Materials Under Extreme

Mapping solute excesses and curvature of grain boundaries

2-D Cr and W Gibbsian interfacial excess maps and variations of the curvature over the grain boundary. The TiO++ map indicates the positions of the nanoclusters which correspond to the black regions in the curvature map.

A novel characterization method has been developed that enables complete characteriza­tions of grain boundaries by atom probe tomography (APT) in terms of the orientation relationship of the adjacent grains, local variations of the habit plane, surface curvature, and the solute excesses over the surface of a grain boundary.

The achieved 1 nm x 1 nm spatial resolution leads to an understanding of the mechanisms that lend certain materials, such as nanostruc­tured ferritic alloys (NFAs) their high creep resistance. In this work, APT results for an ultra-fine grained NFA indicate that the solute segregation of Cr and W over the grain boundary surfaces is correlated with the local curvature of the grain boundary, with signifi­cant variations at the nanometer scale especially near grain boundary precipitates. The solute segregation and the grain boundary precipitates pin the grain boundaries and together with the slow diffusion in this system explain its excellent creep properties. The results also emphasize the importance of nanometer scale characterization methods for a complete understanding of the materials properties.

M. K. Miller and L. Yao, “Limits of detectability for clusters and solute segregation to grain boundaries,” Curr. Opin. Solid State Mater. Sci. 17, 203–210 (2013).

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