F. De Geuser

Advances in the reconstruction of atom probe tomography data

Key to the integrity of atom probe microanalysis, the tomographic reconstruction is built atom by atom following a simplistic protocol established for previous generations of instruments. In this paper, after a short review of the main reconstruction protocols, we describe recent improvements originating from the use of exact formulae enabling significant reduction of spatial distortions, especially near the edges of the reconstruction. We also show how predictive values for the reconstruction parameters can be derived from electrostatic simulations, and finally introduce parameters varying throughout the analysis.

3D atom probe study of solute atoms clustering during natural ageing and pre-ageing of an Al-Mg-Si alloy

The pair-correlation function applied to 3D Atom Probe reconstructed volumes has been used to study the influence of a pre-ageing treatment (363 K) on the early stages of precipitation at 458 K in an Al-Mg-Si 6016 alloy. Mg-Si short-range positive pair correlation (clustering) is shown to form after a pre-ageing treatment. The hetero-atomic clusters are thought to act as preferential nucleation sites and lead to a finer dispersion of precipitates after ageing.

An improved reconstruction procedure for the correction of local magnification effects in three‐dimensional atom‐probe

A new 3DAP reconstruction procedure is proposed that accounts for the evaporation field of a secondary phase. It applies the existing cluster selection software to identify the atoms of the second phase and, subsequently, an iterative algorithm to homogenise the volume laterally. This procedure, easily implementable on existing reconstruction software, has been applied successfully on simulated and real 3DAP analyses.

Clustering and nearest neighbour distances in atom-probe tomography

The measurement of chemical composition of tiny clusters is a tricky problem in both atom-probe tomography experiments and atomic simulations. A new approach relying on the distribution of the first nearest neighbour (1NN) distances between solute atoms in the 3D space composed of A and B atoms was developed. This new approach, the 1NN method, is shown to be an elegant way to get the composition of tiny B-enriched clusters embedded in a random AB solid solution. The theoretical statistical distributions of first neighbour distances P(r) for both random solid solution and solute-enriched clusters finely dispersed in a depleted matrix are established. It is shown that the most probable distance of P(r) gives directly the phase composition. Applications of this model to both one-phase SiGe alloy and boron-doped silicon containing small clusters indicate that this new approach is quite reliable.

Atom Probe Tomography Spatial Reconstruction: Status and Directions

Abstract In this review we present an overview of the current atom probe tomography spatial data reconstruction paradigm, and explore some potential routes to improve the current methodology in order to yield a more accurate representation of nanoscale microstructure. Many of these potential improvement methods are directly tied to extensive application of advanced numerical methods, which are also very briefly reviewed. We have described effects resulting from the application of the standard model and then introduced several potential improvements, first in the far field, and, second, in the near field. The issues encountered in both cases are quite different but ultimately they combine to determine the spatial resolution of the technique.

Application of Fourier transform and autocorrelation to cluster identification in the three-dimensional atom probe

Because of the increasing number of collected atoms (up to millions) in the three‐dimensional atom probe, derivation of chemical or structural information from the direct observation of three‐dimensional images is becoming more and more difficult. New data analysis tools are thus required. Application of a discrete Fourier transform algorithm to three‐dimensional atom probe datasets provides information that is not easily accessible in real space. Derivation of mean particle size from Fourier intensities or from three‐dimensional autocorrelation is an example. These powerful methods can be used to detect and image nano‐segregations. Using three‐dimensional ‘bright‐field’ imaging, single nano‐segregations were isolated from the surrounding matrix of an iron–copper alloy. Measurement of the inner concentration within clusters is, therefore, straightforward. Theoretical aspects related to filtering in reciprocal space are developed.

Investigation of an oxide layer by femtosecond-laser-assisted atom probe tomography

In this letter, we report results obtained from the atom-probe tomography (APT) analysis of an oxide layer developed on a pure iron specimen under low pressure of oxygen. These specimens are generally fragile, and hard to analyze in APT. Here we show that by the use of femtosecond laser pulses, the oxide layer could be field evaporated atom by atom allowing its chemical identification at the atomic scale. The evidence of iron atomic planes through the whole oxide layer suggests that oxygen atoms diffuse though the oxide layer during the first stages of the oxidation process.

A new method for evaluating the size of plate-like precipitates by small-angle scattering

A methodology is presented for extracting the thickness and length of plate-like precipitates from streaking that appears in the small-angle scattering pattern of moderately textured polycrystalline samples. This methodology builds upon existing work on single crystals but is extended to polycrystals through a modeling of the streaking misalignment distribution. It is also shown that it is essential to take into account the Ewald sphere curvature. The protocol is applied to an in situ small-angle X-ray scattering study of the transition between θ′ and T1 in an Al–Li–Cu system, where the contributions of both phases are well separated, and the size, volume fraction and number density are monitored.

Determination of matrix composition based on solute‐solute nearest‐neighbor distances in atom probe tomography

In this study, we propose a fast automatic method providing the matrix concentration in an atom probe tomography (APT) data set containing two phases or more. The principle of this method relies on the calculation of the relative amount of isolated solute atoms (i.e., not surrounded by a similar solute atom) as a function of a distance d in the APT reconstruction. Simulated data sets have been generated to test the robustness of this new tool and demonstrate that rapid and reproducible results can be obtained without the need of any user input parameter. The method has then been successfully applied to a ternary Al‐Zn‐Mg alloy containing a fine dispersion of hardening precipitates. The relevance of this method for direct estimation of matrix concentration is discussed and compared with the existing methodologies. Microsc. Res. Tech., 2010.

Microstructure mapping of a friction stir welded AA2050 Al–Li–Cu in the T8 state

The heterogeneous precipitate microstructure of a AA2050 Al–Li–Cu in the T8 state after friction stir welding has been mapped by small-angle X-ray scattering (SAXS). 2D resolved maps of the fraction and size of both T1 platelets precipitates and clusters/GP zones formed at room temperature are provided. TEM micrographs of selected zone confirm the interpretation of SAXS intensities. This microstructure mapping is compared to microhardness mapping and a direct correlation is shown. Short duration heat treatments made in a salt bath help understanding precipitate stability and suggest that the temperature exploration alone explains to a large extent the distribution of the precipitates microstructure across the welded structure.