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

167 results found

Joseph S, Joseph K, Lindley TC, Dye Det al., 2020, The role of dwell hold on the dislocation mechanisms of fatigue in a near alpha titanium alloy, International Journal of Plasticity, Vol: 131, ISSN: 0749-6419

The dislocation structures appearing in highly mis-oriented soft/hard grain pairs in near-alphatitanium alloy Ti6242Si were examined with and without the application of load holds (dwell)during fatigue. Dislocation pile-up in a soft grain resulted in internal stresses in an adjacent hardgrain which could be relaxed by dislocation multiplication at localised Frank–Read sources, aprocess assisted by the provision of a relaxation time during a load hold. The rate of this processis suggested to be controlled by⟨𝑐+𝑎⟩pyramidal cross-slip and⟨𝑎⟩basal junction formation.A high density of⟨𝑎⟩prism pile-ups was observed with dual slip on two prism planes, togetherwith edge dislocations on the third prism plane in the soft grain of a highly mis-oriented grainpair, increasing the pile-up stress. The stress concentration developed by such pile-ups is foundto be higher in dwell fatigue (single-ended pile-ups) than in LCF (double ended). Analyticalmodelling shows that the maximum normal stress produced on the hard grain in dwell fatigueby this pile-up would be near-basal,≈ 2.5◦to (0002). This provides support for the dominanthypothesis for the rationalisation of dwell fatigue crack nucleation in Ti alloys, which derivesfrom the Stroh pile-up model, and elaboration of the underlying dislocation phenomena thatresult from load shedding and lead to basal faceting.

Journal article

Trant CF, Lindley TC, Martin N, Dixon M, Dye Det al., 2020, Fatigue cracking in gamma titanium aluminide, Publisher: arXiv

Fatigue crack initiation and growth were examined in cast and HIP'ed\textgamma-TiAl 4522XD. It was found that fatigue crack growth rates werehigher at $750\celsius$ than $400\celsius$, but that $\Delta K_\mathrm{th}$ wasalso higher. Temperature excursions between $400$ and $750\celsius$ duringfatigue crack growth resulted in retardation of the crack growth rate, both onheating and cooling; however heating from 400 to $750\celsius$ at a $\Delta K$that would then be below threshold did not result in complete crack arrest. Itwas also found that for notches $0.6$~mm in length and smaller, initiation fromthe microstructure could instead be observed at stresses similar to thematerial failure stress; a microstructural initiation site exists. Secondarycracking around borides could also be observed. A change from trans- to mixedtrans-, inter- and intra-lamellar cracking could be observed where theestimated size of the crack tip plastic zone exceeded the colony size. Changesin fracture surface morphology could not be related to the temperature offatigue crack growth, although this could be observed from the oxide scalecolouration. Compressive pre-loading of a crack results in retardation of thecrack, which could also be observed from the oxide.

Working paper

Dichtl C, Zhang Z, Gardner H, Bagot P, Radecka A, Dye D, Thomas M, Sandala R, da Fonseca JQ, Preuss Met al., 2020, Element segregation and α2 formation in primary α of a near-α Ti-alloy, Materials Characterization, Vol: 164, Pages: 1-10, ISSN: 1044-5803

Alloy TIMETAL®834 is a near-α Ti-alloy typically processed to have a complex bimodal microstructure that provides a good combination of mechanical properties at temperatures in excess of 450 °C. Due to the high Al content, typical ageing procedures result in the formation of intragranular and coherent nano-scale Ti3Al precipitation (α2), which increase strength but also promotes slip planarity. The present study focuses on chemical partitioning as a result of sub-β-transus heat treatment and the consequences for the two different constituents in the bimodal microstructure. The detailed chemical and structural analysis were carried out by combining Electron Probe Micro Analysis (EPMA), Wavelength Dispersive Spectroscopy (WDS), Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT) for investigating local compositional variations and their effect on the formation of α2 precipitates. Detailed microchemical analysis shows a core-shell composition arrangement of α-stabilisers with the shell composition similar to the one of secondary α. Selected area electron diffraction in the TEM and APT analysis demonstrates that those local variations in α stabilisers affect the level of α2 precipitation. In addition, EPMA maps show that while Zr and Sn are often considered to be neutral alloying elements in Ti-alloys, they do segregate to the β-phase during sub-β-transus heat treatment.

