Impact of hydrogen incorporation on electronic and magnetic structure of X2CrNi18-9 stainless steel

Technical Summary: Impact of Hydrogen Incorporation on Electronic and Magnetic Structure of X2CrNi18-9 Stainless Steel

Problem Statement
Hydrogen embrittlement remains a critical barrier to the widespread adoption of hydrogen as an energy carrier, necessitating the development of steels with high resistance to hydrogen-induced failure. While the mechanical effects of hydrogen are well-documented, its influence on the electronic and magneto-structural properties of austenitic stainless steels is less understood. Previous studies suggest that hydrogen incorporation alters the electronic density of states near the Fermi edge, yet the mechanisms linking microstructural features (such as chemical homogeneity, grain size, and dislocation density) to these subtle electronic and magnetic changes require further elucidation. This study investigates how hydrogen incorporation affects the electronic and magneto-structural properties of X2CrNi18-9 (AISI 304L) stainless steel across different microstructural states.

Methodology
The researchers prepared three distinct microstructural states of X2CrNi18-9 steel with identical global chemical compositions but different processing routes:

1. CON-SA: Conventionally processed (casting and forging) followed by solution annealing.
2. PBF-SA: Laser powder bed fusion (PBF) followed by solution annealing.
3. PBF-AB: Laser powder bed fusion in the as-built condition (no further thermal treatment).

Each state was subjected to hydrogen charging (10 bar H₂ at 300 °C) and compared against argon-treated reference samples. The study employed a multi-modal characterization approach:

• Microstructural Characterization: Analytical electron microscopy (EDS mapping) was used to generate thermodynamic stability maps ($\Delta G$) to evaluate chemical homogeneity and phase stability. X-ray diffraction (XRD) with Rietveld refinement was utilized to quantify dislocation densities.
• Electronic Properties: The Seebeck coefficient was measured via the direct method from 300 K to 345 K to probe changes in the electronic density of states near the Fermi edge.
• Magneto-Structural Properties: Diffuse neutron scattering (on the NIMROD instrument) and magnetic Small-Angle Neutron Scattering (SANS, on the ZOOM instrument) were used to investigate structural inhomogeneities and spin misalignment at the nanoscale.

Key Results

• Microstructural Differences: The CON-SA sample exhibited significant mesoscale chemical inhomogeneity (banding) with a standard deviation in $\Delta G$ of 141 J/mol and a grain size of 27.7 µm. In contrast, the PBF samples (both SA and AB) showed high chemical homogeneity (std $\approx$ 87–88 J/mol) and smaller grain sizes (8.2–9.5 µm). The PBF-AB condition possessed the highest dislocation density ($3.88 \times 10^{13}$ m$^{-2}$), which decreased significantly after solution annealing (PBF-SA) to levels comparable to CON-SA.
• Electronic Properties: All samples exhibited negative Seebeck coefficients, indicating electrons as the primary charge carriers. Hydrogen incorporation consistently reduced the absolute values of the Seebeck coefficients across all samples. Notably, while the uncharged PBF and CON-SA samples showed distinct Seebeck values due to their differing microstructures, the hydrogen-charged CON-SA and PBF-SA samples converged to similar values. This suggests that hydrogen incorporation compensates for the electronic differences caused by mesoscale chemical inhomogeneities.
• Magneto-Structural Properties:
  • Diffuse Neutron Scattering: Analysis of the differential pair correlation function $D(r)$ revealed the existence of a short-range ordered (SRO) nanoscale phase with an fcc structure but a correlation length of approximately 3 nm. Upon hydrogen incorporation, this correlation length slightly decreased (from 3.0 nm to 2.7 nm).
  • Magnetic SANS: The PBF samples exhibited signatures of nanoscale inhomogeneities (Guinier radius $R_G \approx 17.9$ nm in PBF-AB) that were reduced after annealing ($R_G \approx 9.4$ nm). Hydrogen incorporation caused these inhomogeneities to grow in size (e.g., $R_G$ increased to 20.3 nm in PBF-AB). This growth indicates that hydrogen preferentially accumulates near these nanoscale segregations, likely at cell boundaries. Furthermore, hydrogen incorporation led to a reduction in spin disorder (spin misalignment), an effect more pronounced in the solution-annealed state.

Key Contributions

• The study demonstrates that hydrogen incorporation induces measurable changes in the electronic structure of austenitic stainless steel, evidenced by shifts in the Seebeck coefficient, despite the low concentration of hydrogen (10–12 ppm).
• It identifies the existence of nanoscale inhomogeneities (3 nm correlation length) within the bulk fcc structure, which act as preferential sites for hydrogen accumulation.
• The research establishes a link between microstructural processing (PBF vs. conventional) and the material's electronic response to hydrogen, showing that hydrogen can effectively "homogenize" the electronic properties of chemically distinct samples.
• It provides evidence that hydrogen accumulation at nanoscale defects reduces spin misalignment, offering new insights into the magnetic response of hydrogen-charged steels.

Significance and Claims
The authors claim that these findings suggest hydrogen preferentially accumulates in the vicinity of nanoscale inhomogeneities, such as cell boundaries in PBF materials, rather than being uniformly distributed. The observed changes in the Seebeck coefficient are attributed to hydrogen's ability to modify the steepness of the electronic density of states near the Fermi edge by attaching to electrically and magnetically active crystal defects.

The paper posits that understanding these nanoscale interactions is crucial for material design. Specifically, the authors suggest that in the medium term, these insights could lead to guidelines for optimizing PBF processing parameters to tune cell boundary characteristics, thereby enhancing hydrogen resistance. Additionally, the sensitivity of the Seebeck coefficient to hydrogen incorporation is proposed as a potential principle for non-destructive sensing of hydrogen uptake in these materials. The study does not claim to have solved hydrogen embrittlement but aims to provide a deeper mechanistic understanding of how hydrogen interacts with the electronic and magnetic structure of the steel.