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

TL;DR

This study investigates how C15 and C36 Laves phases in Mg-Al-Ca alloys enhance strength by affecting deformation mechanisms, revealing that C36 offers a better balance of strengthening and co-deformation.

Contribution

It provides detailed insights into the co-deformation behavior of C15 and C36 Laves phases in Mg alloys, highlighting the superior strengthening effect of C36.

Findings

C15 and C36 phases delay basal slip and twinning, strengthening the matrix.

C36 phase allows some slip transfer, aiding co-deformation.

C36 strengthening is preferable for magnesium alloy applications.

Abstract

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.