AA7050 and AA7010 aluminium alloys in overaged tempers have been a vital material for aerospace applications since their development in the 1970 ' s. Latest generation materials including AA7085, AA7449 and AA7037 have higher levels of Zn and reduced Fe and Si and offered some potential advantages but have suffered from Environmentally Induced Cracking (EIC) in service [1] and now in many cases are being replaced with previous generation materials [2]. Holroyd and Scamans [3] detailed some of the major risk factors associated with the new range of compositions and work by Schwarzenbock et al. [4] showed the stark EIC performance differences however the mechanistic understanding of these differences was still not clear. In this work, in-situ monitoring of 4 point bending tests conducted in conditions of 70 degrees C and 50 % RH was used to compare thick plate AA7050T7651 and AA7085-T7651 as important alloys representative of the two generations of development. In both cases we find brittle intergranular cracks form, identified from the fracture surface, but in the case of AA7085 this is essentially the only mode of EIC whereas for AA7050 we see a mixed mode fracture surface which also includes intergranular modes of microvoid coalescence and transgranular cracking. It appears that cracking in AA7050 is only just viable. Short crack growth behaviour in the two materials allows us to identify an 'intrinsic' resistance of the grain boundaries to EIC and we find this is substantially greater in AA7050 compared to AA7085. This difference in performance is related to the ability of the grain boundary precipitate reactions to sustain hydrogen generation for continuous cracking. We find this process is very much disrupted in the case of AA7050 leading to a mixture of fracture modes and a commensurate reduction in crack growth rates by around an order of magnitude or more.

This study explored the impact of minor Cr and Ag additions on the resistance of a new generation Al-8Zn-2Mg-2Cu-0.1Zr alloy to hydrogen environmentally assisted cracking (HEAC) in humid air. The results show that HEAC in Al-8Zn-2Mg-2Cu-0.1Zr alloy can be suppressed by 0.1 wt% Cr addition, due to the formation of E-Al18Mg3Cr2 dispersoids with a high H binding energy and possibly a less electrochemically active grain boundary (GB) microchemistry. similar to 0.3 wt% Ag addition in Al-8Zn-2Mg-2Cu-0.1Zr alloy had no influence on the HEAC susceptibility, despite 1 at% Ag partitioning in the GB eta-Mg(Zn,Cu,Al)(2) and a minor decrease in the PFZ width.

The mechanical and stress corrosion behaviour of 7xxx aluminium thick-plate alloys are strongly influenced by a dual distribution of η-phase quench-induced and age-induced grain boundary precipitates (Q-GBPs and A-GBPs) that nucleate independently during cooling from solution treatment and artificial ageing, respectively. A novel cold intergranular fracture technique has been used to allow the interaction between these two precipitate distributions, formed on GB faces, in two alloys with different quench sensitivities, AA7050 and AA7085, to be more rigorously compared than previously possible. After a full T76 heat treatment, large widely spaced Q-GBPs are observed, between which high-density patches of small A-GBPs nucleate during subsequent artificial ageing. The A-GBP patches are separated from the Q-GBPs by distinct GB precipitate-free areas. Their respective distributions vary in a non-intuitive way with cooling rate. As the rate increases, the A-GBPs and patch area fractions reduce to a minima before rapidly increasing at very high cooling rates as Q-GBP nucleation becomes fully suppressed. This occurs because, although the size of the Q-GBPS GBPs reduces with cooling rate, there is a more dramatic increase in their nucleation density, which leads to a minima in the GB area where sufficient solute supersaturation is retained for precipitation during ageing. The same relative effects are seen in both alloys, but the Q-GBPs formed in AA7085 are significantly smaller and the density higher due to the lower solvus temperature.

