## 3 Results and Discussion
### Microstructure of the Ti-48 Al-2 Cr Alloy Solidified under Different Processes  
According to the phase diagram of the Ti-Al-Cr ternary alloy [33], the solidification process of the Ti-48 Al-2 Cr alloy is Liquid (L) \(\rightarrow\) \(\beta\) (primary phase), and L + \(\beta\)\(\rightarrow\)\(\alpha\) (peritectic transformation) occurs after continuous cooling. Then, the residual \(\beta\) phase transforms into the \(\alpha\) phase and the B2 phase. During subsequent cooling, the lamellar structure is formed by \(\alpha\)\(\rightarrow\)\(\alpha_{\text{residual}}\) + \(\gamma\)\(\rightarrow\)(\(\alpha_{2}\)+\(\gamma\))\({}_{\text{ lamellar}}\) + \(\gamma\) transformation (\(\alpha_{2}\)-Ti\({}_{3}\)Al, \(\gamma\)-TiAl). Figure 3 shows the microstructure of the Ti-48 Al-2 Cr alloy under different solidification conditions. As shown in Figure 3a, there are a large number of \(\gamma\) phases in the alloy after solidification at ambient pressure. When solidified at 5 GPa, the volume fraction of \(\gamma\) phase decreases while the lamellar structure increases (Figure 3c). Because of the shifted eutectic point to higher Al content and the increased eutectic reaction temperature, the content of the lamellar structure increases, according to the lever rule [34; 35; 36]. The increase of superheat will not only make the element distribution more uniform, but will also improve the thermodynamic undercooling and reduce the content of interdendritic \(\gamma\) phase (Figure 3d) [22]. However, according to [PERSON]'s study [37], the effect of temperature on the size and shape stability of microstructural features has been extensively investigated, the lamellar structure would be coarsened with increasing superheat. When the solidification condition of the Ti-48 Al-2 Cr alloy changed from ambient pressure to 5 GPa at 1873 K and 5 GPa at 1973 K, the volume fraction of \(\gamma\) phase decreased from 43% to 29.4% and 11.2%, respectively (Figure 3a,c,d). Few B2 phase particles can also be found in the Ti-48 Al-2 Cr alloy when solidified under ambient pressure, as shown in Figure 3b (the content of the B2 phase is less and cannot be detected by XRD), but no B2 phase was found after solidification under high pressure. This is because the B2 phase is the result of the ordering of the primary \(\beta\) phase at high temperature [38]; high-pressure solidification can increase the undercooling of the melt and directly lead to the formation of the \(\alpha\) phase from the liquid phase [26]. Meanwhile, high pressure inhibits the diffusion which leads to the increasing of Al content in the liquid phase [26]. The content of Al element in the liquid phase will increase, which can also promote the peritectic phase, direct nucleation, and precipitation. Therefore, the B2 phase cannot be found during high-pressure solidification. Figure 4 shows the EDS analysis result of the Ti-48 Al-2 Cr alloy solidified under different processes. As shown in Figure 4a, the solubility of Cr element in the lamellar structure is about 2.56 at. % under ambient pressure, whereas it increases to 3.09 at. % and 3.26 at. % under 5 GPa pressure at 1873 K and 1973 K respectively (Figure 4b,c). Considering the errors of EDS, it can be seen that high pressure increases the concentration of Cr in the lamellar structure. Further increasing the holding temperature to 5 GPa, the concentration of Cr is nearly unchanged. Al element in the lamellar structure also shows the same phenomenon. The EDS analysis proves that high pressure inhibits the element diffusion. Figure 2: XRD patterns of the Ti-48 Al-2 Cr alloy solidified under different pressures. (a) ambient pressure, 1873 K; (b) 5 GPa, 1873 K; (c) 5 GPa, 1973 K.  
Moreover, the EDS results further confirm the decrease of the \(\gamma\) phase and the disappearance of the B2 phase after high pressure solidified. Figure 4: EDS analysis of the lamellar structure of the Ti-48 Al-2 Cr alloy solidified under different pressures.