Designed fully lamellar microstructures in a γ-TiAl based alloy: adjustment and microstructural changes upon long-term isothermal exposure at 700 and 800 °C

The evolution of microstructure after annealing within the α phase field and subsequent cooling through the (α+γ) phase field has been investigated for a commercial Ti–46.5at.%Al–4at.% (Cr, Nb, Ta)-B alloy. Due to the presence of (Ti, Ta) borides, controlled fully lamellar microstructures can be adjusted exhibiting colony sizes smaller than 150 μm. In order to study the dependence of lamellar spacing on cooling rate, the latter was varied in the range of 1–200 K min−1. The obtained microstructures reached from very fine lamellar structures with a Widmannstätten-type morphology at cooling rates higher than 200 K min−1 through undisturbed fully lamellar structures to a lamellar structure with primary γ-grains at colony boundaries at cooling rates below 4 K min−1. It is demonstrated that the mean interface spacing of the Ti–46.5at.%Al–4at.% (Cr, Nb, Ta)-B alloy decreases with increasing cooling rate following an exponential expression with an exponent of −0.39. Since the adjustment of undisturbed fully lamellar microstructures with narrow lamellar spacing requires relatively high cooling rates, a condition far from thermodynamic equilibrium is obtained which provides one of the driving forces responsible for structural changes in lamellar microstructures upon exposure at elevated temperatures. It is shown that the α2 volume fraction increases significantly with increasing cooling rate. The thermal stability during isothermal exposure to air at temperatures of 700 and 800 °C up to 3500 h was examined by means of optical microscopy, scanning and transmission electron microscopy.