The mechanical properties and stress corrosion cracking (SCC) resistance of 7xxx series aluminium alloys are significantly affected by the composition and distribution of precipitates formed during heat treatment. In particular, their quench sensitivity is related to the formation of η-phase precipitates that nucleate heterogeneously on grain boundaries at lower cooling rates after solution treatment, which has been a key factor restricting the gauge of hot rolled plates in the aerospace industry. To better understand the effects of slower cooling rates on the composition of quench-induced grain boundary precipitates (Q-GBPs) found in thick plate 7xxx alloys, plasma focused ion beam and high-resolution scanning transmission electron microscopy were used to obtain accurate composition data. The η-phase Q-GBPs have a complex- branched morphology, which develops higher aspect ratios and secondary arms as the cooling rate is reduced. Only a small change in average composition of Q-GBPs was found with cooling rate; but a large scatter was observed. This is caused by significant Zn/Cu/Al composition gradients developing along their principal growth directions in both AA7050 and AA7085 alloys. This concentration gradient did not reduce significantly after a T76 treatment. Simulations of Q-GBP growth with different cooling rates using a CALPHAD-informed phase-field model, with the η-phase represented by a two-sublattice model, gave results consistent with experimental observations. Chemical gradients were predicted to develop in the Q-GBPs due to the changing local equilibrium at the growth front during the cooling. The influence of this non-homogeneous microchemistry on the SCC behaviour of 7xxx alloys is briefly discussed.

Recent experience has shown that new-generation 7xxx-series alloys, that have a high Zn content and Zn/Mg ratios, have a greater susceptibility to hydrogen-environmental induced cracking (H-EIC) on exposure to humid air than more established materials, like AA7050. In this study, we report new evidence of the EIC initiation and crack growth behavior of two new-generation alloys, AA7085 and AA7449, when exposed to 50% humidity. In situ, time-lapse, optical imaging over large areas has enabled the exact initiation sites to be identified and investigated with high-resolution fractographic studies, providing evidence for the sequence and mechanisms of initiation and transition to sustained cracking. A consistent behavior was observed for both alloys. This has revealed that minute-scale corrosion reactions, involving highly localized condensed water, are necessary for initiation. The preferred initiation sites are metal ligaments between surface-connected pore clusters and/or intermetallic particles that are subjected to high-stress concentration and undergo mechanical damage with associated higher levels of local oxidation. The growth of short protocracks from these sites is a distinct stage and displays intermittent arrest markings evidenced by localized corrosion. In contrast, in humid air environments, long cracks in these alloys exhibited relatively constant, higher velocity, with extremely limited corrosion commensurate with oxidation of a free surface in this environment resulting in approximately 5 nm oxide layer.

Demand for low carbon energy storage has highlighted the importance of imaging techniques for the characterization of electrode microstructures to determine key parameters associated with battery manufacture, operation, degradation, and failure both for next generation lithium and other novel battery systems. Here, recent progress and literature highlights from magnetic resonance, neutron, X-ray, focused ion beam, scanning and transmission electron microscopy are summarized. Two major trends are identified: First, the use of multi-modal microscopy in a correlative fashion, providing contrast modes spanning length- and time-scales, and second, the application of machine learning to guide data collection and analysis, recognizing the role of these tools in evaluating large data streams from increasingly sophisticated imaging experiments.

Abstract Silicon is a promising negative electrode material for high‐energy‐density Li‐ion batteries (LiBs) but suffers from significant degradation due to the mechanical stress induced by lithiation. Volume expansion and lithiation in Si are strongly anisotropic but associated early interfacial transformations linked to these phenomena and their implications for electrode performance remain poorly understood. Here we develop a novel correlative electrochemical multi‐microscopy approach to study local interfacial degradation at the early stages for three different surface orientations of Si single crystals: Si(1 0 0), Si(1 1 0) and Si(3 1 1), after Li‐ion electrochemical cycling. The experimental strategy combines scanning electrochemical cell microscopy (SECCM) measurements with subsequently recorded scanning transmission electron microscopy images of high‐quality cross sections of Si electrodes, extracted at selected SECCM regions, using a novel Xe+ plasma‐focused ion beam procedure. These studies reveal significant surface orientation–dependent nanoscale degradation mechanisms that strongly control electrode performance. Si(1 0 0) was immune to interfacial degradation showing the best lithiation reversibility, whereas local nanoscale delamination was observed in Si(1 1 0) leading to a lower Coulombic efficiency. Continuous electrochemical deactivation of Si(3 1 1) was associated with delamination across the whole interface, Li trapping and formation of thick (ca. 60 nm) SiO2 structures. These results demonstrate surface crystallography to be a critical factor when designing Si‐based battery materials and strongly suggest that promoting Si(1 0 0) facets could potentially provide longer cycling life and performance due to a higher resistance to degradation.

