Focus review on γ′ coarsening in theorical development and application in Ni-base superalloys and high/medium-entropy alloys

Long-term thermal exposure-induced gamma ' ' coarsening strongly influences the mechanical properties of Ni-base superalloys and high/medium entropy alloys (H/MEAs), which has long been of scientific and industrial concern. In revealing the coarsening behavior, a great deal of theorical research has been made over several decades. One major advance is the development of Ostwald ripening kinetics theories, which allows for a quantitative description of gamma ' ' coarsening kinetics. Nowadays, there have been two types of theorical models in wide acceptance, which advocate the matrix-diffusion controlled (LSW-family models) and trans-interface- diffusion controlled (TIDC model) kinetics mechanisms, respectively. Both of them have been validated in experiments and computational simulations. Besides, another major advance is the theorical revelation of the particle morphology evolution as gamma ' ' coarsening, by means of energetic calculations and phase-field simulations. It has been shown that the morphological evolution depends on the combined effects of interfacial energy, elastic strain energy, and elastic interaction energy. The latter two generally play a dominate role in particle shape changes and regular spatial rearrangements, respectively. Based on these theories, the gamma ' ' coarsening kinetics and morphological evolution patterns in Ni-base superalloys and H/MEAs has been clearly revealed and compared in many studies. Herein, we present a review on the development of these gamma ' ' coarsening theories and their applications. This is not only instructive for alloy design and failure prevention, but also informative for further theorical extensions.