## 3 Results  
### Microstructure and Phase Composition  
According to the equilibrium phase diagram of Al-Ni alloy, the primary \(\alpha\) phase is expected to precipitate first, followed by the eutectic reaction, which is demonstrated by the coupled growth  
Figure 1: Schematic illustration of the assembly sample in the high-pressure solidification process, the structure of the graphite heater, and the final macrostructure. of both \(\alpha\)-Al and \(\beta\)-Al\({}_{3}\)Ni phases during the solidification of Al - 1.5 wt%Ni alloy [19]. The typical backscattered electron (BSE) microstructures of the horizontal sections of each sample solidified under different high pressures, which are displayed in Figure 2. It is worth noting that three distinct regions exist simultaneously in each sample along the directional of the temperature gradient: developed columnar dendrites in the left regions as shown in Figure 2a,d; grey solid solution, black irregular intragranular blocky microstructure, and thinner intergranular layer channels in the intermediate zones as can be seen in Figure 2c1-c3,f1-f3; and cellular dendrites in the right areas as is clear from Figure 2b,e. The conventional directional solidification of hypoeutectic Al - 1.5 wt%Ni alloy has been studied by [PERSON] [9] et al. In their study, the microstructure evolution of the partially melted region was investigated. The transverse section showed that Al grains were bound by thin eutectic layers and Ni-rich intragranular droplets. The Ni-rich droplets and the liquid channels disappeared gradually  
Figure 2: The microstructure of Al - 1.5 wt%Ni alloy solidified under different high pressures. (a–c3) 1 GPa; (d–f3) 3 GPa. after thermal-stable treatment of 2 h 35 min. EDS analysis of the percentage of Ni element along the growth direction showed that after the stationary growth conditions were reached, there was a sharp drop from the nominal concentration C\({}_{0}\) to nearly pure aluminum at the boundary between the non-melted zone and the partially melted region. The content of Ni element in the partially melted region increased first then decreased, whereas the content of Ni element in the fully melted region increased dramatically, and then a plateau with concentration around C\({}_{0}\) was obtained. Therefore, with the analysis of microstructure along the temperature direction under high pressure, it is reasonable to make the conclusion that changing the structure of the graphite heater is an effective way to obtain a mushy zone under high pressure, and, here, the three regions in Figure 2 are defined as fully melted (liquid) area, mushy zone, and non-melted (solid) region. The microstructure of Al - 1.5 wt%Ni alloy solidified at the ambient pressure is shown in Figure 3a. It shows that the eutectic microstructure presented at the grain boundaries in the form of a continuous mesh structure. The eutectic lamellar spacing was very small, and the average dendrite spacing was about 35 \(\upmu\)m. The phase composition of Al - 1.5 wt%Ni alloys solidified under different pressures can be seen in Figure 2b. It demonstrates that the phase composition of all the alloys are the same and are composed of \(\alpha\)-Al and \(\beta\)-Al\({}_{3}\)Ni phases. Figure 3c1-d3 exhibits the BSE microstructures of hypoeutectic Al - 1.5 wt%Ni alloy solidified with temperature gradients and different high pressures. Figure 3c1,d1 shows the fully melted areas. As can be clearly seen, the values of secondary dendrite arm spacing under 1 GPa and 3 GPa were about 13.42 \(\upmu\)m and 11.94 \(\upmu\)m, respectively. Their values are smaller than that of solidification under ambient pressure. Figure 3c2,d2 displays the microstructure of mushy zones; it is clearly shown that microstructures with different brightness co-exist, and more details will be discussed later. Figure 3c3,d3 shows the non-melted regions, where the cellular \(\alpha\)-Al dendrite remained the same, but the eutectic lamellar spacing was obviously larger than that of solidification under ambient pressure.