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 >