PhD defence of Jules Baton

Thanks to its high ductility at room temperature, pure tantalum is ideal for the cold forming of parts with complex geometries with minimal rupture risks. This thesis aims at understanding and providing a description of the physical mechanisms taking place during a plastic deformation and then an annealing.

 

Various samples of pure tantalum were cold deformed by compression and rolling. The deformed microstructures were then characterized by scanning electron microscopy at grain and substructure scales. These characterizations revealed that the substructure development is strongly influenced by the crystallographic orientation of the grains and also by the texture of the initial state. In particular, γ-fiber grains form more substructures than θ-fiber grains. These differences cannot be correctly depicted only by dislocation density values. Parameters for the quantification of substructures have been proposed and their evolutions have been described by models.

 

Recrystallization is greatly impacted by the orientation dependence of the deformed state. Nucleation is promoted in γ-fiber grains due to the more advanced substructure development during deformation. This heterogeneity of behavior can be well described with stored energy estimated at substructure scale. Recovery has been studied directly and indirectly through its effects on recrystallization with various pre-recovery treatments. Two opposite effects on recrystallization were observed. A first non-contributive effect is related to the dislocation annihilation, which involves a decrease in the driving force for recrystallization. A second contributive effect is related to the enhancement of migration ability of subboundaries, which promotes nucleation. The balance between these two effects determines the overall effect of recovery on recrystallization. This balance varies depending on the deformation, the crystallographic orientation and the conditions of the pre-recovery treatment (time and temperature).