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arxiv: 2605.27218 · v1 · pith:P6H46ZDFnew · submitted 2026-05-26 · ❄️ cond-mat.mtrl-sci · cond-mat.mes-hall

Amorphous vs. Short-Range-Ordered Complexions: Consequences for Grain-Boundary-Mediated Plasticity in Nanocrystalline Al-Ni Alloys

Frederic Sansoz , Eve-Audrey Picard This is my paper

Pith reviewed 2026-06-29 16:46 UTC · model grok-4.3

classification ❄️ cond-mat.mtrl-sci cond-mat.mes-hall
keywords nanocrystalline alloysgrain boundary complexionsamorphous complexionsshort-range orderplasticityshear localizationAl-Ni alloysmolecular dynamics
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The pith

Amorphous grain-boundary complexions promote homogeneous plasticity in nanocrystalline Al-Ni alloys while short-range-ordered complexions increase strength at the expense of localization.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper investigates how the atomic structure of grain boundaries affects plastic deformation in nanocrystalline aluminum-nickel alloys. It finds that thick amorphous complexions formed at high annealing temperatures act to absorb dislocations and enable deformation through shear transformation zones across the material. In contrast, complexions with short-range crystalline order from lower temperature annealing create uneven stress patterns that concentrate deformation into narrow bands. This difference holds at both 2 and 4 atomic percent nickel and points to a change in the main control mechanism for plasticity.

Core claim

Amorphous complexions act as dislocation sinks, suppressing shear localization and promoting homogeneous plasticity through shear transformation zones, but at the cost of lower strength. SRO complexions generate higher strength but also promote heterogeneous stress concentrations across the GB network, leading to intense shear localization regardless of Ni concentration. This contrast reflects a fundamental shift in governing mechanism, from shear-transformation-zone-controlled behavior in amorphous complexion alloys to GB-stress-heterogeneity-controlled behavior in SRO complexion alloys.

What carries the argument

The distinction between uniform amorphous intergranular films and semi-amorphous complexions containing FCC-type and BCC-type short-range order, which alters dislocation interactions and stress distribution at grain boundaries.

If this is right

  • Amorphous complexions enable more uniform plastic flow through shear transformation zones.
  • SRO complexions lead to higher strength but intense shear bands due to stress heterogeneity.
  • The governing plasticity mechanism changes with complexion type independent of nickel content.
  • Engineering grain boundary structure via annealing can tune strength and ductility in these alloys.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Controlling the degree of short-range order in grain boundaries could be used to balance strength and ductility in other nanocrystalline materials.
  • Experimental validation at intermediate temperatures might reveal gradual transitions in deformation behavior.
  • Similar complexion effects may influence fatigue or creep resistance in these alloys.

Load-bearing premise

The observed differences in plasticity arise primarily from the structural differences in the complexions produced by the two annealing temperatures rather than from other aspects of the simulation setup.

What would settle it

An experiment showing no change in shear localization behavior when annealing produces different levels of short-range order in the grain boundaries.

Figures

Figures reproduced from arXiv: 2605.27218 by Eve-Audrey Picard, Frederic Sansoz.

Figure 1
Figure 1. Figure 1: Equilibrium microstructure and Ni solute segregation in the Al-4Ni polycrystal predicted by MC/MD simulation at 378 K and then cooled to 300 K. Each grain is highlighted in different colors according to their crystallographic orientation [PITH_FULL_IMAGE:figures/full_fig_p013_1.png] view at source ↗
read the original abstract

Amorphous grain-boundary (GB) complexions in thermally stable nanocrystalline alloys are commonly assumed to be structurally homogeneous, yet their disordered nature makes them susceptible to local short-range ordering (SRO). The influence of local SRO on GB-mediated plasticity mechanisms in such complexions remains poorly understood. This article employs large-scale Monte Carlo and molecular dynamics simulation to address this gap through simulations of nanocrystalline Al-Ni alloys at two Ni concentrations, 2 at.% and 4 at.%. Annealing at 913 K produces thick uniform amorphous intergranular film complexions, while annealing at 378 K produces semi-amorphous complexions containing FCC-type and BCC-type SRO. These two complexion states produce fundamentally different mechanical responses. Amorphous complexions act as dislocation sinks, suppressing shear localization and promoting homogeneous plasticity through shear transformation zones, but at the cost of lower strength. SRO complexions generate higher strength but also promote heterogeneous stress concentrations across the GB network, leading to intense shear localization regardless of Ni concentration. This contrast reflects a fundamental shift in governing mechanism, from shear-transformation-zone-controlled behavior in amorphous complexion alloys to GB-stress-heterogeneity-controlled behavior in SRO complexion alloys. These findings highlight the potential of complexion engineering to tailor the mechanical properties of nanocrystalline materials.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 2 minor

