Matthew S. Dargusch

Influence of microstructure on the corrosion of diecast AZ91D

The corrosion of die cast AZ91D was studied and related to its microstructure. For comparison and to more fully understand the behaviour of die cast AZ91D, corrosion studies and microstructural examinations were also carried out using slowly solidified high purity AZ91, Mg-2%Al, Mg-9%Al, low purity magnesium and high purity magnesium. Corrosion was studied in 1N NaCl at pH 11 by (1) observing the corrosion morphology, (2) measuring electrochemical polarisation curves and (3) simultaneously measuring both the hydrogen evolution rate and the magnesium dissolution rate. The skin of die cast AZ91D showed better corrosion resistance than the interior. This is attributed to a combination of(1) a higher volume fraction of the beta phase, (2) a more continuous beta phase distribution around finer alpha grains, and (3) lower porosity in the skin layer than in the interior of the die casting. This study showed that the casting method can influence the corrosion performance by its influence on the alloy microstructure

Grain-refinement mechanisms in titanium alloys

Despite the importance of the prior-beta grain structure in determining the properties of titanium-based alloys, there are few published studies on methods of controlling the size of these grains in commercial alloys. The existing research raises questions about the relative importance of solute elements in grain-refining mechanisms, particularly the common alloying elements of aluminum and vanadium. The effect of these elements was investigated by producing a series of castings in a nonconsumable arc-melting furnace, and the results were interpreted with the aid of available phase-diagram information and solute-based models of grain refinement. A small reduction in grain size was obtained with increasing solute additions; however, this was not expected from the theoretical analysis. Possible reasons for this discrepancy are discussed.

Biocompatibility and osteoconduction of active porous calcium–phosphate films on a novel Ti–3Zr–2Sn–3Mo–25Nb biomedical alloy

The purpose of this study is to investigate the biocompatibility and osteoconduction of active porous calcium-phosphate films on the novel Ti-3Zr-2Sn-3Mo-25Nb biomedical alloy. The active porous calcium-phosphate films were prepared by the micro-arc oxidation method on the surface of a near β biomedical Ti-3Zr-2Sn-3Mo-25Nb alloy, and then activated in a hydroxyl solution followed by an aminated solution. The phase composition, surface micro-topography and elemental characteristics of the active porous calcium-phosphate films were investigated with XRD, SEM, EDS and XPS. The biocompatibility was assessed using corrosion testing, the in vitro osteoblast cultivation test and implantation in soft tissue (subcutaneous and musculature). The osteoconduction was evaluated using the simulated body fluid test and by implantation in hard tissue. The results show that the active porous films are mainly composed of TiO(2) anatase and rutile. The oxide layer is a kind of porous ceramic intermixture containing Ca and P. Immersion in simulated body fluid can induce apatite formation on the porous calcium-phosphate films resulting in excellent bioactivity. Cell cultures revealed that MC3T3-E1 cells grew on the surface exhibiting favorable morphologies. These results indicate that the Ti-3Zr-2Sn-3Mo-25Nb biomedical alloy coated with an active porous calcium-phosphate film has been shown to have excellent corrosion resistance, good biocompatibility and osteoconduction, which can promote cell proliferation and bone formation.

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.

Review of effect of oxygen on room temperature ductility of titanium and titanium alloys

Abstract Room temperature tensile ductility is an important property of titanium (Ti) and titanium alloys for structural applications. This article reviews the dependency of tensile ductility on oxygen for α-Ti, (α+β)-Ti and β-Ti alloys fabricated via traditional ingot metallurgy (IM), powder metallurgy (PM) and additive manufacturing (AM) or three-dimensional printing methods and recent advances in understanding the effect of oxygen on ductility. Seven mechanisms have been discussed based on case studies of individual titanium materials reported in literature. The dependency of ductility on oxygen is determined by both the composition and microstructure of the titanium alloy. For Ti–6Al–4V (wt-%), as sintered Ti–6Al–4V shows a critical oxygen level of about 0·33 wt-% while additively manufactured Ti–6Al–4V exhibits different critical levels ranging from about 0·22% to well above 0·4% depending on microstructure. Rare earth (RE) elements are effective scavengers of oxygen in titanium materials even just with a small addition (e.g. 0·1 wt-%), irrespective of the manufacturing method (IM, PM and AM). High cycle fatigue experiments revealed no initiation of fatigue cracks from the resulting RE oxide particles over the size range from submicrometres to a few micrometres. A small addition of RE elements offers a practical and affordable approach to mitigating the detrimental effect of oxygen on ductility.

