Fracture of the $C$15 CaAl$_2$ Laves phase at small length-scales

Abstract

The cubic $C$15 CaAl$_2$ Laves phase is a crucial brittle intermetallic precipitate in Mg-Al-Ca alloys. Although knowledge of the mechanical properties of coexisting phases is essential for improved alloy design, the fracture toughness is not yet studied experimentally due to the need for miniaturised testing. Here, micropillar splitting and microcantilever bending are used to experimentally determine the toughness of CaAl$_2$. It is found that the toughness value of ~1 $MPa\cdot\sqrt m$ from pillar splitting is largely insensitive to sample heat treatment, ion beam used for fabrication, micropillar diameter, and surface orientation. From nanoindentation supported by electron channelling contrast imaging and backscatter diffraction, fracture is observed to take place mostly on {011} planes. Atomistic fracture simulations on a model $C$15 Laves phase showed that the preference of {011} cleavage planes over the more energetically favourable {111} is due to lattice trapping and kinetics controlling fracture. Using rectangular microcantilever bending tests where the notch plane was misoriented to the closest possible {112} cleavage plane by ~8{\deg}, and the closest {001}, {011} and {111} plane by >20{\deg}, a toughness of ca. 2 $MPa\cdot\sqrt m$ was determined along with the electron microscopy observation of significant deviations of the crack path, demonstrating that preferential crystallographic cleavage planes determine the toughness in this material. Further investigation using pentagonal microcantilevers with precise alignment of the notch with the cleavage planes revealed similar fracture toughness values for different low-index planes. The results presented here are the first detailed experimental study of fracture toughness of the $C$15 CaAl$_2$ Laves phase, and can be understood in terms of crack plane and crack front dependent fracture toughness.