Microstructure control and mechanical properties of Ti44Al6Nb1.0Cr2.0V alloy by cold crucible directional solidification

The as-cast Ti44Al6Nb1.0Cr2.0V alloy master ingot was prepared by vacuum consumable melting technology. Some bars were cut from this ingot and they were directionally solidified by cold crucible under different pulling velocities. The samples could be well directionally solidified when the power (P) was 45kW and the pulling velocity (V) was 8.33μm/s or 11.67μm/s. The results show that the interlamellar space of the directionally solidified samples decreases from the average 1650nm of as-cast to less than 565nm and is more homogeneous. The microcrack in the master ingot can be eliminated completely and the room temperature (RT) tensile property is also improved after cold crucible directional solidification (CCDS). The ultimate tensile strength (UTS) is 602.5MPa and the elongation is 1.20% as P=45kW and V=11.67μm/s, compared with as-cast 499MPa of UTS and 0.53% of elongation. Trans-granular and trans-lamella fractures are predominant modes. The relationship between CCDS interlamellar space (d) and the pulling velocity can be described as d = 1783.2^{V -0.554} and r12=0.972, where r12 is the corresponding regression coefficient. The CCDS interlamellar space and nanoindentation hardness (H_N) in the lamella region can be described as _{H N} = 17.95^{d -0.145} and r22=0.986, and they are changed as HN'=14.03d'-0.104 and r32=0.975 when the cast condition is considered. The nanoindentation hardness of the B₂ phase and the block γ phase are about 8.89GPa and 8.15GPa, respectively; both of them keep almost the same in different conditions.