Thermodynamics of the Ti–Al–O system

The thermodynamic activities of Ti, Al, and O in the α-Ti(O), α2-Ti3Al(O), and γ-TiAl(O) phases were measured via Knudsen effusion mass spectrometry (KEMS). Measurements were taken at the KEMS facility at Forschungszentrum Jülich, Germany and the KEMS facility at the NASA Glenn Research Center in Ohio, USA. Differences between the measurements at the two facilities are primarily due to the selection of proper ionization cross sections. The measured activities of Ti and Al are compared to a recent CALPHAD assessment of the Ti–Al–O system and agreement is generally good, indicating consistency between our measurements and others. There are some differences in the measured activities of O and those in the assessment; possible reasons for this are discussed. Partial molar enthalpies and entropies of mixing of components Ti, Al, and O are reported for each of the sixteen alloys studied. With constant Ti:Al ratio, the activity of Al increases with increasing O concentration and the activity of Ti decreases with increasing O concentration. These studies indicate that the oxygen potential must be known and controlled for reliable thermodynamic measurements of oxygen active alloys.