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.