1. The CAST CRC, Materials Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia
   
2. The Australian Research Council CoE for design in light metals, Materials Engineering, The University of Queensland, Brisbane, Queensland, 4072, Australia

Correspondence to: C. M. Gourlay¹A. K. Dahle^1,2 Correspondence and requests for materials should be addressed to C.M.G. (Email: c.gourlay@minmet.uq.edu.au) or A.K.D. (Email: a.dahle@minmet.uq.edu.au).

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 faults^{3, 4}, flow in hoppers and silos⁵, and soil mechanics^{6, 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.