Nolwenn RINCE PhD defense

Abstract
Metallic alloys used in medical field exhibit limitations in terms of biocompatibility. Additionally, their mechanical properties often differ from those of bone, which can affect the durability of protheses and implants. Metastable β-titanium alloys are promising for address this issue due to their excellent biocompatibility and elastic modulus close to that of bone. This study focuses on quaternary TiZrNbSn alloys produced by in situ laser powder bed fusion (LPBF), from a blend of pure powder.
After optimizing processing parameters to achieve a dense and homogeneous material, the microstructural and mechanical properties of Ti-22Zr-11Nb-2Sn are characterized, with a particular focus on stress-induced martensitic transformation (SIM) and plastic deformation mechanisms, especially twinning.
The optimization of mechanical properties is then explored through multiple approaches. First, post-fabrication heat treatments are applied to this alloy, revealing a strong sensitivity to cooling rates, whereas treatment temperatures have limited impact. Then, alloy composition is adjusted, demonstrating that niobium content influences material properties. Increasing niobium stabilizes the β phase, while reducing it enables new mechanical behaviors, as a significant strain hardening due to the stress-induced martensitic transformation (TRIP effect) and the twinning (TWIP effect).
Finally, an exploratory study introduces yttria (Y₂O₃) nanoparticles to achieve structural hardening and high mechanical strength. This approach is guided by an in-depth analysis of the precipitates formed and their influence on the alloy’s properties.

Jury
Emmanuel BERTRAND, Associate Professor, Polytech Nantes, Université de Nantes / Reviewer
Guy DIRRAS, Professor, Université Sorbonne Paris Nord / Reviewer
Louise LE BARBENCHON, CNRS Researcher, I2M Bordeaux / Examiner
Patrice PEYRE, CNRS Research Director, PIMM Paris / Examiner
Philippe CASTANY, Associate Professor, ISCR, INSA Rennes, Université de Rennes / PhD Director
Thierry GLORIANT, Professor, ISCR, INSA Rennes, Université de Rennes / PhD Co-Director

Contact
[Philippe Castany](https://iscr.univ-rennes.fr/philippe-castany), [philippe.castany@insa-rennes.fr](mailto:philippe.castany@insa-rennes.fr)