Titanate nanofibers of various sizes and layered structure were prepared from inorganic titanium compounds by hydrothermal reactions. These fibers are different from “refractory” mineral substances ...because of their dimension, morphology, and significant large ratio of surface to volume, and, surprisingly, they are highly reactive. We found, for the first time, that phase transitions from the titanate nanostructures to TiO2 polymorphs take place readily in simple wet-chemical processes at temperatures close to ambient temperature. In acidic aqueous dispersions, the fibers transform to anatase and rutile nanoparticles, respectively, but via different mechanisms. The titanate fibers prepared at lower hydrothermal temperatures transform to TiO2 polymorphs at correspondingly lower temperatures because they are thinner, possess a larger surface area and more defects, and possess a less rigid crystal structure, resulting in lower stability. The transformations are reversible: in this case, the obtained TiO2 nanocrystals reacted with concentrate NaOH solution, yielding hollow titanate nanotubes. Consequently, there are reversible transformation pathways for transitions between the titanates and the titanium dioxide polymorphs, via wet-chemical reactions at moderate temperatures. The significance of these findings arises because such transitions can be engineered to produce numerous delicate nanostructures under moderate conditions. To demonstrate the commercial application potential of these processes, we also report titanate and TiO2 nanostructures synthesized directly from rutile minerals and industrial-grade rutiles by a new scheme of hydrometallurgical reactions.
The evolution of atomistic-level nanostructure during the early stages of elevated temperature ageing of rapid hardening (RH) Al–Cu–Mg alloys has been characterised by a combination of atom probe ...tomography (APT), transmission electron microscopy (TEM) and positron annihilation spectroscopy (PAS). APT analysis confirms that significant dispersions of small solute clusters form during ageing for 60
s at 150
°C. No zone-like precipitate structures were observed by TEM and APT examinations. These small clusters are believed to be responsible for the RH effect. Careful quantitative APT analysis reveals that a high density of Cu–Mg clusters with high Mg:Cu ratio gives the most potent strengthening response. Positron annihilation measurements also show that Cu–Mg clusters provide additional sites for vacancy stabilisation.
In this work, a series of aging treatments has been conducted on AA6111 alloy samples for various times at ambient temperature (so-called natural aging) and at temperatures between 60 and 180°C ...(artificially aged). The time at artificial ageing was chosen such that samples with approximately the same yield stress were produced. The microstructures of these alloy samples have been carefully characterized using atom probe tomography together with advanced cluster-finding techniques in order to obtain quantitative information about the changes in distribution of both the solute clusters and early-stage precipitates that are formed. The size distribution of clusters has been mapped onto the glide plane and then the stress necessary for a dislocation to pass through the range of obstacles has been estimated using an areal glide model where the dislocation–obstacle interaction strength has been assumed to be related to the obstacle size on the glide plane. It is demonstrated that the contribution of cluster strengthening during artificial aging at higher temperatures is dominated by the high number density of small clusters (Guinier radius <1nm), whereas the situation during room temperature natural aging is more complex.
The effects of high-pressure torsion (HPT) processing on an Al–Mg–Si alloy (AA6060) have been investigated comprehensively. We show that the processing temperature has complex effects on the ...strength, grain refinement and solute nanostructures of the alloy. Ten-revolution HPT processing at room temperature produced the highest yield strength of 475MPa, which is similar to a high-strength Al alloy. However, processing at 100°C produced the finest grains due to the strong solute segregation to grain boundaries and the formation of high-density precipitates that pin grain boundaries. Processing at 180°C led to significant decomposition of the alloy and the formation of coarse precipitates. This research demonstrates that solute nanostructures provide key information for unravelling the origins of HPT-induced strengthening and grain refinement, and reveals the important opportunities for “engineering” solute nanostructures to enhance grain refinement in HPT processing.
Processing by equal-channel angular pressing (ECAP) affects the morphology of
η precipitates in an Al–Zn–Mg–Cu (Al-7136) alloy. It is shown by transmission electron microscopy that ECAP changes the ...orientation of precipitates and this influences the atomic configuration and the interfacial energy at the
η/
α-Al interfaces. Consequently,
η precipitates adopt an isotropic growth mode and evolve into equiaxed particles. A three-dimensional atom probe analysis demonstrates that large
η precipitates formed in different numbers of ECAP passes are of similar composition. The coalescence of smaller precipitates, rather than the fragmentation of larger precipitates, dominates the precipitate evolution.
The solute nanostructures formed in the primary α-Al grains of a semi-solid metal cast Al–7Si–0.6Mg alloy (F357) during ageing at 180
°C, and the age-hardening response of the alloy, have been ...systematically investigated by transmission electron microscopy, atom probe tomography and hardness testing. A 120
h natural pre-ageing led to the formation of solute clusters and Guinier–Preston (GP) zones. The natural pre-ageing slowed down the precipitation kinetics 6-fold during 1
h ageing at 180
°C, but this effect diminished after 4
h when the sample reached the same hardness as that without the pre-ageing treatment. It reduced the number density of β″ needles to approximately half of that formed in samples without the treatment, and postponed the peak hardness occurrence to 4
h, four times that of the as-quenched sample. A hardness plateau developed in the as-quenched sample between 1 and 4
h ageing corresponds to the growth of the β″ precipitates and a significant concurrent decrease of solute clusters and GP zones. The average Mg:Si ratio of early solute clusters is <0.7 while that of GP zones changes from 0.8 to 0.9 with increasing size, and that of β″ needles increases from 0.9 to 1.2. β″ needles, GP zones and solute clusters are important strengthening solute nanostructures of the alloy. The partitioning of solutes and precipitation kinetics of the alloy are discussed in detail.
The alloy Mg–1.5Gd has been extruded at different temperatures to produce two significantly different textures. At lower extrusion temperatures there was significant solute clustering in the matrix, ...coupled with segregation of solute to the grain boundaries. At higher temperatures these two phenomena were both less pronounced. It is suggested here that segregation of solute to the grain boundaries plays a significant role in the texture modification effect that rare earth elements have in magnesium alloys.
A recent model developed to predict the smallest grain sizes obtainable by severe plastic deformation has worked well for materials with medium to high stacking fault energies (SFEs) but not for ...those with low SFEs. To probe this issue, experiments were conducted using a Cu–30wt.% Zn alloy with a very low SFE of 7mJ/m2 as the model material. High-pressure torsion was used as the grain refinement technique. The results indicate that stacking faults and twin boundaries play a key role in the grain refinement process such that the smallest achievable grain size is determined by the highest stacking fault and twin density that the system is able to produce. An amorphization of grain boundaries was also observed in the final structure. These observations are very different from those reported for materials having medium to high SFEs and they confirm the operation of a different grain refinement mechanism.
Various sized hollow nanotubes and solid nanorods are synthesized from rutile powder (particle size ≈ 120–280 nm) using a relatively simple chemical approach in alkaline solution. The nanotubes and ...nanorods occur as hydrated phases: TiO2·1.25H2O and TiO2·1.0H2O, respectively. The rutile particles react in concentrated NaOH solution under hydrothermal conditions, yielding layered sodium titanate in the form of either polycrystalline nanotubes or single‐crystal nanorods. The form of the product depends on the temperature and time of hydrothermal reaction: Therefore, this is a report of the template‐free control of the degree of crystallinity, crystal structure, and morphology of these types of nanoscale sodium titanate products. By treating the nanotubes and nanorods with dilute HCl, the sodium ions within them could be exchanged for protons, and the morphology of the nanotubes and nanorods is retained, resulting in hydrogen titanate nanotubes and nanorods. The electrochemical performance of dehydrated hydrogen titanate nanotubes and nanorods is explored in terms of their potential performance as anode materials for lithium‐ion batteries. The discharge capacity is higher for thin anatase nanorods converted from hydrogen titanate nanotubes when compared to the calcined (at 500 °C and 700 °C) products of hydrogen titanate nanorods. The significance of these findings is the possibility of fabricating delicate, nanostructured materials directly from industrial raw materials, because the natural mineral of titanium dioxide and most of the raw industrial TiO2 products exist in the rutile phase.
Hydrogen titanate nanotubes and nanorods (see Figure) have been synthesized in various sizes from rutile powder by a simple chemical approach. The electrochemical performance of dehydrated nanotubes and nanorods is explored in terms of their potential performance as anode materials for lithium‐ion batteries.
Variations in solute element distribution occurring in a commercial 2024 aluminium alloy during isothermal ageing treatments at 170°C for up to 120h have been characterized using atom probe ...tomography. An early (0.5h at 170°C) rapid increase in hardness was correlated with the formation of fine scale (average 24 atom) solute clusters, comprising principally Mg and Cu, but with minor concentrations of Si and Zn. There was, in addition, evidence of significant segregation of Mg, Cu and Si to at least some fraction of grain boundaries and existing matrix dislocations. At peak hardness (80h at 170°C) the microstructure comprised coarse precipitates of S phase, with a composition approaching stoichiometric Al2CuMg, a dense distribution of Guinier–Preston–Bagaryatsky zones elongated parallel to 〈100〉 in a matrix of α-Al and a residual distribution of smaller equiaxed solute clusters. Both the clusters and zones contained predominantly Mg and Cu, with minor concentrations of Si and Zn. The S phase contained small but significant (0.5–1.8at.%) concentrations of Si, which was non-uniformly distributed in elongated domains within the laths of the S phase. In overaged samples (114h at 170°C) the microstructure comprised almost exclusively coarse S phase, Al2Mg(Cu,Si), in assemblies suggestive of a combination of precipitate coarsening and coalescence.
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