Imperial College London

ProfessorDavidDye

Faculty of EngineeringDepartment of Materials

Professor of Metallurgy
 
 
 
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Contact

 

+44 (0)20 7594 6811david.dye

 
 
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Location

 

1.09GoldsmithSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

203 results found

Xu Y, Worsnop F, Dye D, Dunne FPEet al., 2023, Slip intermittency and dwell fatigue in titanium alloys: a discrete dislocation plasticity analysis, Journal of the Mechanics and Physics of Solids, Vol: 179, Pages: 1-13, ISSN: 0022-5096

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 <a>-basal slip are larger in magnitude than those along <a>-prism, which is a consequence of their differing rate sensitivities, previously found from micropillar testing. It is suggested that interstitial oxygen suppresses stress oscillations by inhibiting the thermal activation process. Understanding of these mechanisms is of benefit to the design and safety assessment of jet engine titanium alloys subjected to dwell fatigue.

Journal article

Kwok TWJ, Worsnop FF, Douglas JO, Dye Det al., 2023, Carbon in solution and the Charpy impact performance of medium Mn steels, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, Vol: 54, Pages: 4128-4137, ISSN: 1073-5623

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 (KV10) 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.

Journal article

Pagan DC, Peterson KM, Shade PA, Pilchak AL, Dye Det al., 2023, Using the Ti-Al System to Understand Plasticity and Its Connection to Fracture and Fatigue in <i>α</i> Ti Alloys, METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE, Vol: 54, Pages: 3373-3388, ISSN: 1073-5623

Journal article

Parkes N, Dodds R, Watson A, Dye D, Hardie C, Humphry-Baker SA, Knowles AJet al., 2023, Tungsten-based bcc-superalloys: thermal stability and ageing behaviour, International Journal of Refractory Metals and Hard Materials, Vol: 113, Pages: 1-6, ISSN: 0263-4368

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, β-β’ nanostructures. Alloys were produced by arc melting and the microstructure controlled via thermal heat treatments, by solutionising at 1250 °C, followed by 750 °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 (β-β’) precipitate reinforced structure. Further, here we evaluated ageing at 750 °C, where within the interdendritic domains decomposition consistent with B2TiFe(W) -> B2 + A2 and A2(Ti,Fe,W) -> A2 + A3 is proposed. An experimentally 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.

Journal article

Kwok TWJ, McAuliffe TP, Ackerman AK, Savitzky BH, Danaie M, Ophus C, Dye Det al., 2023, The mechanism of twin thickening and the elastic strain state of TWIP steel nanotwins, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 873, ISSN: 0921-5093

Journal article

Kwok TWJ, Dye D, 2023, A review of the processing, microstructure and property relationships in medium Mn steels, INTERNATIONAL MATERIALS REVIEWS, ISSN: 0950-6608

Journal article

Llewelyn SCH, Owen LR, Playford HY, Jones NG, Dye D, Hardy MC, Stone HJet al., 2023, Influence of Ni:Co ratio and temperature on the lattice misfit of ?-?' Ni-Co-Al-Ti-Cr alloys, JOURNAL OF ALLOYS AND COMPOUNDS, Vol: 937, ISSN: 0925-8388

Journal article

Worsnop FF, Lea SLM, Ilavsky J, Rugg D, Dye Det al., 2023, Crystallographic ordering of Al and Sn in α-Ti, SCRIPTA MATERIALIA, Vol: 226, ISSN: 1359-6462

Journal article

Rogers SR, Daure J, Shipway P, Stewart D, Dye Det al., 2022, Adhesive transfer operates during galling, Scripta Materialia, Vol: 221, ISSN: 1359-6462

In order to reduce cobalt within the primary circuit of pressurised water reactors (PWR’s), wear-resistant steels are being researched and developed. In particular interest is the understanding of galling mechanisms, an adhesive wear mechanism which is particularly prevalent in PWR valves. Here we show that large shear stresses and adhesive transfer occur during galling by exploiting the 2 wt.% manganese difference between 304L and 316L stainless steels, even at relatively low compressive stresses of 50MPa. Through these findings, the galling mechanisms of stainless steels can be better understood, which may help with the development of galling resistant stainless steels.

