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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

Dilation of 15- and 16-grain assemblies in continuous contact at 73 and 93% solid.Position and volumetric strain of each assembly for increasing axial strain at (a) 73% solid (scale bar, 1 mm) and (b) 93% solid (scale bar, 1 mm); 3D rendering of the polyhedron formed by the grain centroids at (c) 73% solid (scale bar, 300 μm) and (d) 93% solid (scale bar, 500 μm); Euler distance travelled by each grain per 2% incremental strain at (e) 73% solid (scale bar, 300 μm) and (f) 93% solid (scale bar, 500 μm); rotation of each grain per 2% incremental strain at (g) 73% solid (scale bar, 300 μm) and (h) 93% solid (scale bar, 500 μm); and internal porosity response at (i) 73% solid (scale bar, 100 μm) and (j) 93% solid (scale bar, 100 μm).
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f4: Dilation of 15- and 16-grain assemblies in continuous contact at 73 and 93% solid.Position and volumetric strain of each assembly for increasing axial strain at (a) 73% solid (scale bar, 1 mm) and (b) 93% solid (scale bar, 1 mm); 3D rendering of the polyhedron formed by the grain centroids at (c) 73% solid (scale bar, 300 μm) and (d) 93% solid (scale bar, 500 μm); Euler distance travelled by each grain per 2% incremental strain at (e) 73% solid (scale bar, 300 μm) and (f) 93% solid (scale bar, 500 μm); rotation of each grain per 2% incremental strain at (g) 73% solid (scale bar, 300 μm) and (h) 93% solid (scale bar, 500 μm); and internal porosity response at (i) 73% solid (scale bar, 100 μm) and (j) 93% solid (scale bar, 100 μm).

Mentions: The micromechanisms leading to the macroscopic volumetric strains in Fig. 3 were investigated by studying the behaviour of individual grains during compression, in a manner similar to the study of the kinematics of sand grains imaged during uniaxial compression1920. Within the resolution limit of this study, there is no statistically significant solidification or remelting of individual grains during an experiment (quantified in Supplementary Fig. 4 and detailed in Supplementary Note 4), and there is no detectable shape change of individual grains (quantified in Supplementary Fig. 5 and detailed in Supplementary Note 5). Grains can therefore be considered quasi-rigid and global deformation occurs by the rearrangement of grains, the flow of liquid and the motion of menisci. Sub-assemblies of 15 and 16 grains were then randomly selected for detailed analysis in 73 and 93% solid samples. 3D renderings of the sub-assemblies are shown in Fig. 4a,b. Note that these grains were in continuous contact during rearrangement and are from regions with no porosity at any strain, so that the in-flow or out-flow of liquid compensated for any changes in local solid packing. The volume of each local 15/16 grain assembly was defined by the polyhedron formed by the centroids of the grains and the development of polyhedron volume was used to calculate the volumetric strain plotted in Fig. 4a,b. The volumetric strain increases with increasing axial strains in both cases and the grain-level behaviour is shown in Fig. 4c,d, where the grains are rendered in grey and the polyhedron is coloured based on volume change. Fig. 4d–h shows the translation vector magnitude and grain rotation, respectively, and indicates that each grain is displacing independently, since neighbouring grains coloured identically at one increment often have different colour in the next and neighbouring grains travelling identical magnitudes often undergo a different rotation. From Fig. 4c–h it can be inferred that shear-induced dilation is due to quasi-rigid grains pushing each other apart as they translate/rotate independently under load, both at 73 and 93% solid. The dilatational volumetric strain and the resulting drawing-in of menisci are then emergent phenomena simply caused by the rearrangement of initially tightly packed quasi-rigid grains, and would not be expected if strain was only accommodated by viscoplastic deformation of the solid phase. The shear-induced dilation of grains/crystallites is also the origin of dilatant shear banding both in semi-solid alloys5 and in other systems such as soils182122, rock2324, magma25 and cornstarch slurries262728.


