R. J. Dashwood

Titanium foams for biomedical applications: a review

Abstract Metals are the oldest of biomedical implant materials and metallic alloys remain the material of choice for applications involving hard tissue replacement. Ti alloy scaffolds are deemed the best among all the metallic alloys. Recently, porous Ti alloy scaffolds have received increasing attention over other metallic counterparts, including monolithic alloys, due to advantages associated with an open porous structure. The main advantages of open porous structures are a low Youngs moduli and enhanced bone ingrowth leading to better fixation with the host tissue. In this paper, the authors first review the suitability of Ti for biomedical applications and then explore the methods for producing highly porous Ti foams. The methods are assessed based on their ability to produce a macro-micro-structure appropriate for biomedical applications. The article concludes with a future outlook on porous Ti production.

Modeling of porosity formation in direct chill cast aluminum–magnesium alloys

Abstract A model of the formation of porosity during the DC casting of Al–Mg alloys was developed and incorporated as a post-processor to a commercial transient macromodel of the three dimensional heat transfer and fluid flow. The porosity model not only predicts the percentage porosity, but also the size, shape and distribution of the pores. The sensitivity of the model to process and alloy variations was evaluated, showing the importance of the cooling rate and hydrogen concentration. An experimental study of the amount of porosity in laboratory scale (250×400 mm cross-section) DC cast ingots of Al 2, 4 and 6 wt.% Mg was performed. The results from these experimental billets were used to validate the model as a function of the location in the ingot and the initial hydrogen and magnesium content. The model correctly predicted the experimentally observed trends, showing good correlation to the measured percentage porosity.

Characterization of the deformation behavior of intermediate porosity interconnected Ti foams using micro-computed tomography and direct finite element modeling.

Under load-bearing conditions metal-based foam scaffolds are currently the preferred choice as bone/cartilage implants. In this study X-ray micro-computed tomography was used to discretize the three-dimensional structure of a commercial titanium foam used in spinal fusion devices. Direct finite element modeling, continuum micromechanics and analytical models of the foam were employed to characterize the elasto-plastic deformation behavior. These results were validated against experimental measurements, including ultrasound and monotonic and interrupted compression testing. Interrupted compression tests demonstrated localized collapse of pores unfavorably oriented with respect to the loading direction at many isolated locations, unlike the Ashby model, in which pores collapse row by row. A principal component analysis technique was developed to quantify the pore anisotropy which was then related to the yield stress anisotropy, indicating which isolated pores will collapse first. The Gibson-Ashby model was extended to incorporate this anisotropy by considering an orthorhombic, rather than a tetragonal, unit cell. It is worth noting that the natural bone is highly anisotropic and there is a need to develop and characterize anisotropic implants that mimic bone characteristics.

Predominance Diagrams for Electrochemical Reduction of Titanium Oxides in Molten CaCl2

Diagrams showing equilibrium potentials (relative to the standard chlorine electrode) plotted vs the negative logarithms of oxide-ion activity (pO 2 - ) have been constructed for titanium species including oxides, chlorides, and metal, in molten calcium chloride over the temperature range 800-1100°C. The utility of such diagrams is particularly evident in the field of the molten salt extractive metallurgy of titanium via recently developed electrochemical processing methods. These plots suggest ideal process routes that produce low-oxygen titanium under controlled melt composition and potential.

Hierarchically structured titanium foams for tissue scaffold applications.

We present a novel route for producing a new class of titanium foams for use in biomedical implant applications. These foams are hierarchically porous, with both the traditional large (>300μm) highly interconnected pores and, uniquely, wall struts also containing micron scale (0.5-5μm) interconnected porosities. The fabrication method consists of first producing a porous oxide precursor via a gel casting method, followed by electrochemical reduction to produce a metallic foam. This method offers the unique ability to tailor the porosity at several scales independently, unlike traditional space-holder techniques. Reducing the pressure during foam setting increased the macro-pore size. The intra-strut pore size (and percentage) can be controlled independently of macro-pore size by altering the ceramic loading and sintering temperature during precursor production. Typical properties for an 80% porous Ti foam were a modulus of ∼1GPa, a yield strength of 8MPa and a permeability of 350 Darcies, all of which are in the range required for biomedical implant applications. We also demonstrate that the micron scale intra-strut porosities can be exploited to allow infiltration of bioactive materials using a novel bioactive silica-polymer composite, resulting in a metal-bioactive silica-polymer composite.

