**Ni/Nisolidsolution/AlNi3/AlNi/Al3Ni2/AlNi/AlNi3/Nisolidsolution/Ni.**

The interfaces of the phases growing due to the reaction in the solid state show a linear character, as it was the case in the samples being annealed for 15 min shown in Figure 4a.

Figure 5 shows the morphology of the growing phases after 1, 3 and 5 h of reaction at 720 ◦C. SEM observations revealed the existence of several intermetallic phases. The measurements of the chemical composition within the joined area indicated that they consisted of:

After 1 h:

```
Ni/Nisolidsolution/AlNi3/AlNi/AlNiNi-deficient/Al3Ni/[(Al)+Al3Ni]eutectics/Al3Ni2/AlNiNi-deficient/AlNi/AlNi3/Nisolidsolution/Ni
```
After 3 h:

**Ni/Nisolidsolution/AlNi3/AlNiNi-rich/AlNi/AlNiNi-deficient[(Al)+Al3Ni]eutectics/AlNiNi-deficient/AlNi/AlNiNi-rich/AlNi3/Nisolidsolution/Ni**

**Figure 5.** Microstructure of the Ni/Al/Ni couples obtained at 720 ◦C after annealing for: 1, 3, 5 h using A- and B- type of Ni substrates. Numbers 1–6 denote particular intermetallic phases: 1-AlNi3, 2-AlNiNi-rich, 3-AlNi, 4-AlNiNi-deficient, 5-(Al) + Al3Ni, 6-Ni solid solution, 7-Al3Ni2.

Obtained results are the same for both types of applied orientations of nickel substrates. However, after 5 h of reaction time one significant difference was noticed. Namely, the sequence of the intermetallic phases for types A and B was not the same (Figure 6). For Ni/Al/Ni interconnection, where the substrates of type B were used, the phase sequence was the same as in the case of 3 h of annealing, whereas, for the joints obtained from substrates of A-type, the AlNi phase deficient in nickel (AlNiNi-deficient) did not appear. Figure 5 presents the phase composition of the interconnection zones for both types of substrates, where particular phases (as previously) are noted by numbers. As can be seen in the equilibrium phase diagram presented in Figure 2, the AlNi phase possesses a wide range of chemical composition. Therefore, there are several types of AlNi intermetallics, namely: stoichiometric, where the ratio of Ni to Al equals (50:50 at. %), AlNi deficient in nickel (45–50 at. % of Ni) and AlNi rich in nickel (50–60 at. % of Ni). Last one was evidenced in the literature data by Lopez et al. [13].

**Figure 6.** SEM micrographs of the cross-sectional view of a Ni/Al/Ni interconnections after 5 h of reaction time at 720 ◦C and EDS line-scan across this area for substrates of A-type (**a**) and B-type (**b**).

A significant expansion of the annealing time to 20 and 72 h resulted in widening (Figure 7a) and disappearance (Figure 7b) of particular phases. After 20 h annealing at 720 ◦C the phases are broadened, however, the location of phases within the interconnection is the same as for samples annealed for 5 h using the substrates of B-type. The interconnection zone is symmetric. The AlNi deficient in nickel is slowly consumed, while the stoichiometric phase expands. 72 h is enough time to fully consume AlNi deficient in nickel and in the interconnection area only AlNi (51.6 at. % Ni), AlNi rich in nickel (60 at. % Ni) and Al3Ni (24.1 at. % Ni) are present. Phases of AlNi type are approximately twice wider in comparison to 20 h of annealing, AlNi3 phase changes its thickness of about 30% (broadening). The sequence of the phases after 72h is as follow:
