Spin cluster expansion approach to the interplay between short-range order and interstitial atoms in austenitic stainless steels

Su, Tianyu

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https://hdl.handle.net/2142/132491 Copy

Description

Title

Spin cluster expansion approach to the interplay between short-range order and interstitial atoms in austenitic stainless steels

Author(s)

Su, Tianyu

Issue Date

2025-11-13

Director of Research (if dissertation) or Advisor (if thesis)

Ertekin, Elif

Doctoral Committee Chair(s)

Krogstad, Jessica Anne

Committee Member(s)

• Sofronis, Petros
• Trinkle, Dallas
• Bellon, Pascal

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• short-range order
• hydrogen embrittlement
• cluster expansion
• austenitic stainless steels

Abstract

Hydrogen embrittlement (HE) severely degrades the mechanical properties of austenitic stainless steels, thus limiting their use in hydrogen transport and storage applications. Despite extensive investigations, the dominant underlying mechanism of HE remains unclear. Recent studies have highlighted short-range order (SRO) as a key microstructural feature that significantly influences alloy properties. In particular, SRO is found to promote slip planarity, a deformation mode frequently associated with HE. Furthermore, interstitial solutes can interact with the matrix, affecting the formation and evolution of SRO. The interplay between SRO and interstitial atoms, especially hydrogen (H) and nitrogen (N), may play a critical role in the HE process. To quantitatively investigate the interactions between SRO and interstitials, thermodynamic modeling approaches are essential for sampling alloy configurations. The cluster expansion (CE) method, combined with Monte Carlo (MC) simulations, provides a powerful computational framework for studying the thermodynamic properties of alloys. We present a detailed mathematical analysis of the conventional CE formalism, including the derivation of SRO parameters and chemical pairwise interactions from the CE model. While the CE model successfully captures the ground states of the Fe–Ni–Cr–H system, its predictions on SRO show noticeable deviations from experimental measurements. These discrepancies highlight the limitations of the conventional model, possibly due to the implicit treatment of magnetic interactions. In addition, the multisublattice CE model introduces entangled interactions among different species, making physical interpretation more challenging. This suggests the need for a more physically informed CE formalism to achieve improved predictive performance. To address these issues, a generalized spin CE model, combining a chemical CE with an Ising model, is developed for Fe–Ni–Cr austenitic alloys. This model explicitly accounts for magnetic exchange interactions, thereby capturing the effects of finite temperature magnetism on SRO. Model parameters are obtained by fitting to a first-principles dataset comprising both chemically and magnetically diverse configurations. The magnetic exchange interactions are found to be of similar magnitude to the chemical interactions, indicating the importance of the magnetism effects. Compared to a conventional implicit magnetism CE, the spin CE shows improved performance on experimental benchmarks over a broad spectrum of compositions, particularly at higher temperatures due to the explicit treatment of magnetic disorder. The Cr content is found to strongly influence SRO, since Cr atoms prefer to align antiferromagnetically with nearest neighbors but become magnetically frustrated with increasing Cr concentration. Increasing the Cr concentration in typical austenitic stainless steels promotes the formation of SRO and increases order-disorder transition temperatures. The mode is further extended to H-incorporated Fe–Ni–Cr alloys to study the interplay between H and SRO. The presence of H only slightly alters the intrinsic ordering preference of the Fe–Ni–Cr alloys.As the temperature decreases, the alloy evolves from disordered to ordered thermodynamic states accompanied by distinct H–metal correlations. 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, in alloys with pre-existing SRO, the hydrogen distribution is markedly altered compared to random alloys, with local H enrichment in SRO domains. Such SRO-driven H accumulation may facilitate slip localization and contribute to the early onset of embrittlement. The effects of N on chemical ordering in Fe–Ni–Cr alloys are also investigated. The atomistic model confirms a strong affinity between N and Cr, which drives the formation of N–Cr SRO and, at higher N concentrations, stabilizes $M_4$N-type ordered phases ($M$ = metal). The simulations reveal that low N concentrations promote local N–Cr or N–N SRO, while increasing N content leads to the emergence of Cr- and N-rich LRO structures. The presence of N suppresses intrinsic Fe–Cr and Ni–Cr SRO by competing with these interactions, particularly at high concentrations. The impact of Cr content on ordering diminishes as N approaches its solubility limit. These findings are consistent with experimental observations in high-N austenitic steels. In summary, we developed a spin CE to overcome the limitations of the conventional CE method in modeling Fe–Ni–Cr austenitic stainless steels, particularly in the presence of interstitial solutes. By explicitly incorporating magnetic interactions, the spin CE model provides a more accurate and physically interpretable description of the chemical order in these alloys. The model reliably predicts the SRO parameters in the presence of interstitials, providing a quantitative understanding of the interplay between H/N interstitial atoms and SRO in the alloys. These results underscore the critical role of magnetic interactions in the thermodynamic properties of transition metal alloys. Furthermore, this framework offers thermodynamic and structural insights into the interactions between interstitials and SRO, highlighting the implications for HE mechanisms in austenitic stainless steels.

Graduation Semester

2025-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/132491

Copyright and License Information

Copyright 2025 Tianyu Su

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