title: Revealing the micromechanisms behind semi-solid metal deformation with time-resolved X-ray tomography.
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.
authors.affiliation: Department of Materials, Imperial College London, Prince Consort Road, London SW7 2AZ, UK.
authors.forename: K M
authors.initials: KM
authors.lastname: Kareh
authors.affiliation: School of Materials, The University of Manchester, Oxford Road, Manchester M13 9PL, UK.
authors.forename: P D
authors.initials: PD
authors.lastname: Lee
authors.affiliation: Diamond Light Source Ltd, Harwell Science & Innovation campus, Didcot OX11 0DE, UK.
authors.forename: R C
authors.initials: RC
authors.lastname: Atwood
authors.affiliation: Diamond Light Source Ltd, Harwell Science & Innovation campus, Didcot OX11 0DE, UK.
authors.forename: T
authors.initials: T
authors.lastname: Connolley
authors.affiliation: Department of Materials, Imperial College London, Prince Consort Road, London SW7 2AZ, UK.
authors.forename: C M
authors.initials: CM
authors.lastname: Gourlay