Microstructure formation and columnar to equiaxed transition during cold crucible directional solidification of a high-Nb TiAl alloy

In order to study the factors of columnar to equiaxed transition (CET) of high-Nb TiAl alloys, Ti46Al7Nb0.4W0.6Cr0.1B alloy has been fabricated by cold crucible directional solidification (CCDS) technique under different pulling rate from 3.3 μm/s to 16.7 μm/s. The marco/micro-structure and phase composition near solid–liquid interface have been characterized. Results show that the CET of the high-Nb TiAl alloy occurs with the increase of the pulling rate at the constant temperature gradient. The microstructure of the columnar grain is composed of α2/γ lamellar matrix and a coupling structure of striped-like B2+γ phases. The lamellar colonies in a columnar grain possess the same orientation, while the arrangement direction between the striped-like B2 phase and growth direction is 0° or 45°. A solidification map for CCDS is established which predicts columnar or equiaxed morphology according to the growth rate (R) and temperature gradient (G). The dendrite morphology at the solid–liquid interface after quenching and the CET is controlled by the actual temperature gradient at the tip of the dendrite. Meanwhile, the increase of growth rate and the satisfaction of heterogeneous nucleation conditions are the main factors for CET. The decrease of actual temperature gradient caused by quenching or the increase of liquidus gradient caused by increasing growth rate can increase the maximum supercooling degree ΔTC. When it reached the supercooling degree ΔTN required to form a new nucleus, equiaxed grains will be produced. In addition, the boride in this alloy can act as a heterogeneous nucleation core to promote CET.