The restoration of the passivity of stainless steel Weldments in pickling solutions observed using electrochemical and surface analytical methods

In this study, a solution containing ammonium fluoride (NH4F) and nitric acid (HNO3) was used as an alternative to the conventional highly toxic pickling solution HF/HNO3 for pickling weldments of selected stainless steels including Type 316 stainless steel (UNS S31600), duplex stainless steel 2205 (UNS S32205), and super duplex stainless steel 2507 (UNS S32750). Electrochemical and surface analytical methods were used to understand the effects of pickling on the stainless steel weldments. Cyclic potentiodynamic polarization (CPP) test results indicated that the restoration of passivity of stainless steel weldments could be achieved by pickling the weldments in both HF/HNO3 solution and NH4F/HNO3 solutions. Scanning electron microscopy observation of the UNS S32750 weldment surface revealed that both the HF/HNO3 solution and the NH4F/HNO3 solution could remove the heat tint on the weldment. X-ray photoelectron spectroscopy analysis indicated that treatment in these two pickling solutions produced passive films with similar characteristics. Thus, this work suggests that the NH4F/HNO3 solution is a promising alternative to HF/HNO3 solution for the pickling of stainless steel weldments, and that the CPP test approach can be used in conjunction with surface analytical methods for further development of safer and environmentally friendly picklingsolutions.

Knitted strain sensor textiles of highly conductive all-polymeric fibers

A scaled-up fiber wet-spinning production of electrically conductive and highly stretchable PU/PEDOT:PSS fibers is demonstrated for the first time. The PU/PEDOT:PSS fibers possess the mechanical properties appropriate for knitting various textile structures. The knitted textiles exhibit strain sensing properties that were dependent upon the number of PU/PEDOT:PSS fibers used in knitting. The knitted textiles show sensitivity (as measured by the gauge factor) that increases with the number of PU/PEDOT:PSS fibers deployed. A highly stable sensor response was observed when four PU/PEDOT:PSS fibers were co-knitted with a commercial Spandex yarn. The knitted textile sensor can distinguish different magnitudes of applied strain with cyclically repeatable sensor responses at applied strains of up to 160%. When used in conjunction with a commercial wireless transmitter, the knitted textile responded well to the magnitude of bending deformations, demonstrating potential for remote strain sensing applications. The feasibility of an all-polymeric knitted textile wearable strain sensor was demonstrated in a knee sleeve prototype with application in personal training and rehabilitation following injury.

Enhanced mechanical response of an ultrafine grained Ti-6Al-4V alloy produced through warm symmetric and asymmetric rolling

An equiaxed ultrafine-grained (UFG) microstructure was successfully produced in a Ti-6Al-4V alloy with an average grain size of 110-230. nm through symmetric and asymmetric warm rolling of a martensitic starting microstructure. The UFG material displayed a combination of ultrahigh strength and ductility at room temperature. Compared with the conventional symmetric rolling, the asymmetric rolling process led to a more pronounced effect of microstructure refinement and a higher tensile ductility. The optimum mechanical response was obtained though the asymmetric rolling at 70% reduction, offering an ultimate tensile strength of 1365. MPa and a total elongation of ~23%. Apart from the magnitude of grain refinement, the inclination of basal texture component from the normal towards the rolling direction during asymmetric rolling and possible strain induced β to martensite transformation may concurrently contribute to a remarkable tensile strength-ductility balance.

Ionic transport through a composite structure of N-ethyl-N-methylpyrrolidinium tetrafluoroborate organic ionic plastic crystals reinforced with polymer nanofibres

The incorporation of polyvinylidene difluoride (PVDF) electrospun nanofibres within N-ethyl-N-methylpyrrolidinium tetrafluoroborate, [C2mpyr][BF4] was investigated with a view to fabricating self-standing membranes for various electrochemical device applications, in particular lithium metal batteries. Significant improvement in mechanical properties and ionic conduction was demonstrated in a previous study, which also demonstrated the remarkably high performance of the lithium-doped composite material in a device. We now seek a fundamental understanding of the role of fibres within the matrix of the plastic crystal, which is essential for optimizing device performance through fine-tuning of the composite material properties. The focus of the current study is therefore a thorough investigation of the phase behaviour and conduction behaviour of the pure and the lithium-doped (as LiBF4) plastic crystal, with and without incorporation of polymer nanofibres. Analysis of the structure of the plastic crystal, including the effects of lithium ions and the incorporation of PVDF fibres, was conducted by means of synchrotron XRD. Ion dynamics were evaluated using VT solid-state NMR spectroscopy. ATR-FTIR spectroscopy was employed to gain insights into the molecular interactions of doped lithium ions and/or the PVDF nanofibres in the matrix of the [C2mpyr][BF4] composites. Preliminary measurements using PALS were conducted to probe structural defects within the pure materials. It was found that ion transport within the plastic crystal was significantly altered by doping with lithium ions due to the precipitation of a second phase in the structure. The incorporation of the fibres activated more mobile sites in the systems, but restricted ion mobility with different trends being observed for each ion species in each crystalline phase. In the presence of the fibres a strong interaction observed between the Li ion and the pyrrolidinium ring disappeared and formation of the second phase was prevented. As a result, an increased number of mobile lithium ions are released into the solid solution structure of the matrix, simultaneously removing the blocking effect of the second phase. Thus, ion conduction was remarkably improved within the Li-doped composite compared to the neat Li-doped plastic crystal.

