Titanium alloys find vast applications as orthopedic implant materials. This is as a result of their excellent mechanical properties and biocompatibility. However, with the recent development in alloys with superior fatigue, strength, corrosion resistance and biochemical impact, there is still an issue to do with high elastic modulus that leads to a mismatch in stiffness between the bone and the implant. The mismatch is known to initiate stress shielding.
A porous structure can be used to reduce the implant’s stiffness, but will reduce its yield strength. It therefore becomes necessary to consider the material’s elastic admissible strain, which is the ratio of the yield strength to elastic modulus. A material with high elastic admissible strain is desirable because it can maintain sufficient strengths even when stiffness is reduced. Thermomechanical methods proposed for improving titanium mechanical properties, are not desirable and therefore it is important to focus on mechanical improvement with minimal processing in the as-cast state.
Spinodal decomposition is one alternative that does not require particle nucleation, thus, allowing uniform nanostructures to be realized without long aging and granting fine control over the microstructure via aging. Researchers led by Professor Cuie Wen investigated the potential value of spinodally strengthened structure in the biomaterial field. They needed to know whether the spinodal effect could be reliably induced in a straightforward approach and whether this would contribute to material strength without increasing the modulus. Their work is now published in Acta Biomaterialia.
The authors developed a novel β-type Titanium-Zirconium-Tantalum (Ti-Zr-Ta) alloy system with varying concentrations of zirconium and tantalum and characterized it both microstructurally and mechanically. As-cast ingots were then cut into rods of 10 mm length and 5mm diameter for compression tests and 2mm thick discs with 10 mm diameter for microstructural analysis and biocompatibility assessment.
The authors cut transmission electron microscopy specimens using a diamond-tipped saw and ground them to about 60nm thickness. They performed uniaxial compression testing on a small rod specimen with a constant compression rate. They measured the strain through a combination of inbuilt strain gauge and a video monitoring system. The researchers used human osteoblast-like sarcoma cells in assessing the biological response of the alloys.
The authors observed that the spinodal decomposition initiated by enthalpy of mixing of the tantalum and zirconium species yielded a uniform nano-scaled dispersion of the tantalum β2 phase in the zirconium β1 matrix. This was however characterized by a cuboidal modulation arranged in the (1 0 0) body centered cubic directions of the lattice. The cuboid volume fraction increased with tantalum content.
Titanium served an important role in moderating the amount of spinodal decomposition, but ensuring that enough β-stabilization remained in the zirconium β1 phase in a bid to retard undue α-precipitation regardless of the low timescales related to cooling in the as-cast state.
The spinodal decomposition contributed to the alloys’ exceptional strength. Compressive yield strengths of 1220MPa and 1400 MPa were achieved in the alloy with the highest tantalum content, with these strengths allowing elastic admissible strains of 1.25-1.48% to be realized in the as-cast state. The strength was realized without sacrificing biocompatibility.
Arne Biesiekierski1,2, Dehai Ping3, Yuncang Li1, Jixing Lin4, Khurram S. Munir1, Yoko Yamabe-Mitarai3, Cuie Wen1,2. Extraordinary high strength Ti-Zr-Ta alloys through nanoscaled, dual-cubic spinodal reinforcement. Acta Biomaterialia, volume 53 (2017), pages 549–558.Show Affiliations
- School of Engineering, RMIT University, Melbourne, Victoria 3001, Australia
- Faculty of Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia
- National Institute for Materials Science, Tsukuba 305-0047, Japan
- College of Materials Science and Engineering, Jilin University, Changchun, Jilin 130025, China
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