Publications
26 results found
Ma K, Blackburn T, Magnussen JP, et al., 2023, Chromium-based bcc-superalloys strengthened by iron supplements, Acta Materialia, Vol: 257, ISSN: 1359-6454
Chromium alloys are being considered for next-generation concentrated solar power applications operating > 800 °C. Cr offers advantages in melting point, cost, and oxidation resistance. However, improvements in mechanical performance are needed. Here, Cr-based body-centred-cubic (bcc) alloys of the type Cr(Fe)-NiAl are investigated, leading to ‘bcc-superalloys’ comprising a bcc-Cr(Fe) matrix (β) strengthened by ordered-bcc NiAl intermetallic precipitates (β’), with iron additions to tailor the precipitate volume fraction and mechanical properties at high temperatures. Computational design using CALculation of PHAse Diagram (CALPHAD) predicts that Fe increases the solubility of Ni and Al, increasing precipitate volume fraction, which is validated experimentally. Nano-scale, highly-coherent B2-NiAl precipitates with lattice misfit ∼ 0.1% are formed in the Cr(Fe) matrix. The Cr(Fe)-NiAl A2-B2 alloys show remarkably low coarsening rate (∼102 nm3/h at 1000 °C), outperforming ferritic-superalloys, cobalt- and nickel-based superalloys. Low interfacial energies of ∼ 40/20 mJ/m2 at 1000/1200 °C are determined based on the coarsening kinetics. The low coarsening rates are principally attributed to the low solubility of Ni and Al in the Cr matrix. The alloys show high compressive yield strength of ∼320 MPa at 1000 °C. The Fe-modified alloy exhibits resistance to age softening, related to the low coarsening rate as well as the relatively stable Orowan strengthening as a function of precipitate radius. Microstructure tailoring with Fe additions offers a new design route to improve the balance of properties in “Cr-superalloys”, accelerating their development as a new class of high-temperature materials.
Ferreiros PA, von Tiedemann SO, Parkes N, et al., 2023, VNbCrMo refractory high-entropy alloy for nuclear applications, INTERNATIONAL JOURNAL OF REFRACTORY METALS & HARD MATERIALS, Vol: 113, ISSN: 0263-4368
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- Citations: 3
King DJM, Knowles AJ, Bowden D, et al., 2022, High temperature zirconium alloys for fusion energy, Journal of Nuclear Materials, Vol: 559, Pages: 1-27, ISSN: 0022-3115
This review considers current Zr alloys and opportunities for advanced zirconium alloys to meet the demands of a structural material in fusion reactors. Zr based materials in the breeder blanket offer the potential to increase the tritium breeding ratio above that of Fe, Si and V based materials. Current commercial Zr alloys might be considered as a material in water-cooled breeder blanket designs, due to the similar operating temperature to fission power plants. For breeder blankets designed to operate at higher temperatures, current commercial Zr alloys will not meet the high temperature strength and thermal creep requirements. Hence, Zr alloys with an operational temperature capability beyond that of current commercial fission alloys have been reviewed, specifically: binary Zr alloy systems Zr-Al, Zr-Be, Zr-Cr, Zr-Nb Zr-Ti, Zr-Si, Zr-Sn, Zr-V and Zr-W; as well as higher order Zr alloys Zr-Mo-Ti, Zr-Nb-Ti, Zr-Ti-Al-V and Zr-Mo-Sn. It is concluded that, with further work, higher order Zr alloys could achieve the required high temperature strength, alongside ductility, while maintaining a low thermal neutron cross-section. However, there is limited data and uncertainty regarding the structural performance and microstructural stability of the majority of advanced Zr alloys for temperatures 500–700 °C, at which they would be expected to operate for helium- and liquid metal-cooled breeder blanket designs.
