Abstract
High temperature phase equilibria were studied in directionally solidified (DS) intermetallic Ti-45.9Al-8Nb (at.%) alloy by the means of quench during directional solidification (QDS) experiments, microstructure analysis and differential thermal analysis (DTA). During directional solidification at constant growth rates V ranging from 5.56x10^-6 to 1.18x10^-4 m s^-1 and three measured temperature gradients in liquid at the solid-liquid interface G_L of 3.5x10³, 5x10³ and 8x10³ K m^-1 the primary solidification phase is identified to be β phase (Ti based solid solution with bcc crystal structure). The microstructural analysis of quenched mushy zones excluded formation of α phase (Ti based solid solution with hcp crystal structure) by a peritectic reaction during directional solidification. Experimentally determined phase transformation sequences L (liquid) → L + β → β → β + α and estimated high temperature phase stability are different from those resulting from thermodynamic calculations using Thermo-Calc software and TiAl database. Due to preferred [001] crystallographic orientation of the β dendrites, lamellar α₂ (Ti₃ Al)/ γ(TiAl) boundaries in DS ingots are observed to be inclined at the angles of 0° and 45° to the growth direction.

Key words
titanium aluminides, TiAl, crystal growth, phase transformations, microstructure

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