Damon Kent

Strength enhancement of a biomedical titanium alloy through a modified accumulative roll bonding technique

The strength of a biomedical β-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 β grains heavily elongated in the rolling direction with a fine dispersion of nanocrystalline α phase precipitates on the β 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.

Effects of phase stability and processing on the mechanical properties of Ti-Nb based β Ti alloys

The influence of β phase stability on mechanical properties, deformation behaviours and phase composition were investigated for a series of Ti-24Nb-3Zr-2Sn-xMo alloys in response to hot and cold rolling. For the hot rolled alloys, the phase composition and deformation behaviours were largely consistent with those predicted on the basis of a Bo-Md plot and the Ms estimates. The deformation mechanisms involve growth and/or reorientation of plate-like martensite and/or twins. However, these are largely restricted in the cold rolled alloys due to the effects of grain refinement and residual stress. The cold rolled alloys exhibit the highest strengthening in combination with more limited ductility, which increased with increasing β phase stability. The moderately stable alloy, B, with e/a around 4.18 and Moeq∼10wt% gave the greatest strengthening in response to cold rolling, which was related to intense localised grain refinement.

Mechanical properties and biocompatibility of porous titanium scaffolds for bone tissue engineering

Synthetic scaffolds are a highly promising new approach to replace both autografts and allografts to repair and remodel damaged bone tissue. Biocompatible porous titanium scaffold was manufactured through a powder metallurgy approach. Magnesium powder was used as space holder material which was compacted with titanium powder and removed during sintering. Evaluation of the porosity and mechanical properties showed a high level of compatibility with human cortical bone. Interconnectivity between pores is higher than 95% for porosity as low as 30%. The elastic moduli are 44.2GPa, 24.7GPa and 15.4GPa for 30%, 40% and 50% porosity samples which match well to that of natural bone (4-30GPa). The yield strengths for 30% and 40% porosity samples of 221.7MPa and 117MPa are superior to that of human cortical bone (130-180MPa). In-vitro cell culture tests on the scaffold samples using Human Mesenchymal Stem Cells (hMSCs) demonstrated their biocompatibility and indicated osseointegration potential. The scaffolds allowed cells to adhere and spread both on the surface and inside the pore structures. With increasing levels of porosity/interconnectivity, improved cell proliferation is obtained within the pores. It is concluded that samples with 30% porosity exhibit the best biocompatibility. The results suggest that porous titanium scaffolds generated using this manufacturing route have excellent potential for hard tissue engineering applications.

Evolution of the microstructure and mechanical properties during fabrication of mini-tubes from a biomedical β-titanium alloy

The processing of Ti-25Nb-3Mo-3Zr-2Sn tubes with outside diameters of 5.6-8.0 mm and wall-thicknesses of 0.7-1.0 mm were investigated in order to study the evolution of microstructure and mechanical properties and their impact on the processing of the tubes. The annealed small tubes with single β phase microstructures exhibit double yielding during tensile tests. The onset of martensitic phase transformation was observed to occur after the lowest point of the strain hardening. Cold rolling also activates the formation of the stress induced martensitic α″ phase. Its volume fraction increased with increasing ε. The rate of strain hardening and the modulus of the tubes are related to the stress induced transformation of the β phase to the α″ phase. The stress induced α″ slightly improves the yield strength of the tubes at low levels of strain. However, larger strains result in grain growth during annealing, which diminishes the mechanical properties.

The Effect of Temperature on the Microstructure of a Metastable β Ti Alloy

A metastable β titanium alloy, BTi-6554 (Ti-6Cr-5Mo-5V-4Al) has been developed for structural applications in aircraft because of its high strength, high toughness, and good fatigue properties. This paper reports on the effect of heat treatment on microstructure and microhardness of the alloy. It has been shown that in the as hot rolled condition, the alloy consists of a single β phase. Heat treatment between 450-750°C results in the precipitation of α laths, while exposure to temperatures between 700-800°C results in the gradual transformation of the α phase back to β phase with larger grain sizes resulting from higher heat treatment temperatures.

Formation of aluminium nitride during sintering of powder injection moulded aluminium

Abstract A detailed transmission electron microscopy study of the structure of aluminium nitride formed during sintering of powder injection moulded aluminium is presented. A polycrystalline layer formed on Al particle surfaces exposed to a nitrogen atmosphere. This layer consisted of fine, rod-like crystallites of hexagonal AlN typically aligned normal to the Al surface. A double layer of AlN separated by a thin layer of Al was observed at the interfaces between Al grains. In this report, the structure of the nitride is characterised and its influence on sintering is discussed.

The Aging Response of a Metastable β Ti Alloy, BTi-6554

In this study the effects of different aging heat treatments on the properties and microstructure of a high strength, high toughness metastable β Ti, BTi-6554 (Ti-6Cr-5Mo-5V-4Al), have been compared. An initial β phase solution treatment was followed by aging at moderate temperatures in the α/β dual phase zone by either step aging directly from the solution treatment temperature or by quenching to room temperature prior to the aging heat treatment. The differing heat treatment methods have significant effects on the microstructure and mechanical properties.

Novel aluminium nitride surface coatings formed on aluminium

A new nitriding method has been devised which requires only a simple vacuum furnace and enables direct nitridation of solid aluminium without any prior surface treatment. It can be used to produce thick aluminium nitride surface layers on aluminium, under nitrogen at atmospheric pressure. A critical element of the process is the use of a magnesium vapour source that reduces/disrupts the natural, protective oxide film on the aluminium surface and facilitates nitriding. The nitride surface layers form through two distinct modes, one growing outward from the aluminium plate surface and the other growing into the aluminium. Studies of the nitride layers utilizing optical microscopy, TEM, SEM, XRD and XPS have been conducted. Details of the composition, structure and growth as well as possible mechanisms for the nitride formation are presented. Understanding of the reaction may have important implications for the production of wear resistant coatings on bulk Al as well as for the production of Al/AlN composites.

The In Situ Fabrication of Al-AlN Composites from Metal Powders and their Resistance to Wear and Cavitation

Recent breakthroughs in the sintering of aluminium alloys under nitrogen have opened the way for the in-situ fabrication of Al-AlN composites in a controllable and reproducible fashion over a wide range of volume fractions of AlN. This work reviews the fundamentals for the in-situ fabrication of the Al-AlN composites from metal powders and highlights their technical potential for niche applications because of their excellent resistance to cavitation erosion in water and their unusually low friction coefficient under oil lubrication.

The cold-rolling behaviour of AZ31 tubes for fabrication of biodegradable stents

Mg alloys are receiving considerable attention for biomedical stents due to their combination of good mechanical properties and high biodegradability. Cold rolling is necessary to process Mg alloy tubes before final drawing and fabrication of the magnesium stents. In this paper, cold-rolled tubes were subjected to a cross-sectional reduction rate (ε) of up to 19.7%, and were further processed at various ratios of wall-thickness to diameter reduction (Q) from 0 to 2.24 with a constant ε of 19.7%. The results show that the cold-rolled tubes exhibited a rise in ultimate tensile strength (UTS), yield strength (YS), and a reduction in elongation as ε increased from 5.5% to 19.7%. UTS, YS and elongation decreased when Q was increased from 0 to 2.24. Mechanical twinning was observed and analysed. Extension twins increased with increasing ε and were almost saturated at a ε of 16.5%. Extension twins play an important role in determining the evolution of mechanical behaviour in the case of increasing ε, whilst contraction/double twins and secondary extension twins have a large effect on mechanical behaviour in the case of varying Q. The results indicate that the proportions and types of twins play a major role in determining the mechanical behaviour of the AZ31 tubes.