Microstructure evolution of TI-SN-NB alloy prepared by mechanical alloying

In the present study, Ti-16Sn-4Nb alloy was prepared by mechanical alloying (MA). Optical microscopy, scanning electron microscopy combined with energy dispersive X-ray analysis (SEM-EDX), and X-ray diffraction analysis (XRD) were used to characterise the phase transformation and the microstructure evolution. Results indicated that ball milling to 8 h led to the formation of a supersaturated hcp α-Ti and partial amorphous phase due to the solid solution of Sn and Nb into Ti lattice. The microstructure of the bulk sintered Ti-16Sn-4Nb alloy samples made from the powders at shorter ball milling times, i.e. 20 min- 2 h, exhibited a primary α surrounded by a Widmanstätten structure (transformed β); while in the samples made from the powders at longer ball milling times, i.e. 5- 10 h, the alloy evolved to a microstructure with a disordered and fine β phase dispersed homogeneously within the α matrix. These results contribute to the understanding of the microstructure evolution in alloys of this type prepared by powder metallurgy.

Synthesis of Ti–Sn–Nb alloy by powder metallurgy

The microstructural evolution and characteristics of the Ti–16Sn–4Nb powder particles and bulk alloys sintered from the powders ball-milled for various periods of time were studied. Results indicated that ball milling to 8 h led to the development of a supersaturated hcp α-Ti and partial amorphous phase due to the solid solution of Sn and Nb into Ti lattice. The bulk Ti–16Sn–4Nb alloy made from the powders ball milled for a short time, up to 2 h, exhibited a primary α and a Widmanstätten structure consisting of interlaced secondary α and β. With an increase in ball milling time up to 10 h, the microstructure evolved into a fine β phase dispersed homogeneously within α phase matrix. The microhardness values of the bulk alloy in both α- and β-phases increased with the increasing of the ball milling time and reached a plateau value at 8 h and longer, i.e. 687 and 550 HV for α- and β-phases, respectively. Likewise, the microhardness of the α phases was always higher than that of the β phases in the bulk alloys made from the powders ball milled for the same milling time.

Inverse opal structure of SnO2 and SnO2 : Zn for gas sensing

In the present work, we propose a low cost synthetic sol-gel route that allows to produce high quality oxide nanostructures with inverse opal architecture which, transferred on alumina substrates provided with Pt interdigitated contacts and heater, are tested as gas sensing devices. An opal template of sintered monodisperse polystyrene spheres was filled with alcoholic solutions of metal oxide precursors and transferred on the alumina substrate. The polystyrene template was removed by thermal treatment, leading to the simultaneous sintering of the oxide nanoparticles. Beside SnO₂, a binary oxide well known for gas sensing application, a Zn containing ternary solid solution (SnO₂:Zn, with Zn 10% molar content) was taken into account for sensor preparation. The obtained high quality macro and meso-porous structures, characterized by different techniques, were tested for pollutant (CO, NO₂) and interfering (methanol) gases, showing that very good detection can be reached through the increase of surface area offered by the inverse opal structure and the tailoring of the chemical composition. The electrical characterization performed on the tin dioxide based sensors shows an enhancement of the relative response towards NO₂ at low temperatures in comparison with conventional SnO₂ sensors obtained with sputtering technique. The addition of Zn increases the separation between the operating temperatures for reducing and oxidizing gases and results in a further enhancement of the selectivity to NO₂ detection.

Lithium doped N-methyl-N-ethylpyrrolidiniumbis(trifluoromethanesulfonyl)amide fast-ion conducting plastic crystals

The incorporation of dopant levels of lithium ions (0.5 to 9.3% by mole) in the N-methyl-N-ethylpyrrolidinium bis(trifluoromethanesulfonyl)amide (P₁₂TFSA) plastic crystalline phase results in increases in the solid state ionic conductivity of more than 3 orders of magnitude at 298 K. Conductivities as high as 10^−-4 S cm⁻¹ at 323 K have been measured in these doped plastic crystal phases. These materials can therefore be classified as fast-ion conductors. Higher levels of Li only marginally increase the conductivity, up to around 33 mol%, followed by a slight decrease to 50 mol%. Thermal analysis behaviour has allowed the partial development of the binary phase diagram for the LiTFSA–P₁₂TFSA system between 0–50 mol% LiTFSA, which suggests the presence of a solid solution single phase at concentrations less than 9.3 mol% LiTFSA. There is also strong evidence of eutectic behaviour in this system with a eutectic transition temperature around 308 K at 33 mol% LiTFSA. A model relating ionic conduction to phase behaviour in this system is presented. The increased conductivity upon doping has been associated with lithium ion motion via⁷Li solid state NMR linewidth measurements.

