Metallic implants used in modern traumatology and orthopaedics are subject to increasing demands due to ageing and physical activity of today's society. The clinically used (α+β) Ti-6Al-4V alloy still has major drawbacks that can lead to implant failure [1-3]: Firstly, it contains allergenic or cytotoxic elements that can cause inflammatory reactions, especially in immunosuppressed patients, and secondly, its high Young's modulus compared to human bone results in stress shielding effects that can lead to bone atrophy and implant loosening. To solve these issues, a design strategy for novel metastable β-type Ti-Cr alloys with excellent biomechanical compatibility and lower elastic modulus is proposed, where Cr is used due to its high cytocompatibility [4] and increased tendency to form a passive film on the implant surface [5].

A critical characteristic of metastable β-Ti alloys is the inevitable formation of ω-phase precipitates leading to embrittlement, or even to a complete loss of ductility. In this work, Sn was added to a β-type Ti-Cr alloy. Sn is known to affect the formation of ω-phase while not reducing the biocompatibility [6] and further passivating the surface of the material [7]. Upon the addition of Sn, we observed an extensive deceleration or even suppression of the ω-formation kinetics which in turn also influences the ω-assisted α-transformation during thermal treatment. The activation energy of the elementary ω-formation process remained almost unchanged, while the formation of β-stabiliser depleted regions susceptible to the transformation appears to be strongly reduced. This suggests, that the addition of Sn drastically slows down the spinodal decomposition prior to the transformation but has most likely only little effect on the thermodynamic stability of the different phases formed.

Our results show, that the addition of Sn can significantly increase the width of the time-temperature process window and the long-term ageing resistance of β-Ti alloys, opening up huge opportunities for advanced alloy design and additive manufacturing routes for ready-to-use parts exploiting the low elastic moduli of the β-phase.

_{[1] M. Niinomi, Sci. Technol. Adv. Mater. 4 (2003) 445-454
[2] M. Geetha et al., Prog. Mater. Sci. 54 (2009) 397-425
[3] Ch. E. Lekka et al., J. Phys. Chem. Sol. 102 (2017) 49-61
[4] R. A. Anderson, Regul. Toxicol. Pharmacol. 26 (1997) 35-41
[5] M. Niinomi et al., Regen. Biomater. 3 (2016) 173-185
[6] M. A.-H. Gepreel et al., J. Mech. Behav. Biomed. Mater. 20 (2013) 407-415
[7] P. E. L. Moraes et al., Mater. Charact. 96 (2014) 273-281
[8] F. Brumbauer et al., Acta Mater. 262 (2024) 119466}