Probing fractal abnormal grain growth at the atomistic level using APT

点击翻译按钮获取中文摘要

Although current models are reasonably successful at explaining normal grain growth in conventional polycrystals, our understanding of grain growth in nanocrystalline (NC) materials is still quite rudimentary. At the nanoscale, coarsening is often abnormal in nature, with a few grains growing orders of magnitude larger than neighboring grains in the surrounding NC matrix. In samples of NC Pd90Au10 produced by inert gas condensation, microstructural coarsening is doubly abnormal, as the rapidly growing grains send forth “tentacles” into the matrix, first encircling nearby grains and then consuming them. The perimeters of the resulting grains are well described by a fractal dimension of about 1.2 (rather than the expected value close to unity).The widely accepted curvature-based mechanism for grain boundary (GB) migration is incompatible with such “fractal” growth, but even “ordinary” abnormal growth—as observed in technical alloys like Fe-Si steels—remains an unsolved mystery. A recent proposal by Hwang et al., however, is the first to subsume these disparate forms of abnormal growth under the single mechanism of “solid-state wetting.” In this picture, the fractal migration of GBs is an extreme manifestation of wetting, rendering NC PdAu an ideal model system for validating the concept.Without Au, grain growth in NC Pd proceeds normally; therefore, we speculate that fractal GB migration results from thermodynamic and/or kinetic effects associated with Au segregation to GBs in NC PdAu. These effects may suppress curvature-driven growth of the matrix grains while, at the same time, promoting (or at least not hindering) solid-state wetting. The key step in testing this hypothesis is to measure the Au concentration in the vicinity of fractally growing grains, comparing the amounts of Au segregation at fractal/matrix and matrix/matrix GBs. Atom-probe tomography (APT) is predestined for this task, capable of quantifying local compositions in a 3D specimen at a spatial resolution comparable to that of the interatomic spacing.In this project, the concentration of Au atoms in NC PdAu will be varied to maximize the fractal dimension. To deconvolute thermodynamic from kinetic aspects of GB migration, a ternary element (hydrogen) having a known tendency to segregate to GBs will be added to the samples. FIB milling will be employed to extract GBs of interest, which will be characterized by APT and correlated with various forms of electron microscopy. Measured values for Au segregation will be input into a phase field model for simulating simultaneous solid-state wetting and curvature-driven growth. The morphology of the simulated grains will be validated against experiment and combined with the local studies of Au and H segregation to determine whether solid-state wetting is the origin of fractal abnormal grain growth in NC PdAu.