The Production of Ti–Mo Alloys from Mixed Oxide Precursors via the FFC Cambridge Process

Ti-10 wt. % W alloys were produced via the electrochemical deoxidation of mixed TiO2+WO3 sintered precursors in a molten CaCl2 electrolyte at 1173 K. The reduction of these ceramic precursors was characterised by analysing several partially reduced samples taken periodically through the deoxidation process. Fully metallic samples were retrieved after 15 h of reduction. This reduction time was longer than that observed by the authors for metallisation of (Ti,Mo)O2 sintered precursors. This was believed to occur as a result of significant differences in the reduction pathway, despite tungsten and molybdenum possessing similar interactions with titanium (group VI elements). It was found that reduction initiated with the rapid reduction of WO3 to a W-Ti particulate. TiO2 then proceeded to reduce sequentially through the lower oxides, with the formation of Ca(Ti,W)O3. Between 1 h and 3 h of reduction the sample is believed to be composed of Ca(Ti,W)O3 and TiO. A comproportion reaction between the two phases is then observed with the formation of W-Ti and CaTi2O4, which then proceeds to reduce to titanium. However homogenisation between the product titanium and W-Ti does not take place until the titanium is sufficiently deoxidised, thus β Ti forms late in the reduction process. It is believed that the lack of formation of β Ti early on in the reduction process, coupled with the lack of formation of a conductive (Ti,Mo)O network, accounts for the relatively slow reduction time.

Production of NiTi via the FFC Cambridge Process

The FFC Cambridge process is a direct electrodeoxidation process used to reduce metal oxides to their constituent metals in a molten CaCl2 salt bath. NiTiO3 was used as a precursor (the first stable oxide to form upon blending and sintering NiO and TiO2 powders) and was successfully reduced using the FFC Cambridge process at 1173 K and a constant cell voltage of -3.1 V to produce a NiTi alloy. This work builds on the literature work [Chinese Science Bulletin, 51, 2535 (2006)] through: (i) a predominance diagram calculated to show the regions of phase stability throughout the usable potential window of the CaCl2 salt; (ii) the investigation of a wide range of reduction times for a fixed cell voltage, elucidating several additional stable phases, to yield a complete and detailed reduction pathway. The reduction pathway for NiTiO3 was identified through the analysis of a series of partial reductions, with fully reduced NiTi formed after a period of 24 h. The first stage of the reaction involved the rapid formation of Ni and CaTiO3. The reduction then proceeded via the formation of the intermediate compounds Ni3Ti and Ni2Ti4O. All the NiTiO3 and Ni were consumed after a period of 6 h, while the intermediate compounds remained until the reaction was near completion. The experimental results related well to the thermodynamic predictions of the predominance diagram. A small variation in stoichiometry of the produced NiTi observed from the edge to the core of the samples was attributed to redeposition of Ti on the sample surface from the salt and a slightly Ti-rich NiTiO3 precursor material. (c) 2008 The Electrochemical Society. [DOI: 10.1149/1.2987739] All rights reserved.

Application of X-ray tomography to quantify the distribution of TiB2 particulate in aluminium

Characterisation of the volume percentage and distribution of reinforcement particles in metal matrix composites with microfocal X-ray tomography was investigated using titanium diboride (TiB2) in an aluminium matrix as a model system. Comparison to other techniques, including scanning electron and optical microscopy, illustrates the benefits of tomography.

Application of novel technique to examine thermomechanical processing of near β alloy Ti–10V–2Fe–3Al

Abstract A novel specimen design and testing strategy has been exploited to determine the effect of thermomechanical processing on the microstructural development of the near β titanium alloy, Ti–10V–2Fe–3Al. A double truncated cone test geometry was isothermally deformed at near βtransus (∼795°C) temperatures, to obtain microstructural information for a range of strains within a single specimen. A finite element modelling package was employed to produce strain profiles, which readily correspond to the equivalent microstructural profiles of the test specimens. A parametric study of the effects of process (e.g. friction) and material (e.g. strain rate sensitivity) parameters on the strain distributions obtained during the test was investigated. Finite element modelling was conducted to interpolate the friction ring compression test. The effectiveness of this testing strategy is illustrated with a qualitative description of the microstructural evolution with strain, for various strain rates, at the sub-βtransus forging temperature of 760°C.

Development of a high strain rate superplastic Al–Mg–Zr alloy

Abstract For superplastic forming of aluminium to break out of the niche market that it currently occupies, alloys will be required to possess a higher strain rate capability, appropriate in service properties, and a significantly lower price and to be capable of volume production. This paper describes an approach that has been developed in an attempt to address these fundamental requirements. A series of Al–Mg–Zr alloys with increasing levels of zirconium (0–1 wt-%)has been prepared via extrusion consolidation of cast particulate (solidification rate ∼103 K s-1). The superplastic properties of the resultant cold rolled sheet have been evaluated as a function of thermomechanical treatment and zirconium addition. It has been found that increasing the level of zirconium has the twofold effect of improving the superplastic properties of the alloy while significantly decreasing the concomitant flow stress. At present the optimum superplastic behaviour has been obtained at strain rates of 10-2 s-1, with the 1%Zr material exhibiting ductilities in excess of 600%. The manufacturing route produces a bimodal distribution of Al3Zr comprising >1 µm primary particles in combination with nanoscale solid state precipitates. The current postulation is that this high strain rate superplasticity is conferred by a combination of particle stimulated and strain induced recrystallisation.