Reducing the environmental embrittlement effect of TiAl alloys exposed to air at high temperatures by a fine-grained surface structure.

The susceptibility of titanium aluminides (TiAl) to environmental embrittlement is one of the factors limiting the use of this class of intermetallic alloys. After being exposed to air at high temperatures, the ductility of the material at room temperature is significantly reduced or even completely lost. In the present work the influence of near surface grain refinement on the example of the β-stabilised TNM alloy on the embrittlement behaviour after exposure for 2 h at 700 °C in air is presented and discussed. The main characteristic of the environmental embrittlement is that cracks during tensile tests at room temperature appear on the surface and lead to rapid failure. One of the reasons given in literature for this behaviour is that high tensile stresses form at the sample edge by exposure. Grain refinement of the surface region with a depth of approximately 350 μm displaces crack initiation during tensile testing well below the surface after exposure. The environmental embrittlement phenomenon after short-term exposure does, thus, not occur in this layer-like structure. Various reasons for this behaviour, which include delayed crack formation, stress relaxation during loading and cooling after loading as well as supporting effects, but also the limitations of the treatment method, are discussed in the work. • Environmental embrittlement of TiAl alloys can be reduced by a fine-grained surface. • Reasons for this behaviour, like stress relaxation, are identified and discussed. • Fine grained surface leads to high strain hardening rate of coarse-grained interior. • Origin of the embrittlement lies in the area close to the surface (max. depth 2 μm). • Removal of the area of visible microstructural changes restores ductility. [ABSTRACT FROM AUTHOR]