Summary. The manuscript uses Monte Carlo and molecular dynamics simulations of nanocrystalline Al-Ni alloys (2 at.% and 4 at.% Ni) to compare two GB complexion states: thick uniform amorphous intergranular films produced by annealing at 913 K versus semi-amorphous films containing FCC- and BCC-type SRO produced by annealing at 378 K. It reports that amorphous complexions act as dislocation sinks, enabling homogeneous plasticity via shear transformation zones at the expense of lower strength, while SRO complexions raise strength but induce heterogeneous GB stress concentrations that drive intense shear localization independent of Ni content, indicating a shift from STZ-controlled to GB-stress-heterogeneity-controlled deformation mechanisms.

Significance. If the reported mechanism shift is robustly attributable to SRO rather than confounding variables, the work would establish that local ordering within complexions can switch the dominant plasticity pathway in nanocrystalline alloys, providing a concrete route for complexion engineering to balance strength and ductility. The direct comparison across two Ni concentrations and the use of large-scale atomistic methods are positive features that support generality within the simulated regime.

major comments (1)
  1. [Abstract/Methods] Abstract and §3 (or equivalent methods/results sections): the two annealing temperatures produce complexions that differ simultaneously in SRO content and in GB film thickness (thick uniform amorphous films vs. semi-amorphous films). Because GB thickness is independently known to modulate dislocation absorption capacity and local stress concentrations, the central attribution of the plasticity-mechanism switch to the presence/absence of FCC/BCC SRO requires explicit isolation of SRO from thickness (e.g., via additional simulations that hold thickness or segregation profiles fixed while varying only ordering). Without such controls the causal claim remains under-determined.
minor comments (2)
  1. [Methods] The interatomic potential used for both Monte Carlo and MD steps should be validated against known Al-Ni ordering tendencies or experimental GB structures; absence of such validation leaves open whether the observed SRO stabilization is potential-specific.
  2. [Methods/Results] Specific system sizes, number of distinct GBs sampled, strain rates, and statistical measures of stress heterogeneity should be stated explicitly to allow assessment of finite-size and sampling effects on the reported localization behavior.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the constructive comment on potential confounding factors in our comparison of complexion states. We address the concern point-by-point below.

read point-by-point responses

  1. Referee: [Abstract/Methods] Abstract and §3 (or equivalent methods/results sections): the two annealing temperatures produce complexions that differ simultaneously in SRO content and in GB film thickness (thick uniform amorphous films vs. semi-amorphous films). Because GB thickness is independently known to modulate dislocation absorption capacity and local stress concentrations, the central attribution of the plasticity-mechanism switch to the presence/absence of FCC/BCC SRO requires explicit isolation of SRO from thickness (e.g., via additional simulations that hold thickness or segregation profiles fixed while varying only ordering). Without such controls the causal claim remains under-determined.

    Authors: We agree that the two annealing temperatures simultaneously alter both SRO content and GB film thickness, and that thickness is known to affect dislocation absorption and stress concentrations. Our structural analysis links the observed stress heterogeneities specifically to the FCC- and BCC-type SRO clusters present only in the lower-temperature films; the high-temperature films are uniformly amorphous without such clusters. The mechanism shift is consistent across both Ni concentrations despite any thickness variation. While dedicated simulations holding thickness fixed would provide stronger isolation, such controls are outside the scope of the present study. We will add a dedicated paragraph in the revised discussion section explicitly acknowledging the thickness difference and justifying the attribution to SRO on the basis of the local structural and stress characterizations already performed. revision: partial

Circularity Check

0 steps flagged

Direct simulation comparison; no derivation reduces to inputs by construction

full rationale

The paper reports outcomes from two distinct Monte Carlo + MD protocols (annealing at 913 K vs. 378 K) on nanocrystalline Al-Ni models, then compares the resulting GB complexions and their plasticity responses. No equations, fitted parameters, or predictions are defined; the claimed mechanism shift is presented as an observed contrast between the two simulation states. No self-citations are invoked as load-bearing uniqueness theorems, and no ansatz or renaming of known results occurs. The central claim therefore remains an empirical finding from the simulations rather than a tautological reduction to prior inputs.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract-only review supplies no information on interatomic potentials, thermostat settings, or boundary selection criteria; therefore the ledger cannot be populated beyond noting that standard molecular-dynamics assumptions are implicitly required.

pith-pipeline@v0.9.1-grok · 5780 in / 1246 out tokens · 33428 ms · 2026-06-29T16:46:40.742854+00:00 · methodology

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