# Corrosion Susceptibility and Microhardness of Al-Ni Alloys with Different Grain Structures
## 3 Results and Discussion
### Microstructure and Microhardness  
Figure 6 shows a portion of the phase diagram of the Al-Ni system (up to approximately 60 wt. %Ni) that is of interest depending on the composition of the alloys studied [20]. It is possible to identify a eutectic transformation that occurs at 629.9 \({}^{\circ}\)C for a composition of Al-5.7 wt. % Ni. Consequently, the microstructures of the Al-Ni alloys in the range of compositions studied are made up of three phases: the \(\alpha\)-phase, rich in aluminum  
Figure 4: (**a**) Position of CET (mm) versus cooling rate (Ve). (**b**) Width of columnar grains (mm) versus cooling rate (\({}^{\circ}\)C/min). Figure 5: Average diameter of the equiaxed grains, \(\rm D_{Ge}\), depending on the position, from the base of the sample. (with a nickel content of up to 0.24% [20]), the intermetallic compound Al\({}_{3}\)Ni, and the eutectic structure consisting of thin Al\({}_{3}\)Ni needles surrounded by the alpha phase [32]. The compositions of these phases were determined by EDS and can be seen in Figure 7 for (a) Al-3 wt. %Ni hypoeutectic alloy and (b) for Al-8 wt. %Ni hypereutectic alloy. Figure 6: Section of the Al-Ni phase diagram showing the alloys studied in the present investigation [20]. Figure 7: _Cont._  
The distribution of the phases in the microstructure depends on the nickel content of the alloy. In the hypoeutectic alloys, Al-1 wt. %Ni and Al-3 wt. %Ni, the microstructure is characterized by an \(\alpha\)-phase (dendritic matrix) and the eutectic structure constituting the interdendritic region (Figure 8a,b). The Al-5.7 wt. %Ni alloy presents the typical eutectic microstructure, as can be seen in Figure 8c. The microstructure of the hypereutectic alloy, Al-8 wt. %Ni is shown in Figure 8d. It presents thick particles of Al\({}_{3}\)Ni intermetallic surrounded by the eutectic structure. In Figure 9, the microhardness values measured for each of the phases of the Al-Ni alloys are presented. It was found that the eutectic region has greater microhardness than the \(\alpha\)-phase, logically associated with the higher Ni-content. It is also observed that the microhardness values increase in each region as the Ni content increases until the eutectic composition is reached. In order to obtain a relationship between the solidification parameters like cooling rate, V\({}_{e}\), and the microhardness (HV) for the directionally solidified Al-Ni alloys, a comparison of the HV obtained in the different grain zones of the samples was carried out. In Figure 10, it is observed that the HV values are smaller in the areas furthest from the base. This is attributed to the fact that in further areas, the heat extraction rate is lower and the grains are less refined; similar values were found by other authors [29]. The highest values of cooling rate, V\({}_{e}\), generate refinement in the secondary dendritic spacing, \(\lambda_{2}\), and lamellar spacing, S\({}_{2}\), obtaining greater microstructural refinement in the zones of the samples close to the base, as seen in Figure 11a-d.  
Figure 7: Chemical composition analysis of Al-Ni alloys using SEM-EDX: (**a**) Al-3 wt. %Ni. (**b**) Al-8 wt. %Ni. Figure 8: Micrographs of the Al-Ni alloys: (**a**) Al-1 wt. % Ni, (**b**) Al-3 wt. % Ni, (**c**) Al-5.7 wt. % Ni, (**d**) Al-8 wt. % Ni. Figure 10: Variation of microhardness, HV, depending on the different positions with respect to the base of the sample. Figure 9: Microhardness, HV, values of the different phases of Al-Ni alloys (Al-3 wt. %Ni and Al-8 wt. %Ni). Figure 9 presents a comparison of the HV values obtained in different positions of the sample for the eutectic phase. In this figure, an increase in HV values was observed in the zone close to the base of the sample where there are columnar grains. These values decrease linearly as we move away from the zone close to the heat extraction device (base of the furnace).