Polycrystalline Ni-based superalloys rely on compositional modifications for high-temperature, structural aerospace applications. However, these changes must be carefully managed to avoid deleterious phases. While the individual effects of Ti and Ta are well documented, their synergistic co-addition has received limited attention in high Co-containing alloys. This study investigates the influence of Ti and Ta on the load partitioning behavior between the γ and γ′ phases using in situ synchrotron diffraction under tensile loads in the temperature range from 600 °C to 800 °C. Our findings show that the ratio of Ti:Ta directly impacts how load is distributed, with a higher ratio leading to a greater load-carrying capacity in the γ′ phase, thereby improving strength at intermediate temperatures (700 °C). However, this benefit seems thermally limited, coinciding with a significant reduction in stiffness in the high-Ti alloy at 800 °C. Conversely, a balanced Ti:Ta ratio maintains consistent load partitioning and strength stability across the entire temperature range. By linking lattice misfit and stiffness evolution to bulk behavior, this work identifies load transfer efficiency as a critical metric for the design of future high-performance superalloys. By understanding this relationship, the compositional limits for optimizing alloy performance in service can be better defined. This work highlights a critical design pathway for future high-performance superalloys by demonstrating the link between controlled alloying, lattice misfit, and load partitioning.

Abstract Titanium alloys owe their superior fatigue performance to a lack of extrinsic nucleation sites for cracking, but this also results in difficulty in developing fine, 10 nm scale precipitates to provide fatigue strength. Conventional Ti alloys used for large components such as jet engine discs must instead develop a hierarchical microstructure through successive waves of nucleation. Here we show that intermediate temperature deformation can result in the nucleation of nanoscale hcp α precipitates in between large μm thick α plates, and observe the precipitation of these in situ in the TEM using 4D-Scanning Transmission Electron Microscopy (4D-STEM) alongside the accompanying partially-relaxed transformation strain fields. This results in an improvement in the high cycle fatigue strength of the material by 95 MPa, to around 920 MPa in un-notched high cycle fatigue at 106 cycles, or 200 MJ kg−1, which is among the highest of all structural materials.

Ti521, a titanium alloy with a tailored composition that provides an excellent balance of strength and ductility, was developed and manufactured. The effects of cooling rate on both quasi-static and dynamic mechanical behaviour were investigated. An extensive experimental campaign was undertaken to characterise the rate dependent properties of Ti521 and to evaluate the influence of manufacturing parameters on its mechanical performance. The alloy exhibited the highest yield strength under rapid cooling conditions, with values exceeding those of Ti6Al4V. Furthermore, the analysis of the energy dissipated during plastic deformation indicates that Ti521 has strong potential to outperform Ti6Al4V in impact containment applications.A novel experimental methodology, incorporating multiple camera setups, was introduced and employed to characterise anisotropy induced by variations in cooling rates and rolling directions. This approach enabled the accurate evaluation of the true stress-strain response in the presence of anisotropy and, consequently, the influence of rolling direction on the mechanical performance of the alloy. Cross rolled specimens exhibited no measurable anisotropic behaviour, whereas unidirectionally rolled material showed slight differences in mechanical properties between the rolling and transverse directions.

Understanding how protective oxide scales evolve over time is necessary for improving the long term resistance of superalloys. This work investigates the time-dependent oxidation behavior of an ingot-processable Co/Ni-based superalloy oxidized in air at 800 degrees C for 20, 100, and 1000 h. Mass-gain and white-light interferometry measurements quantified oxidation kinetics, surface roughness, and spallation, while high-resolution STEM-EDX characterized oxide morphology and nanoscale elemental partitioning. Atom probe tomography captured the key transition regions between the chromia and alumina scales, and X-ray diffraction was used to identify a gradual transition from (Ni, Co)-spinel phases to a compact, chromia and alumina-rich scale. The oxidation rate evolved with parabolic behavior over time, consistent with diffusion-controlled growth once a continuous CrO/AlO scale formed. These observations help link kinetics, structure and chemistry, showing how an originally porous spinel layer transforms into a dense, adherent chromia + alumina scale that provides long-term protection in wrought Co/Ni-based superalloys.

Fusion reactor materials for the first wall and blanket must have high strength, be radiation tolerant and be reduced activation (low post-use radioactivity), which has resulted in reduced activation ferritic/martensitic (RAFM) steels. The current steels suffer irradiation-induced hardening and embrittlement and are not adequate for planned commercial fusion reactors. Producing high strength, ductility and toughness is difficult, because inhibiting deformation to produce strength also reduces the amount of work hardening available, and thereby ductility. Here we solve this dichotomy to introduce a high strength and high ductility RAFM steel, produced by a modified thermomechanical process route. A unique multiscale microstructure is developed, comprising nanoscale and microscale ferrite, tempered martensite containing fine subgrains and a high density of nanoscale precipitates. High strength is attributed to the fine grain and subgrain and a higher proportion of metal carbides, while the high ductility results from a high mobile dislocation density in the ferrite, subgrain formation in the tempered martensite, and the bimodal microstructure, which improves ductility without impairing strength.

