**4. Conclusions**

The diffusion soldering process was successfully applied to join nickel substrates using aluminum as the solder material at 720 ◦C for various periods of time. Nickel substrates, applied in the experiment, differed by size and shape of their grains in order to reveal any influence on the sequence of appearance or growth kinetics of particular intermetallic phases. It was demonstrated that at first the intermetallic phases rich in aluminum were formed (Al3Ni, Al3Ni2) and later replaced by the rich in nickel phases (AlNi, AlNi3). Three variants of AlNi phase were identified, all of them clearly visible in SEM-BSE: deficient in nickel (45 at. % of Ni), stoichiometric (50 at. % of Ni) and rich in nickel (60 at. % of Ni). While the Ni deficient AlNi phase formed separated layer of grains, electron backscattered diffraction map evidenced that the Ni-rich variant was located within the grains of already formed AlNi stoichiometric phase. Moreover, transmission electron microscopy examination revealed local presence of Al3Ni5 phase grains. Growth kinetics data showed that the fastest growing phase was the stoichiometric variant of AlNi phase, growing in a mixed mechanism: at first due to the chemical reaction and later by the volume diffusion. The most interesting results were obtained for two other phases: Ni-rich AlNi and AlNi3. As it was already showed in the literature, the general growth can be attributed to the volume diffusion, however, at early stages significant differences were noticed in this study. In the case of AlNi3 phase, the first stage of growth was due to the chemical reaction and later it slowed down (volume diffusion). For the Ni-rich AlNi phase in the later stage of growth the volume diffusion dominated. However, the first stage of phase growth varied in relation to the applied nickel substrate. In the interconnections, for which the surface of contact with aluminum was composed of the elongated grains, the grain boundary diffusion mechanism occurred, while for the Ni with small, equiaxed grains, the chemical reaction governed the growth of the phase. This work demonstrates that different mechanisms may contribute in the growth of the phases, especially in the beginning of the process and one should be aware of this. The substrate microstructure may have the influence on the intermetallics' growth kinetics and resulting from it differences in the integral diffusion coefficients.

**Author Contributions:** Conceptualization, J.W.-B. and I.K.; methodology, I.K., P.B., A.W.-M., L.L.-D., J.W.-B.; formal analysis, I.K., P.B., J.W.-B.; investigation, I.K., P.B., L.L.-D., J.W.-B.; writing—original draft preparation, I.K.; writing—review and editing, J.W.-B.; visualization, I.K.; supervision, J.W.-B.

**Funding:** The research was co-financed by the European Union from resources of the European Social Fund (Project No.WND-POWR.03.02.00-00-I043/16).

**Acknowledgments:** The research was co-financed by the European Union from resources of the European Social Fund (Project No.WND-POWR.03.02.00-00-I043/16).

**Conflicts of Interest:** The authors declare no conflict of interest.
