Microstructural stability of lamellar TiAl-BASED alloys at high temperatures

The microstructural stability of lamellar TiAl-base alloys at high temperatures was studied by conventional and high-resolution transmission electron microscopy. The influence of substructures on the thermal stability of lamellar structure was emphasized. These substructures produced by thermal mechanical treatments include the interfacial dislocations and ledges, the subgrain boundaries, the impinged T(Q) twins and misorientated lamellar interfaces. The microstructural change of three kinds of lamellar TiAl-base alloys containing differents type and densities of substructures were compared during exposure at 800-1000°C. It was found that the existence of such substructures could accelerate the degeneration of lamellar structure, leading to the rapid necking and break-up of α₂ plates, the coarsening of 7 plates, and the formation of new y grains. As a result, the lamellar structure with substructures started to degenerate after thermal exposure at 800°C for 4. 5 h. While only slight coarsening was observed at the colony boundaries in the lamellar structure without substructures even after exposure at 900°C for 7 d.