Nucleation and Growth of Solid-State Transformations

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9500122 Olson This research program is directed toward a unified view of first-order phase transformations based on quantitative principles established in the prototypical case of martensitic reactions. The research addresses two broad areas (a) the dynamical evolution of hierarchical structure in heterogeneous displacive systems, and (b) competing transformation mechanisms in nonlinear nonlocal solids. Stereological measurements are aimed at identifying a universal form of autocatalytic nucleation site potency distribution for both thermoelastic and non-thermoelastic martensitic systems. Numerical models address the dynamics of growth in a plastic matrix and autocatalytic interactions of multivariant structures, with extension to bainitic reactions as coupled displacive/diffusional transformations. Competing mechanisms of structural transformation are examined by combined theory and experiment employing the weakly first-order cubic-tetragonal transformation in iron-palladium as a model system describable by nonlinear nonlocal continuum models. Small particle experiments address homogeneous nucleation and weak-defect heterogeneous nucleation behavior, while continuum calculations address coherent homogeneous nucleation in three dimensions and heterogeneous nucleation in two dimensions. Theoretical modeling also explores fluctuation-assisted continuous modulation as an alternative to nucleation and growth and contrasts precursor effects of the competing mechanisms. %%% Phase transformations under study here are critical to the properties of iron-base alloys used in structural applications. ***