In-Situ Electron Microscopy Studies of the Effect of Solute Segregation on Grain Boundary Anisotropy and Mobility in an
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In-Situ Electron Microscopy Studies of the Effect of Solute Segregation on Grain Boundary Anisotropy and Mobility in an Al-Zr Alloy Mitra L. Taheri1, Eric Stach2, 4, Velimir Radmilovic2, Hasso Weiland3, and Anthony D. Rollett1 1
Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15232, USA 2
National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA, USA 3
Alcoa Technical Center, Alcoa Center, PA 15609, USA
4
Department of Materials Science & Engineering, Purdue University, West Lafayette, IN
ABSTRACT The presence of impurities in aluminum alloys is of great interest with respect to microstructural properties, specifically, the effect of solute on texture and anisotropy. This paper presents new evidence of the pronounced effect of solute drag based on in-situ annealing and Electron Backscatter Diffraction experiments of Zr-rich Al alloys subject to prior strain. A compensation effect was found for grain boundary mobility maxima for specific boundary types. Trends in activation energy as a function of boundary type support the observations of a compensation effect with respect to temperature. Evidence for irregular motion of boundaries from in-situ observations is discussed in reference to new theoretical results that suggest that boundaries migrating in the presence of solutes should move sporadically provided that the length scale at which observations are made is small enough. A study of both boundary motion and solute segregation to specific boundary types using Scanning Transmission Electron Microscopy and in-situ TEM is presented. INTRODUCTION Although past studies have shown that solutes have a pronounced effect on the mobility of grain boundaries during recrystallization, the migration mechanism is not fully understood. Even at the mesoscopic scale, a consistent view of the dependence of mobility on boundary character as a function of temperature and solute type is lacking. Some authors [1,2,4-7,9,11] claim that these results suggest a selective segregation effect where the decrease in mobility from solute drag for random general boundary types is greater than that of boundaries near 40°. The exceptional behavior of certain boundary types has been ascribed to greater ease of atomic transfer for coincidence site lattice boundaries allied with decreased solute segregation. Many attempts have been made to study the effects of individual solutes on grain boundary migration with respect to velocity [14] and adsorption [15]. In a study of the effect of Zr on Al boundaries by Boutin [14], two competing conclusions with respect to the segregation of Zr emerge: Zr does not segregate at the grain boundaries in solid solution unless the material has been
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annealed at a slow heating rate (or from a prior heat treatment at 400°C), or, alternatively, that the effect of Zr in solid solution is null but as a precipitate on subgrain boundaries, the drag effect is large. Specifically, Boutin observed segregation of Zr accompanied
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