Microstructural evolution and strengthening mechanism of an Al–Si–Mg alloy processed by high-pressure torsion with different heat treatments

Coarse α-Al dendrites and interdendritic networks of lamellar eutectic Si lead to low strength of Al–Si–Mg alloys (e.g. A356). An effective approach involving high-pressure torsion (HPT) processing combined with prior heat treatments is herein proposed to achieve ultra-high strength of A356 alloy. The HPT processing induces the precipitation of nanoscale Si particles, whose number density is strongly related to the heat treatments prior to the HPT. Most importantly, these nanoscale Si particles in turn promote dislocation multiplication and grain refinement, so that the solid solution treated samples with the highest density of nanoscale Si particles after HPT processing, produce the finest microstructures and the highest density of dislocations, thus giving the best yield strength (~440 MPa) and ultimate tensile strength (~560 MPa), with an elongation to failure of 8.8%. As for cast Al-based alloys such as A356 alloy, the number density of nanoscale Si particles provides key information for unravelling the origins of HPT-induced strengthening, and reveals the importance of heat treatments prior to the HPT processing.