Abstract
While extensively studied for heating rates below 1.7 K/s and above 1000 K/s, the solid-state phase transformations in Al/Ni reactive multilayers have not been examined at intermediate heating rates between 100 K/s and 1000 K/s. Combined nanocalorimetry and time-resolved synchrotron x-ray diffraction studies are utilized to address this range of heating rates for multilayers with an overall composition of 10 at. % Ni and a bilayer thickness of 220 nm. It was found that a two-stage phase formation of Al3Ni proceeds up to a heating rate of 1000 K/s. The two growth stages occur in the solid-state and are kinetically separated. The activation energy of the first growth stage is determined to be 137 kJ/mol, which agrees well with the literature data at low heating rates. At 1000 K/s, a transition to a runaway reaction is observed. Unusual for metallic multilayers, the reaction proceeds completely in the solid-state which is also known as “solid flame.” Using nanocalorimetry, a critical input power density for ignition of 5.8 × 104 W/cm3 was determined. The rapid succession of the two Al3Ni formation stages was identified as the underlying mechanism for the self-sustaining reaction.
Highlights
Numerous studies on binary Al-based systems observed phase selection where one particular phase, mostly the most Al-rich one, forms first,3,16,17 suggesting nucleation barrier minimization as the ratelimiting process.15 The Al/Ni system is one of the most intensively studied systems.18–23 During slow heating with maximal 1.7 K/s, Al9Ni24,11,20,24 or Al3Ni21,25–27 appears first depending on fabrication, overall stoichiometry, and bilayer thickness (K)
While extensively studied for heating rates below 1.7 K/s and above 1000 K/s, the solid-state phase transformations in Al/Ni reactive multilayers have not been examined at intermediate heating rates between 100 K/s and 1000 K/s
Combined nanocalorimetry and time-resolved synchrotron x-ray diffraction studies are utilized to address this range of heating rates for multilayers with an overall composition of 10 at. % Ni and a bilayer thickness of 220 nm
Summary
Numerous studies on binary Al-based systems observed phase selection where one particular phase, mostly the most Al-rich one, forms first,3,16,17 suggesting nucleation barrier minimization as the ratelimiting process.15 The Al/Ni system is one of the most intensively studied systems.18–23 During slow heating with maximal 1.7 K/s, Al9Ni24,11,20,24 or Al3Ni21,25–27 appears first depending on fabrication, overall stoichiometry, and bilayer thickness (K). It was found that a two-stage phase formation of Al3Ni proceeds up to a heating rate of 1000 K/s. The solid-state phase selection observed during slow heating does not seem to be critical for the runaway reaction ignited with heating rates > 5000 K/s.
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