Effect of microstructure on thermal stability and mechanical properties of an engineering γ-TiAl based alloy during long-term exposure at 700°C and 800°C in air

This paper summarizes a systematic approach to gain a basic knowledge of the possible microstructural modifications, the stability of the phase, and the underlying mechanisms which induce these changes in gamma-TiAl based alloys during long-term service operations at elevated temperatures. A gamma-TiAl based alloy with the nominal composition of Ti-46.5at.%Al4at.%(Cr,Nb,Ta,B) has been exposed to air at 700degreesC and 800degreesC for up to 10,000 hours. The sheet material exhibits different microstructures prior to static annealing: an equiaxed microstructure which consists of fine grained gamma-TiAl phase with a small volume fraction of the alpha(2)-Ti3Al and beta/B2 phase at grain boundaries and triple junctions and a fully lamellar microstructure with fine lamellar spacing. Evolution of type and composition of constitutional phases for both types of microstructure has been recorded as a function of annealing time and temperature. Additionally, in case of fully lamellar material the impact of thermally induced microstructural changes on mechanical properties was investigated.