Jones RD, Knowles AJ, Clegg WJ, 2021, A binary beta titanium superalloy containing ordered-beta TiFe, alpha and omega, SCRIPTA MATERIALIA, Vol: 200, ISSN: 1359-6462
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- Citations: 3
Knowles A, Dye D, Dodds R, et al., 2021, Tungsten-based bcc-superalloys, Applied Materials Today, Vol: 23, Pages: 1-6, ISSN: 2352-9407
Applications from nuclear energy to rockets and jet engines are underpinned by advanced high temperature materials. Whilst state of the art, the performance of current nickel-based superalloys is fundamentally limited to Ni’s melting point, T. Here, we develop an analogous superalloy concept but with superior high temperature capability by transitioning to a bcc tungsten base, T. This strategy involves reinforcing bcc -W by TiFe intermetallic compound, which results in impressive high temperature compressive strengths of 500 MPa at. This bcc-superalloy design approach has wider applicability to other bcc alloy bases, including Mo, Ta, and Nb, as well as to refractory-metal high entropy alloys (RHEAs). By investigation of the underlying phase equilibria, thermodynamic modelling, characterisation and mechanical properties, we demonstrate the capability of ternary W-Ti-Fe tungsten-based bcc-superalloys as a new class of high temperature materials.
Gong P, Wynne BP, Knowles AJ, et al., 2020, Effect of ageing on the microstructural evolution in a new design of maraging steels with carbon, ACTA MATERIALIA, Vol: 196, Pages: 101-121, ISSN: 1359-6454
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- Citations: 22
Ackerman A, Knowles A, Gardener HM, et al., 2020, The kinetics of primary alpha plate growth in titanium alloys, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, Vol: 51, Pages: 131-141, ISSN: 1073-5623
The kinetics of primary alpha-Ti colony/Widmanstatten plate growth from the beta are examined, comparing model to experiment. The plate growth velocity depends sensitively both on the diffusivity D(T) of the rate-limiting species and on the supersaturation around the growing plate. These result in a maxima in growth velocity around 40 K below the transus, once sufficient supersaturation is available to drive plate growth. In Ti-6246, the plate growth velocity was found to be around 0.32 um min-1 at 850 oC, which was in good agreement with the model prediction of 0.36 um min-1 . The solute field around the growing plates, and the plate thickness, was found to be quite variable, due to the intergrowth of plates and soft impingement. This solute field was found to extend to up to 30 nm, and the interface concentration in the beta was found to be around 6.4 at.% Mo. It was found that increasing O content will have minimal effect on the plate lengths expected during continuous cooling; in contrast, Mo approximately doubles the plate lengths obtained for every 2 wt.% Mo reduction. Alloys using V as the beta stabiliser instead of Mo are expected to have much faster plate growth kinetics at nominally equivalent V contents. These findings will provide a useful tool for the integrated design of alloys and process routes to achieve tailored microstructures.
Knowles A, Gong P, Rahman KM, et al., 2019, Development of Ni-free Mn-stabilised maraging steels using Fe2SiTi precipitates, Acta Materialia, Vol: 174, Pages: 260-270, ISSN: 1359-6454
Computational alloy design has been used to develop a new maraging steel system with low cost, using Mn for austenite reversion and Heusler Fe2SiTi nm-scale precipitates to strengthen the martensite, avoiding high cost alloying elements such as Ni and Co. A pronounced ageing response was obtained, of over 100 HV, associated with the formation of 2–30 nm Fe2SiTi precipitates alongside the development of 10% Mn rich austenite, at the martensite boundaries with the Kurdjumov-Sachs orientation relationship. The precipitates took on different orientation relationships, depending on the size scale and ageing time, with fine precipitates possessing an <100>L21//<100>α orientation relationship, compared to larger precipitates with <110>L21//<100>α. Computational alloy design has been used for the development and demonstration of an alloy design concept having multiple constraints. Whilst in this case computational design lacked the fidelity to completely replace experimental optimisation, it identifies the importance of embedding Thermodynamic and kinetic modelling within each experimental iteration, and vice versa, training the model between experimental iterations. In this approach, the model would guide targeted experiments, the experimental results would then be taken into future modelling to greatly accelerate the rate of alloy development.