The hypothesis is examined that the macrozones that can occur in large Ti-6Al-4V forgings, in combination with the effects of stressed volume and sustained loads, can result in a debit in high cycle fatigue (HCF) performance, which would be of concern in jet engine applications. Here, the effect of placing a notch root in hard vs. soft oriented macrozones, i.e. {0002} parallel or perpendicular to the loading direction, respectively, were examined. The deformation features associated with crack initiation were analysed. A significant (10x) variation in life was observed, as low as 5.6 x 106 cycles in hard macrozones compared to 58.6 x 106 cycles in soft macrozones, at a peak notch root stress of 912 MPa and load ratio R of 0.5. Local macrozones were found neighbouring the initiation facets, which were subsurface. Compared to plain fatigue, LCF or dwell fatigue, the initiating faceted grains possessed low dislocation density, which were predominantly of basal (a) character.

Abstract As well as having suitable mechanical performance, fusion reactor materials for the first wall and blanket must be both radiation tolerant and low activation, which has resulted in the development of reduced activation ferritic/martensitic (RAFM) steels. The current steels suffer irradiation-induced hardening and embrittlement, such that they are not adequate for planned commercial fusion reactors. Producing high strength, ductility and toughness is difficult, because inhibiting deformation to produce strength also reduces the amount of work hardening available, and thereby ductility. Here we solve this dichotomy to introduce a high strength and high ductility RAFM steel, produced by a novel thermomechanical process route. A unique trimodal multiscale microstructure is developed, comprising nanoscale and microscale ferrite, and tempered martensite with low-angle nanograins. Processing induces a high dislocation density, which leads to an extremely high number of nanoscale precipitates and subgrain walls. High strength is attributed to the refinement of the ferrite grain size and the nanograins in the tempered martensite, while the high ductility results from a high mobile dislocation density in the ferrite, the higher proportion of MX carbides, and the trimodal microstructure, which improves ductility without impairing strength.

Here, we introduce a discontinuous spinodal reinforcement strategy in the novel candidate plasma facing material (PFM) of tungsten-chromium alloys. Thermal ageing of a W-34wt%Cr alloy at 1250 degrees C causes nano-scale lamellae 200-600 nm to form heterogeneously from grain boundaries, which progressively grow into the matrix fully, then coarsen to 1-2 mu m after 100 h. The dual-phase microstructure confers exceptional high temperature compressive strength, maintaining 900 MPa at 1000 degrees C- double that of polycrystalline tungsten. Further, the chromium alloying promotes a dense oxide scale that confers a 2 orders of magnitude improvement in resistance against oxidation at 1000 degrees C compared to W, which is an important consideration for PFMs under loss of vacuum accident conditions. The dual-phase W-Cr alloy concept's combination of high strength and oxidation resistance represents a new scalable alternative to tungsten, with wide scope for further alloying and process optimisation.

The galling mechanism of Tristelle 5183, an Fe-based hardfacing alloy, was investigated at elevated tempera- ture. The test was performed using a bespoke galling rig. Adhesive transfer and galling were found to occur, as a result of shear at the adhesion boundary and the activation of an internal shear plane within one of the tribosurfaces. During deformation, carbides were observed to have fractured, as a result of the shear train they were exposed to and their lack of ductility. In the case of niobium carbides, their fracture resulted in the formation of voids, which were found to coalesce and led to cracking and adhesive transfer. A tribologically affected zone (TAZ) was found to form, which contained nanocrystalline austenite, as a result of the shear exerted within 30μm of the adhesion boundaries. The galling of Tristelle 5183 initiated from the formation of an adhesive boundary, followed by sub-surface shear in only one tribosurface, Following further sub-surface shear, an internal shear plane is activated. internal shear and shear at the adhesion boundary continues until fracture occur, resulting in adhesive transfer.

Microstructure evolution during high-strain rate and high-temperature thermo-mechanical processing of a 44MnSiV6 microalloyed steel is investigated using in situ synchrotron high-energy powder X-ray diffraction. The conditions selected replicate a newly developed near solidus high-strain rate process designed for reducing raw material use during the hot processing of steels. High temperatures (exceeding 1300 °C) and high strain rate ε˙ = 9 s-1 processing regimes are explored. The lattice strains and dislocation activity estimated from diffraction observations reveal that the microstructure evolution is primarily driven by dynamic recrystallisation. A steady-state stress regime is observed during deformation, which develops due to intermittent and competing work hardening and recovery processes. The texture evolution during the heating, tension, shear deformation and cooling stages is systematically investigated. The direct observation of phase evolution at high-temperature and high-strain rate deformation enables a comprehensive understanding of new manufacturing processes and provides deep insights for the development of constitutive models for face-centred cubic alloys.

