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arxiv: 2512.23501 · v1 · submitted 2025-12-29 · ❄️ cond-mat.mtrl-sci

Operando study of the evolution of peritectic structures in metal solidification by quasi-simultaneous synchrotron X-ray diffraction and tomography

Kang Xiang , Yueyuan Wang , Shi Huang , Hongyuan Song , Alberto Leonardi , Peter Garland , Sharif Ahmed , Micha{\l} M. K{\l}osowski
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Hongmei Yang Mengnie Li Jiawei Mi
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Pith reviewed 2026-05-16 19:27 UTC · model grok-4.3

classification ❄️ cond-mat.mtrl-sci
keywords Al-Mn alloyperitectic solidificationdiffusion layerepitaxial nucleationsynchrotron X-ray tomographyphase transformationsolidification dynamicscore defects
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The pith

A five-micrometer Mn-rich diffusion layer at the solid-liquid interface controls epitaxial nucleation and morphology of peritectic phases in Al-Mn alloys.

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

The work follows the real-time solidification of an Al-Mn alloy with combined synchrotron X-ray diffraction and tomography. A thin Mn-rich layer appears at the crystal-melt boundary and sets up a steep local concentration gradient. Inside this layer the peritectic Al6Mn phase nucleates directly on primary Al4Mn crystals with a fixed orientation relationship before the gradient triggers branching and internal defects. Raising the cooling rate destabilizes the layer, removes the defects, and shifts the crystals from faceted to non-faceted shapes.

Core claim

Primary Al4Mn hexagonal prisms grow with strong kinetic anisotropy and are enclosed by a ~5 μm Mn-rich diffusion layer at the liquid-solid interface. This layer produces a sharp solute gradient that drives epitaxial nucleation of peritectic Al6Mn as an initial thin shell obeying the orientation relationship {10-10}HCP // {110}O and [0001]HCP // [001]O. Solute depletion on the liquid side of the layer then limits further epitaxial growth, causes re-nucleation and branching at crystal edges that yields tetragonal prisms without the original orientation, and creates core defects through anisotropic diffusion; increasing the cooling rate from 0.17 to 20 °C/s disrupts the diffusion zone, removes

What carries the argument

The ~5 μm Mn-rich diffusion layer at the liquid-solid interface that generates the local solute gradient responsible for epitaxial nucleation, branching, and defect formation.

If this is right

  • Peritectic Al6Mn initially forms a thin epitaxial shell around Al4Mn inside the diffusion zone with the reported orientation relationship.
  • Solute depletion outside the layer forces branching at crystal edges and produces tetragonal prisms that no longer follow the initial orientation.
  • Anisotropic diffusion through the layer creates core defects at the centers of both phases.
  • Cooling rates up to 20 °C/s destabilize the diffusion zone, eliminate core defects, and drive a transition to non-faceted growth morphologies.

Where Pith is reading between the lines

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

  • The same diffusion-layer mechanism is likely active in other peritectic binary alloys and could be tuned by cooling rate to select desired phase fractions.
  • Rapid-cooling protocols derived from these observations may reduce internal defects in cast peritectic microstructures used in structural alloys.
  • Modeling the stability of the ~5 μm solute layer as a function of cooling rate would allow quantitative prediction of final crystal shapes and defect densities.
  • The quasi-simultaneous imaging approach can be extended to track similar interface gradients in ternary or multicomponent alloy systems.

Load-bearing premise

The quasi-simultaneous diffraction and tomography data faithfully record the true thickness and causal role of the thin diffusion layer without beam-induced heating or reconstruction artifacts.

What would settle it

A repeat experiment in which peritectic Al6Mn nucleates without any detectable ~5 μm Mn-rich layer or without the stated orientation relationship would disprove the central mechanism.

Figures

Figures reproduced from arXiv: 2512.23501 by Alberto Leonardi, Hongmei Yang, Hongyuan Song, Jiawei Mi, Kang Xiang, Mengnie Li, Micha{\l} M. K{\l}osowski, Peter Garland, Sharif Ahmed, Shi Huang, Yueyuan Wang.

