Solute effects on deformation mechanisms at nanoscale in Ti-Al intermetallics

Solute atoms dissolved in the crystalline lattice are known to strongly affect the mechanical properties of alloys, by increasing or, on the contrary, by reducing strength and/or ductility of these materials. Despite the fact that the observed phenomena are sometimes spectacular, the mechanisms induced by the solute atoms on the deformation mechanisms at the nanoscale are often very badly understood. Among these intriguing phenomena, the case of embrittlement of the TiAl alloys after exposure to air at high temperatures (from 500°C to 800°C), for durations as short as 1 hour, is particularly striking. Significant increase in yield stress (+25%) accompanied by marked decrease in ductility (-50%) is observed. Therefore, the aim of the post-doctorate will be to investigate how solutes contained in air diffuse into the alloy during the exposure, and modify the microscopic deformation mechanisms. This will concern the major constituents of air, oxygen and nitrogen, but also minor constituents like hydrogen or carbon.

For this purpose, heat treatments in controlled atmospheres will be carried out at CIRIMAT. Then, because the contents of these solutes in the TiAl alloys are very low (below 1000 wt. ppm), specific techniques like SIMS and GDMS concentration profiling will be employed, in collaboration with the GEMAC laboratory in Versailles. The second part of the post-doctorate will concern the interactions of the dislocations contained in the γ phase of TiAl with the solutes, at room and high temperatures. For this purpose, TEM observations will be carried out on pre-deformed specimens (post-mortem investigations), to identify the influence of the solutes on the mechanisms involving dislocations. The electron tomography technique, which is emerging internationally, and has recently been developed at CEMES, will also be employed, to accurately determine the 3D dislocations morphologies from which the habit planes orientations and, hence, the mechanisms, will be obtained. Deformation of specimens at room and high temperatures in the TEM (in-situ experiments) will also be performed, to identify the role of the solutes on the velocity of the dislocations. Finally, one of the potential cause of the decrease in ductility being phenomena occurring at the grain boundaries (like segregation of solutes for example), characterizations at the nanoscale will be carried out at these locations, for example by nano-indentation across the grain boundaries. From the above characterizations, better understanding of the solute effects at the nanoscale in TiAl alloys is expected, from which solutions for decreasing the embrittlement tendency induced by exposure to air at high temperature could be proposed, by selection of suitable alloying elements.