Microstructure evolution and mechanical properties of directionally-solidified TiAlNb alloy in different temperature gradients

Ti44Al6Nb1.0Cr2.0V alloy was directionally solidified (DS) with cold crucible under different input powers (40 kW, 45 kW, 48 kW and 50 kW; corresponding to temperature gradient of 15.2 K/mm, 21.0 K/mm, 22.9 K/mm, 25.5 K/mm) and the same pulling velocity (8.33 μm/s). The results show that all the DS samples have the β-solidification characteristics and when the power is 45 kW or 48 kW, they have the well-DS macrostructure. After DS, the lamellar width becomes smaller and more homogeneous from the as-cast 1650 ± 1200 nm to less than 710 ± 376 nm. The higher the power is, the smaller the lamellar width it has. The original casting microcracks can be eliminated after DS and the percentage of small angle lamellas is higher for the well-DS samples. For the DS samples, the nanoindentation hardness in lamella region and the lamellar width yield the relationships of ^HN=16.12d-^0.127 and ^r2=0.992. When the as-cast and heat treatment (HT) conditions are considered, the relationships are changed as HN'=16.09d'^-0.127 and r'²=0.971. The nanoindentation hardness of B₂ phase and block γ phase keep almost the same respectively under the different conditions. RT tensile property is improved to average UTS 589.4 MPa and average elongation 1.16% after DS (P = 45 kW) comparing with the original as-cast 498.8 MPa and 0.53%. Trans-granular and trans-lamella fracture is the main fracture mode for the DS samples. After annealing (at 1250°C, 1300°C, 1350°C), lamellar width is coarsen to more than 2002 ± 1286 nm; B₂ phase segregation is reduced obviously, but some new microcracks form; HT samples exhibits a poor RT tensile property, in which the HT2 sample has a relatively higher property and it yields the average UTS 440.6 MPa and average elongation 0.45%.