Microstructural factors governing the significant strengthening of Al/Al2Cu mille-feuille structured alloys accompanied by kink-band formation

Development of high-strength Al alloys that can be used at temperatures above 150 °C is strongly desired. To achieve this, Al/Al2Cu mille-feuille structured alloys, in which soft (Al) layer and hard (Al2Cu) layer are alternatively stacked in lamellar form, is recently focused, via inducing kink-band formation as a novel strategy for controlling the mechanical properties. However, the localized formation of large kink bands reduces the yield stress and degrades the ductility of the alloy. There was little information to derive a "general law" for quantitatively predicting an increase in yield stress in mille-feuille structured alloys from a microstructure perspective. In this study, we successfully distinguished the effects of lamellar thickness and colony size on the strength. The results demonstrated that the yield stress governed by kink-band formation is less related to the lamellar thickness in the Al/Al2Cu alloys but it highly depends on the colony size. The decrease in lamellar colony size obtained by increasing the growth rate during the directional solidification of the Al/Al2Cu eutectic alloy can induce the homogeneous formation of tiny kink bands in the alloy, leading to a drastic increment of the yield stress accompanied by ductility. In addition, the formation of tiny kink bands was demonstrated to contribute to tensile deformation. That is, the control of the morphology of kink band changes its role from a fracture mode to a deformation mode. Furthermore, such microstructure control can induce "kink-band strengthening" in the alloys. These experimental results demonstrate that controlling the colony size while maintaining the alignment of lamellae is the key factor for inducing superior mechanical properties of the mille-feuille structured alloy via tiny kink-band formation.