# Corrosion Susceptibility and Microhardness of Al-Ni Alloys with Different Grain Structures
## 2 Materials and Methods
### Directional Solidification Process and Metallographic Characterization  
The experiments were carried out with four concentrations of Al-Ni alloys (Al-1 wt. %Ni, Al-3 wt. %Ni, Al-5.7 wt. %Ni and Al-8 wt. %Ni). The directional solidification process began by pouring the molten metal at a temperature of 800-850 \({}^{\circ}\)C (approximately 200 \({}^{\circ}\)C above the fusion temperature of pure aluminum [20]) into the furnace previously heated to the same temperature. The furnace had a heat extraction system that circulated water at a temperature of 5 \({}^{\circ}\)C. It also had a temperature acquisition system through 6 type K thermocouples coated with refractory ceramics located inside the metal in different positions (that is, every 2 cm from the bottom of the mold) that allowed the recording of the thermal evolution during the entire solidification process. Cooling was initiated by activating the cooling water at the bottom of the mold (cold base). Consequently, the directional solidification took place in a vertically ascending manner.  
Processing the acquired thermal data allowed for obtaining the characteristic temperature versus time curves and the analysis of the thermal parameters involved.  
The solidified samples were cut longitudinally in half and roughened in water with emery papers (#60 to #1200) and then polished with 6 \(\upmu\)m diamond paste (Prazis(r), Buenos Aires, Argentina) for 1 h on a cloth by using ethylene glycol as a lubricant. To reveal the macrostructure, the samples were immersed in a solution of 50 mL of HCl and 500 mL of water for 10-15 min until the appearance of grains was observed [21; 22]. Visual observation of the macrostructures of the samples allowed us to identify the different grain structures obtained in a directional solidification process. That is, columnar at the base of the specimen and equiaxial at the top. Between both structures, there is a transition zone from columnar-to-equiaxed (CET) grain structure. The position of the CET zone was measured with a ruler from the base of the sample [23; 24; 25; 26; 27].  
From the images obtained from the development of the macrostructure, the width of the average columnar grains was measured, as explained in previous work [28]. The size of the equiaxed grains of the equiaxed zone close to the CET and zones close above it was determined.  
Each sample was divided into equal surfaces, and the size of the equiaxed grains was determined at each interval of approximately 10 mm, according to the ASTM 112-96 standard [29]. With these values, the average equiaxed grain diameter, D\({}_{\text{Ge}}\), was calculated.  
The development of the microstructure was carried out using a solution of 1.5 mL HCl, 1 mL of HF, 2.5 mL of HNO\({}_{3}\) and 95 mL of H\({}_{2}\)O [21; 22]. The chemical attack times were 10 s for Al-1 wt. %Ni, 8 s for Al-3 wt. %Ni, and 7 s for the other alloys. After chemical attack, the samples were rinsed with plenty of water and dried with hot air flow.  
The characterization of the surface of Al-Ni alloys was carried out using an FEI Quanta 200(r) Scanning Electron Microscope (SEM) with an energy dispersive spectroscopy (EDS) detector of the Electron Microscopy and Microanalysis Service (SemFi-LIMF, Faculty of Engineering, UNLP, La Plata, Buenos Aires, Argentina). Microhardness measurements were carried out using a Future Tech FM800(r) microviokers durometer, Future Tech Corporation, Tokio, Japon, with a load of 50 g\({}_{\text{f}}\) and a residence time of 10 s [30]. Ten measurements were carried out in each of the phases of each alloy.