Hydrogen embrittlement through the formation of low-energy dislocation nanostructures in nanoprecipitation-strengthened steels

Embrittlement originates from dislocation nanocellular structures stabilized by the presence of hydrogen.

The heat treatment for producing interphase precipitated carbides was performed using the Dilatronic dilatometer at The University of Sheffield. Samples were first reheated to 1250°C again, held for 30 min in a tube furnace, and then water cooled to room temperature. The specimens were then austenitized at 1200°C for 3min and cooled to 630°C at a rate of 10°C s -1 , held isothermally for 90 min, and finally water quenched to room temperature. The rolling and heat treatment schedule is illustrated in Fig. S2.

Tensile experiments
Tensile test specimens were prepared according to ASTM standard (E 8M-04) with the long axes parallel to the rolling direction (shown in Fig. S3). Smooth, tensile specimens were 2.66 mm in thickness with a gauge length of 12.5 mm. Slow strain-rate tensile (SSRT) tests were conducted at a constant strain rate of 10 -5 s -1 at room temperature using a Zwick (BTC T1-FR020 TN A50) universal testing machine immediately after charging conditions (41,42).

FIB sample preparation experiments
Microstructures of tensile fractures from two steels were examined using a field-emission scanning electron microscope (FE-SEM). TEM thin foils were extracted from site-specific locations of the fracture surface using the FIB lift-out technique with the FEI Helios Nanolab 650 SEM/FIB instrument, enabling the examination of deformation microstructures immediately beneath the fracture surface, shows in Fig. S4. During the FIB lift-out from the fracture surface, platinum (Pt) was slowly deposited on the location of interest to preserve the corresponding fracture surface at the location and the microstructure below it. After FIB lift out the samples, it has to be transferred to a suitable support micromanipulator for TEM examination. The sample still needs to be thinned for electron transparency to around 200 nm.
After that, a low-energy final cleaning has been applied using a Ga+ beam accelerated at 5 keV with milling rate to be 10 nm/min with current of 20-40 pA (43).

TDA experiments
Hydrogen was electrochemically charged into the tensile test specimens using 1 g/L in an aqueous solution of 3 wt% NaCl and 0.3 wt% NH4SCN with a current of 10 mA . cm -2 for 48 h at room temperature (44).
To measure the concentrations of diffusible H in the H-charged specimens, thermal desorption analysis (TDA) was conducted using a gas chromatography system (Agilent Technologies 7890A) and a tubular furnace in a He gaseous atmosphere (45,46). TDA was conducted within 10 min after H charging. Continuous heating from room temperature to 700 ºC was used at a rate of 100 ºC h -1 , shown in Fig. S5. The hydrogen gas released from the specimen was analysed at 3 min intervals in a He carrier gas.

Nanomegas experiments
TEM-based automated crystal orientation mapping (ACOM), developed by NanoMEGAS, uses selected area diffraction patterns to determine crystal orientation and allow for rapid collection of diffraction patterns (>200 per second) and reliable indexing of the phase and orientation of the crystal with a spatial resolution of 2 nm (47,48). This allows rapid collection of 2D maps that contain a wealth of information about the microstructure including grain size, grain boundary character and texture.
The ACOM patterns were obtained with a precession angle and frequency of 0.7° and 100 Hz respectively. In order to optimise collection time against pattern quality an exposure time of 40 ms per frame was used. Figure S6 shows typical examples of the precession electron diffraction (PED) pattern obtained, free from Kikuchi lines. These maps were indexed using a simulated diffraction pattern for BCC Iron with lattice parameters of 2.861 nm. A total of 1326 different patterns were generated using the associated DiffGen 2 software. Patterns were generated with step count of 50.