleation bursts leading to the formation of multiple generations (or populations) with different size distributions, of gamma precipitates during the continuous cooling of a commercially used nickel base superalloy. This mechanistic understanding has been developed based on a number of critical experimental results; direct observation of the multiple nucleation bursts during in situ x-ray diffraction studies in the synchrotron, characterization of the size distributions associated with the different populations of gamma precipitates by coupling multiple imaging techniques at the appropriate length scale, and detailed compositional analysis of the gamma' precipitates as well as the gamma matrix using atom probe tomography. These analyses reveal that while local compositional equilibrium appears to have been achieved near the gamma'/gamma interface for the first-generation of precipitates, a non-equilibrium long range diffusion profile in the gamma matrix is retained during the process of continuous cooling which is largely responsible for the subsequent nucleation bursts at larger undercooling...