  The effects of thermal treatments on metallic glass nanolaminates (NLs), with
a composition of Zr$_{24}$Cu$_{76}$ and Zr$_{61}$Cu$_{39}$ and a bilayer period
of 50 nm, were explored to control their mechanical properties through
annealing-induced atomic structure modifications, structural relaxation, and
partial crystallisation. Annealing treatments up to 330 {\deg}C ($T$ < $T_g$,
the glass transition temperature) maintain the amorphous structure of the NLs,
while inducing atomic structural relaxation, densification, and free volume
annihilation, reducing the formation of corrugations on fracture surfaces. Atom
probe tomography measurements reveal that annealing at 330 {\deg}C for 60 mins
also causes intermixing between layers, altering their compositions to
Zr$_{44}$Cu$_{56}$ and Zr$_{55}$Cu$_{45}$ with increased mixing enthalpy.
Moreover, the NLs exhibit superior thermal stability against crystallisation
compared to their monolithic counterparts, remaining amorphous up to 420
{\deg}C, while the monolithic Zr$_{24}$Cu$_{76}$ and Zr$_{61}$Cu$_{39}$ films
are crystalline at 390 {\deg}C, as a result of chemical interdiffusion and the
heterogeneous NL structure of delaying the onset of crystallisation. Annealing
treatments $T$ > $T_g$ ($\sim$420 {\deg}C) induce partial crystallisation,
forming Cu-Zr-based intermetallic and Zr-oxide phases, whereas annealing $T$ <
$T_g$ (330 {\deg}C) retains a visible layer structure. Nanoindentation analyses
show a progressive increase in elastic modulus and hardness for higher
annealing temperatures as a result of structural relaxation and likely
nanocrystal formation, with a maximum hardness equal to 7.6 $\pm$ 0.2 GPa
obtained after heat treatment at 420 {\deg}C for 60 mins and exceeding the
rule-of-mixtures. These results highlight the potential of thermal treatments
to tailor the structural, mechanical and thermal properties of metallic glass
NLs.
