Place: International Seminar series on Time Dependent Multiscale Phenomena of Materials, Tohoku University, Sendai, Japan
Ni is a base component in high-temperature superalloys and Re is one of the key ingredients to improve mechanical properties in Ni-based alloys, in particular creep resistance. It is, however, not fully understood how Re affects creep in Ni-based superalloys.
Previous studies showed that in the dilute limit there is almost no effect of Re on the diffusivity of the atoms or the mobility of the vacancies. In this study, we extend our investigation to the non-dilute limit of Re by including Re-Re interactions. This enables us to investigate atomic mobilities and segregation behaviour over the entire composition range.
As a basis, we use density-functional theory (DFT) calculations to obtain energies for our kinetic Monte Carlo (KMC) approach. However, within the non-dilute limit the number of possible diffusion barriers as a function of the local environment increases considerably. In addition, an on-the-fly evaluation of diffusion barriers using DFT is computationally unfeasible. To retain the accuracy of DFT and speed up the process of calculating energy barriers, we combine the KMC model with a cluster expansion (CE) approach. The CE is parametrised using energies from DFT calculations. The diffusion barriers are determined using the energy difference between the initial and final state of the diffusion process together with a kinetically resolved activation (KRA) energy. For the first part we use a ternary CE for the Ni-Re-vacancy system based on formation energies. The KRA energies are parameterised in a second CE based on DFT diffusion barriers. From the KMC simulations, we then extract diffusion coefficients, and evaluate the vacancy mobility as a function of Re concentration.