Revealing the micromechanisms behind semi-solid metal deformation with time-resolved X-ray tomography

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.
Soil-like granular flow has previously been shown when deforming semi-solid metals. Here, the authors measure bulk and grain-level deformation in semi-solid alloys in three dimensions using X-ray tomography, exploring shear-induced dilation between 64–93% solid and finding hints of a critical state.