Not yet reviewed by Pith; the record is open.

The microstructure of Mg-Al-Ca alloys consists of a hard intra- and intergranular eutectic Laves phase network embedded in a soft $\alpha$-Mg matrix. For such heterogeneous microstructures, the mechanical response and co-deformation of both phases under external load are not yet fully understood. We therefore used nano- and microindentation in combination with electron microscopy to study the deformation behaviour of an Mg-3.68Al-3.8Ca alloy. We found that the hardness of the Mg$_2$Ca phase was significantly larger than the $\alpha$-Mg phase and stays constant within the measured temperature range. The strain rate sensitivity of the softer $\alpha$-Mg phase and of the interfaces increased while activation volume decreased with temperature. The creep deformation of the Mg$_2$Ca Laves phase was significantly lower than the $\alpha$-Mg phase at 170 $^{\circ}$C. Moreover, the deformation zone around and below microindents depends on the matrix orientation and is influenced by the presence of Laves phases. Most importantly, slip transfer from the $\alpha$-Mg phase to the (Mg,Al)$_2$Ca Laves phase occurred, carried by the basal planes. Based on the observed orientation relationship and active slip systems, a slip transfer mechanism from the soft $\alpha$-Mg phase to the hard Laves phase is proposed. Further, we present implications for future alloy design strategies.