Quantitative analysis of diffusion processes associated with gamma'-depletion in Ni-base superalloys

High resolution quantitative EDX-analysis is used to gain new insights into the diffusive processes resulting in gamma'-depletion after high temperature exposure of superalloys. The materials studied are two commercial single crystal alloys (PWA 1484 and PWA 1480) containing substantial amounts of tantalum Ta. It was found that chemical composition of the gamma'-depleted zone reaches a stationary value after moderate exposure times, and that this composition resembles strongly the original composition of the matrix material. The transition between bulk material and gamma'-depleted zone is defined by a diffusive region with concentrations given by a one-dimensional solution of Fick's law. The parameters of this functional relation were determined by solving the corresponding inverse problem. The width of the gamma'-depleted zone can be uniquely defined from the EDX-database. It was shown that a combination of an Arrhenius term and a power law fits well to the dependence of the width of the gamma'-depleted zone on time and temperature, and that the diffusion rates are very much reduced compared to the well-known parabolic rate law. This can be explained by the fact that both material loss by oxidation and continuous extension by diffusion contribute to the measured width of the gamma'-depleted zone.