A. Bostel

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.

Pragmatic reconstruction methods in atom probe tomography

Data collected in atom probe tomography have to be carefully analysed in order to give reliable composition data accurately and precisely positioned in the probed volume. Indeed, the large analysed surfaces of recent instruments require reconstruction methods taking into account not only the tip geometry but also accurate knowledge of geometrical projection parameters. This is particularly crucial in the analysis of multilayers materials or planar interfaces. The current work presents a simulation model that enables extraction of the two main projection features as a function of the tip and atom probe instrumentation geometries. Conversely to standard assumptions, the image compression factor and the field factor vary significantly during the analysis. An improved reconstruction method taking into account the intrinsic shape of a sample containing planar features is proposed to overcome this shortcoming.

Three-dimensional imaging of chemical order with the tomographic atom-probe

Abstract The tomographic atom-probe (TAP) is a high-resolution nanoanalytical microscope, recently developed in our laboratory, which provides three-dimensional maps of chemical heterogeneities in a metallic material on a near-atomic scale. The basic principle of this new generation of apparatus relies on the field evaporation and ionisation of atoms from the material. Chemical species are identified by time-of-flight mass spectrometry. The position of atoms at the specimen surface is determined with the aid of a specially designed position-sensitive multidetector. In this paper, application of the TAP to long-range order in intermetallics is discussed and illustrated through various examples (FeAl B2 structure, L1 2 -ordered precipitates in nickel-based alloys). Thanks to its very high depth resolution, better than 0.1 nm, this new generation of analytical microscope is able to map out the chemical order field in alloys.

Investigation of precipitation in a new maraging stainless steel

Abstract A new type of maraging steel with the composition 12Crue5f89Niue5f84Moue5f82Cu (wt%) was developed by AB Sandvik Steel. According to the analytical electron microscopy (ATEM) investigations preceding this study, the high strengh (3000 MPa) of the material was assigned to a new quasicrystalline phase possessing icosahedral symmetry and of typical composition 35Moue5f840Feue5f816Crue5f82Niue5f87Si (at%). However, only precipitates after prolonged heat treatment could be successfully studied using ATEM technique. To better understand early stages of precipitation APFIM studies of the steels were therefore undertaken. Materials heat treated at 475°C for 5, 25 min, 4 and 400 h were analyzed. The Ni and Cu-rich precipitates were found to be the major precipitates for the short time heat treatments. In the materials after prolonged aging, Cu-rich areas in direct connection to Ni-rich precipitates were observed. Tomographic atom probe (TAP) investigation was performed to clarify the spatial distribution of elements in and close to the precipitates. A Mo-rich phase was detected in the steel after 4 h aging. The distribution of alloying elements in the observed precipitates is discussed.

Improvement of the detection efficiency of channel plate electron multiplier for atom probe application

Improvement of the detection efficiency of microchannel plates (MCPs) is of great importance, especially when it is applied to quantitative measurements. The efficiency is limited by the open area ratio of MCP to a value close to 60%. By applying a small electric field between the input face of the channel plate and a grid above the face, electrons emitted by ions striking the interchannel web are returned to channels and then detected. A few attempts have been made in order to quantify the resulting increase of the detection efficiency. However, the statistical nature of the field evaporation process in the atom probe makes this quantification rather difficult. We have designed a new detector making it possible to quantify easily this improvement. Our results show that the increase in the detection efficiency depends on the electric field strength above the front face of the MCP. With a 95% transmission grid, the maximum detection efficiency obtained with an uncoated MCP is 85%.