Journal article

Worsnop FF, Lim RE, Bernier JV, Pagan DC, Xu Y, McAuliffe TP, Rugg D, Dye Det al., 2022, The influence of alloying on slip intermittency and the implications for dwell fatigue in titanium, Nature Communications, Vol: 13, ISSN: 2041-1723

Dwell fatigue, the reduction in fatigue life experienced by titanium alloysdue to holds at stresses as low as 60% of yield, has been implicated in severaluncontained jet engine failures. Dislocation slip has long been observed to bean intermittent, scale-bridging phenomenon, similar to that seen in earthquakesbut at the nanoscale, leading to the speculation that large stress bursts mightpromote the initial opening of a crack. Here we observe such stress bursts atthe scale of individual grains in situ, using high energy X-ray diffractionmicroscopy in Ti-7Al-O alloys. This shows that the detrimental effect ofprecipitation of ordered Ti_3Al is to increase the magnitude of rare pri<a> andbas<a> slip bursts associated with slip localisation. In contrast, the additionof trace O interstitials is beneficial, reducing the magnitude of bas<a> slipbursts and increasing the homogeneity between basal and prismatic <a> slip.This is further evidence that the formation of long paths for easy basal planeslip localisation should be avoided when engineering titanium alloys againstdwell fatigue.

Journal article

Huang Y, Gao J, Vorontsov V, Guan D, Goodall R, Dye D, Wang S, Zhu Q, Rainforth WM, Todd Iet al., 2022, Martensitic twinning transformation mechanism in a metastable IVB element-based body-centered cubic high-entropy alloy with high strength and high work hardening rate, JOURNAL OF MATERIALS SCIENCE & TECHNOLOGY, Vol: 124, Pages: 217-231, ISSN: 1005-0302

Journal article

Kwok TWJ, Gong P, Rose R, Dye Det al., 2022, The relative contributions of TWIP and TRIP to strength in fine grained medium-Mn steels, Materials Science and Engineering: A, Vol: 855, Pages: 1-13, ISSN: 0921-5093

A medium Mn steel of composition Fe-4.8Mn-2.8Al-1.5Si-0.51C (wt.%) was processed to obtain two different microstructures representing two different approaches in the hot rolling mill, resulting in equiaxed vs. mixed equiaxed and lamellar microstructures. Both were found to exhibit a simultaneous Twinning Induced Plasticity and Transformation Induced Plasticity (TWIP + TRIP) mechanism where deformation twins and -martensite formed independently of twinning with strain. Interrupted tensile tests were conducted in order to investigate the differences in deformation structures between the two microstructures. A constitutive model was used to find that, surprisingly, twinning contributed relatively little to the strength of the alloy, chiefly due to the fine initial slip lengths that then gave rise to relatively little opportunity for work hardening by grain subdivision. Nevertheless, with lower high-cost alloying additions than equivalent Dual Phase steels (2–3 wt% Mn) and greater ductility, medium-Mn TWIP + TRIP steels still represent an attractive area for future development.

Journal article

Vorontsov VA, McAuliffe TP, Hardy MC, Dye D, Bantounas Iet al., 2022, Precipitate dissolution during deformation induced twin thickening in a CoNi-base superalloy subject to creep, Acta Materialia, Vol: 232, Pages: 1-11, ISSN: 1359-6454

The tensile creep performance of a polycrystalline Co/Ni-base superalloy with a multimodal γ distribution has been examined at 800◦C and 300 MPa. The rupture life of the alloy is comparable to that ofRR1000 tested under similar conditions. Microstructural examination of the alloy after testing revealedthe presence of continuous γ precipitates and M23C6 carbides along the grain boundaries. Intragranularly, coarsening of the secondary γ precipitates occurred at the expense of the fine tertiary γ. Longplanar deformation bands, free of γ, were also observed to traverse individual grains. Examination ofthe deformation bands confirmed that they were microtwins. Long sections of the microtwins examinedwere depleted of γ stabilising elements across their entire width, suggesting that certain alloy compositions are susceptible to precipitate dissolution during twin thickening. A mechanism for the dissolutionof the precipitates is suggested based on the Kolbe reordering mechanism.

