science.sciencemag.org/content/367/6474/171/suppl/DC1

Supplementary Materials for

Observation of hydrogen trapping at dislocations, grain boundaries,

and precipitates

Yi-Sheng Chen, Hongzhou Lu*, Jiangtao Liang, Alexander Rosenthal, Hongwei Liu,

Glenn Sneddon, Ingrid McCarroll, Zhengzhi Zhao, Wei Li, Aimin Guo,

Julie M. Cairney*

*Corresponding author. Email: julie.cairney@sydney.edu.au (J.M.C.); luhz@citic.com (H.L.)

Published 10 January 2020, Science 367, 171 (2020)

DOI: 10.1126/science.aaz0122

This PDF file includes:

Materials and Methods

Figs. S1 to S7

Table S1

References

Other Supplementary Material for this manuscript includes the following:

(available at science.sciencemag.org/content/367/6474/171/suppl/DC1)

Movies S1 to S4

1

Table S1. Material composition

Fe C Mn Si Nb S P N

wt.% 98.55 0.23 0.92 0.24 0.049 <20 ppm 0.01 40 ppm

Material production and heat treatment

100 x 60 x 40 mm ingots were cast in a vacuum induction furnace at over 2000°C, heat-

treated at 1200°C for 1 hour, then hot-rolled with 5 steps between 1150°C and 870°C to reduce

the ingot thickness from 40 mm to 6 mm. The hot sheets were laminarly water-cooled to 660°C,

kept at this temperature for 1 hour and then left in the furnace until room temperature was reached.

Finally, the 6 mm sheets were gradually cold-rolled with a step size of approximately 0.5 mm until

they were 1.5 mm thick. For the ferritic samples, the samples were further annealed at 700°C for

5 hours, followed by air cooling. The annealing temperature was determined based on the

calculation (Thermo-Calc Software) shown in Figure S1. The figure on the right shows that an

optimal carbide volume fraction was achieved at approximately 700°C. Martensitic samples were

obtained by austenizing at 930°C for 5 minutes and then water-quenching.

Figure S1. Thermo-Calc carbide volume fraction calculation

Theoretical estimation of the carbide volume fraction at the composition of the investigated

samples (shown in Table S1).

2

TKD experiments

Electron-transparent samples were prepared that were suitable for both TKD and TEM.

Sheets of the steel were cut into 10 x 10 mm squares and ground to approximately 100 μm, thin

enough to punch out 3 mm diameter discs. The discs were then electropolished in a TenuPol-5

electropolisher (Struers) with double-side jets by using a solution of 10% perchloric acid in acetic

acid at approximately 20.5 V until holes formed the sample center, creating thin areas in the

vicinity of the holes.

For TKD, the discs were loaded into a scanning electron microscope (Zeiss ULTRA Plus)

equipped with an electron back-scatter diffraction (EBSD) detector, operating at 20 kV (21). The

grain structures of the ferritic and martensitic samples were analyzed by TKD with 50 and 6 nm

step sizes, respectively. Figure S2 contains band contrast and phase maps for the ferritic and

martensitic samples, complementing the information provided in Figures 1A and 1E. Figures S2B

and S2D show the fraction of BCC/BCT ferrite/martensite. We note that the martensite was able

to be indexed as BCC, as the level of distortion in the lattice in this low carbon steel is very small.

The phase map for the martensitic sample (S2D) reveals a low volume fraction of residual austenite

(blue).

Figure S2. Supplementary TKD maps

(A) and (C) are band contrast maps of the ferritic and martensitic samples shown in Figure 1,

showing the grain configurations, whereas (B) and (D) are phase maps showing the low residual

austenite content (blue).

