INFLUENCE OF COHERENCY STRESS ON MICROSTRUCTURAL EVOLUTION IN MODEL NI-AL-MO ALLOYS

The influence of the sign and magnitude of the misfit strain on two-phase (gamma-gamma') microstructural evolution during coarsening is examined in a series of five low-volume-fraction Ni-Al-Mo alloys using TEM and SAXS. The evolution of the symmetry-conserving, sphere-to-cube transition and the symmetry-breaking cube-to-cuboid transition with increasing particle size were quantified and the general trends were found to be in excellent agreement with two-dimensional calculations of particle shapes. Both types of particle shape transitions and particle alignment occur at smaller particle sizes the larger the magnitude of the misfit strain. A decrease in the coarsening rate constant was measured with increasing Mo content and was attributed to Mo diffusion and partitioning rather than the increasingly negative misfit strain of the alloys with increasing Mo content.