Journal article

Kwok TWJ, Rahman KM, Vorontsov VA, Dye Det al., 2022, Strengthening kappa-carbide steels using residual dislocation content, Scripta Materialia, Vol: 213, ISSN: 1359-6462

A steel with nominal composition Fe-28Mn-8Al-1.0C in mass percent was hot rolled at two temperatures, 1100 °C and 850 °C and subsequently aged at 550 °C for 24 h. The lower temperature rolling resulted in a yield strength increment of 299 MPa while still retaining an elongation to failure of 26%. The large improvement in strength was attributed to an increase in residual dislocation density which was retained even after the ageing heat treatment. A homogeneous precipitation of -carbides in both samples also showed that the high residual dislocation density did not adversely affect precipitation kinetics. These findings demonstrate that the tensile properties of this class of steel can yet be improved by optimising hot rolling process parameters.

Journal article

Tan Q, Yan Z, Wang H, Dye D, Antonov S, Gault Bet al., 2022, The role of β pockets resulting from Fe impurities in hydride formation in titanium, Scripta Materialia, Vol: 213, Pages: 114640-114640, ISSN: 1359-6462

The corrosion potential of commercially pure titanium in NaCl solutions is dramatically affected by trace Fe additions, which cause the appearance of submicron pockets of β phase at grain boundary triple points. Furthermore, the low solubility of hydrogen in hexagonal close-packed α-Ti makes titanium alloys prone to subsequent hydride-associated failures due to stress corrosion cracking. We analyzed α-α and α-β sections of the abutting grain boundary of a β pocket in a Grade 2 CP-Ti, and the α-β phase boundary. Fe and H partition to β and segregate at the grain boundary, but no segregation is seen at the α-β phase boundary. In contrast, a significant Ni (>1 at%) accumulation is observed at the α-β phase boundary. We propose that the β-pockets act as hydrogen traps and facilitate the nucleation and growth of hydrides along grain boundaries in CP-Ti.

Journal article

Joseph S, Kontis P, Chang Y, Shi Y, Raabe D, Gault B, Dye Det al., 2022, A cracking oxygen story: a new view of stress corrosion cracking in titanium alloys, Acta Materialia, Vol: 227, Pages: 117687-117687, ISSN: 1359-6454

Titanium alloys can suffer from halide-associated stress corrosion cracking at elevated temperatures e.g., in jet engines, where chlorides and Ti-oxide promote the cracking of water vapour in the gas stream, depositing embrittling species at the crack tip. Here we report, using isotopically-labelled experiments, that crack tips in an industrial Ti-6Al-2Sn-4Zr-6Mo alloy are strongly enriched (>5 at.%) in oxygen from the water vapour, far greater than the amounts (0.25 at.%) required to embrittle the material. Surprisingly, relatively little hydrogen (deuterium) is measured, despite careful preparation and analysis. Therefore, we suggest that a combined effect of O and H leads to cracking, with O playing a vital role, since it is well-known to cause embrittlement of the alloy. In contrast it appears that in α + β Ti alloys, it may be that H may drain away into the bulk owing to its high solubility in β-Ti, rather than being retained in the stress field of the crack tip. Therefore, whilst hydrides may form on the fracture surface, hydrogen ingress might not be the only plausible mechanism of embrittlement of the underlying matrix. This possibility challenges decades of understanding of stress-corrosion cracking as being related solely to the hydrogen enhanced localised plasticity (HELP) mechanism, which explains why H-doped Ti alloys are embrittled. This would change the perspective on stress corrosion embrittlement away from a focus purely on hydrogen to also consider the ingress of O originating from the water vapour, insights critical for designing corrosion resistant materials.

