Microstructural stability of lamellar TiAl-based alloys at high temperature

The microstructural stability of lamellar TiAl-base alloys at high temperature was studied by conventional and high-resolution transmission electron microscopy. This paper emphasized the influence of substructures on the thermal stability of lamellar structure. These substructures produced by thermal mechanical treatments include lattice dislocations and boundary dislocations, the subgrain boundaries, the impinged T (Q) twins and misorientated multi-ledges lamellar interface boundaries. Three dual-phase TiAl based alloys containing different type and density of substructures were produced by different thermomechanical treatments. The microstructural changes in these dual phase TiAl-base alloys were compared after 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 γ plates, and the formation of new γ grains. A detailed description of various degeneration processes was given in the paper. As a result, the lamellar structure with heavy 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 days.