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 Thom ≈ 0.52 (∼210°C), Ni segregation to Al grain boundaries competes with Al3Ni formation. Hydroxylated ALD-AlOxHy interlayers become mobile, driving swelling and oxidation of adjacent Al. At Thom ≈ 0.77 (∼450°C), κ-Al2O3 crystallises, followed by interlayer rupture, while at Thom ≈ 0.94 (∼600°C) the nanolaminate architecture collapses into a coarsened Al(Ni) matrix with dispersed Al2O3 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.