Journal article

Xu X, Kwok TWJ, Gong P, Dye Det al., 2022, Tailoring the deformation behaviour of a medium Mn steel through isothermal intercritical annealing, Materialia, Vol: 22, Pages: 101422-101422, ISSN: 2589-1529

A novel concept of varying the strain hardening rate of a medium Mn steel with 8 wt.% Mn by varying the duration of the intercritical anneal after hot rolling was explored. It was found that the stability of the austenite phase showed an inverse square root relationship with intercritical annealing duration and that the maximum strain hardening rate showed a linear relationship with austenite stability. The change in austenite stability was attributed to continuous Mn enrichment with increasing intercritical annealing duration. Twinned martensite was also found to be the most likely product of the martensitic transformation during deformation.

Journal article

Kwok TWJ, Gong P, Xu X, Nutter J, Rainforth WM, Dye Det al., 2022, Microstructure evolution and tensile behaviour of a cold rolled 8 Wt Pct Mn medium manganese steel, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, Vol: 53, Pages: 597-609, ISSN: 1073-5623

A novel medium manganese steel with composition Fe–8.3Mn–3.8Al–1.8Si–0.5C–0.06V–0.05Sn was developed and thermomechanically processed through hot rolling and intercritical annealing. The steel possessed a yield strength of 1 GPa, tensile strength of 1.13 GPa and ductility of 41 pct. In order to study the effect of cold rolling after intercritical annealing on subsequent tensile properties, the steel was further cold rolled up to 20 pct reduction. After cold rolling, it was observed that the strain hardening rate increased continuously with increasing cold rolling reduction but without a significant drop in ductility during subsequent tensile tests. The microstructural evolution with cold rolling reduction was analysed to understand the mechanisms behind this phenomena. It was found that cold rolling activated additional twinning systems which provided a large number of potent nucleation sites for strain induced martensite to form during subsequent tensile tests in what can be described as an enhanced TRIP effect.

Journal article

Zhao G, Xu X, Dye D, Rivera-Diaz-del-Castillo PEJ, Petrinic Net al., 2022, Facile route to implement transformation strengthening in titanium alloys, Scripta Materialia, Vol: 208, Pages: 1-5, ISSN: 1359-6462

Developing lighter, stronger and more ductile aerospace metallic materials is in demand for energy efficiency strategies. Alloys with twinning-induced plasticity (TWIP) and/or transformation-induced plasticity(TRIP) effects have been exploited to defeat the conflict of strength versus ductility, yet very few if anyphysically informed methods exist to address the complex interactions between the transitions. Here wereport a facile route to deploy transformation-mediated strengthening in Ti alloys, which particularly focuses on the supervised activation of TRIP and TWIP via a mechanism-driven modelling approach. Newalloys were comparatively developed and presented notable resistances to strain localisation, but interestingly through distinct mechanical characteristics. Specifically, extraordinary strain-hardening rate (dσ/dε)with a peak value of 2.4 GPa was achieved in Ti-10Mo-5Nb (wt.%), resulting from the synergetic activationof hierarchical transformations. An efficient model integrating TRIP and TWIP was applied to understandthe interplays of the transition mechanisms.