3

TEM experiments

After undertaking TKD analyses, the discs were examined in a JEOL2200FS TEM equipped

with a double-tilt holder (Gatan). To image the NbC precipitates in the ferritic sample, the sample

was tilted to the [110] crystallographic orientation. This beam condition allows the observation of

second-phase inclusions in BCC iron with minimal artefacts, and is used to confirm the incoherent

nature of the boundary (no orientation relationship) between the NbC carbide and the ferritic

matrix. To image the dislocations in the martensitic sample, the sample was tilted close to the two-

beam diffraction condition close to [220] to allow centered bright field imaging (CBF) and

centered dark field imaging (CDF) of the observed dislocation lines. Observation of an APT needle

was conducted in a JEOL 2100 TEM with a single tilt holder adapted to hold the needle-shaped

sample. Although double tilt would be required for a full dislocation analysis, imaging at two

different orientations along a single tilt axis (5 degrees difference) showed the presence of a high

density of dislocations. The sample also contained grain boundaries.

Figure S3. TEM images of atom probe needles

TEM bright-field images of an atom probe needle sample with 5 degree difference in tilt. A grain

boundary is labelled in (A). Dislocations, an example of which is labelled in (B) are not visible at

the orientation shown in (A) confirming that they are dislocations, which can only be viewed along

certain orientations with respect to the incident electron beam.

TDS experiments

Thermal desorption spectroscopy (TDS) experiments were undertaken by using custom-

designed equipment, consisting of a glass tube for carrying samples, a vacuum-pumping system,

a heating furnace (Shanghai Jvjing Precision Instrument Manufacturing) and a quadruple mass

spectrometer (INFICON). TDS sheet samples of 1.5 × 5 × 20 mm and were polished and cleaned

on all sides. Hydrogen desorption was tested at a heating rate of 6.66 °C per minute, immediately

after the samples has been charged in 0.5 M H2SO4 + 0.013 M Thiourea (CH4N2S) aqueous

solution at 20 mA/cm2 for 1 hour.

APT experiments

For APT sample preparation, samples were first cut into 1 x 1 x 15 mm matchstick-shaped

bars. Rough electropolishing was carried out halfway along the length of the sample by using 25%

perchloric acid in acetic acid at 10-30 V until the two sides separated to yield two needle-shaped

samples. Fine polishing was conducted with 2% perchloric acid in butoxyethanol under a 40x

optical microscope. Polishing was undertaken at 30 V until a neck forms near the apex, and then

at 10 V until the top part was removed, providing a fresh, sharp sample apex ready for deuterium

charging and subsequent APT analysis.

4

All the atom probe analyses in this work were conducted in a Local Electrode Atom Probe

(LEAP 4000X Si, CAMECA) at a pulse frequency of 200 kHz, a stage temperature of 50 K and a

pulse fraction of 20%.

Atom probe data was reconstructed by using an initial radius estimated by the atom probe

analysis software IVAS (version 3.6.12, CAMECA). For example, 69.16 and 67.54 nm were used

for the data shown in Figure 3A and Figure 4E, respectively. A detector efficiency of 57% and an

image compression factor of 1.25 were also used for reconstruction, where the latter was obtained

after applying the calibration method suggested in (38) to the ferritic dataset shown in Figure 3A.

The data in Figure 4 was reconstructed using the same image compression factor as the ferritic

data of Figure 3A, as we were not able to identify poles in the data in our martensitic sample.

1D z-axis profiles were obtained from cylindrical or cuboid ROIs. A fixed bin width was used

(0.5 nm for the NbC and 1 nm for the grain boundary), without overlapping between the bins.

Proximity histograms (proxigrams) were also used to analyze local compositions in the vicinity of

microstructural features. A proxigram creates a concentration profile by progressively summing

all atoms in bins at set distances away from a defined surface.

