Solidification behavior and microstructure of Ti-(37−52) at% Al alloys synthesized in situ via dual-wire electron beam freeform fabrication

Titanium-aluminide (TiAl) alloys are very promising substitutes for Ni-based superalloys, and additive manufacturing, which is a disruptive technology, is a promising way to fabricate them. Ti-(37−52) at% Al walls were successfully synthesized in situ using dual-wire electron beam freeform fabrication (EBF3), and the solidification behavior was characterized according to their chemical composition, phase identification, microstructure and orientation. The Ti-37 at% Al wall was composed of α2 grains with an average size of 100 µm and a random orientation distribution, indicating that the primary phase was the β phase and that the solid-state β→α transformation occurred. The orientation relationships, namely (0001)α//{111}γ and <112¯0>α//<11¯0]γ, were found in the lamellar structure of the Ti-42 at% Al walls, and the α2 phase and γ grains presented < 0001 > and < 111 > orientations in the deposition direction, respectively. This suggests that the formation of a lamellar structure was the result of the solid-state α → α2 + γ transformation, where the α phase was the primary phase. The solid-state α →γ lamellae and α →γ massive transformations in the Ti-49 at% Al wall caused the massive γ phases to have a random orientation and the lamellar γ phases to be oriented along the < 012 > and < 010 > directions in the building direction. The γ dendritic structure, a characteristic solidification structure, indicated that the solidification path of the Ti-52 at% Al wall was L→γ. Solidification during the EBF3 process deviated from the thermodynamic equilibrium state, and the metastable phase formed in the liquid phase as the primary phase. The solidification conditions, including the thermodynamic and kinetic conditions and cooling rate, were analyzed. The results indicate that the primary phases were selected by undercooling, and the microstructure of Ti-xAl (x = the actual Al content in at%) alloys during the EBF3 process was determined to correspond to an estimated undercooling temperature of 240–247 K. However, the cooling rate during solidification was 214.36 K/s, which was not enough to produce such a high undercooling temperature. The high undercooling temperature was the result of melt superheating and electron beam oscillation. A novel solidification process with a low cooling rate and a high undercooling temperature occurred during EBF3.