Self-assembled V2O5 interconnected microspheres produced in a fish-water electrolyte medium as a high-performance lithium-ion-battery cathode

Interconnected microspheres of V2O5 composed of ultra-long nanobelts are synthesized in an environmental friendly way by adopting a conventional anodization process combined with annealing. The synthesis process is simple and low-cost because it does not require any additional chemicals or reagents. Commercial fish-water is used as an electrolyte medium to anodize vanadium foil for the first time. Electron microscopy investigation reveals that each belt consists of numerous nanofibers with free space between them. Therefore, this novel nanostructure demonstrates many outstanding features during electrochemical operation. This structure prevents self-aggregation of active materials and fully utilizes the advantage of active materials by maintaining a large effective contact area between active materials, conductive additives, and electrolyte, which is a key challenge for most nanomaterials. The electrodes exhibit promising electrochemical performance with a stable discharge capacity of 227 mAh·g–1 at 1C after 200 cycles. The rate capability of the electrode is outstanding, and the obtained capacity is as high as 278 at 0.5C, 259 at 1C, 240 at 2C, 206 at 5C, and 166 mAh·g–1 at 10C. Overall, this novel structure could be one of the most favorable nanostructures of vanadium oxide-based cathodes for Li-ion batteries. [Figure not available: see fulltext.]

Synthesis of an indium oxide nanoparticle embedded graphene three-dimensional architecture for enhanced lithium-ion storage

Indium oxide nanoparticles were synthesised by using a facile and scalable strategy. The as-prepared nanoparticles (20-40 nm) were in situ and homogeneously distributed in a three-dimensional (3D) graphene architecture subsequently during the fabrication process. The obtained nanocomposite acts as a high capacity anode material for lithium-ion batteries and demonstrates good cycle stability. A drastically enhanced capacity of 750 mA h g^-1 in comparison with that of bare In2O3 nanoparticles can be maintained after 100 cycles, along with an improved high rate performance (210 mA h g^-1 at 1 A g^-1 and 120 mA h g^-1 at 2 A g^-1). The excellent performance is linked with the indium oxide nanoparticles and the unique 3D interconnected porous graphene structure. The highly conductive and porous 3D graphene structure greatly enhances the performance of lithium-ion batteries by protecting the nanoparticles from the electrolyte, stabilizing the nanoparticles during cycles and buffering the volume expansion upon lithium insertion.

A facile synthesis of mesoporous silica nanoparticle and its morphology manipulation by varying pH value

 Mesoporous silica nanoparticles (MSNs) with particle size of20 nm have been synthesised through the template directed method at low temperature. The pH value of the reaction solution was found to have a great impact on the morphology of the final products. The morphology of resultant MSNs were investigated through transmission electron microscope. The mesoporous structure was examined by Brunauer-Emmett-Teller and Barrett-Joyner-Halenda methods. The results suggested that the high pH value had a great effect on the morphology of the final MSNs. Higher pH value intensified the interaction between particles. The pH value less than 10 is good for the formation of nanoparticles, while at pH 12, a silica framework with heterogeneous mesopore structure can be obtained.

Increased ion conduction in dual cation [sodium][tetraalkylammonium] poly[4-styrenesulfonyl(trifluoromethylsulfonyl)imide-co-ethylacrylate] ionomers

© The Royal Society of Chemistry. Solid-state polymer electrolytes, as an alternative to traditional liquid electrolytes, have been intensively investigated for energy conversion and storage devices. The transport rate of single ions is the key to their high performance. For application in emerging sodium batteries, we have developed three dual-cation polymeric ionomers, which contain bulky tetraalkylammonium ions in addition to the sodium ion. The sizes and relative contents of the ammonium ions vary relative to the sodium ion contents. Comparative studies of ion dynamics, thermal properties, phase behaviours and ionic conductivities were carried out, taking advantage of various spectroscopic and thermal chemistry methods. The ion conductivities of the ionomers are greatly enhanced by the introduction of bulky counterions, as a result of the additional free volume and decreased sodium ion association. Raman spectroscopy and thermal analysis as well as the solid-state nuclear magnetic resonance studies are used to probe the conductivity behaviour.

Nanopatterning and electrical tuning of MoS2 layers with a subnanometer helium ion beam

We report subnanometer modification enabled by an ultrafine helium ion beam. By adjusting ion dose and the beam profile, structural defects were controllably introduced in a few-layer molybdenum disulfide (MoS2) sample and its stoichiometry was modified by preferential sputtering of sulfur at a few-nanometer scale. Localized tuning of the resistivity of MoS2 was demonstrated and semiconducting, metallic-like, or insulating material was obtained by irradiation with different doses of He(+). Amorphous MoSx with metallic behavior has been demonstrated for the first time. Fabrication of MoS2 nanostructures with 7 nm dimensions and pristine crystal structure was also achieved. The damage at the edges of these nanostructures was typically confined to within 1 nm. Nanoribbons with widths as small as 1 nm were reproducibly fabricated. This nanoscale modification technique is a generalized approach that can be applied to various two-dimensional (2D) materials to produce a new range of 2D metamaterials.

