Strengthening of Eutectic Alloys with Mille-Feuille Structure Accompanied by Kink-Band Formation

Application of the lightweight metal materials to the automobile components has been gaining attentions to mitigate the global warming problems by reduction of CO₂ emissions. Recently, Mg alloys with Long Period Stacking Ordered (LPSO) phase have been gaining attentions because they achieved high strength and high ductility simultaneously by the introduction of characteristic deformation bands; i.e., kink bands. To introduce the kink bands during deformation, the macroscopic shear direction must be restricted along the direction parallel to the loading orientation. Since the LPSO phase consists of periodic stacking of soft Mg layers and hard Y/Zn segregated layers, i.e., mille-feuille structure, it has been considered that this structure brings the strong plastic anisotropy and restricts the shear direction. Thus, by controlling the loading orientation, the kink-bands are expected to be obtained. This microstructure control using mille-feuille structure can be one of the effective ways to achieve high strength and high ductility materials.<br/>In the present study, we have focused on Al-Al₂Cu and Ti-TiFe eutectic alloys to obtain the “micro-scale” mille-feuille structured materials. For the Al-based alloy, to achieve highly-aligned lamellar structure, the directional solidification with growth rates in a range of 5 to 300 mm/h was conducted. On the other hand, for the Ti-based alloy, we made use of the heat flow in the arc melting process to align the lamellar structure. We basically obtained a lamellar microstructure aligned parallel to the growth and heat-flow directions in both alloys. However, in the directionally-solidified Al alloys, as the growth rate increased, the alignment of the lamellae was gradually disturbed and the lamellae tended to incline against the growth direction. Moreover, the thickness of the lamellae and size of the colony, which is the group having the same lamellar orientation, decreased as the growth rate increased.<br/>In the compression tests of the Al-based alloys, for the alloys with large colony diameters, large kink bands formed, which induces buckling of the specimen. On the other hand, it was found that a decrease in the colony diameter induces the homogeneous formation of tiny kink bands, leading to the drastic increases in the yield stress while maintaining ductility as observed in the Mg-based LPSO phase alloys. Moreover, it has been systematically demonstrated that the yield stress proportionally increases as the inverse of the square root of the colony diameter increases. This can be considered because the colony diameter affects the formation stress of the kink bands. From the obtained results, it has been demonstrated that control of the colony size and the alignment of lamellae are the key factors for achieving superior mechanical properties of the mille-feuille structured alloys with homogeneous formation of tiny kink bands.<br/>In the compression tests of the Ti-based alloys, they showed extensively high strength over 2 GPa at room temperature, and kept the high strength at 400^oC. However, the strength decreased drastically at and above 600^oC. In the specimen deformed at 800^oC, the specimen deformed with formation of tiny kink bands. By the crystallographic analysis, rotation axises for the kink-bands formation were different in β-Ti and TiFe phases. This implies that the mechanism of the kink-bands formation in the present eutectic Ti-TiFe alloy was different from the conventional model for the kink-bands formation. In the presentation, the kink-bands formation behavior of the present Al and Ti alloys are presented in detail.