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EE6.5.1/BB6.5.1

Evidence for Solute Drag during Recrystallization of Aluminum Alloys Mitra L. Taheri1, Jason Sebastian2, David Seidman2 and Anthony Rollett1 1

Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15232, USA 2 Department of Materials Science and Engineering, Northwestern University, Evanston, IL, USA ABSTRACT Evidence of both solute drag as well as differences in migration mechanisms of certain boundary types has been found for an Al-Zr alloy. In-situ Transmission Electron Microscopy (TEM) annealing experiments coupled with Scanning Transmission Electron Microscopy (STEM) showed a stark contrast between Zr segregation at small and large scales. Specifically, Zr was found to segregate to, as well as precipitate at boundaries of grains smaller than 2 microns, whereas less segregation and no precipitation was found at large grains sizes, or long annealing times. Motivated by these results and those previously obtained by Orientation Imaging Microscopy (OIM), Local Electrode Atom Probe Microscopy with an Imago™ LEAP® microscope, was used to determine the concentrations of Zr and Al ions at grain boundaries; the results confirmed those from the STEM. INTRODUCTION The effect of solutes on the migration of boundaries has long been a topic of investigation due to its strong impact on industrial materials processing. The lack of measurement of the solute drag effect can be attributed in part to the limitations of current imaging capabilities of electron microscopes. Thus, there exists a limited amount of experimental investigation with respect to solute drag. There has also been an extended discussion of the relative importance of solute drag versus particle pinning in recrystallization. [10] Recent comparisons of experimental data to computer current molecular dynamics simulation show that both microscopy and simulation are limited in terms of size and duration. New theoretical developments [1-3] suggest that boundary motion should be jerky in certain regimes of solute diffusivity and driving force because of repeated pinning and unpinning of boundaries. These results prompted a detailed investigation using in-situ heating in a TEM, which suggested that the 'jerky motion' revealed in velocity profiles taken from SEM experiments are a result of both Zr in solid solution and Al3Zr precipitation at subgrain boundaries. The improved resolution at the atomic level allows for the identification of solute segregation at individual boundaries. To obtain a more detailed analysis of solute segregation, Atom Probe Microscopy in an Imago™ LEAP® microscope was used to prove for concentrations of Zr at grain boundaries. Previous experiments [4, 5] have demonstrated that Atom Probe Microscopy

EE6.5.2/BB6.5.2

is a useful tool for the subnanometer scale investigation of solute segregation. Unfortunately, the limited volumes of material that can be measured at such a scale means that relating segregation to specific boundary types is difficult. In this study, LEAP is combined with EB