  Achieving a combination of high mechanical strength and high electrical
conductivity in low-weight Al alloys requires a full understanding of the
relationships between nanoscaled features and physical properties. Grain
boundary strengthening through grain size reduction offers some interesting
possibilities but is limited by thermal stability issues. Zener pinning by
stable nanoscaled particles or grain boundary segregation are well-known
strategies for stabilizing grain boundaries. In this study, the Al-Ca system
has been selected to investigate the way segregation affects the combination of
mechanical strength and electrical resistivity. For this purpose, an Al-Ca
composite material was severely deformed by high-pressure torsion to achieve a
nanoscaled structure with a mean grain size of only 25 nm. X-ray diffraction,
transmission electron microscopy and atom probe tomography data revealed that
the fcc Ca phase was dissolved for large levels of plastic deformation leading
mainly to Ca segregations along crystalline defects. The resulting
microhardness of about 300 HV is much higher than predictions based on Hall and
Petch Law and is attributed to limited grain boundary mediated plasticity due
to Ca segregation. The electrical resistivity is also much higher than that
expected for nanostructured Al. The main contribution comes from Ca
segregations that lead to a fraction of electrons reflected or trapped by grain
boundaries twice larger than in pure Al. The two-phase state was investigated
by in-situ and ex-situ microscopy after annealing at 200{\textdegree}C for 30
min, where precipitation of nanoscaled Al4Ca particles occurred and the mean
grain size reached 35 nm. Annealing also significantly decreased electrical
resistivity, but it remained much higher than that of nanostructured pure Al,
due to Al/Al4Ca interfaces that reflect or trap more than 85% of electrons.
