Frédéric De Geuser

Advances in the calibration of atom probe tomographic reconstruction

Modern wide field-of-view atom probes permit observation of a wide range of crystallographic features that can be used to calibrate the tomographic reconstruction of the analyzed volume. In this study, methodologies to determine values of the geometric parameters involved in the tomographic reconstruction of atom probe data sets are presented and discussed. The influence of the tip to electrode distance and specimen temperature on these parameters is explored. Significantly, their influence is demonstrated to be very limited, indicating a relatively wide regime of experimental parameters space for sound atom probe tomography (APT) experiments. These methods have been used on several specimens and material types, and the results indicate that the reconstruction parameters are specific to each specimen. Finally, it is shown how an accurate calibration of the reconstruction enables improvements to the quality and reliability of the microscopy and microanalysis capabilities of the atom probe.

Estimation of the Reconstruction Parameters for Atom Probe Tomography

The application of wide field-of-view detection systems to atom probe experiments emphasizes the importance of careful parameter selection in the tomographic reconstruction of the analyzed volume, as the sensitivity to errors rises steeply with increases in analysis dimensions. In this article, a self-consistent method is presented for the systematic determination of the main reconstruction parameters. In the proposed approach, the compression factor and the field factor are determined using geometrical projections from the desorption images. A three-dimensional Fourier transform is then applied to a series of reconstructions, and after comparing to the known material crystallography, the efficiency of the detector is estimated. The final results demonstrate a significant improvement in the accuracy of the reconstructed volumes.

Spatial Resolution in Atom Probe Tomography

This article addresses gaps in definitions and a lack of standard measurement techniques to assess the spatial resolution in atom probe tomography. This resolution is known to be anisotropic, being better in-depth than laterally. Generally the presence of atomic planes in the tomographic reconstruction is considered as being a sufficient proof of the quality of the spatial resolution of the instrument. Based on advanced spatial distribution maps, an analysis methodology that interrogates the local neighborhood of the atoms within the tomographic reconstruction, it is shown how both the in-depth and the lateral resolution can be quantified. The influences of the crystallography and the temperature are investigated, and models are proposed to explain the observed results. We demonstrate that the absolute value of resolution is specimen specific.

Origin of the spatial resolution in atom probe microscopy

Atom-probe microscopy offers unprecedented insights on the subnanometer structure and chemistry of materials in three dimensions. The actual spatial resolution achievable is however still an uncertain parameter, as no comprehensive study has been undertaken to unveil the physics underpinning how key parameters impact the performance. Here, we present a comprehensive investigation of the in-depth and lateral resolution of the technique. We discuss methods to estimate the resolution and show a resolution better than 20 pm in-depth. Models to support our results were developed and are discussed in the present letter.

On the validity of simple precipitate size measurements by small-angle scattering in metallic systems

This paper assesses how simple small-angle scattering particle size evaluation models, such as Porod or Guinier radii, which have a normally limited validity range, may see this range extended to larger q values. This is shown to be particularly true for metallic systems, where the dispersion in particle size is always large. Because of the size dispersion, the relationship between the average particle size and the Guinier radius is shown to change. For systems with relatively large size dispersion, the paper shows that the Porod and Guinier radii, and simple extensions thereof, give valuable information on particle size and particle size distribution. This is demonstrated to be valid for particles with moderate aspect ratios. These simple evaluations are quick and very well adapted to large data sets, such as those originating from time-resolved or scanning small-angle experiments.

Atom probe microscopy investigation of Mg site occupancy within δ′ precipitates in an Al–Mg–Li alloy

The composition and site occupancy of Mg within ordered δ′ precipitates in a model Al–Mg–Li alloy have been characterized by atom probe microscopy and first-principles simulations. The concentration in the precipitates is found to be almost the same as that of the matrix; however, we show evidence that Mg partitions to the sites normally occupied by Li in the L12 structure. Density functional calculations demonstrate that this partitioning is energetically favorable, in agreement with experimental results.

Quantitative Characterization of Precipitate Microstructures in Metallic Alloys Using Small-Angle Scattering

Quantitatively characterizing precipitate microstructures in metals by small-angle scattering poses specific challenges as compared to other areas of application of this technique. In terms of size and morphology evaluation, these include the presence of a significant size distribution, non-isotropic shapes, and interpretation complicated by a partial averaging due to a non-random texture. In terms of volume fraction evaluation, these include the imperfect knowledge of the chemical composition of very small objects. This paper, based on a presentation given at the “Neutron and X-Ray Studies of Advanced Materials V: Centennial” symposium of the 2012 TMS conference, reviews the strategies that can be applied in different characteristic cases to obtain a robust quantification of precipitate microstructures.

Quantitative description of the T1formation kinetics in an Al–Cu–Li alloy using differential scanning calorimetry, small-angle X-ray scattering and transmission electron microscopy

The object of the present study is to design a methodology to follow the kinetics of T1 precipitation, in an AA2198 alloy, in terms of precipitate size, morphology (thickness, diameter) and volume fraction, during a two-temperature isothermal heat treatment. We used in situ small-angle X-ray scattering (SAXS) as a way to measure the evolution of the T1 mean thickness and diameter during the heat treatment. Transmission electron microscopy (TEM) was then performed in order to calibrate these evolutions. Furthermore, we demonstrate that the volume fraction evolution can be described successfully using a simple analysis of the differential scanning calorimetry (DSC) thermograms. The latter was calibrated by selected observations in high angular annular dark field scanning transmission electron microscopy (HAADF-STEM). Microstructure evolution during DSC heating ramps was analysed using in situ SAXS: the T1 phase transformation is found to consist in a two-step thickening process explained by two consecutive diffusion stages. The enthalpy of formation of the T1 phase is deduced from the DSC measurements.

Size distribution and volume fraction of T1 phase precipitates from TEM images: Direct measurements and related correction

Transmission Electron Microscopy (TEM) can be used to measure the size distribution and volume fraction of fine scale precipitates in metallic systems. However, such measurements suffer from a number of artefacts that need to be accounted for, related to the finite thickness of the TEM foil and to the projected observation in two dimensions of the microstructure. We present a correction procedure to describe the 3D distribution of disc-like particles and apply this method to the plate-like T1 precipitates in an Al-Li-Cu alloy in two ageing conditions showing different particle morphologies. The precipitates were imaged in a High-Angular Annular Dark Field Microscope (HAADF-STEM). The corrected size distribution is further used to determine the precipitate volume fraction. Atom probe tomography (APT) is finally utilised as an alternative way to measure the precipitate volume fraction and test the validity of the electron microscopy results.

Relationship Between Microstructure, Strength, and Fracture in an Al-Zn-Mg Electron Beam Weld: Part II: Mechanical Characterization and Modeling

This paper presents an experimental and modeling study of the mechanical behavior of an electron beam welded EN-AW 7020 aluminum alloy. The heterogeneous distribution of mechanical properties is characterized by micro-tensile tests and by strain field measurements using digital image correlation technic. These results are related to the microstructural observation presented in the companion paper. The mechanical behavior of the weld is simulated by a finite element model including a Gurson-type damage evolution model for void evolution. The model is shown to be capable of describing accurately experimental situations where the sample geometry is varied, resulting in stress triaxiality ratios ranging from 0.45 to 1.3.