Hierarchical Interdiffusion Kinetics in Nanoscale Ni/Al Multilayers

Abstract

Nanoscale mass transport governs the onset of intermetallic formation in reactive metallic multilayers, yet the underlying mechanisms remain poorly understood. Here, we combine fast differential scanning calorimetry (FDSC) of free-standing Ni/Al multilayers (20 nm bilayer thickness) with correlative STEM to resolve the early interdiffusion regime. Varying the heating rate over five orders of magnitude (0.1 to 10,000 K/s) enables isoconversional Kissinger-Akahira-Sunose (KAS) analysis, linking heat-flow signatures to microstructural evolution. The as-deposited multilayers are nanocrystalline and exhibit pronounced premixing, with significant Ni enrichment throughout the Al layers. Upon annealing, mass transport proceeds hierarchically: at low temperatures, Ni diffusion is confined to Al grain boundaries (~81 kJ/mol), while at higher temperatures lattice diffusion from grain boundaries into the grain interiors becomes active (~168 kJ/mol), leading to increased mass transport and heat release. These findings identify grain boundaries as the dominant transport pathways controlling reaction onset and as key microstructural design parameters in reactive multilayers. By providing access to transient kinetic regimes and intermediate states, the combined FDSC-microscopy approach opens new opportunities for studying defect-mediated transport and non-equilibrium phase transformations.