Christoph Pauly

The self-propagating reaction in binary Al/Ru multilayers with two different bilayer thicknesses (89 and 178 nm, respectively) forming single-phase AlRu intermetallic compound is investigated experimentally and by means of phase-field simulations. Experimentally, the time-temperature evolution in free-standing films was recorded with a high-speed pyrometer, and the resulting microstructure was determined from electron backscatter diffraction measurements. The phase-field model is constructed based on the minimization of the grand potentials for which the required thermodynamic data are taken from the Calphad database. The simulation of the reaction and subsequent AlRu grain growth starts from Al-rich and Ru-rich layer fillings. After the formation of the AlRu phase is complete, the grain growth during cooling is simulated based on the experimentally recorded time-temperature curves. Finally, the resulting grain sizes obtained from the simulation are found to be in good agreement with the experimental results. Furthermore, it is shown that the final grain sizes in both simulations and experiments depend on the initial bilayer thicknesses.

Reactive multilayer foils (RMFs) for joining processes have attracted a great deal of attention over the last few years. They are capable of exothermic self-propagating reactions and can serve as localized heat sources for joining applications when ignited by suitable means. Using short and ultrashort pulsed lasers with carefully selected parameters, cutting and shaping of RMFs makes it possible to tailor heat release characteristics without triggering the reaction. The present study is an investigation of microstructural changes induced by femtosecond laser machining of a commercially available Ni/Al-based RMF. The effects of the specific laser parameters pulse duration and repetition rate on the heat-affected zone (HAZ) are investigated by scanning and transmission electron microscopy. Debris consisting of oxide deposits can be found at a distance of several tens of microns from the cut edge. A negligible HAZ extending to less than 100 nm was observed for all parameters tested and no signs of ignition of a self-propagating reaction were observed. These results underline the suitability of femtosecond lasers for metal machining with minimal heat input.