Deuterium charging method

The method for deuterium charging of the needle-shaped sample was modified from the

method described in the supplementary section of (13). A charging solution of 0.1 M NaOH in

D2O (Sigma-Aldrich) was used to create a deuterium-rich environment around the steel samples

at the cathode in an electrolytic circuit, with a gold counter-electrode. This was carried out in

custom-designed cryo-transfer glove box (Microscopy Solutions), as shown in Figure S4A. Four

sharp samples were pre-mounted in a four-needle atom probe sample puck with polyether ether

ketone (PEEK) insulation. The four tips protruded from the puck to the same height and were

immersed into the charging solution at a depth that was barely visible, and could only be identified

via the capillary of the liquid solution. The puck and tips were held by an APT sample transfer rod

on a 3D-printed stage as shown in Figure S4B. The deuterium charging was conducted at 2.2 V

(powered by Keysight 2901A) for 30 seconds, followed by immediately immersing the whole puck

into a liquid nitrogen (LN2) bath, as shown in Figure S4B. In the reconstructed data from

electrolytically charged samples, a layer rich in deuterium and oxygen was present at the tip apex.

This region is removed to circumvent data ambiguity with the deuterium in bulk.

Cryo-transfer method

In order to minimize the loss of deuterium due to diffusion, a dedicated cryo-transfer method

was developed, based upon concepts presented in (13, 39). The cryo-transfer method is configured

in three parts: a) a glove box to isolate the samples from ambient moisture condensation once they

have been charged (at room temperature) and cooled in LN2, b) a cryogenic vacuum transfer chain

for keeping the sample cold throughout the process, and c) a procedure to keep the sample cold

while being transferred between the atom probe load lock (i.e. the CAMECA vacuum-cryo-

transfer module (VCTM)) and the built-in cryo-stage of the atom probe instrument. The step-by-

step operation of the glove box, loadlock, gate valve and suitcase are animated in Movie S1.

The cryo-transfer was started by purging the glove box with high purity nitrogen (99.9%)

until it reached 0% relative humidity, measured by a LOG20 by TFA humidity logger, which took

approximately 40 minutes. The atom probe samples were then moved into the glove box via the

loading station shown in Figure S4A, ready for deuteration. All dewers that cool the cryo-stages

involved in the transfer chain were filled with LN2, along with the bath in the glove box. When the

5

whole system had thermally stabilized, the needle-shaped samples were deuterated at room

temperature, and the puck with the samples was immediately moved into the LN2 bath station,

ready to engage the transfer chain. After aligning the LN2 bath station with the transfer arm of the

glove box loadlock, the puck was moved into the loadlock via its transfer arm and placed in its

cryo-stage. Next, the loadlock was evacuated, and the gate valves between the loadlock and the

suitcase were opened after the pressure in the loadlock reached approximately 1x10-5 Pa. Finally,

the puck was transferred into the cryo-stage of the suitcase via its transfer arm, the gate valves

were closed, and the suitcase was detached from the glove box ready for transfer to the atom probe.

As per (34), the components relating to the transfer procedure in the atom probe include a

piggyback puck and a carousel with PEEK insulation at the puck-receiving position, allowing

minimal heat transfer to the cryogenic puck. The piggyback was first placed on the cryo-stage of

the atom probe at 50 K for 30 minutes and was then transferred to the PEEK position of the carousel

for another 30 minutes to pre-cool the position. After attaching the suitcase to the atom probe, the

puck with the deuterated samples was moved to the pre-cooled position and then loaded as quickly

as possible into the analysis stage of the atom probe via its buffer chamber (< 2 minutes).

Confirmation that the cryogenic status of the transferred puck was maintained is carried out by

noting the thermal response of the analysis cryo-stage as the loaded puck is inserted. The analysis

chamber cryo-stage did not exceed 53 K in 10 trials, suggesting the cryo-puck did not heat up

significantly.

6

Figure S4. Cryo-transfer station

(A) The glove box, open, with the suitcase detached, showing the configuration of the cryo-transfer

system. (B) Deuterium charging underway showing the configuration of the charging system and

the LN2 bath.

