Field evaporation behaviour in the gamma phase in Ti-Al during analysis in the tomographic atom probe

A Ti-48 at% Al alloy has been successfully investigated, using atom probe field ion microscopy and transmission electron microscopy. After a specific heat treatment, this alloy has a (alpha2 + gamma) lamellar microstructure. Using the tomographic atom probe (TAP), it has been possible to image the stacking of superlattice planes of gamma and to identify titanium as the highest evaporation field element. In addition, the influence of analysis site on atom probe measurements has been estimated for this phase. A TAP analysis has also made it possible to observe an extremely thin step along a gamma/gamma interface at a near atomic scale.     

New tomographic atom probe at University of Muenster, Germany

Currently atom probe tomography provides the highest spatial resolution compared to all other volume analysis techniques. Owing to its single atom sensitivity, it is specially suited to study nano-structured materials. Therefore, a new atom probe was installed at the Institute for Material Physics at University of Muenster, Germany, to study thin film reactions. Since the available budget was rather limited, a cost-effective non-commercial atom probe was constructed. The instrument is based on a 2D delay line detector system of 120 mm diameter. To achieve a large collecting angle and thus large volumes of analysis, a straight flight tube without a reflectron is used. This way, the flight distance may be reduced down to 160 mm. However, the variable chamber layout allows using a reflectron as an alternative. Furthermore, a laser system is implemented that delivers pulses in the 500 ps range to make possible laser-assisted evaporation of atoms. The article describes instrumental details and presents first characteristic data.     

Impact of laser pulsing on the reconstruction in an atom probe tomography

The implementation of fast pulsed laser has significantly improved the performance of the atom probe technique by enabling near-atomic-scale three-dimensional analysis of poorly conducting materials. This has broadened the range of applications for the atom probe, addressing a major limitation of the technique. Despite this, the implications of lasing on the tomographic reconstruction of atom probe data have yet to be fully characterised. Here, we demonstrate how changes in the shape of the specimen surface, induced by laser pulsing, affect the ion trajectories, and hence the projection parameters used to build the three-dimensional map.     

Laser assisted atom probe analysis of thin film on insulating substrate

We demonstrate that the atom probe analyses of metallic thin films on insulating substrates are possible using laser assisted field evaporation. The tips with metallic thin film and insulating substrate (0.6-3 μm in thickness) were prepared by the lift-out and annular ion beam milling techniques on tungsten supports. In spite of the existence of thick insulating layer between the metallic film and the tungsten support, atom probe tomography with practical mass resolution, signal-to-noise ratio and spatial resolution was found to be possible using laser assisted field evaporation.     

On the many advantages of local-electrode atom probes

Local extraction electrodes offer several crucial advantages for operation of atom probes. Because of the proximity of the local extraction electrode to the specimen, the electric field produced at the specimen apex by a given voltage is enhanced and the voltage required for field evaporation is reduced. In a voltage-pulsed atom probe, the absolute magnitude of the energy uncertainty is correspondingly reduced. High mass resolution (m/deltam > 1000) may therefore be obtained by accelerating the evaporated ions to a greater total potential after the local extraction electrode. The low extraction voltage may also be pulsed rapidly (100 ps rise time) and at high repetition rates (up to 10(5) pulses per second) using currently available solid-state pulsers. Furthermore, a local electrode and intermediate electrodes may be used as optical elements to control the image magnification. All of these benefits may be applied to any type of atom probe. Local-electrode atom probes (LEAP) should be especially advantageous for developing three-dimensional atom probes with high mass resolution and a large field of view. A sample has been developed that consists of many microtips formed on a planar sample using ion beam mask etching. Microtip samples are especially suited to LEAP. Analysis of electrically insulating samples may also be possible with microtip samples in a LEAP. This combination of features suggests flexible, high speed, high mass resolution atom probes that can work with either conventional needle-shaped specimens or the new style of planar microtip specimens.     

Detecting density variations and nanovoids

A combination of simulated and experimental data has been used to investigate the size range of nanovoids that can be detected in atom probe tomography data. Simulated atom probe tomography data have revealed that nanovoids as small as 1 nm in diameter can be detected in atom probe tomography data with the use of iso-density surfaces. Iso-density surfaces may be used to quantify the size, morphology and number density of nanovoids and other variations in density in atom probe tomography data. Experimental data from an aluminum-yttrium-iron metallic glass ribbon have revealed the effectiveness of this approach. Combining iso-density surfaces with atom maps also permits the segregation of solute to the nanovoids to be investigated. Field ion microscopy and thin section atom maps have also been used to detect pores and larger voids.     

