Correlative Atom probe tomography

Correlative atom probe tomography and electron microscopy refers to the combined utilization of a range of microscopy probing techniques to the same region in a single specimen.

This approach is increasingly deployed to understand fundamental aspects in nanoscale material science.

Particularly the combination of transmission electron microscopy (TEM) and atom probe tomography (APT) enables researchers to relate the atomic structure and composition of nanoscale features of interest and has been gaining influence over the past decades.

The correlative use of electron microscopy and atom probe tomography describe an experimental probing methodology which aims at revealing both, all relevant structural features and the chemical composition at exactly the same material position in three dimensions at full atomic scale at ppm chemcial precision. This method is also sometimes referred to as correlative atom probe tomography. 
Typically the method works by preparing needle shaped tips with a tip apex radius of around 50 nm that are suited for atom probe tomography, yet, before doing so these tips are first exposed to electron microscopical observations. 
Then the two data sets from electron microscopy and atom probe tomography or jointly analysed. 

Currently this combination of methods applied to exactly the same material portion represents the highest resolving joint crystallographic and chemical analysis method that can be applied to materials in 3D.

Using correlative electron microscopy and atom probe tomography  enables us to reveal complex structural phenomena in materials and their interplay with chemical features. Most structural effects in complex modern materials, be at the formation of certain phases or the vast hierarchy of different types of lattice defects such as point defects, dislocations, grain boundaries or hetero-phase interfaces are characterized by specific chemical features. Therefore a more holistic understanding of nanostructured materials requires to reveal both, the local chemical composition together with the local structure of phases and defects to highest possible resolution in three dimensions.

Crystallographic atom probe refers to a method where some of the lattice planes can be resolved without the use of electron microscopy only through the adequate analysis of field desorption poles of crystallographic nature from atom probe tomography directly. 
However, different from electron microscopy in many atom probe tomographic experiments only some of the lattice planes can be resolved so that typically a full local crystallographic analysis is not always possible. 
Therefore a combined application of electron microscopy and atom probe tomography to the same sample region gives a more complete structural analysis then using crystallographic atom probe tomography alone. 

Nanoscale solute segregation to or near lattice defects is a coupled diffusion
and trapping phenomenon that occurs in superalloys at high temperatures
during service. Understanding the mechanisms underpinning this crucial
process will open pathways to tuning the alloy composition for improving the
high-temperature performance and lifetime. In this project we introduce an approach combining atom probe tomography with high-end scanning electron microscopy techniques, in transmission and backscattering modes, to enable direct investigation of solute segregation to defects generated during high-temperature deformation such as dislocations in a heat-treated Ni-based superalloy and planar faults in a CoNi-based superalloy. In our project three different protocols were elaborated to capture the complete structural and compositional nature of the targeted defect in the alloy.

In this project correlative scanning transmission electron microscopy, atom probe tomography, and density functional theory calculations are jointly used to resolve the correlation between elastic strain fields and local impurity concentrations on the atomic scale. The correlative approach is applied to coherent interfaces in a κ-carbide strengthened low-density steel and establishes a tetragonal distortion of fcc-Fe. An interfacial roughness of ∼1 nm and a localized carbon
concentration gradient extending over ∼2–3 nm is revealed, which originates from the mechano-chemical coupling between local strain and composition.

Atom probe tomography can also be directly coupled with high resolution electron microscopy to resolve discrete atomic columns. The reason for this is that the needle shaped tips that are produced by using focused ion beam method for instance are so thin so that scanning transmission electron microscopy can resolve the atomic columns in such atom probe specimens prior to evaporation. Therefore atom probe tomography can be at the same region of interest directly combined with atomically resolving scanning transmission electron microscopy for resolving structural features in concert with chemical features at atomic scale.