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A systematic study has been made of the microstructural development in as-cast 30 mm thick Al-3Ti-1Sc (wt%) plate samples that were processed by electron beam (EB) melting at different scanning speeds of 3, 5, 8, 12, 15 and 20 mm/s. The composition of the alloy is designed to capitalize on the potential for significantly extended solubility values of both Sc and Ti in Al when cooled at high cooling rates. The resulting microstructures are characterised using transmission electron microscopy (TEM) and other analytical means, assisted with Thermo-Calc predictions and Vicker's microhardness measurements. EB scanning speed plays a key role in determining the phase formation and microstructural development in the Al-3Ti-1Sc alloy when the beam is applied an accelerating voltage of 50 kV and a current of 30 μA. The major microstructural features in the re-solidified zone include (i) the formation of the primary tetragonal Al3(Ti,Sc) phases and their retention or subsequent transformation to stable cubic Al3(Ti,Sc) phases; (ii) the complete suppression of the primary tetragonal Al3(Ti,Sc) phases at the scanning speed of 20 mm/s; and (iii) the formation of cubic Al3(Ti,Sc) precipitates in the α(Al) matrix supersaturated with both Sc and Ti. These experimental findings are informative for both EB processing of Al-Ti-Sc alloys and the design of new Al alloys for additive manufacturing processes.

Stress corrosion cracking (SCC) of two lower-copper Al–Zn–Mg–Cu alloys, AA7079 and AA7022 (0.6–0.9 wt% Cu), and a higher-copper AA7075 (1.5 wt% Cu) alloy are reported. In aqueous chloride, copper content of grain boundary precipitates is believed to be controlling, whereas in moist air it appears that the hydrogen diffusivity could be evident from the rate of crack growth between crack arrest markings. In moist air, the rate of hydrogen entry may control crack growth rates. X-ray photoelectron spectroscopy showed that the oxide formed in ambient conditions (e.g. ∼50% RH) was more hydrated on the AA7075-T651 than AA7079-T651.

The formation of conjugated precipitates in a ternary Ti–25V–3Cu (wt.%) (Ti–24.4V–2.3Cu (at.%)) alloy subjected to isothermal aging at 500 °C was investigated in this study. X-ray diffraction confirms that the equilibrium precipitates in the alloy are hexagonal α phase and body-centered-tetragonal (bct) Ti2Cu intermetallic compound. These precipitates are composed of two geometrically symmetrical components, which are verified by transmission electron microscopy (TEM) to be twin-related variants of α and Ti2Cu phases. Atomic-level characterizations incorporating high-resolution TEM and atom probe tomography were used to investigate the early-stage nucleation process of the precipitates. The results reveal that precipitation occurs heterogeneously on plate-like (or disc-like) metastable Cu-enriched GP zones that preferentially inhabit {1 1 0}β planes. Crystallographic orientation analyses suggest that the Ti2Cu phase is related to the matrix via a definite orientation relationship of ( 0 1 ¯ 3 ) T i 2 Cu ‖ ( 0 1 1 ) β and [ 1 0 0 ] T i 2 Cu ‖ [ 1 0 0 ] β , while the growth of the α phase is governed by the Potter’s OR such that ( 1 1 ¯ 0 1 ) α ‖ ( 0 1 1 ) β and [ 1 1 2 ¯ 0 ] α ‖ [ 1 1 ¯ 1 ] β . ( 0 1 ¯ 3 ) Ti 2 Cu and ( 1 1 ¯ 0 1 ) α are the twin planes of the respective phases. In spite of highly distinct crystal structures, the conjugated α and Ti2Cu phases attain a nearly perfect lattice correspondence along certain crystallographic planes, which results in a low interfacial energy configuration and favours the co-development of these phases.

This paper presents experimental evidence of homogeneous precipitation of the α-phase within the β matrix of a titanium alloy, and then accounts for this phase transformation by a new, non-classical mechanism involving compositional fluctuations, based on the pseudo-spinodal concept [1] . This mechanism involves local compositional fluctuations of small amplitude which, when of a certain magnitude, can favor thermodynamically certain regions of the β matrix to transform congruently to the α-phase but with compositions far from equilibrium. Subsequently, as measured experimentally using the tomographical atom probe, continuous diffusional partitioning between the parent β- and product α-phases during isothermal annealing drives their compositions towards equilibrium. For a given alloy composition, the decomposition mechanism is strongly temperature dependent, which would be expected for homogeneous precipitation via the compositional fluctuation-mediated mechanism but not necessarily for one based on classical nucleation theory. The applicability of this mechanism to phase transformations in general is noted.

