## 1 Introduction  
As high temperature structure materials, TiAl-based alloys show excellent advantages, such as low density [1], high specific strength [2], good corrosion resistance and creep resistance [3], high hardness and strength [4]. Due to such outstanding properties, TiAl-based alloys have been considered to be promising materials for application in aircraft, gas turbines and automotive engine [5; 6]. However, the low ductility [7; 8; 9] greatly decreases the reliability of the microstructures and limits their application, a problem which has attracted the attention of many researchers [10; 11]. [PERSON] et al. [12] reported that after forging, the cast Ti-43 Al-6 Nb-1B alloy transformed from fully lamellar (FL) structures to duplex (DP) structures. The microstructure was refined and the mechanical properties were improved significantly. Many studies have been done to optimize the mechanical properties of TiAl-based alloys through the addition of a third element. In view of the \(\beta\) phase stabilizer addition, Cr is a notable element to improve ambient temperature, ductility, and mechanical strength of the TiAl-based alloys, which has been widely adopted by many researchers. [PERSON] et al. [13] indicated that the ductility of Ti-48 Al-2 Cr can be improved through the rapid solidification method. At the wheel speed of 30 m/s, the mechanical properties showed a great increase. [PERSON] et al. [14] have found that after spark plasma sintering, the Vickers hardness of the TiAl alloy is increased by 15%.  
Pressure is an important thermodynamic parameter of metallic materials which can influence the solidification process, especially under high pressure [15; 16]. The influence of the alloying element and temperature during solidification processes has been widely studied. However, due to the limitations of traditional technology, the effect of pressure has usually been ignored [17]. High pressure solidification has gained much attention because of its special features, such as the refined microstructure and the decreasing diffusion coefficient [18; 19; 20; 21], and this technique is an important and robust approach widely used in basic and applied science as well as industrial applications. High pressures and high temperatures have revolutionary impacts in modern society, ranging from industrial applications such as the fabrication of superconducting and superhard materials, to exploration of the properties of materials obtained under extreme conditions [22; 23; 24; 25]. These characteristics are different from alloys that have solidified under ambient pressure. [PERSON] et al. [26] confirmed that high pressure increases the eutectic point from 5.69% under ordinary pressure to 26.49% under 2 GPa, and the velocity of dilute solute solidified at interface declines exponentially. [PERSON] et al. [27] reported the tensile strength and yield strength of Al-20 Mg alloy increased to 467 MPa and 245 MPa respectively, because of solution strengthening and deformation strengthening caused by high pressure heat treatment. Nanoindentation is a widespread test technique for measuring the mechanical properties of the nanoscale, the microscale, and the thin-films materials [28; 29]. It makes it possible to determine the hardness of the material, especially the hardness of target phase from hardness-depth data [29; 30].  
Up to now, the microstructure and mechanical properties of TiAl-based alloys solidified under high pressure has been rarely reported. Alloying elements such as Cr, Nb and Zr were added to improve the room temperature brittleness of the TiAl-based alloy [31]. Furthermore, it has been considered that 2% (at. %) Cr additions added to Ti-48 Al alloy have the best room temperature ductility [13]. Therefore, the Ti-48 Al-2 Cr alloy was chosen to study the evolution of the microstructure and mechanical properties when solidified under high pressure in this work. The variation of the phase transformation and nanohardness of Ti-48 Al-2 Cr alloy has been studied.