Evaporation of Al and microstructural variation in TiAl alloy during heat treatment

A TiAl alloy of Ti - 47Al - 2Cr - 2Nb (at.-%) was used to study phase trans formation and microstructure control by heat treatment and a glow discharge spectrometer was applied to analyse the compositional variation from th sp ecimen surface to the interior. It was found that Al tended to evaporate at elevated temperatures. As a result, Al was significantly depleted and a qu ite different microstructure developed in the outer layer compared with tha t in the interior. Results also showed that the outer layer appeared thicke r with increasing heating temperature, time, and cooling rate, and with cha nging the environment from vacuum to air. Since the evaporation of Al leads to microstructure variation and perhaps property deterioration as well, it is suggested that this effect should be considered for alloy design and pr ocessing.