Ni aluminides have technologically attracted much attention as oxidation protective layers on Ni-superalloys for high-temperature and harsh environments applications as well as for reinforcements in metal-matrix composites. Among the Ni aluminides, the AlNi compound exhibits the best combination of oxidation-protective characteristics and hardness; thus there is a progressive demand to produce it particularly through convenient in-situ fabrication processes. Therefore, the evaluation of growth kinetics in Al_xNi_y layers is of crucial importance in determining an optimum compound formation process. To this purpose, Al-Ni intermetallic laminate composites were produced through cold roll bonding and subsequent annealing of aluminum and nickel sheets. The microstructure of the intermetallic layers was investigated in order to specify the controlling mechanisms and subsequently the growth model of the different phases. The Al₃Ni layer was kinetically the first to appear but started to decompose at the expense of the AlNi compound when the direct source of Al disappeared for the reactive diffusion couples. The Al₃Ni layer growth was initially controlled by bulk diffusion, but then at T≥525°C was modified as a function of competition between formation and consumption, whereas the AlNi growth was governed strongly by the interfacial reaction. The time dependence of the growth rate revealed different behaviors of linear and parabolic kinetics. The overall assessment revealed a bulk diffusion-controlled growth for all of the intermetallic layers. Arrhenius parameters could be derived for the Al₃Ni layer, while it was impossible for the AlNi phase because the formation of this layer was caused by a mixture of diffusion mechanism.

The evolution of grain structure during plastic deformation has a significant effect on texture variations and, in turn, the material properties. However, the metal physics leading to a stationary grain size regime in rolled Al combined with a harder phase remains poorly understood. Therefore, the grain and texture evolution in the Al phase and the possible grain coarsening mechanisms operating during accumulative roll bonding (ARB) were investigated in this work. Three ARB cycles were performed at room temperature with the aim of obtaining a bimetallic Al–Ni composite. The microstructure and texture evolutions in this composite were characterized via field emission gun scanning electron microscopy combined with electron backscatter diffraction. With increasing strain, the lamellar grain structure of Al developed into a semi-equiaxed grain structure. Correspondingly, the grain length and thickness decreased from 672 to 0.84 µm and 24.8 to 0.60 µm, respectively. Grain fragmentation was, however, most efficient in the initial stages of rolling, since continuous dynamic recrystallization prevented further grain refinement especially in the last cycle. Consequently, after a strain of 2.7, the refinement continued at decreasing rates, yielding a fragmentation ratio of one at a lower strain than that reported for single-phase Al composites. The mid-section layers of the Al phase were characterized by a mixture of shear and plane strain compression textures. After the third ARB cycle, the Al phase was characterized by a near random texture resulting from grain fragmentation. This fragmentation was induced by local plastic flow in the Al phase, owing to the presence of hard Ni fragments.

In the present work, the influence of strain on phase formation at the Al/Ni interface was investigated during cold roll bonding and annealing. A sandwich sample composed of an Al-Ni-Al stack was cold rolled with reductions in the range of 50% to 90%, followed by annealing at 450 °C for 60 min. The crystallography of the annealed sandwich samples was analyzed by XRD (X-ray diffraction), whereas the microstructure was studied by scanning electron microscopy, equipped with EDS (energy dispersive spectrometer) analysis, and optical microscope. In the annealed samples, the intermetallic phase Al₃Ni has formed at the Ni/Al interface, preferentially on the Al side of the interface. It is found that the applied strains did not have an effect on the type of intermetallic phase that was formed. However, the rolling reduction has a significant effect on the morphology of the intermetallic layer, as it was observed that after the lowest reduction of 50% only some scattered intermetallic nuclei were present, whereas at the highest rolling reduction of 90% a continuous intermetallic layer of 4.1 μm was exhibited. The formation of the intermetallic layer is discussed in terms of Al and Ni diffusion at the interface and irregular nature of the Al/Ni bonded interface after rolling reductions.