An atom probe tomography study of grain boundary segregation in nanocrystalline Ni-W
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An atom probe tomography study of grain boundary segregation in nanocrystalline Ni-W A. J. Detor1, M.K. Miller2, and C.A. Schuh1 1 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 2 Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN ABSTRACT Atom probe tomography is used to observe the solute distribution in electrodeposited nanocrystalline Ni-W alloys with three different grain sizes (3, 10, and 20 nm) and the results are compared with atomistic computer simulations. The presence of grain boundary segregation is confirmed by detailed analysis of composition fluctuations in both experimental and simulated structures, and its extent quantified by a frequency distribution analysis. In contrast to other nanocrystalline alloys, the present Ni-W alloys exhibit only a subtle amount of solute segregation to the intergranular regions. This finding is consistent with quantitative predictions for these alloys based upon a thermodynamic model of grain boundary segregation. INTRODUCTION
Grain size (nm)
A monotonic relationship between nanocrystalline grain size and global solute composition has been observed in alloy systems known to exhibit extensive grain boundary segregation [1, 2]. In these systems, an increase in solute content demands an increase in grain boundary area to accommodate the excess solute, behavior that has been rationalized by thermodynamic models of grain boundary energetics [3-6]. A well-studied example of such a system is Ni-P [2], which exhibits the characteristic grain size-composition relationship plotted using open points in Fig. 1; also shown are recent results from our experiments in the Ni-W system, showing a 50 similar trend. While significant grain boundary Ni-W segregation has been confirmed in the Ni-P Ni-P [2] system [7], it is not clear that the same behavior 40 should be present in the Ni-W system where, unlike Ni-P, the equilibrium phase diagram 30 indicates a high solubility of W in the Ni lattice. In order to accurately assess the solute 20 distribution in nanocrystalline systems, a very highly-resolved analytical technique such as 10 three-dimensional atom probe (3DAP) tomography is required [8, 9]. The present 0 work seeks to determine the distribution of 0 5 10 15 20 25 solute in nanocrystalline Ni-W specimens with Composition (at% P or W) grain sizes ranging from 3-20 nm, and to Figure 1: Nanocrystalline grain sizecompare the findings with an appropriate grain composition relationship for the Ni-P [2] boundary segregation isotherm for this system. and Ni-W systems. To overcome the difficulty in associating 3DAP data with known microstructural features, we
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introduce a new approach whereby a simulated atomistic structure is used to guide the interpretation of the 3DAP data. EXPERIMENTAL AND SIMULATION PROCEDURES Three 100 µm thick nanocrystalline Ni-W specimens with different compositions were produced via an electrodeposition technique similar to that used by a number of autho
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