Limits...
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

Discrete grain response of the whole specimen at 64% (left) and 73% (right) solid.(a) The translation of each grain in the z-direction after subtracting the z-component expected of homogeneous compression, (b) the Euler distances travelled by each grain and (c) the rotation of each grain. Each rendering is for a 2% incremental axial strain (scale bar, 1 mm).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4109016&req=5

f5: Discrete grain response of the whole specimen at 64% (left) and 73% (right) solid.(a) The translation of each grain in the z-direction after subtracting the z-component expected of homogeneous compression, (b) the Euler distances travelled by each grain and (c) the rotation of each grain. Each rendering is for a 2% incremental axial strain (scale bar, 1 mm).

Mentions: At 93% solid, this shear-induced dilation causes not only surface cracking but also the significant opening of pre-existing internal porosity. Figure 5i,j shows 3D renderings of pre-existing gas-filled pores in their grain neighbourhoods. In contrast to the pore at 73% solid, whose volume change can be considered insignificant since it is at the limit of our resolution (detailed in Supplementary Fig. 6 and Supplementary Note 6), the pore at 93% solid opens under compressive strain with a volume increase of ~622% as grains are pushed apart. Other pores shown in Supplementary Fig. 7 also grow into the tortuous liquid channels between grains. Thus, during unfed compression at 93% solid, the expanding interstitial spaces during shear-induced dilation cause the local liquid pressure to drop and small internal pre-existing pores to grow due to the low permeability and lack of feed liquid. This highlights how shear-induced dilation can be another origin of defects in castings.


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)

Discrete grain response of the whole specimen at 64% (left) and 73% (right) solid.(a) The translation of each grain in the z-direction after subtracting the z-component expected of homogeneous compression, (b) the Euler distances travelled by each grain and (c) the rotation of each grain. Each rendering is for a 2% incremental axial strain (scale bar, 1 mm).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f5: Discrete grain response of the whole specimen at 64% (left) and 73% (right) solid.(a) The translation of each grain in the z-direction after subtracting the z-component expected of homogeneous compression, (b) the Euler distances travelled by each grain and (c) the rotation of each grain. Each rendering is for a 2% incremental axial strain (scale bar, 1 mm).
Mentions: At 93% solid, this shear-induced dilation causes not only surface cracking but also the significant opening of pre-existing internal porosity. Figure 5i,j shows 3D renderings of pre-existing gas-filled pores in their grain neighbourhoods. In contrast to the pore at 73% solid, whose volume change can be considered insignificant since it is at the limit of our resolution (detailed in Supplementary Fig. 6 and Supplementary Note 6), the pore at 93% solid opens under compressive strain with a volume increase of ~622% as grains are pushed apart. Other pores shown in Supplementary Fig. 7 also grow into the tortuous liquid channels between grains. Thus, during unfed compression at 93% solid, the expanding interstitial spaces during shear-induced dilation cause the local liquid pressure to drop and small internal pre-existing pores to grow due to the low permeability and lack of feed liquid. This highlights how shear-induced dilation can be another origin of defects in castings.

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