Effects of β-Dendrite Growth Velocity on β→a 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 Widmansta¨tten structure can be observed at large undercooling regime of DT C DT＊, where peritectic reaction cannot proceed and c lamellar precipitation within a 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 b 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.