  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.
