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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

Uniaxial compression at 64, 73, 87 and 93% solid.(a) Transversal (xz) slices at increasing compressive axial strain (scale bar, 1 mm), (b) change in volume of internal voids and surface-connected voids with increasing strain and (c–e) 3D rendering of surface-connected meniscus development at 73 and 93% solid, respectively (scale bar, 300 μm).
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f2: Uniaxial compression at 64, 73, 87 and 93% solid.(a) Transversal (xz) slices at increasing compressive axial strain (scale bar, 1 mm), (b) change in volume of internal voids and surface-connected voids with increasing strain and (c–e) 3D rendering of surface-connected meniscus development at 73 and 93% solid, respectively (scale bar, 300 μm).

Mentions: Figure 2a shows vertical slices approximately halfway through the specimens at three stages during semi-solid uniaxial compression. A striking feature in Fig. 2a is that uniaxial compression causes porosity/cracking in the 73, 86 and 93% solid samples, which increases with increasing solid fraction and increasing ram displacement. We defined porosity as internal or surface connected and tracked it during deformation. Figure 2b shows that most porosity is surface connected (renderings of the porosity at different strains can be seen in Supplementary Fig. 3 and Supplementary Note 3). Figure 2c–e details the process by which air is drawn-in using 3D renderings of surface-connected pores at the sides of the 73 and 93% solid specimens. At 73% solid, the oxidised liquid surface is sucked into the sample both under the ram that is applying compressive load (Fig. 2c) and at the radial free surface (Fig. 2d). The two separate menisci in Fig. 2c develop directly underneath the ram and grow into the liquid during compression. The three radial menisci in Fig. 2d are initially pulled into the liquid before merging into a large surface-connected pore that propagates into the liquid between the grains, producing a complex pore with multiple radii of curvature. This mechanism is the same at 93% solid, but the packing of the solid is such that the propagation of a meniscus into the narrow liquid channel appears as cracking initiating from the surface (Fig. 2e). The drawing-in of menisci indicates that the liquid pressure is decreasing and the grains are moving apart. While this behaviour is common during tensile deformation and hot tearing151617, it is counter intuitive during uniaxial compression and is not predicted by existing theories48. To clarify the underpinning mechanisms, we examined both stress–strain behaviours and discrete grain responses to load.


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)

Uniaxial compression at 64, 73, 87 and 93% solid.(a) Transversal (xz) slices at increasing compressive axial strain (scale bar, 1 mm), (b) change in volume of internal voids and surface-connected voids with increasing strain and (c–e) 3D rendering of surface-connected meniscus development at 73 and 93% solid, respectively (scale bar, 300 μm).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f2: Uniaxial compression at 64, 73, 87 and 93% solid.(a) Transversal (xz) slices at increasing compressive axial strain (scale bar, 1 mm), (b) change in volume of internal voids and surface-connected voids with increasing strain and (c–e) 3D rendering of surface-connected meniscus development at 73 and 93% solid, respectively (scale bar, 300 μm).
Mentions: Figure 2a shows vertical slices approximately halfway through the specimens at three stages during semi-solid uniaxial compression. A striking feature in Fig. 2a is that uniaxial compression causes porosity/cracking in the 73, 86 and 93% solid samples, which increases with increasing solid fraction and increasing ram displacement. We defined porosity as internal or surface connected and tracked it during deformation. Figure 2b shows that most porosity is surface connected (renderings of the porosity at different strains can be seen in Supplementary Fig. 3 and Supplementary Note 3). Figure 2c–e details the process by which air is drawn-in using 3D renderings of surface-connected pores at the sides of the 73 and 93% solid specimens. At 73% solid, the oxidised liquid surface is sucked into the sample both under the ram that is applying compressive load (Fig. 2c) and at the radial free surface (Fig. 2d). The two separate menisci in Fig. 2c develop directly underneath the ram and grow into the liquid during compression. The three radial menisci in Fig. 2d are initially pulled into the liquid before merging into a large surface-connected pore that propagates into the liquid between the grains, producing a complex pore with multiple radii of curvature. This mechanism is the same at 93% solid, but the packing of the solid is such that the propagation of a meniscus into the narrow liquid channel appears as cracking initiating from the surface (Fig. 2e). The drawing-in of menisci indicates that the liquid pressure is decreasing and the grains are moving apart. While this behaviour is common during tensile deformation and hot tearing151617, it is counter intuitive during uniaxial compression and is not predicted by existing theories48. To clarify the underpinning mechanisms, we examined both stress–strain behaviours and discrete grain responses to load.

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