Metastable phase formation in Ti–Al–Nb undercooled melts

Metastable phase formation processes in ternary Ti–Al–Nb alloys were studied by containerless electromagnetic levitation for melt undercooling up to 300 K below the liquidus temperature. Dendrite growth velocities of 15–25 m s^?1 for highly undercooled Ti–Al–Nb melts were consistent with primary β-phase formation, which is promoted by Nb addition. From double-recalescence events in Ti₄₀Al₅₀Nb₁₀ and Ti₄₅Al₅₀Nb₅ melts beyond a critical undercooling a subsequent β to α phase transformation in the semi-solid state was inferred. A second recalescence near 1300 °C, which was attributed to an α to γ solid state transformation, was observed in the pyrometer trace for the Ti₄₅Al₅₀Nb₅ and Ti₄₀Al₅₀Nb₁₀ alloys. The γ phase formation was suppressed in favour of a homogeneous α₂ phase in undercooled Ti₄₅Al₄₅Nb₁₀ samples quenched onto a chill substrate, whereas in Ti₄₀Al₅₀Nb₁₀ high undercooling enabled a direct γ phase solidification.