Additive manufacturing (AM) is redefining the limits of directional solidification (DS) and single-crystal (SX) fabrication for nickel-based superalloys. By reconciling classical Bridgman theory with the extreme thermal gradients (G) and solidification velocities (V) inherent to AM, this review establishes a unified framework for controlled epitaxy. It dissects the kinetics of grain competition and the columnar-to-equiaxed transition (CET), highlighting how scan strategies and thermal management dictate melt pool geometry and the G/V ratio. A comparative assessment of laser powder bed fusion (L-PBF), electron beam powder bed fusion (EB-PBF), and directed energy deposition (DED) delineates di... More >

Intermetallic compounds including TiAl and NiAl suffer from an abrupt brittle-ductile transition at a critical temperature, limiting their higher temperature applications. Conventional viewpoints attribute this transition to thermally activated dislocation actions or dynamic recrystallization. Here, we discover a novel brittle-ductile transition mechanism in polysynthetic twinned TiAl, driven by lamellar degradation, which significantly deviates from the conventional mechanisms. The results demonstrate that the lamellar degradation is primarily caused by the \( \alpha_2 \rightarrow \gamma \) phase transformation at the brittle-ductile transition temperature, which stems from a non-monotonic... More >