Andrew J. Detor

Microstructural evolution during the heat treatment of nanocrystalline alloys

Nanocrystalline alloys often show exceptional thermal stability as a consequence of kinetic and thermodynamic impediments to grain growth. However, evaluating the various contributions to stability requires detailed investigation of the solute distribution, which is challenging within the fine structural-length-scales of nanocrystalline materials. In the present work, we use a variety of techniques to assess changes in the grain size, chemical ordering, grain-boundary segregation, and grain-boundary structure during the heat treatment of Ni–W specimens synthesized over a wide range of grain sizes from 3 to 70 nm. A schematic microstructural evolution map is also developed based on analytical models of the various processes activated during annealing, highlighting the effects of alloying in nanocrystalline materials.

Solute distribution in nanocrystalline Ni–W alloys examined through atom probe tomography

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 previously examined by atom probe tomography, such as Ni–P, the present nanocrystalline Ni–W alloys exhibit only a subtle amount of solute segregation to the intergranular regions.

Measuring grain-boundary segregation in nanocrystalline alloys: direct validation of statistical techniques using atom probe tomography

Atom probe tomography (APT) is used to investigate grain-boundary segregation of W solute atoms in nanocrystalline Ni. For the heat-treated specimens used here, the grain structure can be observed in the APT data, enabling direct composition analyses across individual grain boundaries. These direct measurements are used to validate methods proposed in earlier work, which determine the average segregation state in nanocrystalline materials through statistical analysis of the solute distribution, without knowledge of the boundary positions. Good agreement is demonstrated between the two experimental techniques.

Solute Distribution in Nanocrystalline Ni-W Alloys

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-P2, 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)3. 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