Thermal cycling induced microstructural instability in fully lamellar Ti–45Al–8.5Nb–(W, B, Y) alloys

• The lamellar colony boundaries are coarsened by precipitation of coarse γ grains.
• The coarsening of boundaries is sensitive to the boundary orientation.
• The coarsening occurs only at one side of the primary boundary.
• Precipitated B2 phases always accompany with the formation of new γ grains.
• B2 phases are formed at colony boundaries and at the γ/γ interfaces.

Microstructural instability of fully lamellar Ti–45Al–8.5Nb–(W, B, Y) alloys during long-term thermal cycling (500 and 1000 cycles) between room temperature and 900 °C or 1000 °C was investigated. The coarsening of lamellar colony boundaries and precipitation of B2 particles are produced as a result of the microstructures degradation. The boundaries are coarsened by the precipitation of coarse γ grains. The width of the coarsened boundaries increases with increasing cycles and temperatures. Owing to the large interface energy, the coarsening of colony boundaries prefers to occur at boundaries with high misorientation angles. There is a coincidence-site lattice relationship between the precipitated γ grains and the lamellar colony at one side of the boundary. In addition, the precipitation of B2 phase occurs not only at lamellar colony boundaries by consuming (α2 + γ) lamellae but also at γ/γ interfaces via two possible mechanisms will be discussed in detail.