Microstructural stability of fine-grained fully lamellar XD TiAl alloys by step aging

XD TiAl alloys (Ti-45 and 47Al-2Nb-2Mn+0.8 vol pct TiB₂) (at. pct) were oil quenched to produce fine-grained fully lamellar (FGFL) structures, and aging treatments at different temperatures for different durations were carried out to stabilize the FGFL structures. Microstructural examinations show that the aging treatments cause phase transformation of α ₂ to γ, resulting in stabilization of the lamellar structure, as indicated by a significant decrease in α ₂ volume fraction. However, several degradation processes are also introduced. After aging, within lamellar colonies, the α ₂ lamellae become finer due to dissolution, whereas most of the γ lamellae coarsen. The dissolution of α ₂ involves longitudinal dissolution and lateral dissolution. In addition, at lamellar colony boundaries, lamellar termination migration, nucleation and growth of γ grains, and discontinuous coarsening occur. With the exception of longitudinal dissolution, all the other transformation modes are considered as degradation processes as they result in a reduction in α ₂/γ interfaces. Different phase transformation modes are present to varying degrees in the aged FGFL structures, depending on aging conditions and Al content. A multiple step aging reduces the drive force for phase transformation at high temperature by promoting phase transformation via longitudinal dissolution at low temperatures. As a result, this aging procedure effectively stabilizes the lamellar structure and suppresses other degradation processes. Therefore, the multiple step aging is suggested to be an optimal aging condition for stabilizing FGFL XD TiAl alloys.