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 degrees C or 1000 degrees 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 gamma 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 gamma 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 (alpha(2) + gamma) lamellae but also at gamma/gamma interfaces via two possible mechanisms will be discussed in detail. (C) 2015 Elsevier B.V. All rights reserved.