Skip to content

Role of disordered bipolar complexions on the sulfur embrittlement of nickel general grain boundaries

Summary

Sulfur embrittlement of nickel is a long-standing metallurgical puzzle. The abstract reports aberration-corrected STEM combined with semi-grand-canonical hybrid Monte Carlo / molecular dynamics using a DFT-trained ReaxFF for Ni–S, revealing—unexpectedly—that the same general grain boundaries can exhibit both amorphous-like and bilayer-like facets. Lower-Miller-index grain surface orientation, rather than misorientation alone, is argued to dictate which interfacial structure appears—challenging traditional emphasis on misorientation. Both complexion families show partial bipolar structural order within thermodynamically two-dimensional intergranular phases (“complexions”), linked in the paper to brittle intergranular fracture. The authors also connect the mechanism to abnormal grain growth in S-doped Ni and suggest broader relevance of disordered yet partially ordered interfaces across materials.

Methods

Experiments saturate Ni with S, perform isothermal anneals and quenches across temperature windows including 500–675 °C, and characterize tens of GB facets with AC-STEM (ABF/HAADF) plus EDXS/EELS-based adsorption metrics (Γ_S examples appear in the main text).

Atomistic sampling: Hybrid Monte Carlo / molecular dynamics (MC/MD) with a DFT-trained ReaxFF for Ni–S is carried out in LAMMPS-class software as described in Nat. Commun. Methods, using semi-grand-canonical control to vary S content while exploring grain boundary supercells containing hundreds to thousands of atoms under 3D periodic boundary conditions. Timestep and thermostat settings for the hybrid moves appear in the PDF; N/A — external electric field during the equilibrium sampling emphasized here. NPT vs NVT choices follow the article’s pressure/stress prescriptions for the simulation cells (see PDF tables).

STEM image simulations compare model disorder and segregation to microscopy. Analysis emphasizes bipolar signatures and excess S areal densities for Type A vs Type B facets.

Findings

Outcomes & mechanisms: Type A (amorphous-like) vs Type B (bilayer-like) complexions coexist on general Ni grain boundaries under S segregation; both families show partial bipolar order within thermodynamically two-dimensional interfacial films linked to brittle intergranular fracture.

Comparisons: AC-STEM EDXS/EELS Γ\(_S\) values are compared to hybrid MC/MD + ReaxFF predictions (example paired entries near 34.9 vs 23.9 S/nm² for Type A and 12.6 vs 12.1 S/nm² for Type B facets in Table 1—operators must verify exact rows in pdf_path).

Sensitivity: annealing temperature shifts the fraction of faceted boundaries (article quotes ~54% at 500 °C increasing toward ~84% at 675 °C in the summarized statistics); lower-Miller-index terminating grain surface orientation—not misorientation alone—is argued to dictate which complexion appears.

Limitations / outlook: ReaxFF remains empirical for liquid-like IGFs; kinetics may prevent full equilibrium; STEM gives projected structural information.

Corpus honesty: mechanistic sentences above track the abstract/introduction plus Table 1 visible in normalized/extracts/2018hu-nat-role-disordered_p1-2.txt; deeper MD parameters require the full PDF.

Limitations

ReaxFF remains empirical for liquid-like IGFs; kinetics may impede full equilibrium in experiments. STEM provides projected information; 3D connectivity may differ. Ni–S parameter transferability to alloys is not automatic.

Relevance to group

Demonstrates ReaxFF in open-system hybrid MC/MD for metallurgical segregation and complexion physics—distinct from oxide applications but sharing the same reactive FF toolchain used in materials modeling across the wiki.

Citations and evidence anchors

  • DOI: 10.1038/s41467-018-05070-2 — papers/ReaxFF_others/Hu_NatureComm_2018_ReaxFF_NiS.pdf; extract normalized/extracts/2018hu-nat-role-disordered_p1-2.txt.