Effect of Nb Content on Solidification Characteristics and Microsegregation in Cast Ti-48A1-xNb Alloys

Microstructural evolution in nonequilibrium solidification of Ti-48A1-xNb alloys with Nb contents ranging from 2 to 8 at% has been studied by containerless electromagnetic levitation. Levitated drops of controlled undercooling were quenched onto chill copper substrates and subjected to phase and microstructure analysis. With increasing Nb content, the solidification path changes gradually from hyperperitectic solidification to hypoperitectic solidification and both solidification segregation (S-segregation) and beta-solidification gradually increase. A transition from typical hypoperitectic solidification to a sole solidification of the beta phase beyond a critical undercooling is revealed for the Ti-48A1-8Nb hypoperitectic alloy. For the Ti-48A1-2Nb alloy, the morphologies of the primary beta dendrites are not observed. With increasing undercooling, the coarsening of the lamellar colonies occurs, which can be attributed to the transition of the primary beta dendritic morphology. Furthermore, the solute concentration profiles for the final solidification microstructure are obtained to examine the segregation behaviors of alloying elements. With increasing Nb content, the undercooling eliminating S-segregation gradually increases.