Damon Kent

Iron is employed as an alloying element in powder metallurgy Ti alloys for healthcare applications owing to its fast diffusion which aids sintering, excellent biocompatibility, and cost-effectiveness. In this study, we assess the use of Ge to enhance the sintering and mechanical properties of a Ti-7Fe based powder metallurgy alloy (Ge = 0, 2, and 4 wt.%). Germanium is an α-stabilising element with good solubility in the Ti matrix and possesses better biocompatibility than the commonly used aluminium α-stabiliser. After sintering, all alloys exhibited dual-phase microstructures (α and β phase), while an intermetallic Ti5Ge3 also formed in Ti-7Fe-4Ge. Despite being an α-stabilising element, germanium primarily segregated to the β phase regions in the Ti-7Fe-xGe alloys. Gradual increments of the sintered density of the alloys were observed with increasing Ge additions. The sintered Ti-7Fe-xGe alloys were solution treated to exploit the metastability of the retained β phase. After solution treatment at 1000°C, the alloys predominantly exhibited β phase with minor traces of martensitic athermal ω-phase. Germanium suppressed the formation of the athermal ω-phase and reduced the stability of the β phase. The solution-treated Ti-7Fe-2Ge alloy is most promising for implant applications with a relatively low compressive Young's modulus (∼ 70 GPa) and high yield strength (∼ 1450 MPa), which leads to an outstanding elastic admissible strain of ∼ 2.2%.

Novel microstructural features were found in the Ti−Nb−Zr−Mo−Sn alloy manufactured by Laser Engineered Net Shaping (LENS). Examination of the microstructure showed that the fabricated sample exhibits a layered morphology with arced deposit boundaries. Novel distributions and morphologies of various phases including β, α, α'' and ω were detected in the LENS-manufactured part which substantially differ to conventionally processed alloy counterparts. The β grains and subgrains spread over multiple deposits and layers, aligned to the build direction, forming a complex network microstructure comprising large highly textured columnar grains aligned to β phase <001> orientations. The α precipitates have needle-like shapes and are widely distributed across a majority of the deposited layers, whereas the nanoscale ω particles were present in regions absent of α precipitation. Localised, massively transformed α'' phase with a very long and curved rod-like shape and substantial surface defects was identified. The formation of these novel microstructural features is investigated and discussed in the context of the characteristics of the LENS fabrication process. The microstructures are attributed to the complex thermal history in the unique deposit-by-deposit and layer-by-layer method employed during LENS additive manufacturing in conjunction with the complex precipitation behaviours exhibited by TiNb-based alloys. The characteristics and formation mechanisms of the LENS-manufactured Ti−Nb−Zr−Mo−Sn alloy microstructures revealed here provide a basis to optimize LENS and post-LENS heat treatment processes to optimize microstructures for improved performance.

The potential of using external fields to reduce the cost of emerging magnesium alloys used for the manufacture of medical implant devices while maintaining biocompatibility and mechanical performance is explored. Zirconium (Zr) is often added to high performance magnesium alloys as a grain refiner which is expensive, and there is uncertainty about whether or not Zr is harmful to the biocompatibility of the alloy. This is a significant barrier to the adoption of alloys that contain Zr because of the need to undertake cytotoxicity and haemocompatibility testing, as well as evaluation of long-term clinical effects. An alternative approach is to investigate methods to minimise or eliminate the need for Zr addition. External fields applied during solidification of the alloy provide an opportunity to produce a refined microstructure without the addition of Mg-Zr master alloy presenting a distinct advantage both from biodegradation and biocompatibility perspectives. Preliminary work evaluates the effect of ultrasonic treatment on microstructure and mechanical properties of Mg-Zn-Ca alloys with and without Zr and then compares their in-vivo corrosion performance to that of WE43. A framework for future research is proposed along with opportunities for the development of lower cost alloys with at least equal biocompatibility, mechanical and corrosion performance.

A novel metastable β-Ti alloy Ti-4Mo-3Cr-1Fe with high strength and high ductility was developed through controlling the alloy's stability and deformation mechanisms. The microstructure consists of randomly oriented β-grains containing an even distribution of athermal ω precipitates. Under tensile loading, the alloy exhibits unprecedented and comprehensive mechanical properties including a high yield strength of 870 MPa, excellent total elongation of 41% and an ultrahigh strain hardening rate of 2.5 GPa. Based on investigations of deformation microstructures, the superior mechanical properties are attributed to stress-induced formation of a complex nano-scale hierarchical twin structure which is promoted by reversion of ω precipitates.

There is increasing demand for synthetic bone scaffolds for bone tissue engineering as they can counter issues such as potential harvesting morbidity and restrictions in donor sites which hamper autologous bone grafts and address the potential for disease transmission in the case of allografts. Due to their excellent biocompatibility, titanium scaffolds have great potential as bone graft substitutes as they mimic the structure and properties of human cancellous bone. Here we report on a new thermoset bio-polymer which can act as a binder for Direct Ink Writing (DIW) of titanium artificial bone scaffolds. We demonstrate the use of the binder to manufacture porous titanium scaffolds with evenly distributed and highly interconnected porosity ideal for orthopaedic applications. Due to their porous structure, the scaffolds exhibit an effective Young's modulus similar to human cortical bone, alleviating undesirable stress-shielding effects, and possess superior strength. The biocompatibility of the scaffolds was investigated in vitro by cell viability and proliferation assays using human bone-marrow-derived Mesenchymal stem cells (hMSCs). The hMSCs displayed well-spread morphologies, well-organized F-actin and large vinculin complexes confirming their excellent biocompatibility. The vinculin regions had significantly larger Focal Adhesion (FA) area and equivalent FA numbers compared to that of tissue culture plate controls, showing that the scaffolds support cell viability and promote attachment. In conclusion, we have demonstrated the excellent potential of the thermoset bio-polymer as a Direct Ink Writing ready binder for manufacture of porous titanium scaffolds for hard tissue engineering.

