## 4 Discussion
### Effect of the Curvature of Al/Ni Multilayers on the Phase Transformation  
In previous studies by other research groups, the influence of substrate topography on the phase transformation of reactive multilayers was demonstrated [15]. This occurs because the surface topography of the substrate affects the surface topography, the compactness, and the interphase roughness of as-deposited multilayers. These properties have a great influence in the phase transformation. For this reason, it is necessary to evaluate all the changes generated by the complex topography of the silicon needles. XRD and SAED patterns of the 3D-RMS confirmed the presence of two phases, Al\({}_{0.42}\)Ni\({}_{0.58}\) and AlNi\({}_{3}\) after RTA, while standard RMS after heat treatment became a single phase Al\({}_{0.42}\)Ni\({}_{0.58}\), indicating the uniform transformation of the continuous multilayer. The comparison of DSC measurements for both systems in Figure 8 discloses a sequence of exothermic events that can either correspond to the formation of a new phase or the growth of an existing phase [36]. The number of exothermic events during the phase transformation is the same for both systems. However, DSC curves reveal that for the Ni/Al 3D-RMS a higher amount of enthalpy is released during initial interdiffusion, \(\Delta\)H\({}_{\text{ID}}\), than for the planar RMS. This behavior can be explained considering the curvature of the 3D-RMS, which not only increases the interfacial density per unit mass [15], but also causes the increase of the interfacial roughness [35]. These characteristics of multilayers have been shown to favor the atomic interdiffusion of Ni into the Al region [15; 30]. A high diffusion flow will promote the formation of an oversaturated solid solution of nickel in aluminum at the Al/Ni interface, which is then transformed into the nonequilibrium Al\({}_{0}\)Ni\({}_{2}\) or directly in Al\({}_{3}\)Ni phase interlayer. The metastable Al\({}_{9}\)Ni\({}_{2}\) phase can be transformed into Al\({}_{3}\)Ni or Al\({}_{3}\)Ni\({}_{2}\), which later, with the addition of Ni, will be transformed into B2-AlNi [30]. The crystallization of AlNi in the interface area will act as a physical diffusion barrier blocking the flow of nickel diffusion, even at higher temperatures than the melting temperature of Al [16; 37]. Thus, it can be speculated that the excess of nickel will be trapped between the Al\({}_{0.42}\)Ni\({}_{0.58}\) interlayers instead of diffusing continuously. At the same time, a small amount of aluminum can also diffuse across the interface into the Ni region and distribute heterogeneously [16], with increasing temperature and sufficient time, the high solubility of Al in Ni could promote the formation of AlNi\({}_{3}\). The formation of these two stable phases agrees with the DSC results, which show that at 700 K the reaction was complete and no remaining exothermic events were detected. However, it is difficult to predict the solid-state reaction sequence to form the final Al\({}_{0.42}\)Ni\({}_{0.58}\) and AlNi\({}_{3}\) phases. Therefore, to reveal the sequence of reactions during the transformation of phases, it will be necessary to perform in situ XRD or quenching the phase transformation at selected intermediate temperatures. The presence of pores in the structure, boundaries of the 3D columns, and gaps formed between the pores all act as a diffusion barrier preventing chemical and thermal diffusion during RTA. These defects generated by the shadowing hinder a uniform and complete diffusion throughout the sample, hampering the reaction of the 3D columns with each other. However, the heat provided by the radiation source placed over the sample during the RTA is uniform through the sample. It was also demonstrated with the flat Al/Ni RMS that 30 s and 500 \({}^{\circ}\)C give the Al/Ni system sufficient time and energy to react completely, which was also observed in previous investigations, even with a 300 \({}^{\circ}\)C temperature [18].