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Page 10 of 23 Zhou et al. Microstructures 2023;3:2023043 https://dx.doi.org/10.20517/microstructures.2023.38

Steels
In 2010, Takahashi et al. made a significant breakthrough by reporting the first high-resolution hydrogen
[73]
mapping using APT in ferrite steel containing titanium carbides (TiC), as displayed in Figure 6A . They
achieved this successful analysis by employing a custom gaseous deuteration cell with thermal control on
the APT tip specimens. This allowed for controlled heating during tip deuteration and subsequent
quenching, facilitating the uptake of deuterium and minimizing its desorption thereafter. The study
revealed that the trapped deuterium atoms (represented by red spheres in Figure 6A) were localized in
regions where titanium and carbon atoms clustered, indicating hydrogen trapping at TiC. Subsequently,
Takahashi et al. demonstrated another successful analysis of trapped deuterium atoms in vanadium carbides
[76]
(VC) within a VC precipitation-strengthened steel, as shown in Figure 6B . More recently, Takahashi et al.
utilized cryo-APT to correlate VC-trapped deuterium with the crystal orientation of VC in a peak-aged VC
steel, as depicted in Figure 6C . This study provided valuable insights into the specific (001) plane trapping
[77]
of deuterium in the VC, further expanding the capabilities of cryo-APT in unraveling intricate hydrogen-
microstructure relationships. These studies showcase the potential of cryo-APT in advancing our
understanding of hydrogen behavior in steels.

Apart from the work by Chen et al. have also made contributions to observing hydrogen trapping in
steels . Instead of using a custom gaseous charging deuteration cell, they opted for a route using
[40]
heavy water electrolysis for tip deuteration . This approach generated a substantially higher
[40]
deuterium pressure (or fugacity) compared to the gaseous method used by Takahashi et al., leading to
improved data statistics of deuterium atoms in APT reconstruction . This allowed for a more
[73]
unambiguous observation of hydrogen trapping sites. As shown in Figure 7A, Chen et al. utilized cryo-APT
to demonstrate the presence of trapped deuterium within vanadium-molybdenum-mixed (V-Mo) carbides,
which possess a rocksalt (NaCl) structure in a ferritic steel matrix . The left figure in Figure 7A represents
[40]
a 10-nm-thick slice displaying deuterium and vanadium in red and blue, respectively. The right figure
presents a statistical analysis, superimposing all identified V-Mo carbides from the APT dataset. This
analysis reveals the distributions of deuterium in relation to other carbide-related elements (V, Mo, and C).
The elemental distribution profile indicates that deuterium atoms mainly localize within the V-Mo carbides
rather than at the interfaces of the carbides. As discussed, Chen et al. attributed this result to the presence of
carbon vacancies in the V-Mo carbides .
[15]
Subsequently, Chen et al. applied the same experimental method to examine hydrogen trapping in niobium
carbide (NbC) precipitates, as shown in Figure 7B . Similar to the V-Mo carbides, the NbC also possesses a
[34]
NaCl structure in a ferritic steel matrix but with fewer carbon vacancies. The top right figures in Figure 8B
display 2-D slice views of the NbC precipitates and trapped deuterium (as schematized in the bottom left
figure), revealing the hydrogen trapping at the NbC-ferrite interface. The bottom right figures in Figure 7B
illustrate concentration profiles from two NbC precipitates (as schematized in the bottom left figure),
confirming the interface hydrogen trapping of NbC. The combination of results from Figure 7A and B
provides a comprehensive depiction of hydrogen trapping mechanisms in metal carbides with a NaCl
structure. In addition, Chen et al. utilized cryo-APT to achieve the first-ever high-resolution observations of
hydrogen segregation at dislocations and GBs, further demonstrating the power of cryo-APT in advancing
research on hydrogen trapping and embrittlement in steels . These groundbreaking findings have opened
[34]
up new avenues for understanding the behavior of hydrogen in complex materials and its implications for
material performance and design.

Aluminum alloys
Pristine aluminum (Al) and its alloys have a natural ability to form a protective aluminum oxide layer when
exposed to oxygen, which shields them from further oxidation or corrosion. However, when these metals