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.
Highlights
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
The proposed micromechanisms vary from viscoplastic deformation of a porous solid skeleton[3,6,7] to the deagglomeration of a concentrated flocculated suspension[8,9] or the granular rearrangement of quasi-rigid discrete grains with negligible cohesion[5,10]
Concerning the latter, it might be thought that partially solid alloys containing a solid network would not deform in this way; for example, metallic grains have a yield strength of only up to a few MPa in the solid–liquid two-phase region and grain–grain contacts can be cohesive due to the formation of solid–solid interfaces, which would promote viscoplastic deformation of a porous solid without grain rearrangement
Summary
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. The proposed micromechanisms vary from viscoplastic deformation of a porous solid skeleton[3,6,7] (similar to a liquid-saturated sponge in compression) to the deagglomeration of a concentrated flocculated suspension[8,9] (similar to dispersed clay slurries) or the granular rearrangement of quasi-rigid discrete grains with negligible cohesion[5,10] (similar to a saturated particulate soil where force is transmitted across contacts between grains) Concerning the latter, it might be thought that partially solid alloys containing a solid network would not deform in this way; for example, metallic grains have a yield strength of only up to a few MPa in the solid–liquid two-phase region and grain–grain contacts can be cohesive due to the formation of solid–solid interfaces (grain boundaries), which would promote viscoplastic deformation of a porous solid without grain rearrangement. We show that this granular behaviour induces the formation of casting defects
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