Journal article

Kwok TWJ, Rahman KM, Xu X, Bantounas I, Kelleher JF, Daswari S, Alam T, Banerjee R, Dye Det al., 2020, Design of a high strength, high ductility 12 wt% Mn medium manganese steel with hierarchical deformation behaviour, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 782, ISSN: 0921-5093

Journal article

McAuliffe TP, Ackerman AK, Savitzky BH, Kwok TWJ, Danaie M, Ophus C, Dye Det al., 2020, 4D-STEM elastic stress state characterisation of a TWIP steel nanotwin, Publisher: arXiv

We measure the stress state in and around a deformation nanotwin in atwinning-induced plasticity (TWIP) steel. Using four dimensional scanningtransmission electron microscopy (4D-STEM), we measure the elastic strain fieldin a 68.2-by-83.1 nm area of interest with a scan step of 0.36 nm and adiffraction limit resolution of 0.73 nm. The stress field in and surroundingthe twin matches the form expected from analytical theory and is on the orderof 15 GPa, close to the theoretical strength of the material. We infer that themeasured back-stress limits twin thickening, providing rationale for why TWIPsteel twins remain thin, continuously dividing grains to give substantial workhardening. Our results support modern mechanistic understanding of theinfluence of twinning on crack propagation and embrittlement in TWIP steels.

Working paper

Chang Y, Zhang S, Liebscher CH, Dye D, Ponge D, Scheu C, Dehm G, Raabe D, Gault B, Lu Wet al., 2020, Could face-centered cubic titanium in cold-rolled commercially-pure titanium only be a Ti-hydride?, Scripta Materialia, Vol: 178, Pages: 39-43, ISSN: 1359-6462

A face-centered cubic (FCC) phase in electro-polished specimens for transmission electron microscopy of commercially pure titanium has sometimes been reported. Here, a combination of atom-probe tomography, scanning transmission electron microscopy and low-loss electron energy loss spectroscopy is employed to study both the crystal structural and chemical composition of this FCC phase. Our results prove that the FCC phase is actually a TiHx (x>1) hydride, and not a new allotrope of Ti, in agreement with previous reports. The formation of the hydride is discussed.

Journal article

McAuliffe TP, Foden A, Bilsland C, Daskalaki Mountanou D, Dye D, Britton TBet al., 2020, Advancing characterisation with statistics from correlative electron diffraction and X-ray spectroscopy, in the scanning electron microscope., Ultramicroscopy, Vol: 211, Pages: 1-16, ISSN: 0304-3991

The routine and unique determination of minor phases in microstructures is critical to materials science. In metallurgy alone, applications include alloy and process development and the understanding of degradation in service. We develop a correlative method, exploring superalloy microstructures, which are examined in the scanning electron microscope (SEM) using simultaneous energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD). This is performed at an appropriate length scale for characterisation of carbide phases' shape, size, location, and distribution. EDS and EBSD data are generated using two different physical processes, but each provide a signature of the material interacting with the incoming electron beam. Recent advances in post-processing, driven by 'big data' approaches, include use of principal component analysis (PCA). Components are subsequently characterised to assign labels to a mapped region. To provide physically meaningful signals, the principal components may be rotated to control the distribution of variance. In this work, we develop this method further through a weighted PCA approach. We use the EDS and EBSD signals concurrently, thereby labelling each region using both EDS (chemistry) and EBSD (crystal structure) information. This provides a new method of amplifying signal-to-noise for very small phases in mapped regions, especially where the EDS or EBSD signal is not unique enough alone for classification.

