Quantification of rafting of γ' precipitates in Ni-based superalloys

Rafting, the directional coarsening of y' precipitates in a gamma/gamma' microstructure, can affect the mechanical performance and lifetime of Ni-based superalloys. While rafting has been commonly observed in many systems, a criterion for the onset and completion of rafting has not been fully established. In this work, we employ 2D and 3D moment invariants (MIs) to track the evolution of a synthetic gamma/gamma' microstructure under a tensile load as a function of time. Systems with negative and positive misfit are considered. Under normal conditions, y' precipitates have high MI values characteristic of cuboidal shapes, while rafted precipitates have low MI values. The onset of rafting is defined as the time at which more than half of the y' precipitates have 20 MIs in particular regions of interest of the MI density maps. The rafting completion time is defined as the time at which the initial linear trend slows down with time. The MI approach is also compared to other shape parameters and is shown to provide a convenient all-in-one shape descriptor that can detect the elongation and wavy morphology of the rafted precipitates, as well as the morphological changes in the gamma/gamma' microstructure as a function of time. (c) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.