Recently, iron-based alloys with a π-ferrosilicide phase have emerged as potential alternatives to cobalt-based hardfacing alloys. Here, we present the development of two π-ferrosilicide containing alloys: one with a ferritic matrix and the other with a ferritic-austenitic matrix. In the as-cast condition, both alloys revealed fine Ni- and Si-rich coherent cubic shaped D03 precipitates in the BCC matrix. The π-ferrosilicide phase was found to have an orientation relationship with the ferrite phase, nucleating within ferrite matrix and from ferrite grain boundaries. In contrast to carbide-strengthened hardfacing Fe-alloys, here the dissolution of the π-ferrosilicide phase at 1200°C enables easy thermomechanical processing of these alloys, which results in refinement of the π-ferrosilicide and additional formation of χ-phase precipitates in the ferrite. Nano-scratch tests provided evidence of a resilient silicide-ferrite interface, likely to due to it possessing some coherency. Both alloys also displayed compressive strengths approaching 2 GPa and ductility in compression of approximately 25%. The combination of processability and attractive mechanical properties suggests that these alloys have the potential to serve as alternatives to carbide-reinforced hardfacing Fe-alloys.

Increasing attention is being paid to α2 Ti3(Al,Sn) precipitation from the α phase of titanium alloys owing to its effect on slip band formation, localisation and the implications for fatigue performance in jet engine titanium. However, the early stages of α2 precipitation have historically been difficult to observe by electron microscopy, neutron diffraction or atom probe analysis. Here, small angle X-ray scattering is used to reexamine the phase boundary in binary Ti-Al and Ti-Sn alloys with around 500 ppmw O. It is found that the phase boundaries in the literature are approximately correct, at 6.2 wt.% Al and 16.9 wt.% Sn, and that this favours the use of Al as a solid solution strengthener over Sn for ambient temperature applications. However, once O content and phase partitioning in α+β alloys are taken into account, this implies that Aleq limits for future alloy design of critical rotating parts should be lowered substantially.

A Twinning Induced Plasticity (TWIP) steel with a nominal composition of Fe-16.4Mn-0.9C-0.5Si-0.05Nb-0.05V was deformed to an engineering strain of 6%. The strain around the deformation twins were mapped using the 4D-STEM technique. Strain mapping showed a large average elastic strain of approximately 6% in the directions parallel and perpendicular to the twinning direction. However, the large average strain comprised of several hot spots of even larger strains of up to 12%. These hot spots could be attributed to a high density of sessile Frank dislocations on the twin boundary and correspond to shear stresses of 1–1.5 GPa. The strain and therefore stress fields are significantly larger than other materials known to twin and are speculated to be responsible for the early thickness saturation of TWIP steel nanotwins. The ability to keep twins extremely thin helps improve grain fragmentation, i.e. the dynamic Hall–Petch effect, and underpins the large elongations and strain hardening rates in TWIP steels.

This study investigated the influence of a systematic variation in Ni:Co ratio on the lattice misfit of gamma-gamma' NiCo-5Al-5Ti-15Cr (at%) alloys over a broad temperature range. Seven model superalloys denoted by their nominal Co content, 0Co, 9Co, 19Co, 28Co, 38Co, 47Co and 56Co (at%), were hot-rolled to achieve a fine grain size and then aged at 800 degrees C for 1000 h. Neutron diffraction was performed on all seven alloy compositions at room temperature, 400 degrees C, 600 degrees C, 700 degrees C and 800 degrees C to determine the lattice parameters of the gamma and gamma' phases. Alloys of Co content 0-47 at% showed a reduction in lattice misfit with increasing temperature from ambient to 800 degrees C, with misfit values remaining positive throughout this temperature range. These lattice misfit-temperature profiles demonstrated an almost systematic increase in misfit with increasing alloy Co content from 0 to 38 at%. In contrast, the 47Co alloy exhibited lower lattice misfit values than the 38Co alloy at temperatures ambient to 700 degrees C. Neutron diffraction patterns for the 56Co alloy obtained at each test temperature indicated splitting of gamma' superlattice peaks, plausibly a consequence of the elongation of the gamma' precipitates observed following ageing and of their spatial distribution. The lattice misfit data acquired were compared with elemental partitioning data for the same alloy compositions of the Ni-Co-5Al-5Ti-15Cr (at%) system similarly aged at 800 degrees C. (c) 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