Journal article

Zhou G-H, Xu X, Dye D, Rivera-Diaz-del-Castillo PEJet al., 2020, Microstructural evolution and strain-hardening in TWIP Ti alloys, Acta Materializa, Vol: 183, Pages: 155-164, ISSN: 1359-6454

A multiscale dislocation-based model was built to describe, for the first time, the microstructural evolution and strain-hardening of {332}⟨113⟩ TWIP (twinning-induced plasticity) Ti alloys. This model not only incorporates the reduced dislocation mean free path by emerging twin obstacles, but also quantifies the internal stress fields present at β-matrix/twin interfaces. The model was validated with the novel Ti-11Mo-5Sn-5Nb alloy (wt.%), as well as an extensive series of alloys undergoing {332}⟨113⟩ twinning at various deformation conditions. The quantitative model revealed that solid solution hardening is the main contributor to the yield stress, where multicomponent alloys or alloys containing eutectoid β-stabilisers exhibited higher yield strength. The evolution of twinning volume fraction, intertwin spacing, dislocation density and flow stress were successfully described. Particular attention was devoted to investigate the effect of strain rate on the twinning kinetics and dislocation annihilation. The modelling results clarified the role of each strengthening mechanism and established the influence of phase stability on twinning enhanced strain-hardening. Strain-hardening stems from the formation of twin obstacles in early stages, whereas the internal stress fields provide a long-lasting strengthening effect throughout the plastic deformation. A tool for alloy design by controlling TWIP is presented.

Journal article

Ackerman AK, Vorontsov VA, Bantounas I, Zheng Y, McAuliffe T, Clark WA, Fraser HL, Rugg D, Dye Det al., 2020, Interface characteristics in an α+β titanium alloy, Physical Review Materials, Vol: 4, ISSN: 2475-9953

The alpha/beta interface in Ti-6Al-2Sn-4Zr-6Mo (Ti-6246) is investigated viacentre of symmetry analysis, both as-grown and after 10% cold work.Semi-coherent interface steps are observed at a spacing of 4.5 +/-1.13 atoms inthe as-grown condition, in good agreement with theory prediction (4.37 atoms).Lattice accommodation is observed, with elongation along [-1 2 -1 0]alpha andcontraction along [1 0 -1 0]alpha . Deformed alpha exhibited larger, lesscoherent steps with slip bands lying in {110}beta. This indicates dislocationpile-up at the grain boundary, a precursor to globularisation, offering insightinto the effect of deformation processing on the interface, which is importantfor titanium alloy processing route design.

Journal article

Ackerman A, Knowles A, Gardener HM, Nemeth AAN, Bantounas I, Radecka A, Moody MP, Bagot PAJ, Reed RC, Rugg DR, Dye Det 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.

Journal article

Rogers S, Bowden D, Unnikrishnan R, Scenini F, Preuss M, Stewart D, Dini D, Dye Det al., 2019, The interaction of galling and oxidation in 316L stainless steel, Wear, ISSN: 0043-1648

The galling behaviour of 316L stainless steel was investigated in both theunoxidised and oxidised states, after exposure in simulated PWR water for 850hours. Galling testing was performed according to ASTM G196 in ambientconditions. 316L was found to gall by the wedge growth and flow mechanism inboth conditions. This resulted in folds ahead of the prow and adhesivejunction, forming a heavily sheared multilayered prow. The galling trough wasseen to have failed through successive shear failure during wedge flow.Immediately beneath the surface a highly sheared nanocrystalline layer wasseen, termed the tribologically affected zone (TAZ). It was observed thatstrain-induced martensite formed within the TAZ. Galling damage was quantifiedusing Rt (maximum height - maximum depth) and galling area (the proportion ofthe sample which is considered galled), and it was shown that both damagemeasures decreased significantly on the oxidised samples. At an applied normalstress of 4.2 MPa the galled area was 14 % vs. 1.2 % and the Rt was 780 um vs.26 um for the unoxidised and oxidised sample respectively. This trend waspresent at higher applied normal stresses, although less prominent. Thisdifference in galling behaviour is likely to be a result of a reduction inadhesion in the case of the oxidised surface.