7

8

Figure S5. APT supplementary data - mass spectra

Mass spectra (log scale) with peaks and ranges labelled. (A) Deuterium-charged ferritic sample,

(B) deuterium-free ferritic sample, (C) deuterium-charged martensitic sample and (D) deuterium-

free martensitic sample. The ferritic samples contain NbC precipitates, whereas NbC precipitates

are not present in the datasets from the martensitic samples. The fidelity of the deuterium peaks at

2 Da in (A) and (C) is confirmed by the absence of deuterium peaks in (B) and (D). No peak at 2

Da was observed in the D-free samples, even in localized regions, ruling out the possibility of a

peak from H2. We note that the mass spectrum in (D) is from a deuterium-charged sample that was

left in atom probe storage chamber at room temperature overnight. This confirms that the

deuterium traps do not retain deuterium for long periods at room temperature, and reinforces the

necessity of using cryo-transfer for retaining the deuterium signal.

9

Figure S6. APT supplementary data - proximity histograms of the NbC precipitates in the

ferritic sample in Figure 3

Proximity histograms (30) of (A) NbC#1 and (B) NbC#2 in Figure 3 showing the average

composition as a function of the distance to a 1% niobium isoconcentration surface. The 1%

niobium surface is located where the composition starts to change at the edge of the precipitates.

This 3D information complements the 1D analysis shown in Figure 3. In agreement with the data

from the cylindrical volume, precipitates are enriched in carbon and niobium and the deuterium is

associated with the interfacial regions, but not the carbides themselves.

10

Figure S7. APT supplementary data - proximity histograms of the GB in Figure 4

Proximity histograms of (A) the GB shown in transparent red in Figure 4. The histogram shows

the average composition in the proximity of microstructural features defined by a 2% carbon

isoconcentration surface around the grain boundary. The top figure indicates that a carbon

atmosphere is present. The bottom figure shows that deuterium is also associated with the grain

boundary, indicating hydrogen trapping.

11

Movie S1. Animation of the cryo-transfer procedure

Animation of the cryo-transfer process, including the glove box, the suitcase, the loadlock and the

gate valves.

12

Movie S2. APT supplementary data – animation of the dislocations in Figure 4

Animated 3D reconstruction showing deuterium atoms (red), iron atoms (grey) and a 2% carbon

isosurface (blue) showing the linear morphology of the carbon-decorated dislocations.

13

Movie S3. APT supplementary data – animated slice view of the dislocations in Figure 4

An animation in which a 5 nm slice view moves from the back to the front of the dataset showing

carbon (blue), deuterium (red) and a small fraction of the iron (grey). Both the deuterium and the

carbon are associated with the dislocations.

14

Movie S4. APT supplementary data – animated slice view of the GB region in Figure 4

An animation in which 5 nm slice view of the data along the z plane moves from the top to the

bottom of the grain boundary region, showing carbon (blue), deuterium (red) and a small fraction

of the iron (grey). Both the deuterium and the carbon are associated with the dislocations, but the

amount of segregation varies across the plane of the grain boundaries.

References and Notes

1. W. H. Johnson, On Some Remarkable Changes Produced in Iron and Steel by the Action of

Hydrogen and Acids. Nature 11, 393 (1875).

2. S. Lynch, Hydrogen embrittlement phenomena and mechanisms. Corros. Rev. 30, 105–123

(2012).

3. J. Venezuela, Q. L. Liu, M. X. Zhang, Q. J. Zhou, A. Atrens, A review of hydrogen

embrittlement of martensitic advanced high-strength steels. Corros. Rev. 34, 153–186

(2016).

4. Hydrogen Strategy Group, Hydrogen for Australia's Future, COAG Energy Council,

(Australian Government, 2018).

5. United Nations Framework Convention on Climate Change, The Paris Agreement (UNFCCC,

2015).

6. I. M. Robertson, P. Sofronis, A. Nagao, M. L. Martin, S. Wang, D. W. Gross, K. E. Nygren,

Hydrogen Embrittlement Understood. Metall. Mater. Trans. A Phys. Metall. Mater. Sci.

46, 2323–2341 (2015).

