Growth rate and composition of directionally solidified intermetallic TiAl–Nb alloys with different solidification conditions

Intermetallic Ti–xAl–8Nb (x = 41, 43, 45, 47, 49; at%) alloys were solidified unidirectionally upwards with a constant temperature gradient of G = 3.8 K·mm⁻¹ at wide range of growth rates of v = 10–400 μm·s⁻¹ using a Bridgman directional solidification (DS) furnace. Microstructural parameters including the primary dendrite arm spacing (λ ₁), secondary dendrite arm spacing (λ ₂), dendrite tip radius (R) and mushy zone depth (d) were measured statistically. The values of λ ₁, λ ₂, R and d decrease as the growth rate increases for a given composition (x). The values of λ ₁, λ ₂, R and v increase with the increase in x value, while the value of d firstly increases and then decreases with the increase in x value for a given v. The relationships between λ ₁, λ ₂ and R were analyzed by the linear regression. The average growth rate exponent of λ ₁ is 0.29, which is in accordance with the previous experimental observations, and that of λ ₂ is close to the previous experimental results, while those of R and d are lower than the results in other alloy systems. In addition, theoretical models for λ ₁, λ ₂ and R were compared with the experimental observations, and a comparison of the present experimental results with the theoretical models and previous experimental results was also made.