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. similar to 50% RH) was more hydrated on the AA7075-T651 than AA7079-T651. (C) 2015 Elsevier Ltd. All rights reserved.

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 {110}β planes. Crystallographic orientation analyses suggest that the Ti2Cu phase is related to the matrix via a definite orientation relationship of (01¯3)Ti2Cu‖(011)β and [100]Ti2Cu‖[100]β, while the growth of the α phase is governed by the Potter’s OR such that (11¯01)α‖(011)β and [112¯0]α‖[11¯1]β. (01¯3)Ti2Cu and (11¯01)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.

In this study, corrosion of Zn surfaces caused by fine size acidified droplets (diameter similar to 0.1-5 mu m) was quantified. Fine size droplets of various chemistries were deposited using an unmodified inkjet printer and damage was quantified in terms of volume loss (Delta V) 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. Crown Copyright (C) 2011 Published by Elsevier Ltd. All rights reserved.

The precipitates present in an Al–0.59Mg–0.71Ge (at.%) alloy have been studied using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy. Two types of needle-shaped precipitates growing along 〈0 0 1〉Al were found: a phase isostructural to the trigonal U1 phase found in Al–Mg–Si alloys, and finer precipitates with a hexagonal arrangement of Ge columns. The study revealed the presence of a complex interface structure surrounding the U1-like precipitates, and an explanation based on interatomic distances is proposed.

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.

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 strengthening of an Al-Mg-Si-Cu alloy during natural ageing and subsequent short artificial ageing was investigated using three-dimensional atom probe (3DAP) analysis and tensile testing. The contingency table and Markov chain analyses confirmed that non-random arrangements of atoms already exist after a natural ageing time of only 3.5h. Extensive use of particle analysis tools in the IVAS and PoSAP software packages revealed that whilst the commonly used minimum aggregate size (N(min)) of 10 is a reasonable choice, much more useful information about the system can be gained by additionally employing a wide range of larger and smaller N(min) values. In particular, it was found that the density and volume fraction of solute aggregates increased with increasing natural ageing time in the T4 condition. After a 0.5h artificial ageing treatment at 170 °C (designated as T6), the size, volume fraction and Mg/Si ratio of the aggregates were all found to decrease with increasing prior natural ageing time. These findings are used to discuss the detrimental effect of natural ageing, where the T6 strength has been observed to decrease rapidly with increasing prior natural ageing time before stabilising after several hours of natural ageing.

Rapidly quenched ribbons (˜50m thickness) of Al-5wt.%Ti, Al-lwt.%B and a range of Al-Ti-B alloys have been produced by melt spinning under He atmosphere and the microstructures of the ribbons, following solidification and post-solidification heat treatment, characterized using analytical electron microscopy. In the Al-5Ti alloy, the coarse equilibrium primary phase (b.c.t. Al3 Ti) that is observed following conventional casting is replaced by fine (0.1–0. 2μm), cuboidal particles of a metastable cubic (Ll2) Al3Ti in melt-spun ribbon. These metastable particles form directly from the melt and act as nucleation sites for the solid solution which subsequently forms. A refined microstructure with an average grain size of 1–2μm results. A supersaturation of Ti is retained in matrix solid solution following solidification and a variety of solid state precipitate forms, including fine dispersions of coherent, metastable Al3 Ti particles, is observed to emerge during post-solidification heat treatment. For the Al-1B alloy, the coarse distribution of primary AlB2 particles in a chill-cast ingot is replaced by a fine, uniform dispersion of the metastable boride, α-AlB12, in the melt-spun ribbon. Attempts to induce a refined boride dispersion in melt-spun Al-Ti-B alloys have proved largely unsuccessful.

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 60s 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.

This letter presents a high-throughput methodology to systematically study individual variables involved in atmospheric corrosion. Corrosion caused by droplets, representative of various atmospheric environments, was quantified in terms of volume loss as measured through optical profilometry. Measurements were sensitive to variations in electrolyte chemistry and allowed relatively rapid and reproducible discrimination between the effects of different atmospheric parameters. This approach allows a high degree of experimental control and calculated corrosion rates from optical profilometry were comparable to those obtained using an independent electrochemical method. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3569800] All rights reserved.

Corrosion of cast magnesium-yttrium (Y) alloys with systematic V additions up to a nominal 18 wt.% 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. Mg(24)Y(5) 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. (C) 2011 Elsevier Ltd. All rights reserved.

Among the many considerable challenges faced in developing a rational basis for advanced alloy design, establishing accurate atomistic models is one of the most fundamental. Here we demonstrate how advanced imaging techniques in a double-aberration-corrected transmission electron microscope, combined with ab initio modeling, have been used to determine the atomic structure of embedded 1 nm thick T1 precipitates in precipitation-hardened Al–Li–Cu aerospace alloys. The results provide an accurate determination of the controversial T1 structure, and demonstrate how next-generation techniques permit the characterization of embedded nanostructures in alloys and other nanostructured materials.

Atmospheric corrosion of Zn under a variety of simulated marine aerosols was studied. In-situ monitoring of droplet pH, volume loss measurements, identification and distribution of crystalline and amorphous phases from corrosion under different droplets were used to understand the role of acidification on atmospheric corrosion of Zn. Results for various droplet chemistries are discussed in terms of initiation mechanism, phase distribution and surface morphology in conjunction with chemical equilibrium calculations. Zn exposed to sulphate containing droplets had relatively small corrosion rates and greater coverage with Gordaite as compared to sulphate-free droplets where coupons have relatively more coverage with Simonkolleite. Crown Copyright (C) 2011 Published by Elsevier Ltd. All rights reserved.

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. (C) 2010 Elsevier Ltd. All rights reserved.

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 (theta', theta '' phases) precipitates embedded in binary Al-Cu solid solution are reported and analyzed. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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 3nm 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.