DYNAMITE PROJECT

Under an intense static electric field, the surface of any material can spontaneously lose its cohesion by the expulsion of its constituents in the form of ions. This phenomenon, named “field evaporation” is the physical principle exploited by the Atom Probe Tomography (APT, a nano-analysis quantitative technique). The instrument is one of the breakthrough nano-characterisation techniques used in the study of nuclear materials, one of the main topic of the LABEX EMC3.

Under the effect of temperature and, the bombardment by neutrons produced by the fission reactions and under the effect of the surrounding environment, most of structure materials of the nuclear power industry are shown to degrade significantly. On a macroscopic point of view, this aging phenomenon generally results in a deterioration of mechanical properties (hardening, embrittlement, corrosion awareness ...) and can limit the lifetime of the reactors. This evolution of the properties is due to changes in the microstructure: agglomeration of point defects in the form of dislocation loops or cavities, segregation of solute on these extended defects at the grain boundaries and interfaces, clusters of solute formation, precipitation new phases ... the APT probe remains one of the few analytical techniques able to finely characterize nanoscale chemical heterogeneities in a material and to measure the chemical composition of the matrix in complex structures such as low alloy steels. It is considered as a must in the study of aging of nuclear materials.

The data provided by this technique help us to identify the mechanisms of aging by heat or irradiation, and in a multi-scale approach, to determine the starting point of the used models predicting changes in the macroscopic properties. It is therefore essential to identify the limits of this instrument and to determine how the results provided are close to the reality. Moreover, if APT provides extensive information on chemical heterogeneities, it does not allow to observe the extended defects directly. In addition, the emergence of these surface defects on evaporation by field effect can alter the surface of the sample and affect the three-dimensional reconstructions. It is therefore important to understand how such defects behave in the high electric field existing at the sample surface.

To understand the limitations of the instrument in terms of spatial resolution and reliability composition measurements, we propose to develop a unique multi-physics theoretical approach: the application of molecular dynamics on the surface of a material under an intense electric field. The technology is mature to give a fine interpretation and physical changes of the surface under the effect of intense field required for the technique.

Including several key issues will be addressed by the model:

• Understanding of measurement bias when analyzing APT analyses of low alloy steels (P, C, N, Si ...). Especially, molecular dynamics will provide answers to the origins of atomic diffusion phenomena under the intense field at the sample surface maintained at cryogenic temperature.

• How nanometric cavities formed by irradiating the material, behave in a Apt or field ion microscopy sample? How these cavities can influence the evaporation sequence of solutes located nearby? Can we quantitatively visualize these cavities (minimum size)?

• How point defects and linear defects (dislocations), under the action of the field, behave in the analyzed sample?

Context and objectives

In less than a decade, a tremendous explosion of the field of application of the atom probe tomography (APT) has been observed. This quantitative nano-analysis tool is almost essential in fields as diverse as μelectronic materials, photonics materials and nuclear materials. In all these disciplines, the interest is the same, being able to visualize in 3D, the nanoscale chemical composition with good quantitativity (ie reasonable accuracy on measurement of composition), a sub-nanometer spatial fidelity and a reasonable accuracy (~nanometer). This remarkable instrument, giving an image of the chemical composition on an unprecedented scale in 3D, is often coupled to the field ion microscope (FIM) who shares with the apt its same basic principles, but allows to map the presence of defects at the atomic scale (vacancies, dislocations, cavities, Figure 1 and 2).

 

Figure 1 : 3 examples of correlation between structure features and composition maps obtained by atom probe tomography. On the left, edge dislocation enriched B in FeAl alloy; amid a nuclear power vessel steel irradiated with neutrons (Copper Clusters (green) and phosphorus enriched dislocations (orange) are clearly observed); right correlation between cavities induced by the irradiation of helium observed in tomo TEM, and TiO segregations

 

Figure 2 : 3 examples of visualization of crystalline defects in FIM. At left, an atomic vacancy in a Pt-Au alloy; amid a 3D reconstruction made "by hand" of a vacancy clusters in a sample W irradiated Kr2 + ions ; right nanometer cavity observed by FIM in tungsten (distortions induced by the lens effect of the cavity

The joint use of APT and FIM on nuclear materials, which under the effect of radiation have numerous defects such as nanoscale cavities, dislocation loops, vacancies or excess interstitial atoms, is one of GPM strengths in understanding mechanisms weakening the components and materials of nuclear power plants. Note that the quantitativity and almost atomic resolution of APT / FIM instruments, unparalleled at this scale compared to other tomographic instruments, derives directly from the controlled atom by atom erosion of the atoms of the material under the effect of the intense electric field at the surface of the material maintained at a cryogenic temperature.

Many groups around the world specialized in APT lead similar studies to correlate the presence of structural defects in the segregation / precipitation process in materials using both techniques. Recent results obtained are subject to controversy because the images obtained can be subjected to artifacts difficult to predict (figure 1-3).

 

Figure 3 : (a) Observation of the diffusion of a single atom (N or C) in a low-alloy steel (Ferrite) occuring during the imaging process and (b) Experimental atomic probe image of segregation of atoms C on high-field areas (artifact because the composition should be homogeneous here)

Hence, it was recently observed that chemical solute species seem to migrate in the analysis (Figure 2 ). This phenomenon experimentally observed for nitrogen, or carbon in very low concentrations leads to biased measures of composition in the reconstructed volume. Some ideas were discussed to understand nature, but no theoretical model really exists to predict the nature and magnitude of this bias. The stability of crystalline defects within the atom probe tips is largely unknown in a sample subjected to an electric field of ​​several tens of V / nm, that is to say, under a stress of the order of elastic limit of the material surface. It is not certain that cavities composed of some few vacancies, or small dislocation loops remain still during the erosion process. If these defects move, it can be difficult to correlate the presence of defects in solute segregation. Note that the TEM tomography does not allow to image very small cavities (less that a few nanometers as seen in Figure 1), and only the FIM is able to give a 3D image of clusters of few vacancies (Figure 2 ). Current models developed to understand the erosion APT sample, if they take into account the atomic nature of the process, neglect the high deformations and stresses induced by the electric field. At best some mesoscopic models give an indication of internal deformations average

The development of new theoretical models, coupling the strain at the scale of atoms of a sample surface of a field emitter is required to better understand the coupling and fine structure correlation / composition in atom probe / FIM / MET.