Effect of twinning on microstructural evolution during dynamic recrystallisation of hot deformed as-cast austenitic stainless steel

An as-cast austenitic stainless steel was hot deformed at 1173 K, 1223 K, and 1373 K (900 °C, 950 °C, and 1100 °C) to a strain of 1 with a strain rate of 0.5 or 5 s⁻¹. The recrystallised fraction is observed to be dependent on dynamic recrystallisation (DRX). DRX grains nucleated at the initial stages of recrystallization have similar orientation to that of the deformed grains. With increasing deformation, Cube texture dominates, mainly due to multiple twinning and grain rotation during deformation.

Strain gradients in Cu-Fe thin films and multilayers during micropillar compression

Plastic strain gradients can influence the work-hardening behaviour of metals due to the accumulation of geometrically necessary discolations at the micron/submicron scale. A finite element model based on the conventional theory of mechanism-based strain-gradient plasticity has been developed to simulate the micropillar compression of Cu–Fe thin films and multilayers. The modelling results show that the geometric constraints lead to inhomogeneous deformation in the Cu layers, which agrees well with the bulging of Cu layers observed experimentally. Plastic strain gradients develop inside the individual layers, leading to extra work-hardening due to the accumulation of geometrically necessary dislocations. In the multilayer specimens, the Cu layers deform more severely than the Fe layers, resulting in the development of tensile stresses in the Fe layers. It is proposed that these tensile stresses are responsible for the development of micro-cracks in the Fe layers.

Machining of titanium alloy (Ti-6Al-4V)-theory to application

This article correlates laboratory-based understanding in machining of titanium alloys with the industry based outputs and finds possible solutions to improve machining efficiency of titanium alloy Ti-6Al-4V. The machining outputs are explained based on different aspects of chip formation mechanism and practical issues faced by industries during titanium machining. This study also analyzed and linked the methods that effectively improve the machinability of titanium alloys. It is found that the deformation mechanism during machining of titanium alloys is complex and causes basic challenges, such as sawtooth chips, high temperature, high stress on cutting tool, high tool wear and undercut parts. These challenges are correlated and affected by each other. Sawtooth chips cause variation in cutting forces which results in high cyclic stress on cutting tools. On the other hand, low thermal conductivity of titanium alloy causes high temperature. These cause a favorable environment for high tool wear. Thus, improvements in machining titanium alloy depend mainly on overcoming the complexities associated with the inherent properties of this alloy. Vibration analysis kit, high pressure coolant, cryogenic cooling, thermally enhanced machining, hybrid machining and, use of high conductive cutting tool and tool holders improve the machinability of titanium alloy.

Orientation dependence of the deformation microstructure in a Fe-30Ni-Nb model austenitic steel subjected to hot uniaxial compression

The present work was aimed at a detailed investigation of the orientation dependence of the microstructure characteristics in a Fe-30Ni-Nb austenitic model steel subjected to hot uniaxial compression at 1198 K (925 °C) at a strain rate of 1 s−1 to several strain levels up to 1.0. The quantification of the substructure evolution as a function of strain was performed for the stable 〈011〉 oriented grains. Other grain orientations were also investigated in detail at a strain of 0.2. The 〈110〉 oriented grains contained self-screening arrays of “microbands” (MBs) aligned with high Schmid factor {111} slip planes. The MB crystallographic alignment was largely maintained up to a strain of 1.0, which suggests that the corresponding boundaries kept continuously rearranging themselves during straining and did not follow the sample shape change. The mean MB spacing decreased and misorientation angle increased with strain towards saturation, indicating the operation of the “repolygonization” dynamic recovery mechanism. The non-〈011〉 oriented grains displayed a strong tendency to split during deformation into deformation bands having alternating orientations and being mutually rotated by large angles. The bands were separated by transition regions comprising arrays of closely spaced, extended sub-boundaries collectively accommodating large misorientations across very small distances.

Nanoscale variation in energy dissipation in austenitic shape memory alloys in ultimate loading cycles

Wavy behaviours of hysteresis energy variation in nanoscale bulk of thermomechanical austenitic NiTi shape memory alloy are reported in ultimate nanoindentation loading cycles. One sharp and two spherical tips were used while two loading-unloading rates were applied. For comparison, another austenitic copper-based shape memory alloy, CuAlNi shape memory alloy, and a metal with no phase transition, elastoplastic Cu, were investigated. In shape memory alloys, the hysteresis energy variation ultimately undergoes a linear decrease with internal wavy fluctuations and no stabilisation was observed. The internal energy fluctuation in these alloys was found dissimilar depending on the loading-unloading rate and the indentation tip geometry. In contrast, there was an absence of both overall and internal variations in hysteresis energy for Cu after the second loading cycle. The underlying physics of these variations is discussed and found to be attributed to both the created dislocations and ratcheting thermal-mechanical behaviour of the phase-transformed volume in shape memory alloys.