Direct observation of boron segregation at grain boundaries in astrology by 3D atomic tomography

Rapid, continuous heat treatment for control of microstructure is a widely used technology for ferrous alloys. For example, induction and, to a lesser extent, laser and electron beam methods are common for the surface hardening of steels [1]. Induction heating has also been applied for tempering of quench-hardened steels, solution annealing of stainless steels, and recrystallization annealing of cold worked carbon, electrical, and stainless sheet steels [1,2]. On the other hand, rapid heat treatment applications have been limited for nonferrous materials. Induction heating has been applied for full (and partial) annealing of cold rolled, non-heat treatable aluminum alloys on a production scale [3]. For titanium and titanium aluminide alloys, various rapid heating techniques have been investigated for beta annealing, recrystallization annealing of cold worked sheet alloys, and other special processes, albeit only on a limited laboratory scale and only from an empirical standpoint [4-7]. The objective of the present work was to analyze the kinetics of beta grain growth during rapid, continuous heating of a conventional alpha-beta titanium alloy. The analysis was based on approximate, closed-form theoretical expressions derived by Bourell and Kaysser [8] and Soper and Semiatin [9] as well as a fully numerical, computer-based approach. The problem and approach discussed here differs from previous investigations of grain growth during continuous heating and cooling [10-13], most of which have been for austenite grain growth in the heat-affected zone during welding of steels. In this regard, the main features of the present work are (1) the very high heating rates involved, (2) the avoidance of the application of complex numerical intergration schemes, and (3) the avoidance of using isothermal grain growth kinetic data to fit continuous heating results

Investigation of some selected metallurgical problems with the tomographic atom probe

Abstract The tomographic atom probe is a new instrument which enables a small volume of a metallic material to be reconstructed in 3D on a near-atomic scale. The basic principles on which the tomographic atom probe relies are briefly described. The performance of this new generation of apparatus is illustrated on the ground of some specific experiments. The intrinsic resolution of the spatial detector that was designed and developed is estimated. Several 3D atomic reconstructions of materials are provided. Images related to the investigation of precipitation processes in two-phase nickel-base superalloys, grain-boundary segregation effects as well as G-phase formation related to the spinodal decomposition of the ferrite in duplex stainless steels are given as illustrations. The quantitativity of composition measurements and the mass resolution of the instrument are discussed.

Performance of the multiple events position sensitive detector used in the tomographic atom probe

Abstract The tomographic atom probe (TAP) is the first 3D atom probe based around the use of a parallel position encoding system. This contribution will be focused on the implementation and the study of the TAP position sensitive detector. In the first part, it will be shown why this detector necessarily has to be sensitive to simultaneous events and the detector principle will be described. Then performance and limitations of the TAP will be discussed and related to the detector performance itself. Critical points such as spatial resolution, mass resolution or detection efficiency will be quantified.

Analytic treatment of charge cloud overlaps : an improvement of the tomographic atom probe efficiency

Abstract Although reliable position and composition data are obtained with the Tomographic Atom Probe, the procedure of position calculation by charge centroiding fails when the detector receives two or more ions with close spaced positions and the same mass-to-charge ratio. As the charge clouds of the ions overlap, they form a unique charge pattern on the multianode detector. Only one atom is represented and its position is biased. In order to estimate real positions, we have developed a correction method. The spatial distribution of charges inside a cloud issued from one impact is modelled by a Gaussian law. The particular properties of the Gaussian enable the calculation of exact positions of the two impacts of the overlapped charge patterns and charges of corresponding clouds. The calculation may be generalized for more than two overlapped clouds. The method was tested on a plane-by-plane analysis of a fully ordered Cu3Au alloy performed on a (100) pole.

Improvement of the mass resolution of the atom probe using a dual counter-electrode

As compared to other techniques, the mass resolution of the 3D atom probe is rather poor. This low mass resolution derives from the spread in energy of field-evaporated ions. In this work, the single counter-electrode used to remove atoms from the specimen was replaced with a dual counter-electrode. A positive standing voltage V(PA) is applied on the electrode facing the specimen while the second electrode is grounded. As a result, ions experience a post-acceleration between electrodes that lowers energy deficits of ions resulting in an improvement in the mass resolution. This paper reports the study of the resulting improvement in mass resolution as a function of the post-acceleration voltage. It is also shown that, because of the evaporation pulse, ions also undergo a dynamic post-deceleration in the between electrodes. This post-deceleration contributes to the mass resolution increase. Our results show that this very simple device makes it possible to significantly improve the mass resolution of the atom probe. For a low post-acceleration voltage, the mass resolution is 800 FWHM and 200 at full-width tenth-maximum.