Effects of β-dendrite growth velocity on β → α transformation of hypoperitectic Ti-46Al-7Nb alloy

Solidification characteristics of Ti-46Al-7Nb melts were studied by the electromagnetic levitation technique. A maximum melt undercooling up to 240 K has been achieved. When the undercooling is lower than the critical value DT∗ = 205 K, the alloy possesses typical hypoperitectic solidification characteristic which can be evidenced by a peritectic layer observed in the as-solidified microstructure. However, the Widmanstätten structure can be observed at large undercooling regime of DT C DT∗, where peritectic reaction cannot proceed and γ lamellar precipitation within α plates is suppressed. Based on the BCT dendrite growth model, the dendrite growth velocities were calculated as a function of undercooling. Theoretical analysis indicates that the growth mechanism of the primary β phase transforms from solutaldiffusion- controlled to thermal-diffusion-controlled in the undercooling range of 188-205 K, which can be attributed to the onset of solute trapping at the critical undercooling. Meanwhile, with increasing undercooling, the solute trapping effect becomes more dominant as a consequence.