Journal article

Kwok TWJ, Slater C, Xu X, Davis C, Dye Det al., 2021, A scale-up study on chemical segregation and the effects on tensile properties in two medium mn steel castings, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, Vol: 53, Pages: 585-596, ISSN: 1073-5623

Two ingots weighing 400 g and 5 kg with nominal compositions of Fe–8Mn–4Al–2Si–0.5C–0.07V–0.05Sn were produced to investigate the effect of processing variables on microstructure development. The larger casting has a cooling rate more representative of commercial production and provides an understanding of the potential challenges arising from casting-related segregation during efforts to scale up medium Mn steels, while the smaller casting has a high cooling rate and different segregation pattern. Sections from both ingots were homogenized at 1250 ∘C for various times to study the degree of chemical homogeneity and δ-ferrite dissolution. Within 2 hours, the Mn segregation range (max–min) decreased from 8.0 to 1.7 wt pct in the 400 g ingot and from 6.2 to 1.5 wt pct in the 5 kg ingot. Some δ-ferrite also remained untransformed after 2 hours in both ingots but with the 5 kg ingot showing nearly three times more than the 400 g ingot. Micress modeling was carried out, and good agreement was seen between predicted and measured segregation levels and distribution. After thermomechanical processing, it was found that the coarse untransformed δ-ferrite in the 5 kg ingot turned into coarse δ-ferrite stringers in the finished product, resulting in a slight decrease in yield strength. Nevertheless, rolled strips from both ingots showed >900 MPa yield strength, >1100 MPa tensile strength, and >40 pct elongation with <10 pct difference in strength and no change in ductility when compared to a fully homogenized sample.

Journal article

Ghouse S, Oosterbeek R, Tayub A, Vecchiato F, Dye D, Jeffers Jet al., 2021, Vacuum heat treatments of titanium porous structures, Additive Manufacturing, Vol: 47, ISSN: 2214-8604

Additive manufacturing (AM) of Ti-6Al-4V enables rapid fabrication of complex parts, including porous lattices which are of interest for aerospace, automotive, or biomedical applications, however currently the fatigue resistance of these materials is a critical limitation. Engineering the alloy microstructure provides a promising method for increasing fatigue strength, but conventional heat treatment procedures are known to produce atypical results for AM and porous samples, and must therefore be optimised for these materials. Using vacuum heat treatment, microstructures comparable to those observed for conventional wrought and heat treated alloys were achieved with porous AM Ti-6Al-4V. Fine lamellar microstructures were produced using sub-transus heat treatment at 920 °C, while coarse lamellar microstructures were produced using super-transus heat treatment at 1050 °C or 1200 °C. Increasing the heat treatment temperature increased the elastic modulus from 2552 ± 22 MPa to a maximum of 2968 ± 45 MPa, due to strut sintering increasing the effective strut thickness, and removal of prior β-grain orientation. Heat treatment eliminated the brittle α’ martensite phase in favour of an α + β mixture, where the phase boundaries and β-phase provide greater resistance to crack propagation. Super-transus heat treatments increased the α-lath size which typically reduces crack propagation resistance, however strut sintering reduced surface crack initiation sites, increasing the fatigue strength by 75% from 4.86 MPa for the as-built material to a maximum of 8.51 MPa after 1200 °C heat treatment. This work demonstrates that vacuum heat treatment is effective at tuning the micro- and macro-structure of porous AM Ti-6Al-4V, thereby improving the crucial fatigue resistance.

Journal article

Kim J, Hall D, Yan H, Shi Y, Joseph S, Fearn S, Chater RJ, Dye D, Tasan CCet al., 2021, Roughening improves hydrogen embrittlement resistance of Ti-6Al-4V, Acta Materialia, Vol: 220, Pages: 1-12, ISSN: 1359-6454

Polished surfaces of Ti-6Al-4V, the most commonly used titanium alloy, were observed to suffer from hydride growth and associated embrittlement during hydrogen charging, whereas rough surfaces suffered no such susceptibility. Direct microscopic analyses of recombined hydrogen bubbles and thermal desorption spectroscopy (TDS) revealed that the surface roughening promotes recombination of atomic hydrogen to molecular hydrogen, in turn, reducing the relative amount of atomic hydrogen uptake. Subsurface time-of-flight secondary-ion mass spectrometry (ToF-SIMS) further revealed that the high defect density underneath the roughened surface impedes hydrogen diffusion into the bulk. These combined effects mean that, unexpectedly, roughening significantly reduces hydrogen uptake into Ti-6Al-4V and enhances its resistance against hydrogen embrittlement – all resulting from a simple surface treatment.

