Thermal Instability Pathways of Aluminum‐Based Nanocomposites: The Role of Alloying‐Element‐Driven Precipitation and Amorphous Interlayers

Hybrid physical vapour deposition (PVD) combined with atomic layer deposition (ALD) enables the fabrication of metal/ceramic AlNi/AlO(H) nanolaminates with high room‐temperature strength, controlled architecture, and promising functional performance, yet their thermal stability remains unresolved. Here, we establish design principles for thermally robust Al‐based nanocomposites by systematically assessing their thermal instability through correlated synchrotron X‐ray diffraction, high‐resolution microscopy, atom probe tomography, and nanoindentation following vacuum annealing between 160°C and 600°C. Angle‐resolved X‐ray diffraction (XRD) reveals Al crystallite evolution while high‐resolution microscopy unravels further instability mechanisms: At homologous temperature T hom ≈ 0.52 (∼210°C), Ni segregation to Al grain boundaries competes with Al 3 Ni formation. Hydroxylated ALD‐AlO x H y interlayers become mobile, driving swelling and oxidation of adjacent Al. At T hom ≈ 0.77 (∼450°C), κ ‐Al 2 O 3 crystallises, followed by interlayer rupture, while at T hom ≈ 0.94 (∼600°C) the nanolaminate architecture collapses into a coarsened Al(Ni) matrix with dispersed Al 2 O 3 inclusions. These coupled transformations progressively reduce hardness, transitioning from confined layer slip to matrix–inclusion‐dominated deformation. Beyond elucidating degradation mechanisms, this work shows how hydrogen‐mediated interface dynamics and the dual role of alloyed‐Ni limit the thermal and mechanical stability, precisely identifying the onset temperatures, and providing a pathway to robust Al‐based PVD/ALD nanocomposites.