The combined effects of prestrain and neutron irradiation on reactor pressure vessel (RPV) steel remains an understudied area, particularly for surveillance samples. Here SA508 grade 3 steel is investigated in four conditions: baseline, prestrained (5%), irradiated (approximately 0.1 dpa at 293 ∘C) and irradiated prestrained. Tensile tests showed that whilst both prestrain and irradiation increased yield stress, the combined case experienced a lower increase than the sum of the individual contributions indicating a partial mitigation effect. Atom probe tomography (APT) detected limited solute clustering in the irradiated samples. However, significant amounts of co-segregation was found with Mn, Si, Cu, Ni and P. The limited clustering was attributed to the lower neutron flux of surveillance samples relative to test reactor data of the same material. The APT showed a depletion of solutes around dislocations, indicating that prestrain-induced dislocations act as solute traps and that prestrain and irradiation hardening are not additive.

The use of polymer nanocomposites as gas barrier materials has seen increasing interest, including applications involving hydrogen transport and storage. Better understanding of gas transport through those polymeric systems requires 3D nanoscale detection of distributions and the possible trapping of gas molecules within nanoparticles and polymer/nanoparticle interfaces While atom probe tomography (APT) offers promising means for such nanoscale characterisation, its use for polymers has been mainly limited to thin organic layers deposited onto substrates or pre-fabricated metal needle shaped specimens. This work provides the very first application of APT to bulk polymer nanocomposites. Particularly, site specific atom probe sample preparation by Focused Ion Beam (FIB) liftout has been shown for the first time in a model system of hexagonal boron nanoparticles within a PVDF polymer matrix, using a variety of FIB workflows including Xe FIB, Ga FIB, cryogenic Ga FIB and deuterium charging. Mass spectra from the bulk polymer and the nanoparticle were collected using pulsed laser atom probe using standard conditions and compared. Several challenges encountered during this research including damage of the polymeric matrix during sample preparation were extensively discussed in this paper. Once those challenges have been resolved (e.g. by developing site specific sample preparation protocols), the application of APT to polymer nanocomposites can open new options for nanoscale characterisation of those systems.

As many nations commit to achieving Net Zero, many low carbon scenarios indicate that civil nuclear power generation and the economics thereof are set to play a vital role. To maximise nuclear reactor operation lifetimes, it is essential to develop mechanistic understanding of failure and degradation mechanisms in safety-critical components for increasingly holistic reactor design codes and standards. In this paper, advanced micromechanical testing with in situ digital image correlation is used in combination with crystal plasticity modelling to study various aspects of damage associated with delta hydride precipitates in Zircaloy-4 for reactor fuel cladding applications. Measurements of static and cyclic hydride precipitation strains demonstrate a discernible strain field directionality (associated with intragranular precipitation) which was not previously reported, while cyclic thermomechanical loads are shown to promote the cyclic accumulation of strain due to repeated precipitation and dissolution of hydrides (hydride strain ratcheting) for up to five cycles, leading to the development of networks of geometrically necessary dislocations. Using crystal plasticity finite element modelling of the volumetric expansion associated with hydride precipitation, the strain directionality phenomenon is shown to be linked with hydride morphology. Comparisons with experimental strain fields also suggest that hydride plasticity is an important consideration for damage accumulation during precipitation. Experimental measurements of short fatigue crack propagation through Zircaloy-4 microstructures containing hydrides reveal new crack propagation mechanisms including decohesion, which on average, lead to accelerated rates of crack growth. Twins and hydride precipitation therein are also implicated in even more damaging fatigue behaviour as fatigue cracks are provided a seemingly brittle and direct path for fracture, which was not previously reported in the literature.

This work investigates the effect of alloying elements Mn, Si, and Fe on the corrosion resistance of polycrystalline Ni-based superalloys, at 700 ^∘C in air and 300 vpm SO_2 under 5 μg cm^2 h salt coating. Oxide scale morphology and composition was investigated using SEM–EDX and APT, revealing Mn–Fe-enriched Co_3O_4 particles in the upper outward growing scale as well as Si-Fe-enriched clusters in NiO. Along the NiO– Co_3O_4 phase boundary, a tight network of Fe-decorated dislocations relieving stress between the lattices was identified through APT. In the inward growing scale, Si enrichment was observed in the form of oxides and sulfides, while no Mn was detected.

Borated water is used as a shield in nuclear fusion cooling circuits. General corrosion, activated corrosion products (ACPs) formation and stress corrosion cracking initiation of 316 L steels exposed to ultrapure water (UPW) or 8000 ppm B water at 80 C were tested. A Ni enriched sub-oxide layer, a transition layer and oxide layer were observed using advanced characterisation (STEM-EELS, APT). The oxide formed in UPW was protective (Cr:O 40:45), the oxide formed in 8000 ppm B was non-passivating. 8000 ppm B led to higher release of Fe, Cr and Mo, 316 L was more prone to SCC initiation and enhanced ACPs formation.

