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Revealing the micromechanisms behind semi-solid metal deformation with time-resolved X-ray tomography.

Kareh KM, Lee PD, Atwood RC, Connolley T, Gourlay CM - Nat Commun (2014)

Bottom Line: Here we demonstrate that treating semi-solid alloys as a granular fluid is critical to understanding flow behaviour and defect formation during casting.This leads to the counter-intuitive result that, in unfed samples, compression can open internal pores and draw the free surface into the liquid, resulting in cracking.A soil mechanics approach shows that, irrespective of initial solid fraction, the solid packing density moves towards a constant value during deformation, consistent with the existence of a critical state in mushy alloys analogous to soils.

View Article: PubMed Central - PubMed

Affiliation: Department of Materials, Imperial College London, Prince Consort Road, London SW7 2AZ, UK.

ABSTRACT
The behaviour of granular solid-liquid mixtures is key when deforming a wide range of materials from cornstarch slurries to soils, rock and magma flows. Here we demonstrate that treating semi-solid alloys as a granular fluid is critical to understanding flow behaviour and defect formation during casting. Using synchrotron X-ray tomography, we directly measure the discrete grain response during uniaxial compression. We show that the stress-strain response at 64-93% solid is due to the shear-induced dilation of discrete rearranging grains. This leads to the counter-intuitive result that, in unfed samples, compression can open internal pores and draw the free surface into the liquid, resulting in cracking. A soil mechanics approach shows that, irrespective of initial solid fraction, the solid packing density moves towards a constant value during deformation, consistent with the existence of a critical state in mushy alloys analogous to soils.

No MeSH data available.


Related in: MedlinePlus

Undeformed semi-solid microstructures for nominal solid fractions of 64, 73, 87 and 93%.(a) Cross-sectional (xy) slices (scale bar, 1 mm), (b) 3D rendering of the separated grains (scale bar, 500 μm) and (c) typical grains from each specimen illustrating the decreasing sphericity, ψ, with increasing solid fraction (scale bar, 300 μm).
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f1: Undeformed semi-solid microstructures for nominal solid fractions of 64, 73, 87 and 93%.(a) Cross-sectional (xy) slices (scale bar, 1 mm), (b) 3D rendering of the separated grains (scale bar, 500 μm) and (c) typical grains from each specimen illustrating the decreasing sphericity, ψ, with increasing solid fraction (scale bar, 300 μm).

Mentions: We begin by quantifying the initial semi-solid microstructures. In Fig. 1a, the solid appears dark and the liquid bright, with more crowded solid packing and more tortuous liquid regions at increasing solid fraction. Figure 1b shows 3D renderings of separated and filtered grains in 2 mm3 representative regions and Fig. 1c highlights the increase in surface topology with increasing solid fraction. In these samples, the mean grain sphericity decreases with increasing solid fraction, as the grains fill the interstitial space and acquire a more complex shape with increased surface area (quantified in Supplementary Fig. 1a and detailed in Supplementary Note 1). All specimens have average grain sizes within 15% of each other and the four grain populations are well approximated as self-similar (quantified in Supplementary Fig. 1), which is a consequence of the long-term isothermal semi-solid coarsening (for example, ref. 14) used in preparing the samples. All four specimens contain 10−2–10−3% porosity before loading (shown in Supplementary Fig. 2 and detailed in Supplementary Note 2).


Revealing the micromechanisms behind semi-solid metal deformation with time-resolved X-ray tomography.

Kareh KM, Lee PD, Atwood RC, Connolley T, Gourlay CM - Nat Commun (2014)

Undeformed semi-solid microstructures for nominal solid fractions of 64, 73, 87 and 93%.(a) Cross-sectional (xy) slices (scale bar, 1 mm), (b) 3D rendering of the separated grains (scale bar, 500 μm) and (c) typical grains from each specimen illustrating the decreasing sphericity, ψ, with increasing solid fraction (scale bar, 300 μm).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4109016&req=5

f1: Undeformed semi-solid microstructures for nominal solid fractions of 64, 73, 87 and 93%.(a) Cross-sectional (xy) slices (scale bar, 1 mm), (b) 3D rendering of the separated grains (scale bar, 500 μm) and (c) typical grains from each specimen illustrating the decreasing sphericity, ψ, with increasing solid fraction (scale bar, 300 μm).
Mentions: We begin by quantifying the initial semi-solid microstructures. In Fig. 1a, the solid appears dark and the liquid bright, with more crowded solid packing and more tortuous liquid regions at increasing solid fraction. Figure 1b shows 3D renderings of separated and filtered grains in 2 mm3 representative regions and Fig. 1c highlights the increase in surface topology with increasing solid fraction. In these samples, the mean grain sphericity decreases with increasing solid fraction, as the grains fill the interstitial space and acquire a more complex shape with increased surface area (quantified in Supplementary Fig. 1a and detailed in Supplementary Note 1). All specimens have average grain sizes within 15% of each other and the four grain populations are well approximated as self-similar (quantified in Supplementary Fig. 1), which is a consequence of the long-term isothermal semi-solid coarsening (for example, ref. 14) used in preparing the samples. All four specimens contain 10−2–10−3% porosity before loading (shown in Supplementary Fig. 2 and detailed in Supplementary Note 2).

Bottom Line: Here we demonstrate that treating semi-solid alloys as a granular fluid is critical to understanding flow behaviour and defect formation during casting.This leads to the counter-intuitive result that, in unfed samples, compression can open internal pores and draw the free surface into the liquid, resulting in cracking.A soil mechanics approach shows that, irrespective of initial solid fraction, the solid packing density moves towards a constant value during deformation, consistent with the existence of a critical state in mushy alloys analogous to soils.

View Article: PubMed Central - PubMed

Affiliation: Department of Materials, Imperial College London, Prince Consort Road, London SW7 2AZ, UK.

ABSTRACT
The behaviour of granular solid-liquid mixtures is key when deforming a wide range of materials from cornstarch slurries to soils, rock and magma flows. Here we demonstrate that treating semi-solid alloys as a granular fluid is critical to understanding flow behaviour and defect formation during casting. Using synchrotron X-ray tomography, we directly measure the discrete grain response during uniaxial compression. We show that the stress-strain response at 64-93% solid is due to the shear-induced dilation of discrete rearranging grains. This leads to the counter-intuitive result that, in unfed samples, compression can open internal pores and draw the free surface into the liquid, resulting in cracking. A soil mechanics approach shows that, irrespective of initial solid fraction, the solid packing density moves towards a constant value during deformation, consistent with the existence of a critical state in mushy alloys analogous to soils.

No MeSH data available.


Related in: MedlinePlus