Ultralow Modulus and High Strength Ti-25Nb (at%) Alloy

A critical challenge in the field of orthopedic implants lies in the fact that implant materials typically have a much higher elastic modulus than human bones. This discrepancy can lead to a complication known as "stress shielding," which causes bone weakening over time. Metastable β titanium (Ti) alloys are regarded as the next generation of biomedical structural materials due to their exceptional biocompatibility, superior strength, and impressive wear resistance. Most importantly, they exhibit lower moduli compared to other metallic biomaterials. However, the challenge remains, as the moduli of recently developed metastable β Ti alloys rarely fall below 50 GPa.

The objective of this dissertation is to provide valuable insights into achieving ultralow modulus in metastable β-Ti alloys for biomedical applications. The Ti-25Nb (at%) alloy, which belongs to a unique group known as β-Ti shape memory alloys, is studied in this dissertation. This dissertation consists of two primary sections. Part 1 (Chapter 2) employs a conventional metallurgy approach using thermo-mechanical treatments to develop a desirable microstructure in Ti-25Nb, aiming to achieve an elastic modulus of 30 GPa or lower. This microstructure can initiate stress-induced martensitic transformation (SIMT) at near-zero external stress at room temperature. Part 2 (Chapters 3 and 4) utilizes a modern additive manufacturing (AM) approach, specifically laser powder bed fusion (LPBF). Since the current literature offers limited understanding, Chapter 3 initially investigates the AM microstructure of Ti-25Nb. A novel, unknown orthorhombic phase, as well as the O’ phase, are discovered in the AM Ti-25Nb samples. In Chapter 4, we propose an innovative approach for LPBF that enables the production of Ti-25Nb samples with a low modulus of approximately 30 GPa, while ensuring high strength.