Abstract
Compacted granular materials expand in response to shear1, and can exhibit different behaviour from that of the solids, liquids and gases of which they are composed. Application of the physics of granular materials has increased the understanding of avalanches2, geological faults3,4, flow in hoppers and silos5, and soil mechanics6,7. During the equiaxed solidification of metallic alloys, there exists a range of solid fractions where the microstructure consists of a geometrically crowded disordered assembly of crystals saturated with liquid. It is therefore natural to ask if such a microstructure deforms as a granular material and what relevance this might have to solidification processing. Here we show that partially solidified alloys can exhibit the characteristics of a cohesionless granular material, including Reynolds’ dilatancy1 and strain localization in dilatant shear bands 7–18 mean crystals wide. We show that this behaviour is important in defect formation during high pressure die casting of Al and Mg alloys, a global industry that contributes over $7.3 billion to the USA’s economy alone8 and is used in the manufacture of products that include mobile-phone covers and steering wheels. More broadly, these findings highlight the potential to apply the principles and modelling approaches developed in granular mechanics to the field of solidification processing, and also indicate the possible benefits that might be gained from exploring and exploiting further synergies between these fields.
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Acknowledgements
We thank C. J. Davidson for help recording the volumetric strain, H. Wang for providing a direct shear cell sample, Hydro Aluminium for providing the HPDC samples, and H. I. Laukli for discussions on defect bands in HPDC. This work was supported by the CAST CRC.
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Supplementary Information
This file contains Supplementary Methods, Supplementary Figures 1-20 with legends, Supplementary Tables 1-5, and Supplementary Notes. This file provides an expanded methods section, additional micrographs in support of our findings and a discussion of changes in crystal size and shape during post-deformation solidification. (PDF 3921 kb)
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Gourlay, C., Dahle, A. Dilatant shear bands in solidifying metals. Nature 445, 70–73 (2007). https://doi.org/10.1038/nature05426
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DOI: https://doi.org/10.1038/nature05426
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