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)

Dilation of 15- and 16-grain assemblies in continuous contact at 73 and 93% solid.Position and volumetric strain of each assembly for increasing axial strain at (a) 73% solid (scale bar, 1 mm) and (b) 93% solid (scale bar, 1 mm); 3D rendering of the polyhedron formed by the grain centroids at (c) 73% solid (scale bar, 300 μm) and (d) 93% solid (scale bar, 500 μm); Euler distance travelled by each grain per 2% incremental strain at (e) 73% solid (scale bar, 300 μm) and (f) 93% solid (scale bar, 500 μm); rotation of each grain per 2% incremental strain at (g) 73% solid (scale bar, 300 μm) and (h) 93% solid (scale bar, 500 μm); and internal porosity response at (i) 73% solid (scale bar, 100 μm) and (j) 93% solid (scale bar, 100 μm).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: Dilation of 15- and 16-grain assemblies in continuous contact at 73 and 93% solid.Position and volumetric strain of each assembly for increasing axial strain at (a) 73% solid (scale bar, 1 mm) and (b) 93% solid (scale bar, 1 mm); 3D rendering of the polyhedron formed by the grain centroids at (c) 73% solid (scale bar, 300 μm) and (d) 93% solid (scale bar, 500 μm); Euler distance travelled by each grain per 2% incremental strain at (e) 73% solid (scale bar, 300 μm) and (f) 93% solid (scale bar, 500 μm); rotation of each grain per 2% incremental strain at (g) 73% solid (scale bar, 300 μm) and (h) 93% solid (scale bar, 500 μm); and internal porosity response at (i) 73% solid (scale bar, 100 μm) and (j) 93% solid (scale bar, 100 μm).
Mentions: The micromechanisms leading to the macroscopic volumetric strains in Fig. 3 were investigated by studying the behaviour of individual grains during compression, in a manner similar to the study of the kinematics of sand grains imaged during uniaxial compression1920. Within the resolution limit of this study, there is no statistically significant solidification or remelting of individual grains during an experiment (quantified in Supplementary Fig. 4 and detailed in Supplementary Note 4), and there is no detectable shape change of individual grains (quantified in Supplementary Fig. 5 and detailed in Supplementary Note 5). Grains can therefore be considered quasi-rigid and global deformation occurs by the rearrangement of grains, the flow of liquid and the motion of menisci. Sub-assemblies of 15 and 16 grains were then randomly selected for detailed analysis in 73 and 93% solid samples. 3D renderings of the sub-assemblies are shown in Fig. 4a,b. Note that these grains were in continuous contact during rearrangement and are from regions with no porosity at any strain, so that the in-flow or out-flow of liquid compensated for any changes in local solid packing. The volume of each local 15/16 grain assembly was defined by the polyhedron formed by the centroids of the grains and the development of polyhedron volume was used to calculate the volumetric strain plotted in Fig. 4a,b. The volumetric strain increases with increasing axial strains in both cases and the grain-level behaviour is shown in Fig. 4c,d, where the grains are rendered in grey and the polyhedron is coloured based on volume change. Fig. 4d–h shows the translation vector magnitude and grain rotation, respectively, and indicates that each grain is displacing independently, since neighbouring grains coloured identically at one increment often have different colour in the next and neighbouring grains travelling identical magnitudes often undergo a different rotation. From Fig. 4c–h it can be inferred that shear-induced dilation is due to quasi-rigid grains pushing each other apart as they translate/rotate independently under load, both at 73 and 93% solid. The dilatational volumetric strain and the resulting drawing-in of menisci are then emergent phenomena simply caused by the rearrangement of initially tightly packed quasi-rigid grains, and would not be expected if strain was only accommodated by viscoplastic deformation of the solid phase. The shear-induced dilation of grains/crystallites is also the origin of dilatant shear banding both in semi-solid alloys5 and in other systems such as soils182122, rock2324, magma25 and cornstarch slurries262728.

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