Creep behaviour and related high temperature microstructural stability of Ti-46Al-9Nb sheet material

A high Nb containing γ-TiAl based sheet material with 46 at.%Al and 9 at.%Nb has been tested in order to analyse the influence of microstructure and texture on creep. Tensile test specimens with fine-grained near gamma microstructure and coarse-grained fully lamellar microstructures varying in lamellar spacing were prepared from identical starting materials obtained by a powder metallurgical route. Creep tests up to 350 h at constant load and temperature as well as long-term creep experiments up to 2000 h with load and temperature changes have been carried out. The stress exponent and the apparent activation energy deduced from the creep tests are in the range of 4.2-6.1 and 4.0-4.2 eV, respectively. The results imply diffusion assisted climb of dislocations as the predominant creep mechanism. During long-term creep at high temperatures the fully lamellar microstructure suffers microstructural degradation with a related increase in creep rate. Nevertheless, the creep resistance of the high Nb containing alloy is significantly higher than the creep resistance of low Nb containing, so-called second generation γ-TiAl alloys, such as Ti-46.5 at.%Al-4 at.%(Cr, Nb, Ta, B) and Ti-47 at.%Al-4 at.%(Cr, Mn, Nb, Si, B). This finding is supported by the higher activation energy observed for Ti-46Al-9Nb.