Calvin P, King D, Knowles A, et al., 2019, Data on the annealing of NbTiVZr at 1200°C with slow cooling rate, Data in Brief, Vol: 24, ISSN: 2352-3409
The data presented here is complementary to the publication entitled “High temperature, low neutron cross-section high-entropy alloys in the Nb-Ti-V-Zr system” [1]. A homogenization methodology with slower cooling rate (∼2 °C/min) was performed. X-ray diffraction and scanning electron microscopy (backscattered electron and energy dispersive spectroscopy) data pertaining to annealed high-entropy alloy composition NbTiVZr is presented.
Gao J, Knowles AJ, Guan D, et al., 2019, ω phase strengthened 1.2GPa metastable β titanium alloy with high ductility, SCRIPTA MATERIALIA, Vol: 162, Pages: 77-81, ISSN: 1359-6462
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- Citations: 54
Knowles A, Reynolds L, Vorontsov V, et al., 2019, A nickel based superalloy reinforced by both Ni3Al and Ni3V ordered-fcc precipitates, Scripta Materialia, Vol: 162, Pages: 472-476, ISSN: 1359-6462
A nickel based superalloy has been designed where the fcc γ Ni matrix is reinforced by two different ordered-fcc intermetallic compounds, L12 Ni3Al and D022 Ni3V. Primary ageing at 900–1000 °C precipitated spherical L12 Ni3Al, whose volume fraction and size were controlled by altering the ageing temperature and time. Secondary ageing at 700 °C for 1–1000 h precipitated D022 Ni3V laths. The duplex precipitation increased hardness by up to 85 HV, with ∼ 500 MPa compressive proof strength maintained at 800 °C. Electron microscopy studied the Ni3Al precipitation and confirmed the form of the secondary Ni3V precipitates and their long term stability.
King DJM, Cheung STY, Humphry-Baker SA, et al., 2019, High temperature, low neutron cross-section high-entropy alloys in the Nb-Ti-V-Zr system, Acta Materialia, Vol: 166, Pages: 435-446, ISSN: 1359-6454
High-entropy alloys (HEAs) with high melting points and low thermal neutron cross-section are promising new cladding materials for generation III+ and IV power reactors. In this study a recently developed high throughput computational screening tool Alloy Search and Predict (ASAP) has been used to identify the most likely candidate single-phase HEAs with low thermal neutron cross-section, from over a million four-element equimolar combinations. The selected NbTiVZr HEA was further studied by density functional theory (DFT) for moduli and lattice parameter, and by CALPHAD to predict phase formation with temperature. HEAs of NbTiVZrx (x = 0.5, 1, 2) were produced experimentally, with Zr varied as the dominant cross-section modifier. Contrary to previous experimental work, these HEAs were demonstrated to constitute a single-phase HEA system; a result obtained using a faster cooling rate following annealing at 1200 °C. However, the beta (BCC) matrix decomposed following aging at 700 °C, into a combination of nano-scale beta, alpha (HCP) and C15 Laves phases.
calvert E, Knowles A, Pope J, et al., 2019, Novel high strength titanium-titanium composites produced using field-assisted sintering technology (FAST), Scripta Materialia, Vol: 159, Pages: 51-57, ISSN: 1359-6462
To increase the strength of titanium alloys beyond that achievable with α-β microstructures, alternative reinforcing methods are necessary. Here, field-assisted sintering technology (FAST) has been used to produce a novel Ti-5Al-5Mo-5V-3Cr (Ti-5553) metal-matrix-composite (MMC) reinforced with 0-25 wt.% of a ∼2 GPa yield strength TiFeMo alloy strengthened by ordered body-centred cubic intermetallic and ω phases. The interdiffusion region between Ti-5553 and TiFeMo particles was studied by modelling, electron microscopy, and nanoindentation to examine the effect of graded composition on mechanical properties and formation of α, intermetallic, and ω phases, which resulted in a > 200 MPa strengthening benefit over unreinforced Ti-5553.