Journal article

Goodfellow AJ, Kelleher J, Jones NG, Dye D, Hardy MC, Stone HJet al., 2019, The effect of Mo on load partitioning and microstrain evolution during compression of a series of polycrystalline Ni-Based superalloys, Acta Materialia, Vol: 176, Pages: 318-329, ISSN: 1359-6454

The room temperature deformation behaviour of a series of model polycrystalline Ni-based superalloys with varying Mo content has been studied in compression using in situ neutron diffraction. Initially, it was found that intergranular load partitioning was operative, followed by interphase partitioning at higher applied loads, with yield of the γ phase and associated strain redistribution to the γ′ phase. The initiation of interphase load partitioning was found to be dependent on the lattice misfit, occurring at lower applied stress in alloys with larger lattice misfit, and was influenced by the sign of the lattice misfit. Notably, deformation behaviour was found to be contingent on the complex relationship between lattice misfit and the strength of each phase.

Journal article

Knowles A, Gong P, Rahman KM, Rainforth WM, Dye D, Galindo-Nava EIet 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.

Journal article

Weekes H, Dye D, Proctor JE, Smith DS, Simionescu C, Prior TJ, Wenman Met al., 2019, The effect of pressure on hydrogen solubility in Zircaloy-4, Journal of Nuclear Materials, Vol: 524, Pages: 256-262, ISSN: 0022-3115

The effect of pressure on the room temperature solubility of hydrogen inZircaloy-4 was examined using synchrotron X-ray diffraction on small groundflake samples in a diamond anvil cell at pressures up to 20.9 GPa. Differentcombinations of hydrogen level/state in the sample and of pressure transmittingmedium were examined; in all three cases examined, it could be concluded thatpressure resulted in the dissolution of d hydrides and that interstitialhydrogen retards the formation of w Zr. A pressure of around 9 GPa was requiredto halve the hydride fraction. These results imply that the effect of pressureis thermodynamically analogous to that of increasing temperature, but that theeffect is small. The results are consistent with the volume per Zr atom of thea, d and w phases, with the bulk moduli of a and d, and with previousmeasurements of the hydrogen site molar volumes in the a and d phases. Theresults are interpreted in terms of their implication for our understanding ofthe driving forces for hydride precipitation at crack tips, which are in aregion of hydrostatic tensile stress on the order of 1.5 GPa.

Journal article

Bantounas I, Gwalani B, Alam T, Banerjee R, Dye Det al., 2019, Elemental partitioning, mechanical and oxidation behaviour of two high-gamma-prime W-free gamma/gamma-prime polycrystalline Co/Ni superalloys, Scripta Materialia, Vol: 163, Pages: 44-50, ISSN: 1359-6462

Cr-containing W-free Co alloys that form strengthening precipitates in an fcc γ matrix are presented to examine the effect of Ta and Nb additions to increase the strength, solvus temperature and gamma prime fraction. The alloys are found to have relatively low density, good oxidation resistance (<0.5 μm scale at 800 °C for 100 h) with coherent Al2O3 and Cr2O3 scales, and reasonable yield strengths, ∼800 MPa. The phase partitioning, measured by atom probe tomography, was found to be similar to W-containing Co/Ni superalloys, with Mo partitioning to the matrix providing solid solution strengthening.

Journal article

Knowles A, Reynolds L, Vorontsov V, Dye Det 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.