Experimental investigation of cutting forces and tool wear during laser-assisted milling of Ti-6Al-4V alloy

The improvement of machinability during laser-assisted milling of Ti-6Al-4V alloy was investigated. The effects of laser processing and milling parameters on cutting forces and tool wear have been examined. It is found that local heating and softening of the workpiece by the laser beam in front of the cutting tool significantly reduced the cutting forces, especially the force in the feed direction during up-cut milling. Laser power, tool–beam distance, depth of cut and cutting speed are the parameters influencing the change of feed force during laser-assisted milling. Analysis of the workpiece temperature rise due to laser beam heating shows that the feed force is strongly dependent on the workpiece temperature in front of the cutting zone; significant reduction of feed force occurred when the temperature in front of thecutting zone was in the range 200–450°C. Edge chipping is found to be the tool failure mode for both conventional milling and laser-assisted milling. A significant improvement in tool life during laser-assisted milling was obtained when the workpiece temperature in front of the cutting zone was at an optimum value. Compressed air was used to remove the chip from the cutting tool, which made the milling process more effective. The optimum workpiece temperature in front of the cutting zone with compressed air delivered through the spindle is about 350°C, higher than that with compressed air delivered through a stationary nozzle (about 230°C). The maximum tool life in the former case is much longer than that in the latter case.

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.

Machinability of a near beta titanium alloy

In this paper, the machinability of a near beta Ti25Nb3Mo3Zr2Sn titanium alloy with different heat treatment histories has been investigated in terms of cutting force, chip temperature and chip formation. A solution treatment was performed followed by ageing at different temperatures for the near beta alloy. It has been shown that the same alloy behaves differently during machining at various cutting speeds after different heat treatments. The observations have been explained in terms of friction effects and the hardness of the materials. It has been concluded that the ductility of a workpiece plays a significant role in determining its machinability. High friction at the tool–chip interface in a ductile workpiece resulted in a higher degree of chip segmentation and a larger undeformed surface length in each chip.

Impacts of trace carbon on the microstructure of as-sintered biomedical Ti-15Mo alloy and reassessment of the maximum carbon limit

The formation of grain boundary (GB) brittle carbides with a complex three-dimensional (3-D) morphology can be detrimental to both the fatigue properties and corrosion resistance of a biomedical titanium alloy. A detailed microscopic study has been performed on an as-sintered biomedical Ti-15Mo (in wt.%) alloy containing 0.032 wt.% C. A noticeable presence of a carbon-enriched phase has been observed along the GB, although the carbon content is well below the maximum carbon limit of 0.1 wt.% specified by ASTM Standard F2066. Transmission electron microscopy (TEM) identified that the carbon-enriched phase is face-centred cubic Ti2C. 3-D tomography reconstruction revealed that the Ti2C structure has morphology similar to primary α-Ti. Nanoindentation confirmed the high hardness and high Youngs modulus of the GB Ti2C phase. To avoid GB carbide formation in Ti-15Mo, the carbon content should be limited to 0.006 wt.% by Thermo-Calc predictions. Similar analyses and characterization of the carbide formation in biomedical unalloyed Ti, Ti-6Al-4V and Ti-16Nb have also been performed.

Selective production of hydrogen peroxide and oxidation of hydrogen sulfide in an unbiased solar photoelectrochemical cell

A solar-to-chemical conversion process is demonstrated using a photoelectrochemical cell without external bias for selective oxidation of hydrogen sulfide (H2S) to produce hydrogen peroxide (H2O2) and sulfur (S). The process integrates two redox couples anthraquinone/anthrahydroquinone and I−/I3−, and conceptually illustrates the remediation of a waste product for producing valuable chemicals.