Analysis of the reaction runaway in Al/Ni multilayers with combined nanocalorimetry and time-resolved X-ray diffraction

Self-sustaining runaway reactions in reactive multilayers exhibit heat-up with over 106 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 combine nanocalorimetry with time-resolved synchrotron X-ray diffraction. Nanocalorimetry allows us to perform thermal analysis of ignition as well as the reaction runaway and to develop necessary and mandatory quantitative criterions for ignition. In order to trace the temporal phase evolution, we use time-resolved synchrotron X-ray diffraction. We heat the Al/Ni multilayers with 5000 K/s and find that Ni starts to diffuse into the Al layer at 271 °C. Ignition occurs, dependent on the bilayer thickness, at about 400 °C in the solid state and atomic diffusion is revealed as the dominating mechanism. During the runaway, samples heat up in four stages with maximal 106 K/s to 1100 °C. Ni2Al3 is the first phase to form which starts to nucleate once Al melts. The majority of the intermetallic phase grows after the runaway reaction in the fourth stage and reaches its maximum during cooling. This trend of the temporal phase evolution eventually enables us to propose a mechanism exhibiting conceptual similarities with the exothermic dissolution mechanism recently suggested for self-sustaining reaction fronts in Al/Ni multilayers.