Synthesis of high surface area amorphous tin-zinc oxides by a sol-gel method

A new method to synthesize conducting oxide nanoparticles with low photocatalytic activity was investigated. Initially, the preparation of amorphous ZnO-SnO2 solid solution nanoparticles was studied using a sol-gel technique. It was found that X-ray amorphous nanopowders with low photocatalytic activity were produced when the precipitates were heat treated below 500 °C. However, FT-IR data showed that the sample may not be an oxide semiconductor. A mixture of ZnO and SnO2 crystalline nanoparticles was also produced at 800 °C and found to have much reduced photoactivity than commercial ZnO nanoparticles having a similar specific surface area.

Development of ligand incorporated chitosan nanoparticles for the selective delivery of 5-fluorouracil to colon

Drug delivery systems with active targeting ligand provide improved therapeutic efficiency due to the selectivity towards tumor cells. In this paper we prepared drug loaded nanoparticles (NPs) using folate (FA) incorporated chitosan (FA-CS) based on ionic gelation technology. FA-CS NPs were spherical in shape with an average particle size of 100 nm, while 5-fluorouracil (5-FU) loaded NPs became less circular with average particle size of 100-500 nm. NPs made from FA-CS conjugates exhibited improved capability to encapsulate hydrophilic 5-FU. It was found 5-FU distributed in FA-CS NPs in solid solution state. In vitro release results demonstrated the release of 5-FU from FA-CS NPs was more controllable as compared to that of CS NPs.

Grain refinement of an extruded Mg alloy via na microalloying

The effect of 0.3 wt pct Na on the microstructure of extruded alloy Mg-2Sn-1Zn is examined. We report that Na stabilizes the Mg2Sn phase, resulting in its precipitation during extrusion under conditions where a solid solution is otherwise expected. This effect appears to be thermodynamic in nature and is different from the kinetic enhancement of low- temperature aging reported by Mendis et al. [Phil. Mag. Letters, 86 (2006), 443]. The precipitates of the current study enable useful refinement of the grain size.

Compositional effects on the restoration behaviour in Mg-Zn-Re alloys

Additions of rare earth elements to magnesium alloys are qualitatively reported in the literature to retard recrystallisation. However, their effect in the presence of other (non-rare earth) alloy additions has not been systematically shown nor has the effect been quantified. The microstructural restoration following the hot deformation of Mg-xZn-yRE (x = 2.5 and 5 wt.%, y = 0 and 1 wt.%, and RE = Gd and Y) alloys has been studied using double hit compression testing and microscopy. It was found that, in the absence of rare earth additions, increases in zinc level had a negligible influence on the kinetics of restoration and the microstructure developed both during extrusion and throughout double hit testing. Adding rare earth elements to Mg-Zn alloys was found to retard restoration of the microstructure and maintain finer recrystallised grains. However, in the Mg-Zn-RE alloys, increasing the zinc concentration from 2.5 wt.% to 5 wt.% accelerated the restoration process, most likely due to a depletion of rare earth elements from solid solution and modification of the particles present in the matrix.

Recrystallization kinetic behavior of copper-bearing strip cast steel

In the present study, copper-bearing low carbon steels were produced by direct strip casting (DSC) method on a pilot scale. The effects of copper on mechanical, microstructural, and recrystallization behavior were investigated. As-cast microstructure mainly consists of polygonal ferrite and Widmanstatten ferrite. The increase in Cu increases the amount of Widmanstatten ferrite and induces the formation of bainite in the as-cast condition. It was found that copper increases strength and hardness by solid solution strengthening, grain refinement, and precipitation hardening and the increment is significant above 1% Cu in as-cast condition. Six different compositions were selected for recrystallization study. All the samples were cold rolled to 70% reduction and annealed at three different temperatures, 600, 650, and 700°C for various times. Recrystallization responses were strongly dependent on initial microstructure and Cu content and the effect is dramatic between 1 and 2% Cu. Recrystallization time and temperature were found to be increased with increase in copper content.

Spectroscopic and computational characterizations of alkaline-earth- and heavy-metal-exchanged natrolites

Synchrotron infrared (IR) and micro-Raman spectra of natrolites containing alkaline-earth ions (Ca2+, Sr2+, and Ba2+) and heavy metals (Cd2+, Pb2+, and Ag+) as extra-framework cations (EFCs) were measured under ambient conditions. Complementing our previous spectroscopic investigations of natrolites with monovalent alkali metal (Li+, Na+, K+, Rb +, and Cs+) EFCs, we establish a correlation between the redshifts of the frequencies of the 4-ring and helical 8-ring units and the size of the EFCs in natrolite. Through ab initio calculations we have derived structural models of Ca2+- and Ag+-exchanged natrolites with hydrogen atoms, and found that the frequency shifts in the H - O - H bending mode and the differences in the O - H stretching vibration modes can be correlated with the orientations of the water molecules along the natrolite channel. Assuming that the members of a solid solution series behave as an ideal mixture, we will be able to use spectroscopy to probe compositions. Deviation from ideal behavior might indicate the occurrence of phase separation on various length scales. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Comparative study of the microstructures and mechanical properties of direct laser fabricated and arc-melted AlxCoCrFeNi high entropy alloys