Journal article

Chang Y, Lu W, Guenole J, Stephenson LT, Szczpaniak A, Kontis P, Ackerman AK, Dear FF, Mouton I, Zhong X, Zhang S, Dye D, Liebscher CH, Ponge D, Korte-Kerzel S, Raabe D, Gault Bet al., 2019, Ti and its alloys as examples of cryogenic focused ion beam milling of environmentally-sensitive materials, Nature Communications, Vol: 10, ISSN: 2041-1723

Hydrogen pick-up leading to hydride formation is often observed in commercially pure Ti (CP-Ti) and Ti-based alloys prepared for microscopic observation by conventional methods, such as electro-polishing and room temperature focused ion beam (FIB) milling. Here, we demonstrate that cryogenic FIB milling can effectively prevent undesired hydrogen pick-up. Specimens of CP-Ti and a Ti dual-phase alloy (Ti-6Al-2Sn-4Zr-6Mo, Ti6246, in wt.%) were prepared using a xenon-plasma FIB microscope equipped with a cryogenic stage reaching −135 °C. Transmission electron microscopy (TEM), selected area electron diffraction, and scanning TEM indicated no hydride formation in cryo-milled CP-Ti lamellae. Atom probe tomography further demonstrated that cryo-FIB significantly reduces hydrogen levels within the Ti6246 matrix compared with conventional methods. Supported by molecular dynamics simulations, we show that significantly lowering the thermal activation for H diffusion inhibits undesired environmental hydrogen pick-up during preparation and prevents pre-charged hydrogen from diffusing out of the sample, allowing for hydrogen embrittlement mechanisms of Ti-based alloys to be investigated at the nanoscale.

Journal article

Wang Y, Liu B, Yan K, Wang M, Kabra S, Chiu YL, Dye D, Lee PD, Liu Y, Cai Bet al., 2019, Corrigendum to ‘Probing deformation mechanisms of a FeCoCrNi high-entropy alloy at 293 and 77 K using in situ neutron diffraction’ [Acta Mater. 154C (2018) 79–89], Acta Materialia, Vol: 163, Pages: 240-242, ISSN: 1359-6454

The authors regret that there were errors in Figs. 3 and 4, which in turn meant there were errors in Table 2. In Figs. 3 and 4, the lattice strain ((d-d0)/d0, where d is the lattice spacing) as a function of strain/stress should have plotted. Please find below the corrected versions of the figures and table. The authors would like to apologise for any inconvenience caused. Fig. 3. The evolution of elastic lattice strains along the axial and radial directions in grain families having {111}, {200}, {220}, {311} and {222} crystallographic planes during tensile loading at (a) 77 K and (b) 293 K.[Figure presented] Fig. 4. The (111) first order and (222) second order reflections together with the stacking fault probability as a function of true strain at (a) 77 K, (b) 293 K. [Figure presented] Table 2. Uniaxial materials properties of FeCoCrNi HEA at 77 and 293 K. [Table presented]

Journal article

calvert E, Knowles A, Pope J, Dye D, Jackson Met 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.

Journal article

Yang R, Rahman KM, Rakhymberdiyev AN, Dye D, Vorontsov VAet al., 2019, Mechanical behaviour of Ti-Nb-Hf alloys, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 740, Pages: 398-409, ISSN: 0921-5093

Journal article

Tong VS, Knowles AJ, Dye D, Ben Britton Tet 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.

Journal article

Dear FF, Kontis P, Gault B, Ilavsky J, Gardner H, Bagot PAJ, Moody MP, Rugg D, Dye Det al., 2019, Combined APT, TEM and SAXS characterisation of nanometre-scale precipitates in titanium alloys, Microscopy and Microanalysis, Vol: 25, Pages: 2516-2517, ISSN: 1431-9276

Journal article

Joseph S, Lindley T, Dye D, 2018, Dislocation interactions and crack nucleation in a fatigued near-alpha titanium alloy, International Journal of Plasticity, Vol: 110, Pages: 38-56, ISSN: 0749-6419

Dislocation interactions at the crack nucleation site were investigated in near-alpha titanium alloy Ti-6242Si subjected to low cycle fatigue. Cyclic plastic strain in the alloy resulted in dislocation pile-ups in the primary alpha grains, nucleated at the boundaries between the primary alpha and the two-phase regions. These two phase regions provided a barrier to slip transfer between primary alpha grains. We suggest that crack nucleation occurred near the basal plane of primary alpha grains by the subsurface double-ended pile-up mechanism first conceived by Tanaka and Mura. Superjogs on the basal dislocations were observed near the crack nucleation location. The two phase regions showed direct transmission of dislocations between secondary alpha plates, transmitted through the beta ligaments as , which then decompose into dislocation networks in the beta. The beta ligaments themselves do not appear to form an especially impenetrable barrier to slip, in agreement with the micropillar and crystal plasticity investigations of Zhang et al.