Recently, hollow thermoplastic microspheres, have emerged as an innovative filler material for use in polymermatrix composites. The resulting all-polymer syntactic foam takes on excellent damage tolerance properties, strong recoverability under large strains, and favourable energy dissipation characteristics. Aside from syntactic foams, thermoplastic microspheres are finding increasing usage in a variety of applications and industries. Despite this, there is an absence of statistical geometrical and mechanical data for certain classes of thermoplastic microspheres. In this work we characterise two classes of thermoplastic microsphere using X-ray computed tomography, focused ion beam and electron microscopy. We observe the spatial distribution of these microspheres within a polyurethane-matrix syntactic foam and show that the volume-weighted polydisperse shell diameters follow a normal distribution. Interestingly, polydispersity of the shell wall thickness is not observed and furthermore the wall thickness is not correlated to the shell diameter. We utilise the geometrical information obtained in analytical micromechanical techniques in the small strain regime to determine, for the first time, estimates of the Young's modulus and Poisson's ratio of the microsphere shell material itself. Our results contribute to potential future improvements in the design and fabrication of materials that employ thermoplastic microspheres, including syntactic foams.

While the general characteristics of various peening techniques have been established, there have been few comparative studies. Here we compare the variation of the residual stresses and microstructural characteristics with depth for 316L austenitic stainless steel treated by cavitation peening (CP), shot peening (SP) and laser peening (LP) all peened to similar intensity levels. While the plastically affected depths were similar in all cases (similar to 400 mu m), the SP specimen showed the most extensive near surface plastic deformation, deformation twinning, dislocation density and compressive residual stress. To counterbalance this, the compressive residual stresses extended deeper for the LP and CP. Across the three treatments, a similar dependency was found between diffraction peak broadening and hardness. The dislocation density at the surface determined by the diffraction line profile analysis (LPA) for the SP specimen (4.9 x 10(15) m(-2)) was approximately 2.5 times that for the CP and LP specimens (2.0 x 10(15) and 2.1 x 10(15) m(2)). Electron backscatter diffraction (EBSD) shows that the extensive work introduced by the SP had generated planar defects near to the surface. The increase in yield stress estimated from the hardness corresponded with the increase in dislocation density obtained by the LPA.

Here we describe the first automated fully integrated in-microscope broad ion beam (BIB) system. Ar+-BIB has several advantages over Ga+ focused ion beam (FIB) and Xe+ plasma-FIB (PFIB) methods inducing less beam damage, especially for ion beam sensitive materials. It can mill areas several orders of magnitude larger (up to millimetre scale), and is not confined to the edge of the sample with associated curtaining issues. BIB is shown to have sputter rates up to five times higher than comparable FIB techniques. This new coupled BIB-SEM system (commercial name 'iPrep (TM) II') enables in-microscope surface polishing to remove contaminants or damage for two dimensional (2D) imaging, as well as automated serial section tomography (SST) by milling and imaging hundreds of slices, cost and time efficiently. The milled slice thickness can be controlled from a few nanometers up to a micrometre. A novel sample transfer, handling and interlock system allows automated and sequential BIB polishing, scanning electron microscopy (SEM) and analysis by secondary electron (SE) imaging, electron back scatter diffraction (EBSD) and energy dispersive spectroscopy (EDS) for 3D microstructure analysis. Furthermore, insulating surfaces can be sputter coated after milling each slice to reduce charging during SEM analysis. The performance of the instrument is demonstrated through a series of case studies across the materials, earth and life sciences exploiting the imaging, crystallographic and chemical mapping capabilities. These include the study of butterfly defects in bearing steels, meta-stable intermetallic phases in bronze bearings, shale gas rock, aluminium plasma electrolytic oxide (PEO) coatings as well as liver and mouse brain tissues.