To optimize both the mechanical and biological properties of titanium for biomedical implants, a highly flexible powder metallurgy approach is proposed to generate porous scaffolds with graded porosities and pore sizes. Sugar pellets acting as space holders were compacted with titanium powder and then removed by dissolution in water before sintering. The morphology, pore structure, porosity and pore interconnectivity were observed by optical microscopy and SEM. The results show that the porous titanium has porosity levels and pore size gradients consistent with their design with gradual and smooth transitions at the interfaces between regions of differing porosities and/or pore sizes. Meanwhile, the porous titanium has high interconnectivity between pores and highly spherical pore shapes. In this article we show that this powder metallurgy processing technique, employing the novel sugar pellets as space-holders, can generate porous titanium foams with well-controlled graded porosities and pore sizes. This method has excellent potential for producing porous titanium structures for hard tissue engineering applications.

For multicomponent near-β alloys, we have investigated the mechanisms responsible for the β-to-ω and ω-to-α phase transformations upon isothermal ageing at 573 K. Experimental evidence from atom probe tomography and aberration-corrected high-resolution transmission electron microscopy indicates that the formation of isothermal ω involves a structural reconstruction assisted by nanoscale spinodal decomposition of the β matrix, prior to the specific chemistry change required to form ω, rather than a mixed-mode process with structure and chemistry changes occurring simultaneously as has been previously suggested. First, incommensurate embryonic ω evolve via a displacive mechanism within Mo-lean regions created by second-order coherent spinodal decomposition of the β matrix. The subtle spinodal decomposition in β and chemistry of embryonic ω are carefully analysed by an advanced atom probe data analysis algorithm. When the size of embryonic É· exceeds a critical value, commensurate isothermal É· forms through the exit of the other alloying solutes. O-rich regions present at the isothermal ω/β interface provide potent sites for the formation of α. The concurrent compositional partitioning of solutes in É· and α indicates the transformation from ω to α involves both a rapid lattice reconstruction at the ω/α interface and a slow Al diffusion at the α/β, therefore a mixed-mode displacive-diffusive process. This study provides novel experimental evidence to understand the much-disputed transformation processes and elucidate the mechanisms responsible for these important phase transformations.

This study demonstrates that trace boron addition to Ti-6Al-4V coupons produced by additive layer manufacturing is an effective way to eliminate the deleterious anisotropic microstructures often encountered with this manufacturing technique. Trace boron additions (up to 0.13 wt.%) to this alloy eliminate grain boundary-a and colony-a, and instead produce a homogeneous a-microstructure consisting of fine equiaxed a-grains in both as-deposited and heat treated coupons. Prior-b grains remain columnar with boron addition but become narrower due to the wider solidification range and growth restricting effect of the boron solute. Compared to unmodified Ti-6Al-4V alloy, Ti-6Al-4V modified with trace boron additions showed up to 40% improvement in plasticity with no loss in strength under uniaxial compression at room temperature. Boron additions were found to inhibit twinning transmission that causes sudden large load drops during deformation of the unmodified Ti-6Al-4V alloy in the heat treated condition.

Cold-deformability and mechanical compatibility of the biomedical β-type titanium alloy are the foremost considerations for their application in stents, because the lower ductility restricts the cold-forming of thin-tube and unsatisfactory mechanical performance causes a failed tissue repair. In this paper, β-type titanium alloy (Ti-25Nb-3Zr-3Mo-2Sn, wt%) thin-tube fabricated by routine cold rolling is reported for the first time, and its elastic behavior and mechanical properties are discussed for the various microstructures. The as cold-rolled tube exhibits nonlinear elastic behavior with large recoverable strain of 2.3%. After annealing and aging, a nonlinear elasticity, considered as the intermediate stage between "double yielding" and normal linear elasticity, is attributable to a moderate precipitation of α phase. Quantitive relationships are established between volume fraction of α phase (Vα) and elastic modulus, strength as well as maximal recoverable strain (εmax-R), where the εmax-R of above 2.0% corresponds to the Vα range of 3-10%. It is considered that the "mechanical" stabilization of the (α+β) microstructure is a possible elastic mechanism for explaining the nonlinear elastic behavior.

The strength of a biomedical ββ-type alloy, Ti-25Nb-3Zr-3Mo-2Sn, was enhanced through severe plastic deformation using a modified accumulative roll bonding technique. Incremental strength increases were observed after each cycle, while ductility initially fell but showed some recovery with further cycles. After 4 cycles there was a 70% improvement in the ultimate tensile strength to 1220 MPa, a two-fold increase in the 0.5% proof stress to 946 MPa and the ductility was 4.5%. The microstructure comprised of ultrafine grain ββ grains heavily elongated in the rolling direction with a fine dispersion of nanocrystalline αα phase precipitates on the ββ grain boundaries. Shear bands formed in order to accommodate large plastic strains during processing and the grains within the bands were significantly finer than the surrounding matrix.