7. H. W. Liu, Hydrogen assisted intergranular cracking in steels. Eng. Fract. Mech. 78, 2563–

2571 (2011).

8. C. D. Beachem, New model for hydrogen-assisted cracking (hydrogen embrittlement). Metall.

Trans. 3, 437 (1972).

9. I. M. Robertson, The effect of hydrogen on dislocation dynamics. Eng. Fract. Mech. 68, 671–

692 (2001).

10. H. K. D. H. Bhadeshia, Prevention of Hydrogen Embrittlement in Steels. ISIJ Int. 56, 24–36

(2016).

11. M. Koyama, M. Rohwerder, C. C. Tasan, A. Bashir, E. Akiyama, K. Takai, D. Raabe, K.

Tsuzaki, Recent progress in microstructural hydrogen mapping in steels: Quantification,

kinetic analysis, and multi-scale characterisation. Mater. Sci. Technol. 33, 1481–1496

(2017).

12. W. Y. Choo, J. Y. Lee, Thermal-Analysis of Trapped Hydrogen in Pure Iron. Metall. Trans.,

A, Phys. Metall. Mater. Sci. 13, 135–140 (1982).

13. Y.-S. Chen, D. Haley, S. S. A. Gerstl, A. J. London, F. Sweeney, R. A. Wepf, W. M.

Rainforth, P. A. J. Bagot, M. P. Moody, Direct observation of individual hydrogen atoms

at trapping sites in a ferritic steel. Science 355, 1196–1199 (2017).

14. J. Takahashi, K. Kawakami, Y. Kobayashi, Origin of hydrogen trapping site in vanadium

carbide precipitation strengthening steel. Acta Mater. 153, 193–204 (2018).

15. J. Takahashi, K. Kawakami, Y. Kobayashi, T. Tarui, The first direct observation of hydrogen

trapping sites in TiC precipitation-hardening steel through atom probe tomography. Scr.

Mater. 63, 261–264 (2010).

16. J. Takahashi, K. Kawakami, T. Tarui, Direct observation of hydrogen-trapping sites in

vanadium carbide precipitation steel by atom probe tomography. Scr. Mater. 67, 213–216

(2012).

17. M. Kuzmina, M. Herbig, D. Ponge, S. Sandlöbes, D. Raabe, Linear complexions: Confined

chemical and structural states at dislocations. Science 349, 1080–1083 (2015).

18. M. K. Miller, Atom probe tomography characterization of solute segregation to dislocations

and interfaces. J. Mater. Sci. 41, 7808–7813 (2006).

19. J. Wilde, A. Cerezo, G. D. W. Smith, Three-dimensional atomic-scale mapping of a cottrell

atmosphere around a dislocation in iron. Scr. Mater. 43, 39–48 (2000).

20. A. T. Paxton, I. H. Katzarov, Quantum and isotope effects on hydrogen diffusion, trapping

and escape in iron. Acta Mater. 103, 71–76 (2016).

21. Materials and methods are available as supplementary materials.

22. P. W. Trimby, Y. Cao, Z. Chen, S. Han, K. J. Hemker, J. Lian, X. Liao, P. Rottmann, S.

Samudrala, J. Sun, J. T. Wang, J. Wheeler, J. M. Cairney, Characterizing deformed

ultrafine-grained and nanocrystalline materials using transmission Kikuchi diffraction in

a scanning electron microscope. Acta Mater. 62, 69–80 (2014).

23. D. B. Williams, C. B. Carter, Transmission Electron Microscopy: A Textbook for Materials

Science (Springer, ed. 2, 2009), pp. 277.

24. M. Nagumo, Hydrogen related failure of steels - a new aspect. Mater. Sci. Technol. 20, 940–

950 (2004).

25. K. Takai, H. Shoda, H. Suzuki, M. Nagumo, Lattice defects dominating hydrogen-related

failure of metals. Acta Mater. 56, 5158–5167 (2008).

