Prolonged work hardening in bimodal grain structured aluminum matrix composites: a sequential heterostructure effect

High-strength aluminum matrix composites (AMCs) suffer from poor ductility, due to the limited work hardening capacity. In this study, a remarkable prolonged work hardening is sustained in ultrastrong Al-5Mg matrix composites via an optimized bimodal grain heterostructure, with triple or even fourfold uniform elongation and raised tensile/yield strength. The prolonged work hardening proceeds through two sequential deformation stages. In the first stage with minor strains (<2.5%), a high gradient of geometrically necessary dislocations in soft coarse-grained (CG) zones generates strong back stress, which promotes not only hetero-deformation induced (HDI) hardening but also dislocation multiplication in hard ultrafine-grained (UFG) zones. The work hardening of UFG is thus improved with higher density of dislocations interacting with some nanoparticles. Subsequently, the stress of UFG zones rises sufficiently to induce dispersed microvoids formation within UFG zones, instead of localized cracking at hetero-zone boundaries. Therefore, an effective HDI hardening depending on the well-bonded hetero zones is sustained in the second stage (strain >2.5%). Such a sequential heterostructure effect is analyzed to obtain an appropriate width range of soft zones for bimodal grained AMCs, improving the conventional empirical heterostructure design principle. This work advances the understandings on heterostructured AMCs that when employing intermediate-sized soft zones, the hard UFG zones play a key role in obtaining good ductility, instead of only providing high strength.