Atom probe tomography (APT) is commonly used to study solute clustering and precipitation in materials. However, standard techniques used to identify and characterize clusters within atom probe data, such as the density-based spatial clustering applications with noise (DBSCAN), often underperform with respect to small clusters. This is a limitation of density-based cluster identification algorithms, due to their dependence on the parameter Nmin, an arbitrary lower limit placed on detectable cluster sizes. Therefore, this article attempts to consider the characterization of clustering in atom probe data as an outlier detection problem of which k-nearest neighbors local outlier factor and learnable unified neighborhood-based anomaly ranking algorithms were tested against a simulated dataset and compared to the standard method. The decision score output of the algorithms was then auto thresholded by the Karcher mean to remove human bias. Each of the major models tested outperforms DBSCAN for cluster sizes of <25 atoms but underperforms for sizes >30 atoms using simulated data. However, the new combined k-nearest neighbors (k-NN) and DBSCAN method presented was able to perform well at all cluster sizes. The combined k-NN and seven methods are presented as a new approach to identifying clusters in APT.

A novel γ – γ ′– γ ″ dual-superlattice superalloy, with promising mechanical properties up to elevated temperatures was recently reported by Mignanelli et al . (in: Proceedings of the 9th International Symposium on Superalloy 718 & Derivatives: Energy, Aerospace, and Industrial Applications, pp 679–690, 2018). The present work employs state-of-the-art chemical and spatial characterization techniques to study the effect systematic additions of Mo, W, and Fe and variations in Nb and Al contents have on the phase fraction, thermal stability, elemental partitioning, and mechanical properties of alloys from this system. Alloys were produced through arc melting followed by heat treatment. Multi-scale characterization techniques and hardness testing were employed to characterize their microstructure, thermal stability, and mechanical properties. Alterations in such properties or in elemental partitioning behavior were then explained through thermodynamic modeling. A modest addition of 1.8 at. pct Mo had a strong effect on the microstructure and thermal stability: it minimized microstructural coarsening during heat treatments while not significantly decreasing the γ ′ solvus temperature. A reduction of Nb by 0.6 at. pct strongly reduced the γ ″ volume fraction, without affecting the γ ′ volume fraction. The reduced precipitate fraction led to a significant reduction in alloy hardness. Fe, added to achieve better processability and reduced material cost, decreased the γ ′ solvus temperature and caused rapid microstructural coarsening during heat treatments, without affecting alloy hardness. A reduction of Al by 0.4 at. pct reduced the γ ′ volume fraction and the γ ′ solvus temperature, also without affecting alloy hardness. The addition of 0.9 at. pct W decreased the γ ′ solvus temperature but increased both precipitate volume fractions. These data will be invaluable to optimize current alloy design and to inform future alloy design efforts. Graphical Abstract

This review aims to improve our understanding of the important factors which influence the susceptibility of thermomechanical controlled processed (TMCP) steels to sulfide stress cracking (SSC). Mechanisms involved in hydrogen embrittlement (HE) from three perspectives are focused on: the microstructure constituents of TMCP steels; environmental factors; and fracture mechanism of SSC. Microstructures are reviewed as they affect the diffusion and trapping of hydrogen that can reduce the resistance to fracture. Environmental factors discussed highlight that when exposed to an aqueous H2S environment, a sulfide layer can form and influence the ingress of hydrogen, and this is affected by pH, temperature, and H2S partial pressure. Fracture is influenced by the nature of the crack tip and the crack tip plastic zone during crack propagation, and hydrogen can significantly affect crack tip growth. This review provides a critical assessment of the interplay between these three factors and aims to provide understanding to enhance our engineering approaches to manage and mitigate against fracture of TMCP steels.

The current work presents a systematic study of two alloy compositions (RR3010 and CMSX-4) and two types of coatings: inward grown (pack) and outward grown (vapour) deposited aluminides, exposed to 98Na 2 SO 4 –2NaCl mixture. Grit blasting was used on some of the samples, prior to coating, to mimic in-service procedures and remove oxides from the surface prior to coating. Two-point bend tests were then performed on the coated samples, with and without applied salt at 550 °C for 100 hours. Samples were pre-strained at 0.6 pct strain to deliberately pre-crack the coating and then strained at 0.3 pct for the heat treatment. Exposure to 98Na 2 SO 4 –2NaCl under applied stress of vapour-aluminide coated samples of both alloys, revealed significant coating damage in the form of secondary cracks in the intermetallic-rich inter-diffusion zone, although only CMSX-4 exhibited cracks propagating further into the bulk alloy while RR3010 proved more resistant. The pack-aluminide coating proved more protective for both alloys, with cracks propagating only into the coating and never into the underlying alloy. In addition, grit blasting proved beneficial in reducing spallation and cracking for both types of coating. The findings were used to propose a mechanism based on thermodynamic reactions, to explain the crack width changes through the formation of volatile AlCl 3 in the cracks.

