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 ℃ 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. 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 irrelevant to the lamellar thickness in the Al/Al₂Cu alloys but it 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. 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.