Journal article

Wang Y, Liu B, Yan K, Wang M, Kabra S, Chiu Y-L, Dye D, Lee PD, Liu Y, Cai Bet al., 2018, Probing deformation mechanisms of a FeCoCrNi high-entropy alloy at 293 and 77 K using in situ neutron diffraction, Acta Materialia, Vol: 154, Pages: 79-89, ISSN: 1359-6454

The deformation responses at 77 and 293 K of a FeCoNiCr high-entropy alloy, produced by a powder metallurgy route, are investigated using in situ neutron diffraction and correlative transmission electron microscopy. The strength and ductility of the alloy are significant improved at cryogenic temperatures. The true ultimate tensile strength and total elongation increased from 980 MPa to 45% at 293 K to 1725 MPa and 55% at 77 K, respectively. The evolutions of lattice strain, stacking fault probability, and dislocation density were determined via quantifying the in situ neutron diffraction measurements. The results demonstrate that the alloy has a much higher tendency to form stacking faults and mechanical twins as the deformation temperature drops, which is due to the decrease of stacking fault energy (estimated to be 32.5 mJ/m2 and 13 mJ/m2 at 293 and 77 K, respectively). The increased volume faction of nano-twins and twin-twin intersections, formed during cryogenic temperature deformation, has been confirmed by transmission electron microscopy analysis. The enhanced strength and ductility at cryogenic temperatures can be attributed to the increased density of dislocations and nano-twins. The findings provide a fundamental understanding of underlying governing mechanistic mechanisms for the twinning induced plasticity in high entropy alloys, paving the way for the development of new alloys with superb resistance to cryogenic environments.

Journal article

Gao J, Huang Y, Guan D, Knowles AJ, Ma L, Dye D, Rainforth WMet 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.

Journal article

Gao J, Nutter J, Liu X, Guan D, Huang Y, Dye D, Rainforth WMet al., 2018, Segregation mediated heterogeneous structure in a metastable β titanium alloy with a superior combination of strength and ductility, Scientific Reports, Vol: 8, ISSN: 2045-2322

In β titanium alloys, the β stabilizers segregate easily and considerable effort has been devoted to alleviate/eliminate the segregation. In this work, instead of addressing the segregation problems, the segregation was utilized to develop a novel microstructure consisting of a nanometre-grained duplex (α+β) structure and micrometre scale β phase with superior mechanical properties. An as-cast Ti-9Mo-6W alloy exhibited segregation of Mo and W at the tens of micrometre scale. This was subjected to cold rolling and flash annealing at 820 oC for 2 and 5 mins. The solidification segregation of Mo and W leads to a locally different microstructure after cold rolling (i.e., nanostructured β phase in the regions rich in Mo and W and plate-like martensite and β phase in regions relatively poor in Mo and W), which play a decisive role in the formation of the heterogeneous microstructure. Tensile tests showed that this alloy exhibited a superior combination of high yield strength (692 MPa), high tensile strength (1115 MPa), high work hardening rate and large uniform elongation (33.5%). More importantly, the new technique proposed in this work could be potentially applicable to other alloy systems with segregation problems.

