Microscopic phase-field study for mechanisms of directional coarsening and the transformation of rafting types in Ni-Al-V ternary alloys

Directional coarsening (rafting) of precipitates and transformation on rafting type under stress are investigated by computer simulations for Ni-Al-V ternary alloys. The simulation technique is based on a microscopic phase-field model that characterizes simultaneously the spatiotemporal evolution of both precipitate microstructures and occupation probabilities. The initial configurations consisting of cubical γ′ and tetragonal θ are constructed according to experimental observations and microscopic phase-field simulation. For a given state of the lattice misfit and external load, the predicted rafted precipitates and the transformation from P-type rafting to N-type rafting with stress agree well with the experimental observations. This indicates that stress plays an important role in directional coarsening, which may promote or block the diffusion for atoms in different directions. The evolution of directional coarsening both in space and in time for solute atoms caused by the coupling of external load and misfit stress and precipitation is evaluated from morphologies and occupation probabilities, from which the diffusion of atoms is analyzed.