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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

Bulk mechanical response at 64, 73, 87 and 93% solid.(a) True axial stress–true axial strain with the inset zooming in on the strain range at which all four stress responses reach a similar stress, (b) volumetric strain–true axial strain and (c) volume fraction of solid–true axial strain.
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f3: Bulk mechanical response at 64, 73, 87 and 93% solid.(a) True axial stress–true axial strain with the inset zooming in on the strain range at which all four stress responses reach a similar stress, (b) volumetric strain–true axial strain and (c) volume fraction of solid–true axial strain.

Mentions: Figure 3a shows axial true stress–true strain curves where in situ imaging has been used to measure the true specimen area in contact with the moving ram. The 73, 86 and 93% solid samples all exhibit a peak whereas the 64% sample does not (see the inset in Fig. 3a), and the peak stress increases with increasing solid fraction. The samples that have a peak stress exhibit strain softening before a final period of deformation occurs at relatively low constant stress (115±17 kPa). Following granular mechanics, we define the strain with respect to the solid assembly rather than the whole material, such that a contractive volumetric strain occurs if grains move closer together and liquid/gas is expelled and a dilatational volumetric strain occurs if grains move apart and liquid/gas is drawn into the expanding interstitial spaces18. Here, the sum of the developing surface-connected porosity, internal porosity and the liquid phase was defined as the ‘interstitial fluid’ and the change in the volume of solid plus interstitial fluid was used to calculate the volumetric strain in Fig. 3b. When viewed in this way, all specimens undergo a volumetric expansion during deformation and the maximum volumetric strain increases with increasing solid fraction.


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)

Bulk mechanical response at 64, 73, 87 and 93% solid.(a) True axial stress–true axial strain with the inset zooming in on the strain range at which all four stress responses reach a similar stress, (b) volumetric strain–true axial strain and (c) volume fraction of solid–true axial strain.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: Bulk mechanical response at 64, 73, 87 and 93% solid.(a) True axial stress–true axial strain with the inset zooming in on the strain range at which all four stress responses reach a similar stress, (b) volumetric strain–true axial strain and (c) volume fraction of solid–true axial strain.
Mentions: Figure 3a shows axial true stress–true strain curves where in situ imaging has been used to measure the true specimen area in contact with the moving ram. The 73, 86 and 93% solid samples all exhibit a peak whereas the 64% sample does not (see the inset in Fig. 3a), and the peak stress increases with increasing solid fraction. The samples that have a peak stress exhibit strain softening before a final period of deformation occurs at relatively low constant stress (115±17 kPa). Following granular mechanics, we define the strain with respect to the solid assembly rather than the whole material, such that a contractive volumetric strain occurs if grains move closer together and liquid/gas is expelled and a dilatational volumetric strain occurs if grains move apart and liquid/gas is drawn into the expanding interstitial spaces18. Here, the sum of the developing surface-connected porosity, internal porosity and the liquid phase was defined as the ‘interstitial fluid’ and the change in the volume of solid plus interstitial fluid was used to calculate the volumetric strain in Fig. 3b. When viewed in this way, all specimens undergo a volumetric expansion during deformation and the maximum volumetric strain increases with increasing solid fraction.

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