## 3 Results and Discussion
### Nanohardness of High Pressure Solidified Ti-48 Al-2 Cr Alloy  
A nanoindenter was used to measure the mechanical property of high pressure solidified Ti-48 Al-2 Cr alloy under different processes. Figure 6 shows the hardness-depth curves of the lamellar structures of the Ti-48 Al-2 Cr alloy solidified under different solidification processes. As can be seen, the fluctuations of the curves are small when the indentation is above 900 nm, which means the hardness of the lamellar structures remains nearly constant with the increase in indentation depth. The average hardness values are displayed in Table 1. When solidified at 5 GPa and 1873 K, the nanohardness is 5.54 GPa. Compared with the samples solidified at ambient pressure, the nanohardness increases about 60.5%. Further increasing the holding temperature led to a decrease in nanohardness. The nanohardness of the high pressure solidified samples depends on the solidification process. As we mentioned, high pressure leads to an increase in Cr concentration and to dislocations in the microstructures8. Hence, the increase in nanohardness can be attributed to the solution strengthening and dislocation strengthening. When increasing the temperature to 1973 K at 5 GPa, the change of Cr content in the lamellar structures is very small. Combined with the microstructure analysis, the main  
Figure 5: TEM images of the Ti-48 Al-2 Cr alloy solidified at 5 GPa pressure under different temperatures. (**a**) 1873 K; (**b**) and (**c**) 1973 K; (**d**) SAED of \(\gamma\) phase in (**c**). reasons for the decrease in nanohardness are the coarsening of the lamellar structure and the decrease in dislocation volume; thus, dislocation strengthening is the dominant mechanism. The Mechanism of the Relationship between Dislocation Density, Microstructure Refinement, and Properties under High Pressure  
Figure 7 shows a diagram of the dislocation structure relationship of the Ti-48 Al-2 Cr alloy, which reveals the grain size and dislocation movement in the grain under high pressure and different solidification temperatures. High pressure can inhibit the diffusion of the element and lead to solute enrichment at solidification interface, and supercooling can affect the nucleation and growth of grains and the microstructure evolution mechanism of the alloy [17; 22]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Sample** & **Nanohardness (GPa)** \\ \hline Ambient pressure, 1873 K & 3.45 \(\pm\) 0.19 \\
5 GPa, 1873 K & 5.54 \(\pm\) 0.21 \\
5 GPa, 1973 K & 4.48 \(\pm\) 0.27 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The average nanohardness of the Ti-48 Al-2 Cr alloy solidified under different pressures. Figure 6: Hardness-depth curves obtained from solidified Ti-48 Al-2 Cr alloy under different pressures. Figure 7: The strengthening mechanism in the microstructure of the Ti-48 Al-2 Cr alloy (**a**) ambient pressure, 1873 K; (**b**) 5 GPa, 1873 K; (**c**) 5 GPa, 1973 K.  
Under high pressure, the pile-up of dislocations between grains is relatively close, and the stress produced by high pressure causes the dislocation source to start, release a large number of dislocations, and pile up at the grain boundary (Figure 7a,b). Due to the increased pressure, the dislocations pass through the grain boundary and enter the adjacent grain, which leads to increasing dislocation density and the improvement of mechanical properties [39]. (Figure 5a,b). Compared with the samples solidified at ambient pressure, the nanohardness of the alloy after high-pressure solidification increased to 5.45 GPa and 4.48 GPa, respectively. This is because when solidified at 5 GPa and 1873 K, dislocations cross the grain boundary and enter the adjacent grains, which can activate internal dislocations and release the dislocation pile-ups (Figure 7b). The nanohardness of the alloy was increased by increasing the dislocation density. The nanohardness of the alloy decreased after being solidified under high pressure at higher superheating temperature.