Abstract
The recalescence behaviour of undercooled melts of binary Ti 55Al 45, Ti 50Al 50, Ti 45Al 55, and ternary Ti 45Al 50Nb 5, Ti 45Al 45Nb 10, Ti 40Al 50Nb 10 alloys have been studied by a containerless electromagnetic levitation technique in a He/H 2 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 Ti 45Al 50Nb 5 and Ti 40Al 50Nb 10 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.
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