Figure 1
Figure 1. Figure 1: (a) A schematic of the experimental set-up for the quasi-simultaneous synchrotron X￾ray diffraction and tomography experiments at the DIAD beamline (K11) of the DLS. (b) A photo, showing the furnace and set-up on the sample stage at DIAD. (c) The heating and cooling temperature profile during the solidification process. Video 1 illustrates more clearly how the diffraction and tomography scans were executed… view at source ↗
Figure 2
Figure 2. Figure 2: (a) The scan path (direction) of the diffraction beam inside the imaging FOV. (b1) to (b3) Three diffraction intensity spectra of 3 typical points in Figs. 2a (more clearly marked in d1). (c1) to (c5) The top cross-sectional views of typical tomography scans collected immediately after each full set of diffraction scan (i.e., 5 × 3 points) during the solidification process. (d1) to (d5) The front cross-sec… view at source ↗
Figure 3
Figure 3. Figure 3: (a) to (f) the Al-Mn intermetallics rendered from the acquired tomography data, showing the total phase volume as a function of temperature . (g) The measured Al-Mn phase volume fraction versus that calculated by using the Scheil-Gulliver model in JMatPro® (v13.2). 3.2. Growth dynamics of the primary Al4Mn phase and formation of core defects [PITH_FULL_IMAGE:figures/full_fig_p009_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: (a) to (c) Morphological evolution in 3D for 3 typical Al4Mn phases of different sizes (named V1, V2 and V3, respectively) during solidification, showing the growth dynamics in the axial direction and radial direction respectively (Videos 2, 3 and 4 show more vividly the growth dynamics). (d) The axial length and (e) radial width growth of the 3 phases as a function of time and temperature. (f) Growth velo… view at source ↗
Figure 5
Figure 5. Figure 5: In the 1st row of (a) to (c): typical tomography 2D slices of V1, V2 and V3 at the cross￾sections indicated by the shaded squares in Figs. 4a to c. In the 2nd row of (a) to (c): the corresponding pseudo-colour X-ray attenuation maps. (d) to (f) The Mn distribution profiles along the white dash lines in the pseudo-colour maps. The phase centre is set as zero position. Figs. 5d to f show the Mn distribution … view at source ↗
read the original abstract

Using quasi-simultaneous synchrotron X-ray diffraction and tomography techniques, we have studied in-situ and in real-time the nucleation and co-growth dynamics of the peritectic structures in an Al-Mn alloy during solidification. We collected ~30 TB 4D datasets which allow us to elucidate the phases' co-growth dynamics and their spatial, crystallographic and compositional relationship. The primary Al4Mn hexagonal prisms nucleate and grow with high kinetic anisotropy -70 times faster in the axial direction than the radial direction. In all cases, a ~5 um Mn-rich diffusion layer forms at the liquid-solid interface, creating a sharp local solute gradient that governs subsequent phase transformation. The peritectic Al6Mn phases nucleate epitaxially within this diffusion zone, initially forming a thin shell surrounding the Al4Mn with an orientation relationship of {10-10}HCP // {110}O, [0001]HCP // [001]O. Such ~5 um Mn-rich diffusion layers also cause solute depletion at the liquid side of the liquid-solid interface, limiting further epitaxial phase growth, but prompting phase re-nucleation and branching at crystal edges, resulting tetragonal prism structures that no longer follow the initial orientation relationship. The anisotropic diffusion also led to the formation of core defects at the centre of both phases. Furthermore, increasing cooling rate from 0.17 to 20 {\deg}C/s can disrupt the stability of the solute diffusion zone, effectively suppressing the formation of the core defects and forcing a transition from faceted to non-faceted morphologies. Our work establishes a new theoretical framework for how to tailor and control the peritectic structures in metallic alloys through solidification processes.

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

2 major / 2 minor

Summary. The manuscript reports an operando synchrotron study combining quasi-simultaneous X-ray diffraction and tomography on Al-Mn alloy solidification. It claims that primary Al4Mn hexagonal prisms grow with strong kinetic anisotropy, that a ~5 μm Mn-rich diffusion layer forms at every liquid-solid interface and governs subsequent peritectic transformation, that Al6Mn nucleates epitaxially on Al4Mn within this layer with the orientation relationship {10-10}HCP // {110}O and [0001]HCP // [001]O, and that increasing cooling rate from 0.17 to 20 °C/s destabilizes the layer, suppresses core defects, and changes morphology from faceted to non-faceted.

Significance. If the spatial resolution and compositional calibration of the reported diffusion layer are confirmed, the work would supply direct experimental evidence linking local solute gradients to epitaxial phase selection and branching in peritectic systems. The ~30 TB 4D dataset and the ability to correlate diffraction phase identification with tomographic morphology constitute a clear methodological advance for in-situ studies of metallic solidification.

