Solidification behaviour and strengthening–toughening mechanisms of HPDC Al–Si composites reinforced with micron–nano dual-scale high-entropy alloy particles

• Proposes a micro–nano dual-scale HEA strategy for heat-treatment-free HPDC Al–Si composites, achieving σ UTS = 366.7 MPa and ε f = 7.2%. • Clarifies a size-controlled porosity route: micron particles − shrinkage, nano particles − gas; excessive loading leads to porosity-dominated failure. • Develops a porosity-corrected strengthening–defect coupling model to guide particle architecture and densification design. Micron–nano dual-scale FeCoNiCrAl high-entropy alloy (HEA) particles were introduced into a hypoeutectic Al–Si alloy by high-pressure die casting (HPDC) to fabricate heat-treatment-free composites and to clarify how particle size and content jointly govern solidification, porosity and tensile properties. The particles exhibit good interfacial bonding with the Al–Si matrix, and a Mg2Si-enriched shell is observed on the surface of some particles. HEA addition markedly refines the as-cast microstructure: primary α-Al grains decrease from 90 μm (base alloy) to 20.5 μm in the optimised dual-scale composite, while eutectic Si size reduces from 14.2 μm to 5.6 μm. Both micron and nano HEA promote heterogeneous nucleation of α-Al, whereas nanoparticles dominate eutectic-Si refinement. Micron HEA blocks interdendritic feeding and promotes large shrinkage pores, while nano HEA increases dispersed gas porosity. Excessive HEA content superposes these mechanisms, causing porosity escalation and pore-dominated fracture. Dual-scale reinforcement outperforms single-scale designs, achieving UTS 372.5 MPa with 7.4% elongation. A yield-strength model combining load transfer, thermal-mismatch dislocations, grain refinement and Orowan strengthening, with a porosity-degradation term, reasonably captures the measured strengthening.