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.