Due to their highly favourable thermal, mechanical, and acoustic properties in extreme environments, syntactic foams have emerged as a popular material choice for a broad variety of applications. They are made by infiltrating a polymeric matrix, in either a glassy or polymeric state, with hollow microspheres made from a wide range of materials. In particular, hollow plastic microspheres, such as Expancel made by Nouryon have recently emerged as an important filler medium, with the resulting all-polymer composites taking on excellent damage tolerance properties, strong recoverability under large strains, and very favourable energy dissipation characteristics. There is however, a near-complete absence of statistical information on the diameter and shell-thickness distributions of these microspheres. In this work, using X-Ray computed tomography, focused ion beam, and scanning electron microscopy, we report on these quantities and observe the spatial distribution of microspheres within a syntactic foam. We then employ this data to predict the effective stress-strain response of the foams at small strains, using both analytical micromechanical methods and computational finite element methods, where the latter involves the construction of appropriate representative volume elements. We find excellent agreement between the predictions of the effective Young's modulus and Poisson ratio from the computational and theoretical methods and good agreement between these predictions and experimental results for the macroscopic response.

Butterfly defects initiate from inclusions in the subsurface of steel bearing components subject to rolling contact. The white etching matter (WEM) microstructure is a characteristic of butterflies and is related to the dissolution of carbides and thus generally believed to be enriched with carbon, in supersaturated solid solution, relative to the parent microstructure. Here, several butterflies are investigated using wavelength dispersive spectroscopy (WDS), soft x-ray emission spectroscopy (SXES) and electron microscopy (EM). Contrary to established thinking, in all cases investigated the butterfly-neighbouring WEM was found to be depleted in carbon, relative to parent material, by around 27% (measured in counts). Furthermore, the carbon level was shown to be lower than the matrix itself, suggesting that solute carbon is also expelled from the WEM during its formation, possibly due to the low level of solubility of carbon in ferrite. This was observed in both AISI 52100 and 18NiCrMo14-6 bearing steels. In spite of this depletion, nano-indentation found that WEM in both alloys was ~17% harder than the parent material. This may explain the strings of micro-voids observed near the WEM-parent interface, which appear to play a role in the growth of the butterfly cracks. It is suggested that the increased hardness of the WEM is mainly due to microstructural changes, rather than changes in solute carbon concentration.

White etching cracks (WECs) initiate subsurface in bearings and can propagate to cause premature failure. These cracks are bordered by an altered microstructure known as white etching matter (WEM), which is thought to form via a crack-rubbing mechanism. However, WEM is often observed bordering a single side of a meandering crack. In search of a microstructural difference to justify the observed WEM asymmetry, regions of untransformed material, adjacent to cracking in a bearing inner ring, which had undergone hydrogen charging prior to testing, were studied using correlative electron microscopy (EM), electron backscattered diffraction (EBSD), electron probe microanalysis (EPMA) and nano-indentation techniques. The investigations found no significant differences between the untransformed material neighbouring the cracks and the parent material; both were found to have similar: grain size and shape; crystallographic texture; carbon concentration; carbide population and hardness, which questions why only one rubbed surface of the crack has formed WEM. The initiation of WEM ahead of the crack is suggested, however more investigations are needed to build another WEM formation model. EPMA characterisation revealed evidence of carbide dissolution in the WEM. Despite this, significant variation in the carbon concentration of the WEM was found; ranging from +13% enrichment to 42% depletion (in counts), relative to the parent material. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd.