**Ni/Al3Ni2/Al3Ni/[(Al)+Al3Ni]eutectics/Al3Ni/Al3Ni2/Ni.**

The interface between nickel and Al3Ni2 phase is smooth and approximately parallel to the surface of the applied substrates. This type of planar interfaces is characteristic for solid/solid reaction. Similar observation was noticed in other papers [14,26].

**Figure 4.** The SEM microstructures after (**a**) 15 min and (**b**) 30 min of reaction at 720 ◦C. The SEM microstructures in (**<sup>c</sup>**–**<sup>e</sup>**) present the magnified area of Al3Ni2/Ni interface. Numbers 1–6 denote various phases: 1-Ni, 2-Ni solid solution, 3-AlNi3, 4-AlNi, 5-Al3Ni2, 6-(Al)+Al3Ni.

The obtained interconnection zone after 15 min of annealing at 720 ◦C may be compared with joint described by Tumminello and Sommadossi [14], for the same time of annealing (15 min) but at higher temperature of 776 ◦C. The sequence of the intermetallics creation is in both cases compatible. In present work, the interconnection zones are much broader in comparison to the ones shown in [14], being approximately twice as wide. Table 2 contains the comparison of the results from both papers. In present work the whole joint consists of Al3Ni in 21% and Al3Ni2 in 59%, the rest is the eutectics (Al) + Al3Ni. In Tumminello's work [14] the amount of the phases were 13% and 47%, respectively. The observed differences between these two works could arise mostly from the various thickness of applied aluminum foil (much broader in [14]) and also possibly from the used atmosphere (here vacuum, while argon in [14]).

**Table 2.** Average thickness of the whole joint and particular intermetallic phases—both sides of the joint.


Elongation of the reaction time to 30 min, allowed observing significant difference of the interconnection zones in comparison to the sample after 15 min of annealing. Figure 4b shows the interconnection at this stage of annealing, where the middle of the joint is completely filled by Al3Ni2 phase, which was confirmed by EDS. The line scan through the area inside the pink frame, allowed

excluding the presence of the eutectics (Al3Ni–Al) and Al3Ni phase in this area. The next important microstructural feature exists at the interface between Al3Ni2 phase and Ni substrate and has occurred in early stages of solid/solid reaction. The EDS measurements indicate high variability of chemical composition in the areas marked with yellow frames. Average content of aluminum in Al3Ni2 equals 60.5 at. %, while in the close neighborhood of the Al3Ni2/Ni interface it is about 57.6 at % and decreases in the direction toward the nickel substrate. The next measurements of the chemical composition showed the presence of narrow zones of about 1.5 μm, where the content of aluminum equals 51.3 at. %, which can be attributed to AlNi phase, then 30.3 at. % of Al, standing for AlNi3 phase. The presence of Ni solid solution was also observed (0.8 at. % of Al). In the mentioned area the differences in BSE contrast can be observed, the last one being distinguishable at the Al3Ni2/AlNi interface. Changes of the chemical composition near the phases' boundaries indicate progressive diffusion processes leading to the initial stages of the AlNi growth after 30 min of annealing. Interconnection zone constituents can be this time summarized as follows:
