Peculiar microstructural evolution and tensile properties of β-containing γ-TiAl alloys fabricated by electron beam melting

The microstructure and mechanical properties of β-containing Ti-44Al-4Cr alloy rods manufactured by electron beam melting process were examined focusing on input energy density determined by the processing parameters. We found for the first time that two types of fine microstructures have been obtained in the alloys by varying the energy density during the fabrication process. A uniform α₂/β/γ mixed structure was formed at high energy density conditions. On the other hand, a peculiar layered microstructure perpendicular to the building direction consisting of a ultrafine α₂/γ lamellar grain layer and a α₂/β/γ mixed structure layer was obtained in the rods built at lower energy density conditions. This difference in the microstructures originates from the difference in the solidification microstructure and the temperature distribution from the melt pool, which are dependent on the energy density. Moreover, it was also found that the strength of the alloys is closely related to the volume fractions of the β phase and the ultrafine α₂/γ lamellar grains which originates from the massive α grains formed by rapid cooling. The alloys with high volume fractions of these peculiar microstructures exhibit higher strength than the conventional β-containing γ-TiAl at 1023 K.