Journal article

Shi Y, Joseph S, Saunders EA, Sandala RS, Walker A, Lindley TC, Dye Det al., 2021, AgCl-induced hot salt stress corrosion cracking in a titanium alloy, Corrosion Science, Vol: 187, Pages: 1-11, ISSN: 0010-938X

The mechanism of AgCl-induced stress corrosion cracking of Ti-6246 was examined at 500 MPa and 380 °C for 24 h exposures. SEM and STEM-EDX examination of a FIB-sectioned blister and crack showed that metallic Ag was formed and migrated along the crack. TEM analysis also revealed the presence of SnO2 and Al2O3 corrosion products mixed into TiO2. The fracture surface has a transgranular nature with a brittle appearance in the primary α phase. Long, straight and non-interacting dislocations were observed in a brittle appearance fractured primary α grain, with basal and pyramidal traces. This is consistent with a dislocation emission view of the cracking mechanism.

Journal article

Dear FF, Kontis P, Gault B, Ilavsky J, Rugg D, Dye Det al., 2021, Mechanisms of Ti3Al precipitation in hcp α-Ti, Acta Materialia, Vol: 212, ISSN: 1359-6454

Nucleation and growth of Ti3Alα2ordered domains inα-Ti–Al–X alloys were characterised using a combination of transmission electronmicroscopy, atom probe tomography and small angle X-ray scattering. Model alloys based on Ti–7Al (wt.%) and containing O, V and Mowere aged at 550◦C for times up to 120 d and the resulting precipitate dispersions were observed at intermediate points. Precipitates grewto around 30 nm in size, with a volume fraction of 6–10% depending on tertiary solutes. Interstitial O was found to increase the equilibriumvolume fraction ofα2, while V and Mo showed relatively little influence. Addition of any of the solutes in this study, but most prominentlyMo, was found to increase nucleation density and decrease precipitate size and possibly coarsening rate. Coarsening can be described by theLifshitz-Slyozov-Wagner model, suggesting a matrix diffusion-controlled coarsening mechanism (rather than control by interfacial coherency).Solutionising temperature was found to affect nucleation number density with an activation energy ofEf=1.5±0.4 eV, supporting the hypothesisthat vacancy concentration affectsα2nucleation. The observation that all solutes increase nucleation number density is also consistent with avacancy-controlled nucleation mechanism.

Journal article

Knowles A, Dye D, Dodds R, Watson A, Hardie C, Humphry-Baker Set 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.

Journal article

Cann JL, De Luca A, Dunand DC, Dye D, Miracle DB, Oh HS, Olivetti EA, Pollock TM, Poole WJ, Yang R, Tasan CCet al., 2021, Sustainability through alloy design: Challenges and opportunities, PROGRESS IN MATERIALS SCIENCE, Vol: 117, ISSN: 0079-6425

Journal article

McAuliffe TP, Bantounas I, Reynolds LR, Foden A, Hardy MC, Britton TB, Dye Det al., 2021, Quantitative precipitate classification and grain boundary property control in Co/Ni-base superalloys, METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE, Vol: 52, Pages: 1649-1664, ISSN: 1073-5623

A correlative approach is employed to simultaneously assess structure and chemistry of (carbide and boride) precipitates in a set of novel Co/Ni-base superalloys. Structure is derived from electron backscatter diffraction (EBSD) with pattern template matching, and chemistry obtained with energy dispersive X-ray spectroscopy (EDS). It is found that the principal carbide in these alloys is Mo and W rich with the M6C structure. An M2B boride also exhibiting Mo and W segregation is observed at B levels above approximately 0.085 at. pct. These phases are challenging to distinguish in an SEM with chemical information (EDS or backscatter Z-contrast) alone, without the structural information provided by EBSD. Only correlative chemical and structural fingerprinting is necessary and sufficient to fully define a phase. The identified phases are dissimilar to those predicted using ThermoCalc. We additionally perform an assessment of the grain boundary serratability in these alloys, and observe that significant amplitude is only obtained in the absence of pinning intergranular precipitates.

