Decoupling microstructural refinement and heat release in reactive Ni–Al powders processed by low-energy ball milling

Despite extensive research on mechanically activated reactive Ni–Al powders, the relationship between microstructural refinement and thermal output remains incompletely understood, particularly under systematically controlled milling conditions. This study investigates the mechanical activation of equiatomic Ni–Al powders using low-energy ball milling (LEBM), focusing on the effects of milling duration (4–13 h) and initial Ni particle size (∼ 3 μm and ∼ 50 μm) on particle characteristics, microstructural evolution, phase formation, and thermal behavior. Progressive milling led to particle size reduction, enhanced interfacial mixing, and the development of increasingly refined lamellar microstructures, with smaller starting Ni particles significantly accelerating these processes. For example, after 4 h of milling, powders prepared using ∼3 μm Ni particles exhibited an interface density approximately three times higher than that of powders prepared using ∼50 μm Ni particles (4.53 vs. 1.49 μm/μm²). X-ray diffraction (XRD) analysis revealed progressive peak broadening alongside the gradual emergence of NiAl intermetallic phases with increasing milling duration. Spark-ignition sensitivity increased with microstructural refinement, indicating enhanced reactivity and reduced ignition barriers. However, the total exothermic heat release did not exhibit a monotonic dependence on refinement. Powders prepared from smaller Ni particles showed a progressive decline in heat output with prolonged milling, while still maintaining higher overall heat release, reaching a maximum of 1130 J/g compared to 919 J/g for powders prepared from larger Ni particles, which showed an initial increase in heat release followed by a decrease. Notably, comparable exothermic heat release was observed for batches exhibiting approximately two-fold differences in interface density, demonstrating that microstructural refinement alone does not fully determine thermal output. These results demonstrate that although milling-induced refinement strongly influences ignition behavior, interfacial development alone cannot fully explain trends in heat release. The results highlight the importance of combining quantitative microstructural and particle descriptors with thermal characteristics to achieve a more comprehensive understanding of the reactivity of mechanically activated Ni–Al powders.