26. I. J. Park, S. M. Lee, H. H. Jeon, Y. K. Lee, The advantage of grain refinement in the

hydrogen embrittlement of Fe-18Mn-0.6C twinning-induced plasticity steel. Corros. Sci.

93, 63–69 (2015).

27. E. Wallaert, T. Depover, M. Arafin, K. Verbeken, Thermal Desorption Spectroscopy

Evaluation of the Hydrogen-Trapping Capacity of NbC and NbN Precipitates. Metall.

Mater. Trans., A Phys. Metall. Mater. Sci. 45, 2412–2420 (2014).

28. F. G. Wei, K. Tsuzaki, Quantitative analysis on hydrogen trapping of TiC particles in steel.

Metall. Mater. Trans., A Phys. Metall. Mater. Sci. 37, 331–353 (2006).

29. H. Suzuki, K. Takai, Summary of Round-robin Tests for Standardizing Hydrogen Analysis

Procedures. ISIJ Int. 52, 174–180 (2012).

30. B. Gault, M. P. Moody, J. Cairney, S. P. Ringer, Atom Probe Microscopy (Springer, 2012).

31. O. C. Hellman, J. B. du Rivage, D. N. Seidman, Efficient sampling for three-dimensional

atom probe microscopy data. Ultramicroscopy 95, 199–205 (2003).

32. X. X. Yu, C. R. Weinberger, G. B. Thompson, Ab initio investigations of the phase stability

in group IVB and VB transition metal carbides. Comput. Mater. Sci. 112, 318–326

(2016).

33. D. Di Stefano, R. Nazarov, T. Hickel, J. Neugebauer, M. Mrovec, C. Elsässer, First-

principles investigation of hydrogen interaction with TiC precipitates in α-Fe. Phys. Rev.

B 93, 184108 (2016).

34. M. Ohnuma, J. I. Suzuki, F. G. Wei, K. Tsuzaki, Direct observation of hydrogen trapped by

NbC in steel using small-angle neutron scattering. Scr. Mater. 58, 142–145 (2008).

35. A. Turk, D. San Martin, P. E. J. Rivera-Diaz-del-Castillo, E. I. Galindo-Nava, Correlation

between vanadium carbide size and hydrogen trapping in ferritic steel. Scr. Mater. 152,

112–116 (2018).

36. M. Enomoto, D. Hirakami, T. Tarui, Thermal desorption analysis of hydrogen in high

strength martensitic steels. Metall. Mater. Trans. A Phys. Metall. Mater. Sci. 43, 572–581

(2012).

37. A. P. A. Subramanyam, A. Azócar Guzmán, S. Vincent, A. Hartmaier, R. Janisch, Ab initio

study of the combined effects of alloying elements and H on grain boundary cohesion in

ferritic steels. Metals 9, 291 (2019).

38. A. De Backer, D. R. Mason, C. Domain, D. Nguyen-Manh, M.-C. Marinica, L. Ventelon, C.

S. Becquart, S. L. Dudarev, Multiscale modelling of the interaction of hydrogen with

interstitial defects and dislocations in BCC tungsten. Nucl. Fusion 58, 016006 (2018).

39. B. Gault, M. P. Moody, F. de Geuser, G. Tsafnat, A. La Fontaine, L. T. Stephenson, D.

Haley, S. P. Ringer, Advances in the calibration of atom probe tomographic

reconstruction. J. Appl. Phys. 105, 034913 (2009).

40. L. T. Stephenson, A. Szczepaniak, I. Mouton, K. A. K. Rusitzka, A. J. Breen, U. Tezins, A.

Sturm, D. Vogel, Y. Chang, P. Kontis, A. Rosenthal, J. D. Shepard, U. Maier, T. F.

Kelly, D. Raabe, B. Gault, The Laplace Project: An integrated suite for preparing and

transferring atom probe samples under cryogenic and UHV conditions. PLOS ONE 13,

e0209211 (2018).