Nanoscale brittle-to-ductile transition of the C15 CaAl$_2$ Laves phase

Abstract: The influence of temperature on the deformation behaviour of the C15 CaAl$_2$ Laves phase, a key constituent for enhancing the mechanical properties of Mg alloys up to service temperatures of 200 {\deg}C, remains largely unexplored. This study presents, for the first time, the nanoscale brittle-to-ductile transition (BDT) of this intermetallic phase through in situ testing including nanoindentation, scratch testing, and micropillar splitting conducted at elevated temperatures. By correlating observations from these techniques, changes in deformation of CaAl$_2$ were identified in relation to temperature. High-temperature nanoindentation quantitatively determined the temperature range for the BDT, and revealed that CaAl$_2$ undergoes a BDT at ~0.55T$_m$, exhibiting an intermediate region of microplasticity. A noticeable decrease in nanoindentation hardness was observed at ~450-500 {\deg}C, accompanied by an increase in residual indent size, while indentation cracking was not observed above 300 {\deg}C. Results from high-temperature micropillar splitting revealed cracking and brittle pillar splitting up to 300 {\deg}C, with an increase in apparent fracture toughness from 0.9 $\pm$ 0.1 MPa$\cdot\sqrt m$ to 2.8 $\pm$ 0.3 MPa$\cdot\sqrt m$, and subsequent crack-free plastic deformation from 400 {\deg}C. Transmission electron microscopy analysis of the deformed material from nanoindentation revealed that the BDT of CaAl$_2$ may be attributed to enhanced dislocation plasticity with increasing temperature.