Solute Distribution in Nanocrystalline Ni-W Alloys

Abstract

The stability of nanocrystalline metals can be vastly improved through the addition of alloying elements. Previous works have suggested that this enhanced stability may be due to a thermodynamic phenomenon, where grain boundary energy is reduced upon solute segregation to the intercrystalline regions. Atom probe studies have confirmed strong segregation in nanocrystalline alloys such as Ni-P <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , where grain boundaries were clearly decorated with a high concentration of P atoms. Recently, Ni-W alloys have shown behavior reminiscent of this segregation-based stabilization, although tungsten is expected to exhibit a relatively weak preference for grain boundary segregation. In order to appreciate the segregation behavior, this study examines the solute distribution in nanocrystalline Ni-W alloys, produced over a range of nanocrystalline grain sizes, with the chemical and spatial resolution of the three-dimensional atom probe (3DAP) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . Nanocrystalline Ni-W alloys have been produced using a pulsed electrodeposition method at the Massachusetts Institute of Technology (MIT). Specimens with average grain sizes of 3, 10, and 20 nm were examined in the local electrode atom probe. No clear tendency for W segregation was observed in the atom maps due to the high solute content and large extent of the data. Therefore, a more detailed statistical analysis was performed in order to infer the existence and extent (if any) of segregation in the experimental specimens. To aid in this analysis, and directly compare with the experimental 3 nm specimen, an atomistic simulation was conducted with the same grain size and composition. The equilibrium solute distribution of the simulated structure was determined with a Monte Carlo energy minimization procedure coupled with a conjugate gradient relaxation routine. With the unique knowledge of the grain boundary locations in the simulated cell, the existence and extent of grain boundary segregation was directly confirmed and measured from composition profiles, autocorrelation functions, and a two-constituent composition distribution model. These same techniques were applied to the experimental data, revealing a subtle amount of W segregation to the intercrystalline regions in comparison to previous studies of alloys exhibiting segregation-based stabilization. From the classic McLean grain boundary segregation isotherm, it was shown that nanocrystalline Ni-W is best described by a low segregation energy on the order of 1 kJ/mol. This subtle degree of grain boundary segregation has important consequences for the thermal stability of these alloys