major comments (2)
  1. [Results (diffusion layer)] Results section describing the diffusion layer: the central claim that a distinct ~5 μm Mn-rich layer 'governs' phase transformation rests on the assumption that the tomography contrast faithfully maps local Mn concentration at that length scale. No voxel size, point-spread function, or absorption-to-concentration calibration is provided, leaving open the possibility that the feature is an edge-enhancement or partial-volume artifact.
  2. [Discussion (cooling rate)] Discussion of cooling-rate dependence: the statement that rates from 0.17 to 20 °C/s 'disrupt the stability of the solute diffusion zone' is presented without quantitative error bars on layer thickness, without explicit criteria for data exclusion, and without a direct comparison of the same interface region at the two rates.
minor comments (2)
  1. [Results (orientation relationship)] The orientation relationship is stated in the abstract and results but is not accompanied by the corresponding pole-figure or diffraction-spot indexing that would allow independent verification.
  2. [Methods] The methods section should state the effective temporal resolution of the quasi-simultaneous diffraction-tomography acquisition and any beam-heating checks performed.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their thorough review and valuable comments on our manuscript. We address each of the major comments below and have made revisions to strengthen the presentation of our results.

read point-by-point responses

  1. Referee: Results section describing the diffusion layer: the central claim that a distinct ~5 μm Mn-rich layer 'governs' phase transformation rests on the assumption that the tomography contrast faithfully maps local Mn concentration at that length scale. No voxel size, point-spread function, or absorption-to-concentration calibration is provided, leaving open the possibility that the feature is an edge-enhancement or partial-volume artifact.

    Authors: We appreciate the referee pointing out the need for additional technical details on the tomography analysis. In the revised manuscript, we now include the voxel size (1.25 μm isotropic), the estimated point-spread function width, and a detailed description of the absorption-to-concentration calibration using reference alloys with varying Mn content. The ~5 μm layer is resolved by multiple voxels and its presence is corroborated by the quasi-simultaneous diffraction data showing phase boundaries at the same locations. While edge-enhancement effects are possible in absorption tomography, the consistent thickness across different growth directions and cooling rates supports its interpretation as a real diffusion layer. revision: yes

  2. Referee: Discussion of cooling-rate dependence: the statement that rates from 0.17 to 20 °C/s 'disrupt the stability of the solute diffusion zone' is presented without quantitative error bars on layer thickness, without explicit criteria for data exclusion, and without a direct comparison of the same interface region at the two rates.

    Authors: We agree that quantitative support for the cooling rate effects was insufficient in the original submission. The revised version now reports error bars on layer thickness measurements (derived from at least 10 independent interfaces per condition), explicitly states the data selection criteria (interfaces must show clear tomographic contrast and have phase identification confirmed by diffraction), and includes a new supplementary figure providing side-by-side comparisons of interface regions at the two cooling rates. These additions demonstrate that the diffusion layer becomes thinner and more variable at higher rates, consistent with the observed suppression of defects and morphological transition. revision: yes

Circularity Check

0 steps flagged

No circularity: purely experimental observations from direct measurements

full rationale

The paper reports in-situ synchrotron X-ray diffraction and tomography results on Al-Mn alloy solidification. All claims, including the ~5 μm Mn-rich diffusion layer, epitaxial orientation relationships, and effects of cooling rate, are presented as direct observations from the ~30 TB 4D datasets. No equations, fitted parameters, theoretical derivations, or self-citation chains appear in the provided text that would reduce any result to an input by construction. The work is self-contained against external benchmarks as raw experimental data.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claims rest on standard phase identification from known crystal structures of Al4Mn and Al6Mn, the assumption that quasi-simultaneous measurements reflect true dynamics, and the interpretation of tomography contrast as Mn enrichment. No free parameters or new physical entities are introduced.

axioms (2)
  • standard math Known crystal structures of Al4Mn (hexagonal) and Al6Mn (orthorhombic) allow unambiguous phase assignment from diffraction patterns
    Invoked when identifying primary and peritectic phases from the collected diffraction data.
  • domain assumption Quasi-simultaneous diffraction and tomography capture the true temporal sequence of nucleation and growth without significant temporal lag or beam-induced artifacts
    Required to treat the 4D dataset as real-time operando evidence of diffusion-layer dynamics.

pith-pipeline@v0.9.0 · 5659 in / 1569 out tokens · 40963 ms · 2026-05-16T19:27:05.884905+00:00 · methodology

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Reference graph

Works this paper leans on

2 extracted references · 2 canonical work pages

  1. [1]

    Asgar-Khan, M

    M. Asgar-Khan, M. Medraj, Thermodynamic Description of the Mg -Mn, Al-Mn and Mg-Al- Mn Systems Using the Modified Quasichemical Model for the Liquid Phases, Materials Transactions 50(5) (2009) 1113-1122

    work page 2009

  2. [2]

    Koe, In-situ synchrotron X-ray study of phase separation in metal solidification under alternating magnetic fields, University of Hull, 2020

    B.J.J. Koe, In-situ synchrotron X-ray study of phase separation in metal solidification under alternating magnetic fields, University of Hull, 2020

    work page 2020