Titanium alloys are particularly sensitive to temperature during additive manufacturing processes, due to their dual phase microstructure and sensitivity to oxygen uptake. In this paper, laser powder bed fusion (LPBF) was used in conjunction with a heated substrate bed at 100 °C, 570 °C and 770 °C to produce specimens of Ti–6Al–4V, to investigate the change in mechanical properties and segregation of alloying elements. An initial increase in ductility was observed when increasing the temperature from 100 °C to 570 °C, followed by a significant loss in ductility when samples were produced at 770 °C. A suite of multi-scale characterisation techniques revealed that the as-printed microstructure was drastically different across the range of temperatures. At 100 °C, α + α ′ phases were identified. Deformation twinning was extensively observed in the a phase, with Al and V segregating at the twin interfaces. At 570 °C (the most ductile sample), α ′, α and nano-particles of β were observed, with networks of entangled dislocations showing V segregation. At 770 °C, no martensitic α ′ was identified. The microstructure was an α + β microstructure and an increased volume fraction of tangled dislocations with localised V segregation. Thermodynamic modelling based on the Gibbs-free energy of formation showed that the increased V concentration at dislocations was insufficient to locally nucleate β phase. However, b-phase nucleation at grain boundaries (not dislocations) caused pinning of grain boundaries, impeding slip and leading to a reduction in ductility. It is likely that the increased O-content within specimens printed at increased temperatures also played a key role in high-temperature embrittlement. Building operations are therefore best performed below sub-transus temperatures, to encourage the growth of strengthening phases via solute segregation, and the build atmosphere must be tightly controlled to reduce oxygen uptake within the samples.

Carbon or low alloy steel tubes in steam generators (or boilers) are potentially vulnerable to under deposit corrosion (UDC), arising from the formation of porous magnetite deposits on the waterside heat-transfer surfaces. Beneath these deposits, “wick-boiling” causes a concentration of contaminants (such as chlorides), which eventually leads to rapid corrosion. In this work, the corrosion of carbon steel has been investigated in hot acid chloride solutions that simulate the concentrated local environments formed during UDC. Trisodium phosphate (TSP) is sometimes dosed into boilers for pH control. This work has shown that TSP addition such that the phosphate concentration equals the chloride concentration dramatically reduces the corrosion rate in these simulated environments from >20 mm/y to <0.1 mm/y. Additionally, a model of wick-boiling beneath deposits has been used to analyze the concentration of chlorides and phosphates during the initiation stages of UDC, suggesting that dosing of only 100 ppb of TSP into bulk boiler water should be sufficient to increase the critical deposit thickness (required for UDC) by >100 µm across a wide range of operational scenarios.

Atom probe tomography has been raising in prominence as a microscopy and microanalysis technique to gain sub-nanoscale information from technologically-relevant materials. However, the analysis of some functional ceramics, particularly perovskites, has remained challenging with extremely low yield and success rate. This seems particularly problematic for materials with high piezoelectric activity, which may be difficult to express at the low temperatures necessary for satisfactory atom probe analysis. Here, we demonstrate the analysis of commercial BaTiO3 particles embedded in a metallic matrix. Density-functional theory shows that a metallic coating prevents charge penetration of the electrostatic field, and thereby suppresses the associated volume change linked to the piezoelectric effect.

Additive manufacturing (AM) allows for the fabrication of custom orthopaedic implant devices which have complex geometries and similar mechanical properties to bone. This paper reviews the corrosion, fatigue and wear properties of AM Ti alloys to confirm their safety for use in orthopaedic implants. Specifically, AM Ti lattice geometries are highlighted due to their improved osseointegration and better modulus matching with that of bone, making them an attractive option for more durable implant devices. Finally, the properties of current implants made via AM are compared with that made via conventional manufacturing methods to confirm their overall safety.

