Phase transformations in a β-solidifying γ-TiAl based alloy during rapid solidification

In this study, the solidification behavior of an intermetallic gamma-TiAl based alloy, the so-called beta-solidifying TNM alloy, Ti-43.5Al-4Nb-lMo-0.1B [at.%], was investigated regarding the evolving phases, morphology and microstructure as a result of the high solidification rate during gas-atomization. By means of X-ray diffraction combined with electron back-scatter diffraction as well as at a higher resolution by transmission electron microscopy and transmission Kikuchi diffraction, conclusions regarding the ongoing processes during cooling were withdrawn. Furthermore, differential scanning calorimetry measurements on the powder condition served as supportive tool for the understanding of the microstructural development during heating of the initial state of disequilibrium. The investigations revealed a cellular/dendritic structure of the primary solidified beta-grains and microsegregations along the former solidification front. The former beta-grains, examined by energy dispersive Xray spectroscopy and atom probe tomography, showed an increasing Al-content and a depletion of refractory metals when moving from the center toward the interface. Depending on the local chemical composition, the high cooling rate results in a massive type transformation of alpha-Ti(Al)/alpha(2)-Ti3Al- and gamma-TiAl phase in the Al enriched areas, and to a martensitic transformation in the center of the retained beta-phase. Furthermore, a partial suppression of the ordering reaction of the majority alpha-phase is detected. During annealing the microstructure evolves toward equilibrium condition, i.e. the ordering of the retained alpha-phase and the formation of nanolamellar gamma-phase are observed.