Page 296 - Read Online

Zhou et al. Microstructures 2023;3:2023043 https://dx.doi.org/10.20517/microstructures.2023.38 Page 13 of 23

referred to as the S phase, shown in Figure 8C and D). Figure 8A reveals co-located clusters of Zr, a
component of the Al Zr dispersoids, along with hydrogen (H) and deuterium (D), indicating that hydrogen
3
and deuterium are trapped within these dispersoids. Figure 8B displays a 1-D concentration profile across
the dispersoid, confirming the presence of trapped hydrogen and deuterium. Figure 8C presents atom maps
showing the distribution of Al (matrix), Mg (S phase), and the trapped hydrogen and deuterium. Figure 8D
shows the corresponding 1-D concentration profile from the region marked in the Mg map of Figure 8C.
These findings offer valuable insights into the specific microstructures in aluminum alloys that have the
capability to trap hydrogen, thus contributing to the development of materials that are more resistant to
hydrogen embrittlement.

Titanium alloys
Titanium (Ti) alloys are highly sought after for aerospace applications due to their exceptional strength-to-
weight ratio and corrosion resistance. However, Ti alloys are susceptible to hydrogen embrittlement, a
phenomenon where hydrogen combines with the metal to form brittle hydride, leading to crack initiation
[65]
and propagation . Characterizing the hydride and its impact in Ti alloys using conventional FIB and APT
has been challenging, primarily because of the rapid formation of hydrides during specimen preparation,
such as in FIB, where hydrogen is often present as a residual gas in the environment. This tendency to
absorb environmental hydrogen and form artifact hydrides has caused significant confusion in material
characterization when trying to identify the true origin of hydrides.

As such, Chang et al. developed specimen preparation methods using cryo-PFIB and cryo-APT to analyze
commercially pure Ti (CP Ti) and a Ti-6Al-2Sn-4Zr-6Mo alloy (referred to as Ti6246) in their pristine
[46]
states . The researchers employed hydrogen-sensitive APT to examine tip specimens prepared by
electropolishing, conventional Ga FIB, Xe PFIB operating at room temperature, and cryo-PFIB. Their
findings revealed that only cryo-PFIB provided hydride-free APT tips, as depicted in Figure 9.
Figure 9A and B displays the CP-Ti and Ti6246 samples prepared using conventional FIB, showing the
presence of artifact hydrides or significant hydrogen uptake from the FIB environment. On the other hand,
Figure 9C and D demonstrates that using cryo-PFIB produced hydride-free APT results for CP-Ti and
Ti6246, allowing for the analysis of the beta phase, which is prone to hydrogen uptake, with low hydrogen
content [Figure 9D]. These results suggest a promising path to enhance the quality of high-resolution
hydrogen analysis in Ti-based alloys by utilizing cryo-APT and cryo-FIB.

Liquid and organic specimens
The research in analyzing mobile hydrogen atoms in metals has showcased the capabilities of cryo-APT and
cryo-FIB workflows to achieve low temperatures. This breakthrough has paved the way for their application
in the analyses and preparation of biological specimens, which predominantly consist of water and have
poor conductivity. However, the existing knowledge of APT and its instrumentation is primarily geared
towards conductive materials, especially metals. How organic and ice specimens can behave in APT
evaporation experiments is still largely unknown, which could impede the data interpretation in the 3-D
atom reconstruction.

As such, Schwarz et al. conducted APT analysis on ice using cryo-specimen fabrication and laser-pulsed
APT, resulting in the collection of several tens of millions of atoms, as shown in the mass spectrum in
Figure 10A . The analysis identified several primary peaks related to ice evaporation, with the top five
[80]
+
+
+
2+
peaks in relative intensity being H O at 19 m/z, (H O)H O at 37 m/z, (H O) H O at 55 m/z, (H O) H O
3
2
3
2
3
3
3
2
2
+
at 36.5 m/z, and (H O) H O at 73 m/z (note the logarithmic scale). Building upon this achievement,
2
3
3
Schwarz et al. performed another analysis on an aqueous solution saturated with glucose (chemical formula:
C H O ) to demonstrate the ability of APT to distinguish glucose peaks from ice peaks . Figure 10B serves
[81]
6
12
6