A new approach to atomic level characterization of grain boundaries by atom probe tomography
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A new approach to atomic level characterization of grain boundaries by atom probe tomography L. Yao1 and M. K. Miller1 1 Oak Ridge National Laboratory Oak Ridge, TN 37831-6139, U.S.A. ABSTRACT A novel atom probe tomography (APT) method has been developed that enables a full description of the orientation relationship between individual grains to be determined together with estimates of the extents of solute segregation for all elements over the surface of the grain boundary with 1 nm by 1 nm spatial resolution. This approach also enables variations in the solute excess for the elements with the habit plane and curvature of the grain boundary to be evaluated. The method has been applied to a mechanically-alloyed nanostructured ferritic alloy (NFA) after high dose heavy ion irradiation. The innovative high-resolution two-dimensional mapping of the solute segregation across the surface of grain boundaries in the NFA clearly demonstrates that the distributions of chromium and tungsten are not uniform across the grain boundaries, and the distributions correlate with changes in its local curvature and the position of the grain boundary precipitates. These features pin the grain boundary against grain growth and provide the stability for excellent creep properties. INTRODUCTION The stability of grain boundaries in NFAs is a major factor in their excellent response to high temperature creep and their tolerance to high dose radiation under extreme conditions. [1,2] In ultra-fine grained (UFG) materials, such as these NFAs, the grain boundaries exhibit complex non-equilibrium shapes and curvatures as well as many triple lines. UFG materials are excellent candidates for analysis by APT due to its high spatial resolution, sensitivity to all elements, and ability to analyze several grain boundaries in a single specimen. A novel APT method based on a 3D Hough transformation has been developed that enables the full five degrees of freedom of the orientation relationship between the individual grains to be measured.[3-5] In addition, the extent of solute segregation for all elements over the surface of the grain boundary may be estimated with 1 nm by 1 nm spatial resolution. This approach enables variations in the solute excess for the elements with the habit plane and curvature of the grain boundary to be evaluated. The new method also enables low levels of solute segregation, such as could be observed after ion irradiation, to be quantified. In favorable cases, the method could also be applied to triple junctions and possibly interphase interfaces. EXPERIMENTAL The APT method has been applied to a 14YWT NFA that was produced by mechanical alloying a gas-atomized Fe-13.1 wt.% Cr-0.54 W-0.19 Ti master alloy with 0.25% of yttria powder. The resulting 100-200-μm-diameter and ~30-μm-thick flakes were consolidated by hot extrusion into solid bar form at 850 °C and then annealed for 1 h at 1000 °C. The resulting grain
size varied between 20 and 400 nm. A small rectangular coupon was cut from the bar and polished to a flat surface. Thi
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