Short-range order influences H distribution in Fe-Ni-Cr austenitic stainless steels

TL;DR

This work tackles how hydrogen interacts with short-range order (SRO) in Fe–Ni–Cr austenitic stainless steels to influence hydrogen embrittlement via HELP. It develops a spin cluster expansion (CE) framework augmented with Monte Carlo (MC) sampling to quantify SRO using Warren–Cowley parameters and to capture H–metal interactions within a thermodynamic, magnetically active lattice, with validation against experimental benchmarks. The main findings show that hydrogen modestly perturbs intrinsic SRO, but pre-existing SRO domains, especially Ni-rich regions, drive local H enrichment, potentially promoting slip localization and early embrittlement. The study provides atomistic thermodynamic insights into the coupling between H and SRO, suggesting microstructure design strategies to mitigate HELP by controlling SRO characteristics and H segregation pathways.

Abstract

Hydrogen embrittlement (HE) in austenitic stainless steels is advanced by hydrogen enhanced localized plasticity (HELP), typically accompanied by a transition from homogeneous to localized slip. Short-range order (SRO) in face-centered cubic (FCC) alloys is known to promote slip planarity, and recent studies suggest that H may amplify this localization behavior linked to inherent SRO. However, the manner in which the introduction of H affects SRO properties and, conversely, the manner that pre-existing SRO may affect H behavior, are not fully understood. In this work, a spin cluster expansion model combined with Monte Carlo simulation is employed to study the interplay between H and SRO in Fe-Ni-Cr alloys. Chemical order is quantified using Warren-Cowley SRO parameters, and the model predictions are validated against experimental data. We find that the presence of H only slightly alters the intrinsic ordering preference of the Fe-Ni-Cr alloys. As temperature decreases and the alloy evolves from disordered to ordered thermodynamic states, distinct H-metal correlations emerge. In particular, H-Ni and H-Cr pairs exhibit stronger ordering tendencies than H-Fe pairs, suggesting a selective affinity of H for certain atomic environments. On the other hand, we also find that compared to random alloys, when pre-existing SRO is present, it significantly affects the resulting H distribution by promoting local H enrichment in SRO domains. Such SRO-driven local H accumulation may facilitate slip localization and contribute to the early onset of embrittlement. These findings provide thermodynamic and structural insights into the interaction between H and SRO in austenitic stainless steels, highlighting possible implications on how the interaction between HELP and SRO brings about hydrogen embrittlement in austenitic stainless steels.

Figures (4)

• Figure 1: (a) Scheme for interstitial H atoms occupying octahedral sites within the FCC lattice, with the center octahedral interstitial site highlighted in the cell. Two interpenetrating FCC sublattices are present. The yellow atoms label the sites on the metal sublattice, while the pink atoms label the sites on the interstitial sublattice. (b) The comparison between DFT-calculated formation energies and predictions from the spin CE model for the Fe–Ni–Cr–H dataset.
• Figure 2: (a,b) The effect of H on Fe–Cr and Ni–Cr 1NN SRO of the Fe$_{70}$Ni$_{10}$Cr$_{20}$ alloy. The H concentration incorporated in the alloy varies from 0 at.% to 10 at.%. (c,d) The effect of Cr on H–Ni and H–Cr 1NN SRO for different alloys with 10 at.% H atoms. The Ni concentration of the alloy is fixed at 10 at.% while the Cr concentration varies from 5 at.% to 20 at.%. All panels illustrate thermodynamic ordering tendencies.
• Figure 3: Thermodynamic ordering tendencies characterized by (a) H–metal 1NN, and (b) H–metal 2NN SRO parameters for the Fe$_{70}$Ni$_{10}$Cr$_{20}$ alloy incorporated with 10% H atoms. (c,d) The corresponding representative MC snapshots generated at 1500 K and 500 K, visualized using the OVITO software stukowski2009visualization. Fe, Ni, Cr, and H atoms are marked as red, green, blue, and white, respectively. (e) H concentration for ordered (500 K) and random (1500 K) configurations plotted here along [100] direction to highlight the compositional variation. (f) The distribution of H atoms as a function of the number of H–H pairs within two FCC unit cells.
• Figure 4: (a,b) H–metal 1 NN SRO parameter as a function of temperature for the fixed-lattice Fe$_{70}$Ni$_{10}$Cr$_{20}$ alloy in ordered and random states, respectively. (c,d) H–metal 2 NN SRO parameter as a function of temperature for the fixed-lattice Fe$_{70}$Ni$_{10}$Cr$_{20}$ alloy in ordered and random states, respectively. The H concentration is 10 at.% in the alloys.