D. Blavette

The tomographic atom probe: A quantitative three‐dimensional nanoanalytical instrument on an atomic scale

The physical architecture and the performance of a quantitative three‐dimensional atom probe recently constructed are described. The development of such an instrument relies on the design of a multi‐impact position sensitive detector. The multidetection system that we have developed is based on the use of a 10×10 anode array placed behind a two microchannel plate assembly in a chevron arrangement. The spread of charge between the microchannel plate and the multianode is used to derive the position of ion striking the detector. Spatial coordinates can be calculated for multiple and simultaneous time‐of‐flight events. The procedure used for the derivation of ion positions from charge measurements is given. Specific experiments were carried out in order to determine the intrinsic spatial resolution of the multidetector. Three‐dimensional reconstruction of two‐phase materials are provided and illustrate the performance of this new apparatus. The reconstructed images demonstrate that atoms are positioned with ...

Trajectory overlaps and local magnification in three-dimensional atom probe

Local magnification effects related to the presence of a second phase in three-dimensional atom probe have been investigated using a simulation of ion trajectories from the analyzed sample surface. Spherical precipitates containing only B atoms embedded in pure A solid solution were considered. The magnification was found to vary drastically from 0.5 to 2.0 times when the evaporation field of B (EB) was varied from 1.15 EA to 0.85 EA. The trajectories were found to overlap over distances close to 1 nm only when the reduced evaporation field (eB=EB/EA) is outside of a gap ranging from 0.9 to 1.1. Simulations indicate that the “measured” composition in the inner core of precipitates is not biased in this gap. This is also the case for particles which have a diameter larger than a critical value of 2 nm.

Design of a femtosecond laser assisted tomographic atom probe

A tomographic atom probe (TAP) in which the atoms are field evaporated by means of femtosecond laser pulses has been designed. It is shown that the field evaporation is assisted by the laser field enhanced by the subwavelength dimensions of the specimen without any significant heating of the specimen. In addition, as compared with the conventional TAP, due to the very short duration of laser pulses, no spread in the energy of emitted ions is observed, leading to a very high mass resolution in a straight TAP in a wide angle configuration. At last, laser pulses can be used to bring the intense electric field required for the field evaporation on poor conductive materials such as intrinsic Si at low temperature. In this article, the performance of the laser TAP is described and illustrated through the investigation of metals, oxides, and silicon materials.

A general protocol for the reconstruction of 3D atom probe data

Abstract Data collected with 3D atom probes have to be carefully analysed in order to give reliable composition data precisely positioned in the probed volume. Indeed, the large analysed surfaces of 3D atom probes require the development of reconstruction methods taking into account the tip geometry. When the analysis does not take place in the close vicinity of the tip axis, the analysis direction is no longer perpendicular to the evaporated surface. The influence of this effect on the local magnification and atom positioning must be taken into account. The proposed procedure will be validated by studying the effects of calculations on a long-range-ordered phase.

Solid state amorphization in cold drawn Cu/Nb wires

The microstructure of cold drawn Cu/Nb nanocomposite wires was investigated using a three dimensional atom probe (3D-AP) and transmission electron microscopy (TEM). Although there is no solubility between Nb and Cu in the equilibrium state, atom probe analysis results revealed that intermixing occurs between Nb and Cu filaments as a result of cold drawing with a large strain. High resolution transmission electron microscopy (HRTEM) results revealed that an amorphous layer is formed along some Cu/Nb interfaces. This solid state amorphization is compared with similar reactions observed in Cu-Nb multilayers.

Atomic-scale APFIM and TEM investigation of grain boundary microchemistry in Astroloy nickel base superalloys

Electron microscopy and atom-probes techniques, including 3D (three-dimensional) atom-probe, were applied to the investigation of grain-boundary (GB) microchemistry and interfacial segregation phenomena in nickel base superalloys, Astroloy. Diffraction patterns and EDX microanalyses exhibit the presence of intergranular M23C6 chromium enriched carbides as well as intragranular TiC precipitates. Complex phases containing Zr, C and S were also observed. Three-dimensional images as provided by the tomographic atom-probe revealed the presence of a strong segregation of both boron and molybdenum at grain boundaries. Considerable chromium enrichment at γ′-γ′ grain boundaries and slight carbon segregation to GBs, whatever their chemical nature, were also detected. All these segregants are distributed in a continuous manner along the boundary over a width close to 0.5 nm. Experiments show that segregation occurs during cooling and more probably between 1200 and 800°C. Boron, chromium and molybdenum GB enrichments are thought to be mainly controlled by an equilibrium segregation process. No segregation of zirconium was detected.

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.

Structural analyses in three-dimensional atom probe: a Fourier transform approach

The three‐dimensional atom probe (3DAP) technique gives the elemental identities and the position of atoms within the small volume analysed (on the order of 10 × 10 × 100 nm3). The large number of atoms collected (up to two million) and the excellent spatial resolution of this instrument allows the observation of some crystallographic features of phases chemically identified. This paper shows that the application of a discrete Fourier transform algorithm to a 3DAP dataset provides information that is not easily accessible in real space. The derivation of the mean size of particles from Fourier intensities is an example. Using 3D ‘dark‐field’ imaging, single ordered grains were isolated from the disordered matrix of a ternary alloy. Moreover, the intrinsic spatial resolution of the instrument was evaluated by this method for pure metal; the resolution reaches 0.2 nm laterally and 0.06 nm in depth. This excellent resolution is shown to be sufficient to give access to the crystalline lattice. The use of image filtering in the reciprocal space enables for atomic columns to be imaged the first time from 3DAP data.

Modeling Image Distortions in 3DAP

A numerical model has been developed to simulate images obtained from the three-dimensional atom probe. This model was used to simulate the artefacts commonly observed in two-phase materials. This model takes into account the dynamic evolution of the atomic-scale shape of the specimen during field evaporation. This article reviews the model and its applications to some specific cases. Local magnification effects were studied as a function of the size, the shape, and the orientation of precipitated phases embedded in the matrix. Small precipitates produce large aberrations in good agreement with experiments. The magnification from such precipitates, as measured from the simulation, is only found to match the theoretical value for mesoscopic scale precipitates (size similar to the specimen size). Orientation effects are also observed in excellent agreement with experiments. The measured thickness of a grain-boundary-segregated film in the simulation is found to decrease with the angle between the normal to the grain boundary and the tip axis. Depth scaling artefacts caused by variation in the evaporation field of atoms in multilayer structures were successfully simulated and again showed good agreement with effects observed experimentally.

Ordering and phase separation in Ni–Cr–Al: Monte Carlo simulations vs three-dimensional atom probe

Ordering and phase separation in Ni–Cr–Al alloys were studied using Monte Carlo (MC) simulation and three-dimensional atom probe. Excellent agreement was found. Simulation showed that: (i) in the γ′ phase, Cr substitutes for both Al and Ni sublattices; (ii) in the γ phase, a “Ni3Cr”-type and an L12-type short range order (SRO) develop, and transient Al-rich L12 ordered zones exist. In low supersaturated alloy a nucleation and growth mechanism was observed. Simulations indicated that phase transformation occurred in three stages: I—SRO develops: a “Ni3Cr”-type and an L12-type; II—formation of Al enriched and ordered zones; III—growth and coarsening of γ′ particles. Simulations of high supersaturated alloys showed that a high density of L12 ordered zones nucleates before unmixing.