Recalescence behaviour of binary Ti–Al and ternary Ti–Al–Nb undercooled melts

The recalescence behaviour of undercooled melts of binary Ti55Al45, Ti50Al50, Ti45Al55, and ternary Ti45Al50Nb5, Ti45Al45Nb10, Ti40Al50Nb10 alloys have been studied by a containerless electromagnetic levitation technique in a He/H2 atmosphere in order to reveal phase formation and microstructure evolution processes. The time–temperature traces of the pyrometer exhibit steep single recalescence events for undercooling values of ∼300 K below the liquidus temperature. Dendrite growth velocities of the order of 10–20 m/s for highly undercooled ternary Ti–Al–Nb melts are consistent with a primarily formed bcc β-phase, which is promoted by Nb addition. For ternary Ti45Al50Nb5 and Ti40Al50Nb10 alloy compositions, a second recalescence near 1300 °C was observed, which can be attributed to a solid state transformation. The microstructure of levitated melt drops subsequently quenched onto a chill substrate shows more evidence for the predicted phase transformations.