Correlative APT-TEM analysis of precipitates

400 nm b

The combination of APT and TEM provides a unique glimpse into the atomic world, helping to better understand and tailor materials’ properties.

Davit Melkonyan

Materials characterisation with atomic-scale spatial resolution and chemical compositional measurements are driving the engineering of new functional materials. Advanced materials research has now evolved towards astonishing insights into the atomic composition of complex 3-dimensional (3D) nanostructures with unprecedented resolution. Although there are many characterisation techniques providing spatial and compositional information, atom probe tomography (APT) is the only one offering extensive capabilities for both 3D-imaging and chemical composition measurements at the atomic scale. In addition, correlative microscopy – whereby the power and advantages of several techniques such as electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and APT are combined – can provide extra information extending the atomic-scale insight into the materials composition and crystallography.

At OCAS, we started using APT in metalrelated research in 2016. Over the past several years, APT has supported us with an advanced characterisation of precipitates, coatings, grain and phase boundaries. Below we present a case study highlighting the TEM-APT correlative analysis of precipitates in InterMetallic Composite Alloys (IMCA). IMCAs are a specific type of martensitic ultra-high strength steels which get their strength by the nanoprecipitation of (complex) intermetallics. The typical alloying elements are Ni, Co, Mo, Al and Ti. The formation of a certain type of strengthener depends on the chemical composition and processing route of the IMCA alloys. In this process, the characterisation of these precipitates plays a crucial role in understanding and controlling their formation and impact. APT is a useful technique for addressing this need. APT analysis requires a needle-shaped specimen with an apex diameter smaller than 100 nm. This specimen was prepared using a focused ion beam/scanning electron microscopy (FIB/SEM) dual beam system, and a TEM image of this specimen was taken afterwards. Next, the sample was analysed using APT. APT is a destructive technique, evaporating the needle-shaped specimen atom by atom. This is demonstrated by the comparison of the TEM images before and after APT analysis. Indeed, the overlay of these two images reveals that the first 400 nm of the specimen was analysed. This information helps us achieve more accurate APT reconstruction, improving the data accuracy. The APT analysis allows us to obtain the morphology size distribution composition and volume fraction of these precipitates. In this example, the TEM was used to improve APT data accuracy, but it can also be used to obtain crystallographic information, which, combined with compositional information of APT data, can help us better understand kinetics and mechanisms of precipitation.