Carbon is a well known austenite stabiliser and can be used to alter the stacking fault energy and stability against martensitic transformation in medium Mn steels, producing a range of deformation mechanisms such as the Transformation Induced Plasticity (TRIP) or combined Twinning and Transformation Induced Plasticity (TWIP + TRIP) effects. However, the effect of C beyond quasi-static tensile behaviour is less well known. Therefore, two medium Mn steels with 0.2 and 0.5 wt pct C were designed to produce similar austenite fractions and stability and therefore tensile behaviour. These were processed to form lamellar and mixed equiaxed + lamellar microstructures. The low C steel had a corrected Charpy impact energy (KV _10 ) of 320 J cm ^-2 compared to 66 J cm ^-2 in the high C steel despite both having a ductility of over 35 pct. Interface segregation, e.g., of tramp elements, was investigated as a potential cause and none was found. Only a small amount of Mn rejection from partitioning was observed at the interface. The fracture surfaces were investigated and the TRIP effect was found to occur more readily in the Low C Charpy specimen. Therefore it is concluded that the use of C to promote TWIP + TRIP behaviour should be avoided in alloy design but the Charpy impact performance can be understood purely in terms of C in solution.

Tungsten is considered as a primary material for the divertor and first wall in many fusion reactor designs. There has been further interest in nano-structured multi-phase tungsten alloys and composites, such as oxide dispersion strengthened alloys, where interfaces may be harnessed as defect sinks to improve irradiation resilience, whilst also improving base mechanical strength, and potentially ductility. Here we further investigate the concept of tungsten-based 'bcc-superalloys' within the W-Ti-Fe ternary system, comprising W-TiFe, A2-B2, beta-beta' nano-structures. Alloys were produced by arc melting and the microstructure controlled via thermal heat treatments, by solutionising at 1250 degrees C, followed by 750 degrees C ageing.The alloys were characterised using electron microscopy, including composition measurements, alongside hardness measurements. Building on our previous work, we have demonstrated that nano-scale B2 TiFe(W) forms within A2(W,Ti,Fe) in the W-Ti-Fe alloys, creating localised regions of the targeted A2-B2 (beta-beta') precipitate reinforced structure. Further, here we evaluated ageing at 750 degrees C, where within the interdendritic domains decomposition consistent with B2TiFe(W) -> B2 + A2 and A2(Ti,Fe,W) -> A2 + A3 is proposed. An experi-mentally validated preliminary W-Ti-Fe ternary phase diagram has been produced, helping to understand the stable phases present and instructing onward optimisation of W-superalloys as a candidate material for fusion energy.

Medium Mn steels are an emerging class of 3rd generation advanced high-strength steels. These steels have received significant attention due to their high strengths, large ductilities and also lower cost compared to their predecessor high Mn Twinning Induced Plasticity (TWIP) steels. Additionally, medium Mn steels have been found to exhibit TWIP and/or Transformation Induced Plasticity (TRIP) effects which can be harnessed to give a high strain hardening rate. Many thermomechanical processing concepts in the literature have been developed, producing multiple microstructure types with differentmechanical properties. The present review therefore aims to summarise the current knowledge of medium Mn steel alloy design especially on the processing, microstructure and property relationships in medium Mn steels. It complements the review of Sun et al. [Physical metallurgy of medium-Mn advanced high-strength steels, Int Mater Rev. 2023.], written independently and in parallel, which focusses more on the phase interfaces and thermodynamics.

The Ti–Al system, specifically with Al wt pct ranging from approximately 4 to 14 pct, has served as a model system for exploring the deformation behavior of the hexagonal close-packed α phase of Ti alloys. The system has enabled exploration of nearly all aspects of deformation and failure across length scales from atoms to the continuum, in addition to being of value for both experimental and model method development. Here, we provide a review of what has been learned over the past 60 years on fundamental Ti deformation behavior using the model Ti–Al system including topics such as dislocation motion, crystallographic slip, grain-scale deformation, creep, and fracture.

Slip intermittency and stress oscillations in titanium alloy Ti-7Al-O that were observed using in -situ far-field high energy X-ray diffraction microscopy (ff-HEDM) are investigated using a discrete dislocation plasticity (DDP) model. The mechanistic foundation of slip intermittency and stress oscillations are shown to be dislocation escape from obstacles during stress holds, governed by a thermal activation constitutive law. The stress drop events due to -basal slip are larger in magnitude than those along -prism, which is a consequence of their differing rate sensitiv-ities, previously found from micropillar testing. It is suggested that interstitial oxygen suppresses stress oscillations by inhibiting the thermal activation process. Understanding of these mecha-nisms is of benefit to the design and safety assessment of jet engine titanium alloys subjected to dwell fatigue.