θ′ is an effective strengthening precipitate phase in high-strength Al alloys; unfortunately its nucleation is difficult and usually requires assistance, such as that provided by Sn in Sn-microalloyed Al–Cu. In order to clarify the mechanisms by which Sn promotes the nucleation of θ′, we investigated the structure and thickness of θ′ precipitates in a Al–1.7 at.% Cu alloy with trace additions of Sn (0.01 at.%). Scanning transmission electron microscopy imaging reveals that θ′ platelets recently nucleated at 160 and 200 °C exhibit a discrete distribution of specific, or “magic”, thicknesses, corresponding to minima in the residual volumetric and shape misfit strain. This observation is unique to the Sn-assisted nucleation of θ′: θ′ platelets that undergo growth or form in the Sn-free alloy do not display this discrete distribution, although preference for the magic thicknesses persists. Direct evidence is presented that Sn does not accommodate volumetric misfit strain. Instead, it is shown that Sn in its solute form reduces either the interfacial energy of θ′ and/or the transformation shape strain associated with thicknesses intermediate to the magic thicknesses.

In this study, corrosion of Zn surfaces caused by fine size acidified droplets (diameter ∼0.1–5 μm) was quantified. Fine size droplets of various chemistries were deposited using an unmodified inkjet printer and damage was quantified in terms of volume loss (ΔV) as determined through optical profilometry (OP). SEM–EDS and FIB milling were used to characterise corrosion products and perform cross-sectional analysis of surface oxides. Results show synergistic interactions between chloride concentration and the types of acids used for acidification. Corrosion under fine size droplets was found to be dependent on the initial volume of aerosols, oxygen diffusion, surface area to volume ratio and likely the microstructural features of the underlying metal.

The impact velocity of particles during the cold spray process is crucial to the optimisation of coating quality and spraying costs. In the present investigation, both underexpanded and overexpanded impinging jets are employed to accelerate Aluminium particles towards a substrate. The impact velocity and angle statistics are generated by injecting polydisperse particles into the jet and the particle dynamics are characterised using the velocity and trajectories of the particles. The optimum particle size corresponding to the maximum impact speed is recast in terms of the Stokes number and shown to have a value of approximately one. Finally, a normal shock model is proposed which may be employed to estimate the particle impact speed using the nozzle exit conditions. It is shown that owing to artificial viscosity associated with the total variation diminishing scheme, this model tends to underestimate the speed.

The θ′ phase is a well-known intermediate phase that forms as part of the classic decomposition sequence of an aluminium–copper solid solution. It is also the archetypal precipitate phase of high-strength aluminium alloys. Here we use atomic-resolution electron microscopy imaging to reveal that the coherent interface of θ′ with aluminium can adopt a structure that differs from that previously assumed based on the bulk crystal structures. These observations, combined with first-principles energetics calculations for realistic models of embedded θ′ precipitates, show that this new interfacial structure constitutes an intermediate state in the atomistics of precipitate thickening. These findings will require revision of theoretical models of θ′ precipitate growth and nucleation.

The effects of pre-stretching following solution treatment on the hardness and microstructures of aged 2524 aluminium alloy at 170 °C were studied. Ageing hardness values under different pre-stretching conditions were measured, and the corresponding microstructures were characterized by transmission electron microscopy (TEM). The results show that compared with unstretched samples, the peak hardness is increased and the time to reach the peak hardness is reduced with the increase of pre-strain; the number density of S (Al2CuMg) phases is increased and the length is shortened in pre-stretched alloy. Additionally, the number density of GPB zones is decreased with the increase of pre-strain in peak-aged samples. When the pre-strain is up to 5%, S phases play the predominant contribution to the peak hardness. Fine and uniformly distributed S phases lead to a higher hardness than GPB zones together with S phases existing in conventionally aged 2524 alloy.

The thermal stability of constituent particles in both as-cast and homogenised alloy AA7150 has been investigated by means of scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The results indicate that the as-cast eutectic starts to melt at 474°C and that the melting onset temperatures of the η- and S-phase particles after homogenisation are 474°C and 484°C, respectively. Two types of homogenisation treatments were used to dissolve eutectic particles: a two-step homogenisation treatment (40 h at 465°C + 4 h at 480°C) can completely dissolve the η- and S-based constituents, whereas a significant amount of S-phase still exists after a single-step homogenisation treatment of 40 h at 465°C. After utilising the two-step homogenisation treatment to dissolve all the η- and S-based constituents without overheating, samples were cooled at different controlled cooling rates and analysed by SEM and DSC to understand the effect of cooling rate on the type, size, distribution and density of precipitates formed during the cooling process. Slow cooling was found to result in the precipitation of coarse η-phase particles, with both the amounts and sizes of these η-phase particles increasing with decreasing cooling rate.

