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Analysis of the Reaction Runaway in Al/Ni Multilayers with Combined Nanocalorimetry and Time-Resolved X-Ray Diffraction

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Abstract: Self-sustaining runaway reactions in reactive multilayers exhibit heat-up with over 1,000,000 K s to temperatures of higher than 1000 °C, which defines unprecedented kinetic regimes for metallurgical phase transformations. The latter allows for developing alternative concepts for microstructure design. In order to explore the phase transformations in these kinetic regimes, we combined nanocalorimetry with time resolved synchrotron X-ray diffraction. While generally suitable for reactive metallic multilayers, we study the phase transformations leading to the runaway reaction in Al Ni multilayers. The overall composition was 25 at. Ni. Nanocalorimetry allows us to perform thermal analysis of ignition as well as the reaction runaway and develop necessary and mandatory quantitative criterions for ignition. In order to trace the temporal phase evolution and to infer trends for phase growth with sub-millisecond time resolution during the nanocalorimeter experiment, we use time-resolved synchrotron X-ray diffraction. In detail, we heated the Al Ni multilayers with 5000 K s and found that Ni starts to diffuse into the Al layer at 271 °C. We evaluated the accessible phase transformations prior to ignition via a pulsed thermal heat treatment with 5,000 K s. Ignition occurs, dependent on the bilayer thickness, at about 400 °C in the solid state. During the runaway, samples heat up in three stages with maximal 1,000,000 K s to 1100 °C. The diffraction results suggest that atomic intermixing is the dominating mechanism up to ignition. Ni2Al3 is the first phase to form once the melting temperature of Al (Tm = 660 °C) has been exceeded. The majority of the intermetallic phase grows after the runaway reaction in a fourth stage and reaches its maximum during cooling. The trend of the temporal phase evolution eventually enables us to propose a mechanism which exhibits conceptual similarities with the exothermic dissolution mechanism recently suggested for self-sustaining reaction fronts in Al Ni multilayers.

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