High entropy alloys (HEA) are a relatively new metal alloy system that have promising potential in high temperature applications. These multi-component alloys are typically produced by arc-melting, requiring several remelts to achieve chemical homogeneity. Direct laser fabrication (DLF) is a rapid prototyping technique, which produces complex components from alloy powder by selectively melting micron-sized powder in successive layers. However, studies of the fabrication of complex alloys from simple elemental powder blends are sparse. In this study, DLF was employed to fabricate bulk samples of three alloys based on the AlxCoCrFeNi HEA system, where x was 0.3, 0.6 and 0.85M fraction of Al. This produced FCC, FCC/BCC and BCC crystal structures, respectively. Corresponding alloys were also produced by arc-melting, and all microstructures were characterised and compared longitudinal and transverse to the build/solidification direction by x-ray diffraction, glow discharge optical emission spectroscopy and scanning electron microscopy (EDX and EBSD). Strong similarities were observed between the single phase FCC and BCC alloys produced by both techniques, however the FCC/BCC structures differed significantly. This has been attributed to a difference in the solidification rate and thermal gradient in the melt pool between the two different techniques. Room temperature compression testing showed very similar mechanical behaviour and properties for the two different processing routes. DLF was concluded to be a successful technique to manufacture bulk HEA[U+05F3]s.

Atomic scale analysis of light alloys using atom probe tomography

The present paper reviews recent progress in atomic-scale characterisation of composition and nanostructure of light alloy materials using the technique of atom probe tomography. In particular, the present review will highlight atom-by-atom analysis of solid solution architecture, including solute clustering and short-range order, with reference to current limitations of spatial resolution and detector efficiency of atom probe tomography and methods to address these limitations. This leads to discussion of prediction of mechanical properties by simulation and modelling of the strengthening effect exerted by solute clusters and the role of experimental atom probe data to assist in this process. The unique contribution of atom probe tomography to the study of corrosion and hydrogen embrittlement of light alloys will also be discussed as well as a brief insight into its potential application for the investigation of solute strengthening of twinning in Mg alloys.

The competition between metastable and equilibrium S (Al2CuMg) phase during the decomposition of AlCuMg alloys

Abstract The decomposition sequence of the supersaturated solid solution leading to the formation of the equilibrium S (Al2CuMg) phase in AlCuMg alloys has long been the subject of ambiguity and debate. Recent high-resolution synchrotron powder diffraction experiments have shown that the decomposition sequence does involve a metastable variant of the S phase (denoted S1), which has lattice parameters that are distinctly different to those of the equilibrium S phase (denoted S2). In this paper, the difference between these two phases is resolved using high-resolution synchrotron and neutron powder diffraction and atom probe tomography, and the transformation from S1 to S2 is characterised in detail by in situ synchrotron powder diffraction. The results of these experiments confirm that there are no significant differences between the crystal structures of S1 and S2, however, the powder diffraction and atom probe measurements both indicate that the S1 phase forms with a slight deficiency in Cu. The in situ isothermal aging experiments show that S1 forms rapidly, reaching its maximum concentration in only a few minutes at high temperatures, while complete conversion to the S2 phase can take thousands of hours at low temperature. The kinetics of S phase precipitation have been quantitatively analysed for the first time and it is shown that S1 phase forms with an average activation energy of 75 kJ/mol, which is much lower than the activation energy for Cu and Mg diffusion in an Al matrix (136 kJ/mol and 131 kJ/mol, respectively). The mechanism of the replacement of S1 with the equilibrium S2 phase is discussed.

Synthesis of Al-doped Mg2Si1-xSnx compound using magnesium alloy for thermoelectric application

Abstract Mg2Si1-xSnx thermoelectric compounds were synthesized through a solid-state reaction at 700 °C between chips of Mg2Sn-Mg eutectic alloy and silicon fine powders. The Al dopants were introduced by employing AZ31 magnesium alloy that contains aluminum. The as-synthesized Mg2Si1-xSnx powders were consolidated by spark plasma sintering at 650-700 °C. X-ray diffraction and scanning electron microscopy revealed that the Mg2Si1-xSnx bulk materials were comprised of Si-rich and Sn-rich phases. Due to the complex microstructures, the electrical conductivities of Mg2Si1-xSnx are lower than Mg2Si. As a result, the average power factor of Al0.05Mg2Si0.73Sn0.27 is about 1.5 × 10^-3 W/mK² from room temperature to 850 K, being less than 2.5 × 10^-3 W/mK² for Al0.05Mg2Si. However, the thermal conductivity of Mg2Si1-xSnx was reduced significantly as compared to Al0.05Mg2Si, which enabled the ZT of Al0.05Mg2Si0.73Sn0.27 to be superior to Al0.05Mg2Si. Lastly, the electric power generation from one leg of Al0.05Mg2Si and Al0.05Mg2Si0.73Sn0.27 were evaluated on a newly developed instrument, with the peak output power of 15-20 mW at 300 °C hot-side temperature.

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.