## 1 Introduction  
Reactive multilayer systems (RMS), typically forming binary or ternary intermetallic phases, have been thoroughly investigated due to their energetic properties and potential applications [1]. RMS consist of alternating layers of two or more components which react exothermically when a localized pulse of energy is applied. The released heat of the reaction promotes the reaction in the neighboring zones, establishing a self-sustained and self-propagating reaction [1, 2]. Due to the high amount of stored chemical energy and the large energy release rate of RMS, they can be used in technological applications such as welding, brazing, or in thermal batteries [3, 4, 5, 6]. More recently, Al/Ni RMS were used as an ultrafast heat source to produce high entropy alloy films [7]. Therefore, much effort has been made in order to study the influence of different RMS characteristics, such as bilayer thickness, intermixing thickness, and chemical composition, on its energetic properties [4; 8; 9; 10; 11]. For the equiatomic Al/Ni RMS, the phase transformation process has already been widely investigated in literature [12]. During a self-propagating reaction, the formation of AlNi intermetallic phase occurs directly from a semi liquid-solid state [13]. However, during annealing with low heating rates the formation of Al\({}_{9}\)Ni\({}_{2}\) or Al\({}_{3}\)Ni at the early stage is observed, and later a sequence of phases with the formation of AlNi as the final product [12; 14]. Recently, more attention has been given to the substrate topography, since it will impact the interface roughness of the RMS and on the phase transformation [15]. A rough interface with a high density of defects favors the atomic diffusion of Ni in Al [15; 16]. These studies were carried out using multilayer foils. However, it is also possible to produce reactive multilayer particles by using structured substrates such as nylon fibers or black Si [17; 18]. Particles produced on nylon fibers were separated from the substrate and funneled into glass tubes in order to measure the propagation front velocity. It was 200 times slower than a RMS-foil with the same characteristics and the slower propagation was attributed to a reduction of the heat transfer due to the thermal resistances at the particle-particle interfaces [17; 19]. Nanostructured Si has interesting properties such as surface enlargement that can be exploited in chip assembly [20]. For this reason, it was used as a substrate during the deposition of Al/Ni RMS [18]. The needle-shaped topography of Si disturbs the morphology of the deposited multilayers, giving rise to a complex structure of 3D reactive multilayers (3D-RMS). Furthermore, the black Si surface enhanced the adhesion of the RMS on the substrate in comparison with a flat surface, even after heat treatment [18]. During rapid thermal annealing (RTA) at a maximum temperature of 550 \({}^{\circ}\)C, 3D-RMS on Si needles reacted only partially forming B2-AlNi leaving unreacted Ni residue, unlike RMS foils on flat Si which fully transform the precursor materials into the B2-AlNi phase. The complex morphology of microscale structures with nanoscale reactive layers and its influence in the phase transformation and energetic properties were not studied thus far in literature to our knowledge; hence, this paper is focused on the study of the novel morphology of multilayers caused by the silicon needles and its influence on the phase transformation process. By using scanning and transmission electron microscopy (SEM/TEM), geometric parameters of the microstructures were calculated. Local morphology before and after RTA is investigated by X-ray diffraction analysis (XRD) and selected area electron diffraction (SAED). All results are compared with results from differential scanning calorimetry (DSC) experiments.