Contents lists available at ScienceDirect Journal of the Mechanical Behavior of Biomedical Materials journal homepage: www.elsevier.com/locate/jmbbm Research Paper Increasing strength of a biomedical Ti-Nb-Ta-Zr alloy by alloying with Fe, Si and O Josef Stráský a, ⁎ , Petr Harcuba a , Kristína Václavová a , Klaudia Horváth a , Michal Landa b , Ondřej Srba c , Miloš Janeček a a Department of Physics of Materials, Faculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 5, 121 16 Prague 2, Czech Republic b Institute of Thermomechanics, Academy of Sciences of the Czech Republic, Dolejskova 5, 182 00 Prague 8, Czech Republic c Structural and system diagnostic, Research Centre Rez, Hlavni 130, Husinec-Rez, Czech Republic ARTICLE INFO Keywords: β-Ti alloys Orthopaedic implants Elastic modulus Strengthening mechanisms Ductility Ultrasound spectroscopy ABSTRACT Low-modulus biomedical beta titanium alloys often suffer from low strength which limits their use as load- bearing orthopaedic implants. In this study, twelve different Ti-Nb-Zr-Ta based alloys alloyed with Fe, Si and O additions were prepared by arc melting and hot forging. The lowest elastic modulus (65 GPa) was achieved in the benchmark TNTZ alloy consisting only of pure β phase with low stability due to the ‘proximity’ to the β to α’’ martensitic transformation. Alloying by Fe and O significantly increased elastic modulus, which correlates with the electrons per atom ratio (e/a). Sufficient amount of Fe/O leads to increased yield stress, increased elongation to fracture and also to work hardening during deformation. A 20% increase in strength and a 20% decrease in the elastic modulus when compared to the common Ti-6Al-4V alloy was achieved in TNTZ-Fe-Si-O alloys, which proved to be suitable for biomedical use due to their favorable mechanical properties. 1. Introduction Replacement of large joints is considered as a major achievement in the orthopaedic surgery. However, an appropriate implant material is also a big challenge for material scientists. Along with knee arthro- plasty, the hip endoprosthesis is the most demanded joint implant. One of the most delicate issues in hip implant design is the femoral stem that is crucial to prevent the implant from loosening. In fact, the loosening of the implant is one of the most frequent causes of implant failure (Chu et al., 2002). 152,000 hip joint replacements were performed in the US in 2000, thereof almost 13% were revisions and reoperations of previous hip replacement (Long and Rack, 1998). The percentage of reoperations will rise due to the longer life expectation and more active life-style. Therefore, the demand for implants with enhanced life-time will be increasing. Development of orthopaedic implants is a complex and multi-field scientific issue. Titanium alloys have been extensively applied in orthopaedics for several decades due to their superior mechanical properties, excellent corrosion resistance and favourable biocompat- ibility (Geetha et al., 2009, Katti, 2004, Long and Rack, 1998, Rack and Qazi, 2006). Elastic modulus of the implant material determining its stiffness is currently a widely discussed topic. Typical elastic modulus of Ti and common Ti alloys is around 100 GPa, while elastic modulus of the cortical bone ranges from 20 to 30 GPa and elastic modulus of cancellous bone is even lower (7–15 GPa) (Niinomi et al., 2012, Rho et al., 1993, Zysset et al., 1999). This difference in stiffness of implant and surrounding bone leads to the transmission of the applied load through the implant stem and consequently, the surrounding bone is not loaded (so-called stress-shielding effect). The bone tissue that is not regularly loaded becomes atrophied and is prone to failure. Therefore, materials with reduced elastic modulus are being developed. The relationship between the elastic moduli (E) of different phases in Ti can be expressed as follows E β ≈ E α’’ ≈ 60–85 GPa < E α ≈ 100 GPa <E ω ≈ 130 GPa (Niinomi, 1998, Nejezchlebová et al., 2016, Sun et al., 2007, Tane et al., 2013), which demonstrates the interest in β-Ti alloys. Metastable β-Ti alloys have been developed since 1960s (Lütjering and Williams, 2007). The dominant area of application is the aerospace industry. However, two decades ago, specialized biocompatible alloys also emerged. The most used β stabilizing alloying elements are vanadium, chromium, iron, molybdenum and niobium. Nb and Zr are regarded as biocompatible alloying elements, whereas V, Cr and Co are considered inappropriate (Steinemann, 1998). The design of biomedical alloys for orthopaedic use therefore faces several limitations. Firstly, only biotolerant elements can be used. Secondly, sufficient strength level must be achieved. And thirdly, elastic modulus should be reduced well below 100 GPa. Note that the latter two requirements are often in a trade-off relationship. The Ti-Nb-Ta-Zr alloying system is a highly biocompatible material http://dx.doi.org/10.1016/j.jmbbm.2017.03.026 Received 23 October 2016; Received in revised form 4 March 2017; Accepted 25 March 2017 ⁎ Corresponding author. Journal of the mechanical behavior of biomedical materials 71 (2017) 329–336 Available online 02 April 2017 1751-6161/ © 2017 Elsevier Ltd. All rights reserved. MARK