Tong VS, Knowles AJ, Dye D, et al., 2019, Rapid electron backscatter diffraction mapping: Painting by numbers, Materials Characterization, Vol: 147, Pages: 271-279, ISSN: 1044-5803
Microstructure characterisation has been greatly enhanced through the use of electron backscatter diffraction (EBSD), where rich maps are generated through analysis of the crystal phase and orientation in the scanning electron microscope (SEM). Conventional EBSD analysis involves raster scanning of the electron beam and serial analysis of each diffraction pattern in turn. For grain shape, crystallographic texture, and microstructure analysis this can be inefficient. In this work, we present Rapid EBSD, a data fusion approach combining forescatter electron (FSE) imaging with static sparse sampling of EBSD patterns. We segment the FSE image into regions of similar colour (i.e. phase and crystal orientation) and then collect representative EBSD data for each segmented region. This enables microstructural assessment to be performed at the spatial resolution of the (fast) FSE imaging whilst including orientation and phase information from EBSD analysis of representative points. We demonstrate the Rapid EBSD technique on samples of a cobalt based superalloy and a strained dual phase titanium alloy, comparing the results with conventional analysis. Rapid EBSD is advantageous for assessing grain size distributions in time-limited experiments.
Ikeuchi D, King DJM, Laws KJ, et al., 2019, Cr-Mo-V-W: a new refractory and transition metal high-entropy alloy system, Scripta Materialia, Vol: 158, Pages: 141-145, ISSN: 1359-6462
Cr-Mo-V-W high-entropy alloy (HEA) is studied, with 2553 K equilibrium solidus and high Cr content to promote protective oxide scale formation, suggesting potential applications in hot, oxidising environments. Alloy Search and Predict (ASAP) and phase diagram calculations found a single phase, body-centred cubic (BCC) solid solution at elevated temperatures, across the range of compositions present within the system - uncommon for a HEA of refractory and transition metals. Density functional theory identified solubility of 22 at.% Cr at solidus temperature, with composition-dependent drive for segregation during cooling. An as-cast, BCC single-phase with the composition 31.3Cr-23.6Mo-26.4 V-18.7 W exhibiting dendritic microsegregation was verified.
Humphry-Baker S, Harrison R, Greaves G, et al., 2018, A candidate fusion engineering material, WC-FeCr, Scripta Materialia, Vol: 155, Pages: 129-133, ISSN: 1359-6462
A new candidate fusion engineering material, WC-FeCr, has been irradiated with He ions at 25 and 500 °C. Ions were injected at 6 keV to a dose of ~15 dpa and 50 at. % He, simulating direct helium injection from the plasma. The microstructural evolution was continuously characterised in situ using transmission electron microscopy. In the FeCr phase, a coarse array of 3–6 nm bubbles formed. In the WC, bubbles were less prominent and smaller (~2 nm). Spherical-cap bubbles formed at hetero-phase interfaces of tertiary precipitates, indicating that enhanced processing routes to minimise precipitation could further improve irradiation tolerance.
Gao J, Huang Y, Guan D, et al., 2018, Deformation mechanisms in a metastable beta titanium twinning induced plasticity alloy with high yield strength and high strain hardening rate, Acta Materialia, Vol: 152, Pages: 301-314, ISSN: 1359-6454
Metastable β titanium alloys with both twinning (TWIP) and martensite transformation (TRIP)usually exhibit a low yield strength of between 200 and 500MPa, but high strain hardening rateand large uniform elongation. Alloys that exhibit twinning on a single system provide a higheryield strength, but a lower strain hardening rate. Here, for the first time, we report a new alloy(Ti-7Mo-3Cr wt%) with both high yield strength (695 MPa) and high work hardening rate(~1900 MPa) and a substantial 33.3% uniform elongation. The deformation mechanisms weresystematically investigated using EBSD and TEM for samples strained to 1.3%, 5% and 16%.The high yield strength was achieved through initial deformation mechanisms of two twinsystems, namely both {332}<113> and {112}<111> twinning. Importantly, the martensitetransformation was suppressed at this stage of deformation. The combination of two twinsystems, with approximately the same intensity, resulted in a high strain hardening rate(1600MPa to 1900MPa), much greater compared to alloys that exhibit a single twin system.Moreover, the TRIP effect was observed at strains greater than 5%, which also contributed tothe high strain hardening rate large uniform elongation.