Sulfur, a common impurity arising from atmospheric and environmental contamination, is highly corrosive and detrimental to the lifespan of nickel superalloys in jet engines. However, sulfur-scavenging coatings have yet to be explored. Our study presents the successful development of a stable, uniform, impurity-scavenging Ni-Mn coating on Ni-based superalloy CMSX-4, through electroplating. The coating was characterised via combined scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy. An optimal coating thickness of > 600 nm was deposited. The coated alloy was exposed to corrosive salt mixture 98% Na 2 SO 4 –2% NaCl at 550 °C for 100 h, mimicking engine exposure conditions, thereby proving that the coating successfully trapped sulfur and prevented its diffusion into an underlying alloy. This work presents a promising development for the prevention of sulfur-induced corrosion in industrial setting such as gas turbine engine, where the effects of sulfur diffusion into the bulk alloy could lead to premature failure. Graphical Abstract

Under Deposit Corrosion (UDC) presents a significant issue to carbon steel tubes in industrial boiler systems, however, the fundamental mechanisms are not fully understood. This work uses time-resolved high-speed tomography to investigate iron corrosion products in-situ, in aerated and deaerated environments at 200°C in 0.1M NiCl2 solution. Laminated corrosion structures were observed, in agreement with previous observations and field examples. The oxidation rate increased by 55% in aerated specimens, confirming the presence of oxygen causes an increase in the oxidation rate. This experimental set up provides an opportunity for investigations into complex corrosion mechanisms, for a range of aggressive environments.

This article discusses challenges faced in the development of new Ni-based superalloys for applications in the hottest sections of turbine engines and the use of atom probe tomography and correlative microscopy for characterization of these complex alloys with regards to microstructural and compositional design. The two strengthening phases γ and γ′ are introduced and the precipitation of topologically close-packed phases and their potential detrimental effects on superalloy properties are reviewed. Mechanisms of environmental degradation, namely oxidation and hot corrosion, are elucidated and recent research studies on a new phenomenon of hot corrosion at relatively low temperatures below 600°C are discussed. The effect of individual alloying elements on superalloy properties is reviewed, with a focus on Mo and W. The use of atom probe in correlation with state-of-the-art microscopy, spectroscopy and diffraction techniques to study and understand oxidation and corrosion of Ni-based superalloys, including crack tip investigations, is presented. Graphical abstract

The Divertor Tokamak Test (DTT) vacuum vessel (VV) is a toroidal chamber that ensures an enclosed vacuum environment for the plasma, a confinement barrier, and temperature control. Water flows in the double-shell D-shaped VV cross-sectional structure made from stainless steel to operate at a set temperature. Beyond temperature control, the water cooling circuit needs to act as a neutron shielding media to protect the structures installed outside of the VV, namely, the superconducting coils. The shielding function is achieved due to the addition of boric acid in the water. The requirement on the borated water for the DTT VV is to have 8000-ppm B solution highly enriched in (SUPERSCRIPT)-S-1 ZEROB (95% (SUPERSCRIPT)-S-1 ZEROB). Given the lack of water chemistry guidelines for fusion power plants, the water chemistry requirements from fission power plants were investigated. In this work, general corrosion of stainless steel, 316L type family, in concentrated borated water solutions was investigated experimentally using metal release test. Samples were exposed to ultrapure water (UPW) and 8000-ppm B borated water at 80 degrees C for one week to quantify the amount of ions released in solution. 316L general corrosion was studied considering the different water chemistries, UPW versus borated water, and steels microstructures. DTT VV presents many welded joints, so general corrosion of welding-induced microstructure was here investigated compared to 316L base microstructure. The release of ions from general corrosion was found to be more influenced by water chemistry than microstructure.

Coated carbide-rich martensitic stainless steels are typically used for sharp edges because of their high wear and corrosion resistance. Yet they fail under the combined chemo-mechanical actions of the harsh environment they operate in, and under the mixed mode II-III stress they are subjected to. To investigate this complex failure process, we exposed sharp edges to increasing corrosion severity and carried out in situ electron microscopy cutting experiments with two micro-mechanical testing setups: a Deben Gatan micro-tensile deformation stage and a Hysitron PI 88 Picoindenter. At low corrosion levels, byproducts formed on the surface are removed during cutting, and cracks propagate at an angle with respect to the sharp edge to form a chip. As corrosion severity increases, a percolated void structure develops in the substrate and cracks propagate perpendicularly to the sharp edge, with portions of the material bending out-of-plane. At high corrosion, a chromium-rich oxide forms in place of the percolated void structure and the material becomes brittle and unable to withstand any applied stress. Although increasing corrosion severity increases the cutting force, our numerical investigation suggests that the formation of the percolated porous structure, which in turn increases substructural heterogeneity in the material, is responsible for the change in crack propagation direction and failure mechanism.(c) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The service lives of nickel superalloys are often limited by environmental degradation. The present study compares oxidation, sulfidation and hot corrosion at 750 degrees C of three variants of a polycrystalline superalloy: a baseline alloy, a variant containing 1 wt% Mn and one containing 0.5 wt% Si. Mn reduced the oxidation rate without changing the scale morphology. The MnCr2O4 scale formed proved more protective against sulfidation and hot corrosion, but internal sulfides extended the damage depth. Si modified the oxide morphology to a continuous Cr2O3-Al2O3 dual layer. This provided improved protection, reducing the sulfidation depth by 2/3 and the hot corrosion depth by 1/2.