Corrosion of cast magnesium-yttrium (Y) alloys with systematic Y additions up to a nominal 18wt.% were studied. Corrosion performance was related to the quantitative alloy microstructure and found to increase significantly with the level of alloying and volume fraction of the Mg-Y intermetallic present. In the alloy microstructures, Mg24Y5 was principally formed; the electrochemistry of which was characterised using the electrochemical microcell method. Electrochemical testing revealed the fundamental corrosion behaviour of Mg-Y alloys and elucidated the corrosion mechanisms at play. © 2011 Elsevier Ltd.

The precipitation of a-phase has been investigated in a concentrated b-alloy of the Ti-V-Cu system. a-precipitates in geometrically coupled forms were developed in the alloy when subject to isothermal ageing at 500°C. High-resolution transmission electron microscopy (HRTEM) revealed that a-phase embryos tend to nucleate in a symmetrical manner directly from an early-stage solute-partitioned diffusional product. The a-precipitates so developed constitute twin-related variants characterized by a twin plane lying on one of the {0111}a planes. The results are discussed with respect to the role of Cu on the formation of heterogeneous nucleation sites for a-phase.

The formation of the ω phase in the presence of simultaneous development of compositional modulations (or phase separation) within the body-centered cubic β matrix phase of a Ti–10V–6Cu (wt.%) alloy during continuous cooling has been investigated using a combination of transmission electron microscopy and atom probe tomography. While a water quench from the high-temperature β phase field allows apparently athermal formation of ω domains without any significant partitioning of solute or modulation in matrix composition, subsequent annealing at 500 °C for just 60 s leads to unusually rapid growth of the ω domains concurrent with, but apparently independent of, a slower development of finer-scale modulations in solute composition occurring apparently uniformly across both ω and β phases. In contrast, on slower air cooling from the solution treatment temperature, there are pronounced compositional fluctuations within the β phase, presumably as a product of spinodal decomposition, that are detectable prior to the formation of ω phase. The ω phase subsequently forms preferentially in solute-depleted regions of the matrix β, with a composition reflecting the local matrix composition and a solute content significantly lower than the average matrix composition. As a result, it has a cuboidal morphology, distinguishably different from the elliposoidal form that is observed in samples water-quenched and annealed at 500 °C.

A genuine non-destructive imaging of embedded nanoparticles is demonstrated using the novel technique of momentum-transfer X-ray diffraction imaging. For the first time, the method yields direct estimation of the statistical properties of the nanoparticle ensemble. Statistical analysis of the experimental data reveals quantitative information about the size distribution of the diffracting objects. Experimental results defining the statistically average form, orientation and dimensions of metastable Al2Cu (θ″, θ′ phases) precipitates embedded in binary Al–Cu solid solution are reported and analyzed.

This article discusses the competing mechanisms of martensite formation vs eutectoid decomposition via pearlitic or bainitic mechanisms during continuous cooling of a Ti-5 wt pct Cu hypoeutectoid alloy, which falls under the category of active eutectoid systems. Faster cooling rates result in a mixed microstructure of nanoscale bainite consisting of a far-from-equilibrium Ti2Cu phase and martensitic alpha plates, as determined from three-dimensional atom probe (3DAP) coupled with energy-filtered transmission electron microscopy (EFTEM). Slower cooling resulted in near-equilibrium eutectoid-based microstructures.

The discontinuous precipitation (DP) in a supersaturated Al-14.6at.%Zn alloy in relation to different forms of deformation structures has been investigated with optical and electron microscopy. It has been found that intense surface scribing, followed by short-term ageing at 65 °C, resulted in a recrystallized duplex structure with nanoscale equiaxed β-Zn particles and α-Al grains. In the absence of recrystallization and shear banding, moderate surface grinding increased the transformation kinetics of DP on the alloy surface by an order of magnitude compared with that of the undeformed counterpart. The enhanced transformation kinetics is attributed to intragranular nucleation and growth of DP colonies associated plausibly with dislocation cell wall structures induced by the surface strain. In contrast, bulk deformation by means of cold-rolling (13–66% reduction) and in situ stress-ageing (∼1% strain) both suppressed the development of DP in the alloy. The role of deformation bands as nucleation sites of DP and the driving force determining the development of DP colonies in deformed matrices are discussed.

Variations in solute element distribution occurring in a commercial 2024 aluminium alloy during isothermal ageing treatments at 170 °C for up to 120 h have been characterized using atom probe tomography. An early (0.5 h 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 (80 h 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 〈1 0 0〉 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 (0.5–1.8 at.%) concentrations of Si, which was non-uniformly distributed in elongated domains within the laths of the S phase. In overaged samples (114 h 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.

Studies for Al-alloys normally employ specimens subject to a fixed solution heat treatment (SHT), and hence the specific role of solutionising is overlooked. It is revealed that SHT has a major role in corrosion of AA7150 as judged by electrochemical, microscopic and profilometry studies. SHT dictates the constituent particle type and population remaining in the alloy for subsequent processing, and is thus not only a principal factor in corrosion, but a factor that can be engineered in developing more damage tolerant AA7150. Effect of an optimised SHT minimising the residual MgZn 2 and Al 2 MgCu content on corrosion resistance is demonstrated.

There is little or no correlation between grain-boundary, or matrix, microstructures and stress corrosion cracking (SCC) susceptibility. Grain-boundary microchemistry, especially the copper content of grain-boundary precipitates (GBP) is the most important factor. Further evidence for this is a correlation between the initial open-circuit potential of ‘fresh’ brittle intergranular fracture surfaces, the copper content of GBP, and the plateau SCC velocities of overaged 7079 and 7075 alloys. Preliminary comparisons of overaged (T7) and peak-aged (T651) material have also been made, and support the view that beneficial effects of overaging on SCC resistance is associated with increasing copper content of GBPs.

Interfacial characterisation of Kinetic Metallization (KM) sprayed Cu coatings applied on metal substrates was performed using optical and electron microscopy, as well as microindentation hardness testing and microchemical analysis. The interfacial characterisation of KM coatings remains scarce to date. Cross sectional observations of KM coatings on light metal substrates revealed an undulating, patelliform profile with thin-lipped cusps at the interface. Pure Al and Mg substrates exhibited a mechanically impinged zone

Aluminium alloys possess a unique combination of strength and low density, which historically have made these alloys ideal for many structural applications. However, addition of alloying elements in appreciable concentrations, in particular Cu and Mg, make Al-alloys inherently susceptible to localised corrosion. In this work, we adopt a bottom-up approach in an attempt to quantify the critical microstructural feature size (viz. precipitate size) that is capable of triggering a cascade of pitting events and eventual degradation of corrosion resistance. This is accomplished by exploiting the well characterised hardening response in a model alloy, Al–1.1Cu–1.7Mg (at.%), for which pitting resistance of the alloy was tracked with aging time and hence microstructural evolution. Corrosion performance and microstructural characterisation were carried out using a combination of electrochemical testing, coupled with high-resolution scanning transmission electron microscopy (HRSTEM) and atom probe tomography (APT). Results indicate, at least for this particular alloy, that second phase features below a critical width of approximately 3 nm can be tolerated from a corrosion perspective. This study has potentially wide consequences in the understanding of aluminium alloy corrosion initiation and the development of highly corrosion resistant aluminium alloys.

In this study, voids commonly associated with tin particles in two aluminium alloys containing microalloying additions (0.01 at.%) of tin have been observed by transmission electron microscopy. The voids were generated by quenching the alloys at moderate rates (102–103 K s−1) from a temperature (718 K) in excess of the melting temperature (501 K) of elemental tin in tin–microalloyed aluminium. Estimates of the void volume as a function of the volume of the associated tin particle reveal a linear relationship consistent with the excess volume resulting from the solidification of the tin particle. The formation and stabilisation of shrinkage voids in metallic alloys are suggested to arise from a combination of high vacancy supersaturation, the large volumetric misfit strain of the solidifying tin particle and a reduction in void surface energy associated with segregation of alloying elements.

Two novel β-titanium alloys, of compositions Ti–10V–2Ni and Ti–10V–6Cu (wt.%) and with distinct potential for athermal ω-phase formation, have been selected and the sensitivity of the age-hardening responses to the cooling rate post solution treatment investigated. Upon rapid water quenching, the solution-treated Ti–10V–2Ni alloy contained a significantly higher volume fraction of ω-phase than the Ti–10V–6Cu alloy. It was observed that the Ti–10V–6Cu alloy is remarkably insensitive to cooling rates in the range of water quenching to air cooling, in terms of both microstructural evolution and age-hardening behaviour, when aged isothermally at 350 °C and 500 °C. The microstructure is thermally stable and the hardness remains approximately constant for extended aging (48 h) at both temperatures, implying that it may have potential as a deep-hardenable alloy. In comparison, the aging response of the Ti–10V–2Ni alloy was marginally influenced by cooling rate when aged at 500 °C, the effect being ascribed to autocatalytic formation of α-phase in the course of slow air cooling. The results are discussed with respect to the β-phase stability and the time-temperature-transformation (TTT) characteristics of metastable beta-titanium alloys.

Scandium is a light but highly potent hardening element for aluminium. However, the amount of Sc that can be precipitated for hardening is rather limited because of its limited solubility. Electron beam direct manufacturing (EBDM) is a novel metal fabrication technique. We show that electron beam processing substantially increases the solubility of Sc in α(Al) and the attendant hardening effect is about twice that attained from solid solution and ageing. These findings provide a basis for new alloy design for EBDM.

Plate samples of Al-2Sc were processed with an electron beam (EB) gun in a vacuum of 10-3 Pa at 50kV and 30 µA. EB processing suppressed the formation of primary Al3Sc, altered the morphology of the eutectic (Al)-Al3Sc, and increased the solubility of Sc in the eutectic (Al). In addition, its cyclic heating effect induced the precipitation of Al3Sc. EB processing resulted in more profound hardening than conventional high temperature solid solution and ageing.

A good combination of high strength and hardenability makes the alloy Ti 10V-2Fe-3Al a prime candidate for applications in the aerospace arena. However, these properties are very dependent on a post-forming heat treatment. The overall objective of this work is to determine the effect of prior deformation on the aging behaviour. In this particular study, the influence of the heat treatment, either solution and/or aging, on the microstructures, and consequently on the mechanical properties, without introduced strain is reported. Various solution heat treatments have been conducted, either in the β phase or in the (α+β) phase field, followed by rapid quenching or slow cooling, and aging treatments at different temperatures (250, 350, 400, 500C°) above and below the ω-transus temperature. Vickers hardness indentations were used to follow the precipitation hardening behaviour, and mechanical properties were determined using a shear punch test. The aging response is dependent not only on the presence of the athermal ω phase but also on the proximity of the aging temperature to the ω-transus. Most treatments showed an unusual initial softening behaviour prior to age hardening, however this appears to be related again to the composition and fraction of the β phase retained after solution treatment.

The effect of different pre-ageing treatments on the subsequent artificial ageing response of Al-Mg-Si(-Cu) alloys have been investigated using hardness, tensile and electrical conductivity testing. The microstructural evolution was characterised by differential scanning calorimetry (DSC) and 3-dimensional atom probe (3DAP) analysis. Pre-ageing treatments were carried out at 160-250°C for short times. Results show that the early stage artificial ageing response after 30 minutes at 170°C is strongly influenced by the pre-ageing and natural ageing conditions. A pre-ageing treatment performed for a short time at a high temperature and within a short delay after solution treatment and quenching was found to give a promising hardening response during subsequent artificial ageing. The mechanisms by which pre-ageing can reduce the detrimental effect of natural ageing on the artificial ageing response will be discussed in relation to the formation and distribution of clusters, GP zones and/or precipitates.

In this work, the influence of multi-step solution (MSS) treatments on the constituent particle dissolution, overheating and associated quench cracking behaviour in room temperature water-quenched 7150 Al alloy has been investigated. For comparison, the microstructure and quench cracking behaviour of single step solution treated samples water-quenched from 505°C were also investigated. Based on optical microscopy of differently quenched samples, the quench cracking mode and the influence of overheating of constituents on the quench cracking behaviour have been demonstrated. The results reveal that the constituent particles can be effectively dissolved in the MSS-505°C samples. When the quench temperature of MSS-505°C samples is equal to or higher than 485°C , macro quench cracks can be clearly observed. Moreover, the density and length of the quench cracks increase with increasing quench temperature. Etched microstructures indicate that the quench crack propagation mode is intergranular. However, for samples directly heated to 505°C , typical overheating can be observed at the triple junctions and these regions preferentially act as crack propagation routes.

The green strength of a powder compact results from the mechanical interlocking of the irregularities on the particle surfaces. During compaction, particle rearrangement, plastic deformation and particularly surface deformation of powders occur. Titanium powder is susceptible to interstitial element contamination, which may lead to solid solution strengthening of the particles and/or the formation of non-metallic compounds on the surface. However, the influence of these various impurities, namely oxygen and nitrogen, on the green strength has not been investigated. This work investigates and quantitatively evaluates the factors influencing the green strength of the powder compacts. The indirect tensile test was applied for the determination of the green strength of the powder compacts, and test results were compared to that of a more conventional 3-point bending test. The substantial dependence of green strength on both the amount of impurity element in the core of the powder particles and the compaction pressure is demonstrated. The effect of the surface condition of the powder particles on green strength is also reported.

Measurement of the electrochemical properties phase-by-phase on the nanoscale in real (commercial) alloys is critical to understanding the microstructure-corrosion relationship and subsequently controlling it. This work presents a novel AFM based in situ corrosion probing methodology (for the first time) that is capable of resolving the electrochemical activity (impedance response) into the nanometer range; the method subsequently having major ramifications in the study of aluminium alloy corrosion, the interpretation of corrosion propagation, and the subsequent development of corrosion resistant aluminium alloys.

Electron beam (EB) direct manufacturing is an additive near-net shape digital fabrication process developed recently. The process offers a promising route for the fabrication of intricate speciality aluminium alloy parts for aircraft and aerospace applications because of the excellent energy coupling between an electron beam and aluminium. As part of a fundamental study on EB manufacturing of Al alloys, this work investigates the effect of the EB processing parameters on the development of the molten pool and the solidification characteristics of Al 2219 and Al 6061 alloys. The samples were processed using a 50kV electron beam gun over a wide range of beam currents (10-40mA) and welding speeds (0.3-0.86m/min) in both the static and oscillation focus modes. In the static focus mode, the molten pool is wedge-shaped; while in the oscillation focus mode, the molten pool is hemispherical, wider and shallower. In both cases, the depth and width of the molten pool increase with increasing beam current but they are less affected by the moving speed of the EB gun in the range 0.3-0.86m/min. Electron beam re-melted and subsequently re-solidified Al 2219 and Al 6061 alloys show microstructural features distinct from those obtained under sand casting and direct chill casting conditions. In particular, fine intermetallic precipitates in the size range 100-200nm are prominent in the equiaxed grains formed in the re-solidified weld beads of Al 6061 compared to the coarse intermetallic particles up to 10m in size prior to EB processing. EB processing offers opportunities for aluminium alloy development.

Rod-shaped Zr–Zn particles are commonly observed within individual α-Mg grains in magnesium alloys containing both Zr and Zn elements. In the present study, the identity, morphology, and orientation of such solid-state precipitate particles formed in a Mg–1Ca–1Zn–1Zr (wt.%) alloy have been characterized using analytical transmission electron microscopy and electron microdiffraction. It is found that these particles have a tetragonal crystal structure similar to that of Zn2Zr3 (P42/mnm, a = 0.768 nm, c = 0.699 nm). Three distinguishably different orientation relationships are observed between the Zn2Zr3 and matrix phases. But the long axes of the rods are invariably parallel to the [001] direction of the Zn2Zr3 phase.

We report a novel approach to X-ray diffraction data analysis for nondestructive determination of the shape of nanoscale particles and clusters in three dimensions with a spatial resolution of a few nanometers. The advantage of the proposed approach is that it does not require a coherent X-ray source and therefore is suitable for almost any synchrotron radiation beamline and many laboratory sources. The technique is insensitive to the coherence of the X-rays, and 3D reconstruction of a modal image is possible without tomographic synthesis, rendering the approach suitable for laboratory facilities. Successful application of the technique to the characterization of nanoparticulate in a range of dispersed-phase nanocomposite structures illustrates this novel approach.

The mechanical properties of bulk and nanoscale TiO 2 phases are examined with a view to assess the available bulk modulus and hardness data, and to understand the size-dependent behaviors. The bulk modulus values of thermodynamically stable bulk TiO 2 phases show a general correlation with Ti–O coordination number. As with the cotunnite-structured (OII) phase, it is likely that the seven-coordinated OI and eight-coordinated fluorite forms of TiO 2 are ultrahard substances. Of the nanoscale phases investigated thus far, nanocrystalline anatase displays the strongest size dependence of bulk modulus values, with possible stiffening behavior effected by incipient grain boundary amorphization under pressure. Nanocrystalline rutile and baddeleyite phases do not show appreciable size dependence in their compression behaviors.

Equal channel angular extrusion (ECAE), with simultaneous application of back pressure, has been applied to the consolidation of 10 mm diameter billets of pre-alloyed, hydride–dehydride Ti–6Al–4V powder at temperatures ≤400 °C. The upper limit to processing temperature was chosen to minimise the potential for contamination with gaseous constituents potentially harmful to properties of consolidated product. It has been demonstrated that the application of ECAE with imposed hydrostatic pressure permits consolidation to in excess of 96% relative density at temperatures in the range 100–400 °C, and in excess of 98% at 400 °C with applied back pressure ≥175 MPa. ECAE compaction at 20 °C (back pressure = 262 MPa) produced billet with 95.6% relative density, but minimal green strength. At an extrusion temperature of 400 °C, the relative density increased to 98.3%, for similar processing conditions, and the green strength increased to a maximum 750 MPa. The relative density of compacts produced at 400 °C increased from 96.8 to 98.6% with increase in applied back pressure from 20 to 480 MPa, while Vickers hardness increased from 360 to 412 HV. The key to the effective low-temperature compaction achieved is the severe shear deformation experienced during ECAE, combined with the superimposed hydrostatic pressure.

A variation of the Orowan equation is developed for aluminium alloys strengthened by dispersed, deformation-resistant {1 0 0} α precipitate plates, and the quantitative effects of precipitate volume fraction, number density and aspect ratio on increments in Orowan strengthening are examined. The validity of the model is examined by a combination of mechanical property measurements and rigorous quantitative stereology of the size and distribution of {1 0 0} α plates of θ ′ phase in an Al–4Cu–0.05Sn (wt.%) alloy aged isothermally at 200 °C. For the ageing conditions selected, the predictions of the model for Orowan strengthening increments are in good quantitative agreements with those observed experimentally. The model predicts that, for a given volume fraction and number density of precipitate plates, an increase in plate aspect ratio can lead to a substantial increase in strength. In addition, it is not necessary to invoke a transition from precipitate deformation to Orowan strengthening to account for the form of the precipitation-strengthening response of the alloy. The maximum strengthening increment is associated with the minimum effective inter-particle spacing, rather than a critical thickness of the precipitate plates.

The application of wide field-of-view detection systems to atom probe experiments emphasizes the importance of careful parameter selection in the tomographic reconstruction of the analyzed volume, as the sensitivity to errors rises steeply with increases in analysis dimensions. In this article, a self-consistent method is presented for the systematic determination of the main reconstruction parameters. In the proposed approach, the compression factor and the field factor are determined using geometrical projections from the desorption images. A three-dimensional Fourier transform is then applied to a series of reconstructions, and after comparing to the known material crystallography, the efficiency of the detector is estimated. The final results demonstrate a significant improvement in the accuracy of the reconstructed volumes. Copyright © Microscopy Society of America 2008.

Powder metallurgy is widely used to produce alloys with low cost of production. The main drawback using powders is the level of residual porosity of final product which often implies the application of a complicated and costly hot isostatic pressing process. However, this issue can be overcome by using equal channel angular pressing (ECAE) with back pressure (BP). The use of severe shear deformation, with imposed hydrostatic pressure, allows a reduction in the range of compaction temperatures compare to those used in conventional practice. The compaction of Ti-6Al-4V powder by the ECAE method has been investigated. The compaction has been performed at temperatures starting from room temperature (RT) and increasing up to 400°C with various back pressures ranging from 0 to 350MPa. A billet processed by ECAE with 43MPa back-pressure at 400°C was found to have improved relative density of 97.5% and increased Vickers hardness of 369HV, compared to values of 96.7% and 325HV respectively obtained at RT. A relative density of 98.2% and 426HV hardness were measured for billets processed with BP = 262MPa at 400°C. A fully compact billet was obtained by applying 350 MPa of BP at 400°C.

The range of commercial titanium alloys available is currently extremely restricted, with one alloy (Ti-6Al-4V), and derivatives of it, accounting for a very large proportion of all applications. High performance alloys are costly to fabricate and limited to low-volume applications that can sustain the cost. With the emergence of new processing technologies that promise to reduce significantly the cost of production of titanium metal, especially in powder form, there is an emerging imperative for cost-effective near net shape powder processing techniques to permit the benefit of reduced metal cost to be passed on to higher-volume applications. Equally, there is a need for the design and development of new alloys that are intrinsically low-cost and lend themselves to fabrication by novel cost-effective net shape processing. The approaches that might be used to select, design and process both conventional alloys and novel alloy systems will be reviewed, with a focus on innovation in design of low-cost alloys amenable to new processing paths and increasingly tolerant of variability in composition.

A magnesium–8 wt% lithium alloy was prepared to give a microstructure of hcp (α) and bcc (β) phases, with the α phase appearing as plates in the β matrix. A near Burgers OR was observed between the two phases, i.e., [ 0 0 0 1 ] α / / [ 0 1 ¯ 1 ] β , ( 1 ¯ 1 0 0 ) α / / ( 2 ¯ 1 1 ) β . An additional ∼0.5° rotation of the bcc lattice around [ 0 0 0 1 ] α / / [ 0 1 ¯ 1 ] β axis, as well as the α/β interface orientation, were both predicted by a moire plane approach and observed experimentally.

The form and crystallography of precipitate S phase have been examined in overaged Al–Cu–Mg alloys of low Cu:Mg atomic ratio using high-resolution electron microscopy and diffraction. Orthorhombic S takes the form of laths elongated parallel to [1 0 0] S //[1 0 0] α in face-centred cubic (fcc) matrix α solid solution and, in any given sample, may adopt a continuous or near-continuous range of orientation relationships, distinguished by rotation of up to 7° about these common axes between two discrete limits defined by the relationships: ( 0 0 1 ) S / / ( 0 2 1 ) α , [ 1 0 0 ] S / / [ 1 0 0 ] α and ( 0 2 ¯ 1 ) S / / ( 0 1 4 ) α , [ 1 0 0 ] S / / [ 1 0 0 ] α Such variation in orientation is accompanied by accommodating changes in interface orientation and structure, and by a systematic variation in the lattice parameters of the S phase. It is demonstrated that these changes are, in turn, associated with, at least local, changes in matrix α lattice parameter, likely to be attributable to heterogeneous coherent nucleation in association with pre-existing defects and/or solute or solute/defect clusters.

A novel approach to X-ray diffraction data analysis for nondestructive determination of the shape of nanoscale particles and clusters in three-dimensions is illustrated with representative examples of composite nanostructures. The technique is insensitive to the X-ray coherence, which allows 3-D reconstruction of a modal image without tomographic synthesis and in situ analysis of large (over a several cubic millimeters) volume of material with a spatial resolution of few nanometers, rendering the approach suitable for laboratory facilities.

The effect of Si additions on vacancy behavior following solution treatment and quenching was studied for an Al-Cu-Mg-Ag alloy system using positron annihilation lifetime spectroscopy (PALS). Both the initial positron lifetime and steady state positron lifetime increase as the Si concentration increases. This behavior is interpreted in terms of Si interacting with vacancies, leading to their retention in the alloy. The ability of PALS to monitor vacancy behavior after solution treatment should allow the improved prediction of phase transformation kinetics and hence the tailoring of ageing treatments.

The effects of multiple ageing treatments on the precipitation hardening response of aluminium alloy AA6111 has been studied by varying the first stage ageing temperature and period systematically. The effects of intermediate low temperature ageing dwell time were also studied. Throughout this work, the final ageing temperature was restricted to 180°C in order to coincide with the temperature used for the paint-baking cycle in the industry. It is found that with proper multiple ageing treatments, the kinetics of the age hardening response can be reduced from 8 hours to 4 hours and that the peak hardness can be increased from 128HV to 132HV.

The ability to improve mechanical properties through trace alloying additions that enhance the precipitation-hardening response has been a cornerstone of aluminium research and alloy development. It is now known that many of these alloying additions are beneficial (or deleterious) because of their influence on the competitive precipitation kinetics, in many cases arising from the formation of chemical and structural heterogeneities in the matrix. In order to better our understanding of this phenomenon, the earliest stages of microstructural evolution of indium precipitates and θ' plates in an Al-Cu-In ternary alloy have been investigated. Quantitative microstructural characterization reveals that there is a correlation between the size of the pre-existing indium particle and the resulting heterogeneous precipitation kinetics, with larger indium particles favouring θ' precipitation.

Nuclear magnetic resonance (NMR) was used to systematically monitor the amount of copper in the matrix of various age hardenable Al-Cu alloys as a function of ageing treatment. These NMR results were used to compare the measured amount of copper residing in the matrix during microstructural evolution with that suggested from the equilibrium phase diagrams. Our results indicate that the matrix copper composition for under- and peak-aged microstructures is generally far from equilibrium, with much more copper in solution than expected based on equilibrium phase diagrams. NMR is shown to be a sensitive and efficient metallurgical characterization technique to monitor the partitioning of solute in both non-equilibrium and equilibrium phases.

The interaction between vacancies and Sn and Cu solute atoms in an Al-1.7at.%Cu- 0.01at.%Sn alloy was investigated by exploring the effect of incorporating natural ageing into conventional age hardening treatment. It was found that provided the artificial ageing temperature does not exceed a critical value between 160°C and 200°C, a narrow window of natural ageing (3-100 h) will result in a significant acceleration of the age hardening response and no decrease in peak hardness. Transmission electron microscopy showed that this effect reflects a large and rapid increase in number density of Cu GP(I) zones, and, to a lesser extent, of θ". The distribution and number density of θ' are essentially unaffected. Three-dimensional atom probe provided strong evidence that refinement of GP(I) zone distribution is not due to clustering of Cu atoms onto pre-existing Sn clusters. Instead it appears to be caused by a subtle interaction between vacancies, Sn and Cu atoms.

Regardless of whether it is cast microstructure, the grain structure that is the product of thermomechanical processing or the nanoscale dispersions of strengthening second-phase particles, it is inescapable that the structural scale that controls mechanical properties in Al alloys is determined primarily by processes of nucleation during either solidification, recrystallisation or solid-state phase transformation. In those advanced alloys with bulk amorphous or nanocrystalline structure, production of an amorphous precursor is reliant on initial suppression of the nucleation of crystallisation, and subsequent controlled nucleation of dispersed nanocrystals within amorphous matrix. The processes of nucleation that control structural scale in modern Al alloys are briefly reviewed, with a focus on potential for further structural refinement and advances in properties.