Contributions from elastic inhomogeneity and from plasticity to γ′ rafting in single-crystal Ni–Al Original Research Article

Directional coarsening (rafting) of γ′ precipitates in single-crystal Ni–Al alloys under external load is investigated by three-dimensional computer simulations. Contributions from modulus mismatch between the precipitate and matrix phases and from channel plasticity are considered. The simulation technique is based on a phase field model that captures spatiotemporal evolution of precipitate microstructure in elastically anisotropic and inhomogeneous systems. Channel plasticity is incorporated in the model through the introduction of an effective plastic strain that is formulated based on dislocation contents in the γ channels. Experimental data on lattice misfit, elastic moduli, applied stress and dislocation density in γ channels are used as model inputs. The driving force for rafting is monitored continuously by tracking the change in chemical potential difference between different types of γ channels after an external stress is applied until rafting completes. Quantitative comparisons of the rafting driving force and rafting completion time obtained from the simulations confirm the dominant role of channel plasticity, indicating that the rafting direction is determined by channel plasticity rather than by modulus inhomogeneity. However, elastic inhomogeneity also makes a significant contribution to the driving force for rafting, especially during the first several hours after the external load is applied.