The dynamic response of a β titanium alloy to high strain rates and elevated temperatures

The stress-strain behaviour and microstructural evolution of the Ti-6Cr-5Mo-5V-4Al (Ti6554) alloy was systematically investigated using Split Hopkinson Pressure Bar (SHPB) tests over a wide range of strain rates from 1000s-1 to 10,000s-1 and initial temperatures from 293K to 1173K. Dislocation slip is the main deformation mechanism for plastic flow of the Ti6554 alloy at high strain rates. The flow stress increases with increasing strain rate and decreasing temperature. Also the flow stress is more sensitive to temperature than to strain rate. For high strain rate deformations, the strain hardening rate is found to be negative at 293K and increases with increasing temperatures. Flow softening observed at 293K is potentially caused by adiabatic heating. The increment in the strain hardening rate with increasing temperatures may be the result of interactions between thermally activated solute Cr atoms and mobile dislocations. When the temperature is raised to 873K, a novel α precipitate morphology consisting of globular α aligned in strings was observed in specimens deformed at strain rates of 4000 and 10,000s-1. It has hardening effects on the β matrix and is purported to nucleate on dislocations introduced by the high strain rate deformation. Adiabatic shear bands were observed in specimens deformed at higher temperatures (873K). The microstructure inside the shear bands is harder than that outside of the shear bands in the Ti6554 alloy

Constitutive equation for the hot deformation behavior of Csf/AZ91D composites and its validity for numerical simulation

The flow stress behavior of 10 vol. % short carbon fibers reinforced AZ91D composites (C-sf/AZ91D) were investigated by hot compression test. The results show the flow stress reach the peak value at small strain and then decrease monotonically until the end of the large strain, which exhibits an obvious dynamic strain softening. The decrease of stress level with deformation temperature increasing or strain rate decreasing can be represented by Zener-Hollomon parameter in a hyperbolic sine equation. By considering the effect of strain on material constants, a modified viscoplastic constitutive equation was established to characterize the dependence of flow stress on the deformation temperature, strain, and strain rate. The stress-strain values calculated by the constitutive equation are in consistent with the experimental results. Applying the constitutive equation, the plastic deformation of C-sf/AZ91D) composites during the hot compression process were analyzed by finite element simulation. The calculated punch force-stroke curves agree well with the measured ones. The results confirmed that the established constitutive equation can accurately describe the hot plastic deformation behavior of C-sf/AZ91D composites, and can be used for the finite element analysis on the hot forming process. (C) 2016 Elsevier Ltd. All rights reserved

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

The effect of ultrasonic treatment on the mechanisms of grain formation of as-cast high purity zinc

The potential for producing a large refined equiaxed zone by ultrasonic treatment (UST) of high purity zinc was investigated in order to improve the mechanical performance and formability. The macrostructure of cast ingots changed from large columnar grains without UST to three zones of fine columnar grains adjacent to the mould walls of the ingot, a refined equiaxed zone and a zone of a mixture of coarse equiaxed and columnar grains. A small zone of equiaxed grains was obtained when UST was applied during cooling from 440 °C to 419 °C for 2 min. The size of the equiaxed zone increased from about 20% of the casting's cross section to 50% when UST was applied for 3 or 4 min. In contrast, the application of UST for a longer time from a higher temperature (450–419 °C for 4 min) resulted in a smaller equiaxed zone of 18% indicating that a specific combination of UST time and temperature is required for the formation of a large equiaxed zone. The factors affecting the formation of the equiaxed zone throughout the solidification cycle are described

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

Finite element analysis of porous commercially pure titanium for biomedical implant application

In biomedical implant applications, porous metallic structures are particularly appealing as they enhance the stiffness compatibility with the host tissue. The mechanical properties of the porous material are critically affected by microstructural features, such as the pore shape, the distribution of porosity, and the level of porosity. In this study, mechanical properties of porous commercially pure titanium structures with various porosity levels were investigated through a combination of experiments and finite element modelling. Finite element simulations were conducted on representative volume elements of the microstructure to assess the role of pore parameters on the effective mechanical properties. Modelling results indicated that the shape of the pore, in addition to porosity level, play a significant role on the effective behaviour. Finite element simulations provide reasonably accurate prediction of the effective Young's modulus, with errors as low as 0.9% for porosity of 35%. It was observed that the large spread in yield strength produced by the simulations was most likely due to the random pore distribution in the network, which may lead to a high probability of plastic strain initiation within the thin walls of the porous network

Corrosion and stress corrosion cracking of ultra-high-purity Mg5Zn

This paper reports on the measurement of the corrosion rate, and the stress corrosion cracking threshold stress, for ultra-high-purity Mg-5Zn. The corrosion rate was higher than the intrinsic corrosion rate of ultra-high-purity Mg, attributed to the quality of the corrosion product film. The threshold stress for stress corrosion cracking, at an applied stress rate of 0.00016 MPa s(-1), was equal to 0.7 times the yield stress in air. The ductility of the cracking indicated that the stress corrosion cracking mechanism was probably hydrogen enhanced localized plasticity. (C) 2015 Elsevier Ltd. All rights reserved

Optimising degradation and mechanical performance of additively manufactured biodegradable Fe–Mn scaffolds using design strategies based on triply periodic minimal surfaces

Additively manufactured lattices based on triply periodic minimal surfaces (TPMS) have attracted significant research interest from the medical industry due to their good mechanical and biomorphic properties. However, most studies have focussed on permanent metallic implants, while very little work has been undertaken on manufacturing biodegradable metal lattices. In this study, the mechanical properties and in vitro corrosion of selective laser melted Fe–35%Mn lattices based on gyroid, diamond and Schwarz primitive unit-cells were comprehensively evaluated to investigate the relationships between lattice type and implant performance. The gyroid-based lattices were the most readily processable scaffold design for controllable porosity and matching the CAD design. Mechanical properties were influenced by lattice geometry and pore volume. The Schwarz lattices were stronger and stiffer than other designs with the 42% porosity scaffold exhibiting the highest combination of strength and ductility, while diamond and gyroid based scaffolds had lower strength and stiffness and were more plastically compliant. The corrosion behaviour was strongly influenced by porosity, and moderately influenced by geometry and geometry-porosity interaction. At 60% porosity, the diamond lattice displayed the highest degradation rate due to an inherently high surface area-to-volume ratio. The biodegradable Fe–35Mn porous scaffolds showed a good cytocompatibility to primary human osteoblasts cells. Additive manufacturing of biodegradable Fe–Mn alloys employing TPMS lattice designs is a viable approach to optimise and customise the mechanical properties and degradation response of resorbable implants toward specific clinical applications for hard tissue orthopaedic repair

Effect of tool wear on chip formation during dry machining of Ti-6Al-4V alloy, part 2: effect of tool failure modes

Variation in the geometric and surface features of segmented chips with an increase in the volume of material removed and tool wear has been investigated at cutting speeds of 150 and 220 m/min at which the cutting tools fail due to gradual flank wear and plastic deformation of the cutting edge, respectively. Among the investigated geometric variables of the segmented chips, slipping angle, undeformed surface length, segment spacing, degree of segmentation and chip width showed the different variation trends with an increase in the volume of material removed or flank wear width, and achieved different values when tool failed at different cutting speeds. However, the chip geometric ratio showed a similar variation trend with an increase in the volume of material removed and flank wear width, and achieved the similar value at the end of tool lives at cutting speeds of both 150 and 220 m/min regardless of the different tool failure modes. Plastic deformation of the tool cutting edge results in severe damage on the machined surface of the chip and significant compression deformation on the undeformed surface of the chip