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

Abstract

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.