Atom probe tomography and transmission electron microscopy characterisation of precipitation in an Al-Cu-Li-Mg-Ag alloy

State-of-the art atom probe tomography (APT) combined with transmission electron microscopy (TEM) were used to investigate the microstructure at different stages of the ageing process of an alloy of composition (at%) Al-1.68%Cu-4.62%Li-0.33%Mg-0.1%Ag. These alloys were shown to exhibit a complex microstructure of T₁ plates and several metastable phases, including θ′ and S. We will highlight the early stages of clustering, precipitate interactions and possible solute segregation at the matrix/precipitate interfaces and detail the chemical composition of the different phases.     

Characterisation of the early stages of solute clustering in 1Ni–1.3Mn welds containing Cu

Microstructural characterisation of neutron irradiated low alloy steels is important for developing mechanistic understanding of irradiation embrittlement. This work is focused on the early stages of irradiation-induced clustering in a low Cu (0.03 wt%), high Ni (∼1 wt%) weld. The weld was irradiated at a very high dose rate and then examined by atom probe (energy-compensated position-sensitive atom probe (ECOPoSAP) and local electrode atom probe (LEAP)) with supporting microstructural information obtained by small angle neutron scattering (SANS) and positron annihilation (PALA).     

Analysis conditions of an industrial Al-Mg-Si alloy by conventional and 3D atom probes.

Industrial 6016 Al-Mg-Si(Cu) alloys are presently regarded as attractive candidates for heat treatable sheet materials. Their mechanical properties can be adjusted for a given application by age hardening of the alloys. The resulting microstructural evolution takes place at the nanometer scale, making the atom probe a well suited instrument to study it. Accuracy of atom probe analysis of these aluminium alloys is a key point for the understanding of the fine scale microstructural evolution. It is known to be strongly dependent on the analysis conditions (such as specimen temperature and pulse fraction) which have been widely studied for ID atom probes. The development of the 3D instruments, as well as the increase of the evaporation pulse repetition rate have led to different analysis conditions, in particular evaporation and detection rates. The influence of various experimental parameters on the accuracy of atom probe data, in particular with regard to hydride formation sensitivity, has been reinvestigated. It is shown that hydrogen contamination is strongly dependent on the electric field at the specimen surface, and that high evaporation rates are beneficial. Conversely, detection rate must be limited to smaller than 0.02 atoms/pulse in order to prevent drastic pile-up effect.     

Analysis of clustering in Al-Mg-Si alloy by density spectrum analysis of atom probe data

Early stages of cluster formation in an Al-Si-Mg alloy were investigated by atom probe tomography and evaluated by a newly developed statistical method based on the nearest neighbour distributions. After solutionising and quenching, an alloy sample was naturally aged for one week. The atom probe data then measured was analysed for Mg, Si or Mg-Si clusters. For comparison specimen artificial aged with well developed precipitates was also investigated. A general approach for the analysis of density spectra was set up, which reduced the problem to the solution of an integral equation. Application of the method to the atom probe data set allowed us to detect clusters and to evaluate the atomic fractions within these clusters. This is also possible for an arbitrary number of nucleated phases. The higher-order next nearest neighbour distributions were used for the estimation of cluster sizes. Combining the density distribution method with a Monte Carlo simulation we found very small Si-Si and Mg-Mg clusters consisting of only a few atoms in the naturally aged state.     

Investigation of wüstite (Fe1-xO) by femtosecond laser assisted atom probe tomography

In this paper, we report results obtained from laser assisted three-dimensional (3-D) atom probe tomography (APT) on wüstite (Fe_1−xO). Oxides are generally insulating and hence hard to analyse in conventional electrical assisted APT. To overcome this problem, femtosecond laser pulses are used instead of voltage pulses. Here we discuss some aspects of pulsed laser field evaporation and optimization of parameters to achieve better chemical accuracy.     

Shaping the lens of the atom probe: fabrication of site specific, oriented specimens and application to grain boundary analysis

The random sampling provided by classical atom probe sample preparation methods is one of the major factors limiting the types of problems that can be addressed using this powerful technique. A focused ion beam enables not only site-specific preparation, but can also be used to give the specimen, which acts as the lens in an atom probe experiment, a specific shape. In this paper we present a technique that uses low accelerating voltages (10 and 5 kV) in the focused ion beam (FIB) to reproducibly produce specimens with selected grain boundaries <100 nm from the tip at any desired orientation. These tips have a high rate of successfully running in the atom probe and no Ga contamination within the region of interest.This technique is applied to the analysis of grain boundaries in a high purity iron wire and a strip-cast steel. Lattice resolution is achieved around the boundary in certain areas. Reconstruction of these datasets reveals the distribution of light and heavy elements around the boundary. Issues surrounding the uneven distribution of certain solute elements as a result of field-induced diffusion are discussed.     

