Phase segregation and rafting in nickel base superalloys: Models and experiments

When a material such as a Ni-Al alloy is cooled rapidly from a high temperature homogeneous phase to a low temperature, it segregates into two or more different phases, a process known as coarsening. In many commercially important materials, a lattice mismatch between the phases creates long ranged, anisotropic elastic interactions. These have a significant effect on both the coarsening dynamics as well as the shape and arrangement of the phase domains, resulting in lattice-like arrangements of cuboidal precipitates, and considerable slowing down of coarsening. Understanding the effect of elastic interactions on the coarsening kinetics and mechanical properties of such multicomponent materials is important for their design.; In the Introduction we give general background information about experimental and theoretical approaches to the dynamics of phase segregation in the presence of elastic interactions. In Chapters 2--5, we introduce and study a three-dimensional microscopic model for phase separating alloys with elastic interactions, whose basic constituents are atoms on a coherent lattice, coupled by springs. Effects of external stress and elastic inhomogeneity are also included. In Chapters 3--4, Monte Carlo simulations of the model under external stress show the formation of lattice-like networks of cylindrical or plate like precipitates at the mesoscale, as observed in real alloys. The dynamics of the defects in these mesoscopic lattices mediate the coarsening, bearing some similarities and some striking differences to atomic lattice defects. The coarsening rate is much slower than the conventional R(t) ∼ t⅓ behavior, which is explained in terms of the different coarsening mechanisms (compared to the usual Ostwald ripening) when elastic strains and external stresses are present. In Chapter 5, simulations of the model with zero external stress and increasing lattice mismatch show a cross-over from a spherical, self-similar morphology, exhibiting dynamical scaling and coarsening as t⅓, to a spatially ordered, lattice like arrangement, accompanied by a breakdown of dynamical scaling and near halt of precipitate growth.; In Chapter 6, mesoscopic measurements of the mechanical properties of the gamma' and gamma phases in Ni-Al-Mo alloys are made with an atomic force microscope-nanoindentation apparatus. With this technique the mechanical response of sub-micron sized phase domains can be measured individually. The gamma'-phase is consistently harder than the gamma-phase, but the elastic moduli show no significant difference. Estimates of the gamma' supersaturation and effect of adjoining phases are measured.