Knowles A, Tea-Sung J, Bhowmik A, et al., 2018, Data on a new beta titanium alloy system reinforced with superlattice intermetallic precipitates, Data in Brief, Vol: 17, Pages: 863-869, ISSN: 2352-3409
The data presented in this article are related to the research article entitled “a new beta titanium alloy system reinforced with superlattice intermetallic precipitates” (Knowles et al., 2018) [1]. This includes data from the as-cast alloy obtained using scanning electron microscopy (SEM) and x-ray diffraction (XRD) as well as SEM data in the solution heat treated condition. Transmission electron microscopy (TEM) selected area diffraction patterns (SADPs) are included from the alloy in the solution heat treated condition, as well as the aged condition that contained < 100 nm B2 TiFe precipitates [1], the latter of which was found to exhibit double diffraction owing to the precipitate and matrix channels being of a similar width to the foil thickness (Williams and Carter, 2009) [2]. Further details are provided on the macroscopic compression testing of small scale cylinders. Of the micropillar deformation experiment performed in [1], SEM micrographs of focused ion beam (FIB) prepared 2 µm micropillars are presented alongside those obtained at the end of the in-situ SEM deformation as well as videos of the in-situ deformation. Further, a table is included that lists the Schmidt factors of all the possible slip systems given the crystal orientations and loading axis of the deformed micropillars in the solution heat treated and aged conditions.
Knowles A, Bhowmik A, Purkayastha S, et al., 2017, Data on a Laves phase intermetallic matrix composite in situ toughened by ductile precipitates, Data in Brief, Vol: 14, Pages: 489-493, ISSN: 2352-3409
The data presented in this article are related to the research article entitled “Laves phase intermetallic matrix composite in situ toughened by ductile precipitates” (Knowles et al.) [1]. The composite comprised a Fe2(Mo, Ti) matrix with bcc (Mo, Ti) precipitated laths produced in situ by an aging heat treatment, which was shown to confer a toughening effect (Knowles et al.) [1]. Here, details are given on a focused ion beam (FIB) slice and view experiment performed on the composite so as to determine that the 3D morphology of the bcc (Mo, Ti) precipitates were laths rather than needles. Scanning transmission electron microscopy (S(TEM)) micrographs of the microstructure as well as energy dispersive X-ray spectroscopy (EDX) maps are presented that identify the elemental partitioning between the C14 Laves matrix and the bcc laths, with Mo rejected from the matrix into laths. A TEM selected area diffraction pattern (SADP) and key is provided that was used to validate the orientation relation between the matrix and laths identified in (Knowles et al.) [1] along with details of the transformation matrix determined.
Knowles AJ, Bhowmik, Purkayastha S, et al., 2017, Laves phase intermetallic matrix composite in situ toughened by ductile precipitates, Scripta Materialia, Vol: 140, Pages: 59-62, ISSN: 1872-8456
Laves phase based materials are of interest for elevated temperature applications for their high melting points and strengths but are critically limited by their low fracture toughness. Here, a Laves phase intermetallic matrix composite toughened by ductile precipitates has been studied. This microstructure was produced in situ by heat treating a Fe2(Mo,Ti) based alloy to precipitate ∼ 12% volume fraction of fine ∼ 250 nm bcc, A2 (Mo,Ti), phase, with an orientation relationship of . The precipitated A2 phase increased the indentation fracture toughness from 1.1 to 2.2 MPa m1/2 while maintaining a high hardness of HV0.5 = 8.9 GPa similar to monolithic Laves phases.
Knowles AJ, Jun T-S, Bhowmik A, et al., 2017, A new beta titanium alloy system reinforced with superlattice intermetallic precipitates, Scripta Materialia, Vol: 140, Pages: 71-75, ISSN: 1872-8456
Titanium alloys traditionally lack a nm-scale intermetallic precipitate that can be exploited for age-hardening from solid solution. Here such a strengthening concept is developed in the Ti-Fe-Mo system, with it being found that a high temperature β (bcc A2) single-phase field for homogenisation can be obtained, which following ageing (750 °C/80 h) precipitated B2 TiFe <100 nm in size. The orientation relationship was found to be ⟨100⟩A2//⟨100⟩B2, {100}A2//{100}B2, with a misfit of −6.1%. The alloy was found to be very hard (HV0.5 = 6.4 GPa) and strong (σy, 0.2 = 1.9 GPa) with a density of 6.68 g cm−3. TEM observation and micropillar deformation showed that the precipitates resist dislocation cutting.