Journal article

Collins CR, Dear FF, Rugg D, Dye Det al., 2021, The effect of dissolved nitrogen on the fatigue behavior of Ti-6Al-4V, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, Vol: 52, Pages: 1596-1608, ISSN: 1073-5623

The effect of nitrogen additions on fatigue behavior has been examined in near-equiaxed, rolled Ti-6Al-4V bar. This is the first-time nitrogen content that has been systematically explored with respect to monotonic and cyclic properties in a Ti-6Al-4V alloy base composition. Nitrogen additions were found to increase the β-transus temperature and strength, but they decreased ductility, even in microstructures where some β phase remained. This carried across into both the low- and high cycle fatigue behavior; even small contents of 240 and 560 ppmwN caused reductions in both low cycle fatigue life and high cycle fatigue strength. In samples containing 240 and 560 ppmwN, a conventional striated fractographic appearance was observed, but a dramatic change to a macroscopically brittle fracture surface was observed at 1800 and 3600 ppmwN, but still with substantial evidence of plasticity at the microscale. Therefore, neither microstructure or fractographic examination, nor EDX-based compositional analysis in the electron microscope are necessarily a reliable indicator of an absence of deleterious nitrogen contamination. This is significant for the investigation of potentially nitrogen-contaminated surface-initiated cracks, either due to service or processing exposures.

Journal article

Coakley J, Higginbotham A, McGonegle D, Ilavsky J, Swinburne TD, Wark JS, Rahman KM, Vorontsov VA, Dye D, Lane TJ, Boutet S, Koglin J, Robinson J, Milathianaki Det al., 2020, Femtosecond quantification of void evolution during rapid material failure, Science Advances, Vol: 6, Pages: 1-10, ISSN: 2375-2548

Understanding high velocity impact, and the subsequent high strain rate material deformation and potential catastrophic failure, is of critical importance across a range of scientific and engineering disciplines that include astrophysics, materials science and aerospace engineering. The deformation and failure mechanisms are not thoroughly understood, given the challenges of experimentally quantifying material evolution at extremely short time-scales. Here, copper foils are rapidly strained via picosecond laser ablation and probed in situ with femtosecond x-ray free electron (XFEL) pulses. Small angle x-ray scattering (SAXS) monitors the void distribution evolution while wide angle scattering (WAXS) simultaneously determines the strain evolution. The ability to quantifiably characterize the nanoscale during high strain rate failure with ultrafast-SAXS, complementing WAXS, represents a broadening in the range of science that can be performed with XFEL. It is shown that ultimate failure occurs via void nucleation, growth and coalescence, and the data agree well with molecular dynamics simulations.

Journal article

McAuliffe TP, Dye D, Britton TB, 2020, Spherical-angular dark field imaging and sensitive microstructural phase clustering with unsupervised machine learning., Ultramicroscopy, Vol: 219, Pages: 1-11, ISSN: 0304-3991

Electron backscatter diffraction is a widely used technique for nano- to micro-scale analysis of crystal structure and orientation. Backscatter patterns produced by an alloy solid solution matrix and its ordered superlattice exhibit only extremely subtle differences, due to the inelastic scattering that precedes coherent diffraction. We show that unsupervised machine learning (with principal component analysis, non-negative matrix factorisation, and an autoencoder neural network) is well suited to fine feature extraction and superlattice/matrix classification. Remapping cluster average patterns onto the diffraction sphere lets us compare Kikuchi band profiles to dynamical simulations, confirm the superlattice stoichiometry, and facilitate virtual imaging with a spherical solid angle aperture. This pipeline now enables unparalleled mapping of exquisite crystallographic detail from a wide range of materials within the scanning electron microscope.

Journal article

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