Dynamic reconstruction for atom probe tomography

Progress in the reconstruction for atom probe tomography has been limited since the first implementation of the protocol proposed by Bas et al. in 1995. This approach and those subsequently developed assume that the geometric parameters used to build the three-dimensional atom map are constant over the course of an analysis. Here, we test this assumption within the analyses of low-alloyed materials. By building upon methods recently proposed to measure the tomographic reconstruction parameters, we demonstrate that this assumption can introduce significant limitations in the accuracy of the analysis. Moreover, we propose a strategy to alleviate this problem through the implementation of a new reconstruction algorithm that dynamically accommodates variations in the tomographic reconstruction parameters.     

A sensitivity analysis of the maximum separation method for the characterisation of solute clusters

Variants of the maximum separation method have become the de-facto methodologies for the characterisation of nanometre scale clusters in atom probe tomography (APT) data obtained from dilute solid solutions. All variants rely on a number of parameters and it is well known that the precise values for these parameters strongly influence estimates of cluster size and number density. Quantitative analyses require an improved understanding of the inter-relationship between user-defined parameters, experimental parameters such as detection efficiency and the resultant parameterisation of the microstructure.A series of simulations has been performed to generate clusters with a range of compositions (50-100%) and diameters (1.5-2.5 nm) in a dilute solid solution. The data were degraded to simulate the effects of the finite detection efficiencies and positioning uncertainties associated with the ECOPoSAP and LEAP-3000X HR.An extensive analysis of each resultant dataset, using a range of values for the maximum separation parameters was then performed. Optimum values for each material condition were identified and it is shown that it is possible to characterise cluster size, number density and matrix chemistry. However, accurate estimates of cluster compositions are more difficult and absolute measurements must be treated with caution. Furthermore, it is shown that D_MAX must increase with decreasing detection efficiency and consequently clusters of a specific size will appear slightly larger in atom probes with a lower detection efficiency.     

Qualification of the tomographic reconstruction in atom probe by advanced spatial distribution map techniques

New and improved spatial distribution map (SDM) methods are developed to identify and extract crystallographic information within atom probe tomography three-dimensional (3D) reconstructions. Detailed structural information is retrieved by combining z-SDM offset distribution analyses computed in multiple crystallographic directions, accurately determining inter-planar spacings and crystallographic angles. The advantages of this technique in comparison to applying the complete z-SDM and complementary xy-SDM analysis to a single crystallographic direction are investigated. Further, in determining these multidirectional z-SDM and xy-SDM profiles, background noise reduction and automatic peak identification algorithms are adapted to attain increased accuracy and is shown to be particularly effective in cases where crystal structure is present but poorly resolved. These techniques may be used to calibrate the reconstruction parameters and investigate their dependence on the design of individual atom probe experiments.     

Quantitative laser atom probe analyses of hydrogenation-disproportionated Nd-Fe-B powders

We report a successful atom probe tomography of hydrides in hydrogenation-disproportionated Nd-Fe-B powder using a green femtosecond laser. The atom probe specimens were prepared from one particle of powder using the focused ion beam lift-out method. The atom probe tomography taken from an α-Fe/NdH₂ structure suggested that B and Ga (trace added element) were partitioned in the NdH₂ phase. The hydrogen concentration of 64 at% determined from the atom probe analysis was in excellent agreement with the stoichiometry of the NdH₂ phase.     

Microstructural investigation of Sr-modified Al-15 wt%Si alloys in the range from micrometer to atomic scale

Strontium-modified Al-15 wt%Si casting alloys were investigated after 5 and 60 min of melt holding. The eutectic microstructures were studied using complementary methods at different length scales: focused ion beam-energy selective backscattered tomography, transmission electron microscopy and 3D atom probe. Whereas the samples after 5 min of melt holding show that the structure of eutectic Si changes into a fine fibrous morphology, the increase of prolonged melt holding (60 min) leads to the loss of Sr within the alloy with an evolution of an unmodified eutectic microstructure displaying coarse interconnected Si plates. Strontium was found at the Al/Si eutectic interfaces on the side of the eutectic Al region, measured by 3D atom probe. The new results obtained using 3D atom probe shed light on the location of Sr within the Al-Si eutectic microstructure.     

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.     

Materials analysis with a position-sensitive atom probe

A position-sensitive detector has been combined with time-of-flight mass spectrometry in the atom probe field-ion microscope to yield a system in which both chemical identity and spatial information are obtained for individual ions field-evaporated from the specimen surface. This allows the variations in composition originally present in the sample to be reconstructed in 3-D with sub-nanometre resolution. The prototype position-sensitive atom probe is being used to study phase chemistry in a number of metallurgical alloys, including accurate composition determination of 1–2 nm Cu-rich precipitates formed in Fe–1.3% Cu–1.4%Ni aged to peak hardness. Other applications of the position-sensitive atom probe (POSAP) include the analysis of surface layers on superconductors and atom probe studies of semiconductor multiple quantum wells. These initial applications of the instrument are reported, and the limitations and intended improvements to the instrument are discussed.