Knowles AJ, Jones NG, Jones CN, et al., 2017, Phase Equilibria in the Fe-Mo-Ti Ternary System at 1173 K (900 A degrees C) and 1023 K (750 A degrees C), Metallurgical and Materials Transactions A - Physical Metallurgy and Materials Science, Vol: 48A, Pages: 4334-4341, ISSN: 1073-5623
Alloys with fine-scale eutectic microstructures comprising Ti-based A2 and TiFe B2 phases have been shown to have excellent mechanical properties. In this study, the potential of alloys with further refined A2-B2 microstructures formed through solid-state precipitation has been explored by analyzing a series of six alloys within the Fe-Mo-Ti ternary system. Partial isothermal sections of this system at 1173 K (900 °C) and 1023 K (750 °C) were constructed, from which the ternary solubility limits of the A2 (Ti, Mo), B2 TiFe, D85 Fe7Mo6, and C14 Fe2Ti phases were determined. With these data, the change in solubility of Fe in the A2 phase with temperature, which provides the driving force for precipitation of B2 TiFe, was determined and used to predict the maximum potential volume fraction of B2 TiFe precipitates that may be formed in an A2 (Ti, Mo) matrix.
Johnstone DN, Knowles AJ, Krakow R, et al., 2016, Crystallographic mapping in engineering alloys by scanning precession electron diffraction, Pages: 211-212
<jats:p>Crystallographic, compositional and morphological complexity in modern engineering alloys necessitates the use of sophisticated tools for multi‐scale materials characterisation. Here, we develop scanning precession electron diffraction (SPED) for mapping crystalline phases in engineering alloys. SPED involves scanning the electron beam across the specimen and recording a PED pattern at each point by rocking a focused probe in a hollow cone above the specimen and de‐rocking the beam back to the optic axis below. In this way, integrated diffraction intensities are recorded in the geometry of a conventional electron diffraction pattern [1]. A 4D dataset is obtained comprising a 2D PED pattern at each position in the 2D scan region, which can be analysed in a number of ways. Most simply, ‘virtual diffraction images’ can be formed by plotting the intensity of a sub‐set of pixels in each PED pattern as a function of probe position to elucidate variations in the diffraction condition in a versatile post‐acquisition scheme. Phase and orientation maps can also be formed by matching each PED pattern to a library of simulated patterns [2]. Here, we use this approach to determine the phases of precipitates in a nickel base superalloy and to identify orientation relationships existing between these phases. To do this we explore the orientation data in disorientation space where the rotation axis and angle between the two crystallographic bases is plotted (Figure 1). This automated analysis enabled treatment of multiple precipitates yielding a more representative view of the microstructure compared to conventional SAED methods.</jats:p> <jats:p>New methods for strain mapping and phase characterisation based on machine learning were developed as part of this work to extract further insight into microstructural features. Strain maps were obtained by comparing each pattern to an unstrained reference and used to explore the strain distri
Knowles AJ, Jones NG, Messe OMDM, et al., 2016, Phase equilibria in the Fe-Mo-Ti ternary system at 1000 degrees C, INTERNATIONAL JOURNAL OF REFRACTORY METALS & HARD MATERIALS, Vol: 60, Pages: 160-168, ISSN: 0263-4368
Knowles AJ, Jones NG, Jones CN, et al., 2016, Phase Equilibria and Properties of Ti‐Fe‐Mo Alloys With Ultra‐Fine Lamellar Microstructures, Publisher: Wiley, Pages: 1229-1236, ISBN: 9781119283263
Knowles AJ, Jiang X, Galano M, et al., 2014, Microstructure and mechanical properties of 6061 Al alloy based composites with SiC nanoparticles, JOURNAL OF ALLOYS AND COMPOUNDS, Vol: 615, Pages: S401-S405, ISSN: 0925-8388
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- Citations: 137
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