# Corrosion Susceptibility and Microhardness of Al-Ni Alloys with Different Grain Structures
## 3 Results and Discussion
### Cooling Rates and Macrostructure  
The cooling rates, Ve, in the liquid were determined by taking the average values of the slopes of the temperature versus time curves in the zone before the plateau for each thermocouple position. On the other hand, the solidification rates were obtained as the average value of the temperatures within the plateau in the cooling curves. These thermal parameters are presented in Table 1.  
The temperatures decrease faster in the regions closest to the bottom of the water-cooled furnace. This caused the cooling rates to gradually decrease towards the completion of local solidification located in the upper zone of the sample; see Figure 3.  
The analysis of macrostructures obtained allowed us to determine the width of columnar grains, the height in the sample at which the CET zone occurred (positions of the CET\({}_{\text{Min}}\) and CET\({}_{\text{Max}}\)) and the average diameter of equiaxed grains, D\({}_{\text{Ge}}\) (Table 1). The CET transition zone is characterized due to the columnar grains coexisting with the equiaxed grains [23; 24; 25; 26; 27].  
Figure 2: (**a**) Thermal gradients indicating the position of the CET zone. (**b**) Macrostructure and (**c**) microstructures corresponding to columnar and equiaxed zones of the samples. Al-3 wt. %Ni.  
Figure 4a,b represent the CET position in the sample and the columnar grain width, respectively, with respect to the cooling rate. In this work, a proportional relationship of the cooling rate with respect to the position in which the CET occurs was observed. At a higher cooling rate, the CET moves away from the base of the sample (considering the base the only point of heat extraction). It should be noted that this quasi-linear relationship is not fulfilled when the solute content comes into play so that for a higher V\({}_{\rm e}\), the CET changes its behavior; that is to say, the value of the distance with respect to the base decreases. This behavior could be attributed to the fact that there are more solute particles; these could generate greater solidification sites and favor the nucleation of the equiaxed grains despite the fact that the V\({}_{\rm e}\) is higher. This behavior was observed for the Al-8 wt. %Ni alloy.  
On the other hand, it is also evident that, at higher V\({}_{\rm e}\) values, a columnar grain refinement effect is generated. That is, higher cooling rates produce more refined columnar grains. A non-linear adjustment was made for this relationship because it was observed that at very low velocities, very wide columns tend to form.  
Figure 4: \(Cont.\)  
Figure 3: Cooling rates, V\({}_{\rm e}\), versus position for Al-Ni alloys.  
According to the data presented in Table 1, it is possible to observe the effect of solidification velocity with respect to the \(\rm D_{Ge}\). It is observed that, at a lower solidification velocity, the equiaxed grains presented a higher \(\rm D_{Ge}\) value. This is expected because a lower \(\rm V_{s}\) generates longer solidification times, allowing the grains to increase their average size. As seen in Figure 2, the heat extraction rate is lower in the areas furthest from the base of the sample. Consequently, it is expected that being in a position further from the base, the \(\rm D_{Ge}\) will increase. This behavior can be seen in Figure 5, where the \(\rm D_{Ge}\) values for the four alloys studied are shown, depending on the distance from the base of the samples.