Strengthening of Mg-Al-Ca alloys with C15 and C36 Laves phases

The Laves phase skeleton in cast Mg-Al-Ca alloys is known to provide considerable strengthening. Laves phases such as CaMg$_2$ (C14), Ca(Al,Mg)$_2$ (C36), and CaAl2 (C15) have high melting points, high hardness at room and elevated temperatures, but unfortunately are inherently brittle. Mg-Al-Ca alloys thus have good creep properties but limited ductility. An understanding of the co-deformation behaviour of $\alpha$-Mg and Laves phases is essential for optimising the strength-ductility balance of these alloys. Here, we study the mechanical behaviour of a Mg-4.65Al-2.82Ca alloy using micropillar compression in the $\alpha$-Mg matrix, at $\alpha$-Mg/C36 and $\alpha$-Mg/C15 interfaces and in the C15 phase in combination with scanning electron microscopy (SE imaging), electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD), and low-kV scanning transmission electron microscopy (STEM). We show that both, C15 and C36, Laves phases provide considerable strengthening to the $\alpha$-Mg matrix by delaying the onset of basal slip and extension twinning, while only the C36 phase appears to allow a certain extent of slip transfer/ plastic co-deformation, in spite of its greater anisotropy compared with the cubic C15 phase. We therefore conclude based on these results that strengthening of the $\alpha$-Mg matrix by the C36 Laves phase is preferable given that it combines easy skeleton formation with some co-deformation and considerable stability at common application temperatures of magnesium alloys.