Journal article

Chang Y, Breen AJ, Tarzimoghadam Z, Kürnsteiner P, Gardner H, Ackerman A, Radecka A, Bagot PAJ, Lu W, Li T, Jägle EA, Herbig M, Stephenson LT, Moody MP, Rugg D, Dye D, Ponge D, Raabe D, Gault Bet al., 2018, Characterizing solute hydrogen and hydrides in pure and alloyed titanium at the atomic scale, Acta Materialia, Vol: 150, Pages: 273-280, ISSN: 1359-6454

Ti has a high affinity for hydrogen and are typical hydride formers. Ti-hydride are brittle phases which probably cause premature failure of Ti-alloys. Here, we used atom probe tomography and electron microscopy to investigate the hydrogen distribution in a set of specimens of commercially pure Ti, model and commercial Ti-alloys. Although likely partly introduced during specimen preparation with the focused-ion beam, we show formation of Ti-hydrides along α grain boundaries and α/β phase boundaries in commercial pure Ti and α+β binary model alloys. No hydrides are observed in the αphase in alloys with Al addition or quenched-in Mo supersaturation.

Journal article

Joseph S, Lindley TC, Dye D, Saunders EAet al., 2018, The mechanisms of hot salt stress corrosion cracking in titanium alloy Ti-6Al-2Sn-4Zr-6Mo, Corrosion Science, Vol: 134, Pages: 169-178, ISSN: 0010-938X

Hot salt stress corrosion cracking in Ti 6246 alloy has been investigated to elucidate the chemical mechanisms that occur. Cracking was found to initiate beneath salt particles in the presence of oxidation. The observed transgranular fracture was suspected to be due to hydrogen charging; XRD and high-resolution transmission electron microscopy detected the presence of hydrides that were precipitated on cooling. SEM-EDS showed oxygen enrichment near salt particles, alongside chlorine and sodium. Aluminium and zirconium were also involved in the oxidation reactions. The role of intermediate corrosion products such as Na2TiO3, Al2O3, ZrO2, TiCl2 and TiH are discussed.

Journal article

Coakley J, Radecka AE, Dye D, Bagot PAJ, Martin TL, Prosa TJ, Chen Y, Stone HJ, Seidman DN, Isheim Det al., 2018, Characterizing nanoscale precipitation in a titanium alloy by laser-assisted atom probe tomography., Materials Characterization, Vol: 141, Pages: 129-138, ISSN: 1044-5803

Atom-probe tomography was performed on the metastable β-Ti alloy, Ti-5Al-5Mo-5V-3Cr wt% (Ti-5553), aged at 300 °C for 0 to 8 h, to precipitate the embrittling isothermal ω phase. Accurate precipitate quantification requires monitoring and controlling suitable charge-state ratios in the mass spectrum, which in turn are closely related to the laser pulse energy used. High ultraviolet laser pulse energies result in significant complex molecular ion formation during field-evaporation, causing mass spectral peak overlaps that inherently complicate data analyses. Observations and accurate quantification of the ω-phase under such conditions are difficult. The effect is minimized or eliminated by using smaller laser pulse energies. With a small laser pulse energy, Ti-rich and solute depleted precipitates of the isothermal ω phase with an oxygen enriched interface are observed as early as after 1 h aging time utilizing the LEAP 5000X S (77% detection efficiency). We note that these precipitates were not detected below a 2 h aging time with the LEAP 4000X Si (58% detection efficiency). The results are compared to the archival literature. The Al concentration in the matrix/precipitate interfacial region increases during aging. Nucleation of the α-phase at longer aging times may be facilitated by the O and Al enrichment at the matrix/precipitate interface (both strong α-stabilisers). The kinetics and compositional trajectory of the ω-phase with aging time are quantified, facilitating direct correlation of the APT data to previously published mechanical testing.

Journal article

Bagot PAJ, Radecka A, Magyar AP, Bell DC, Smith GDW, Moody MP, Dye D, Rugg Det al., 2018, The effect of oxidation on the subsurface microstructure of a Ti-6Al-4V alloy, Scripta Materialia, Vol: 148, Pages: 24-28, ISSN: 1359-6462

Atom Probe Tomography (APT) and Transmission Electron Microscopy (TEM) are combined for examining α-case formation in Ti-6Al-4V, generated by air exposure at 800 °C. Below the oxide surface, the microstructure separates into a nanoscale mixture of the α-Ti and α2 Ti3Al phases, of compositions Ti70-O30 and Ti65-O10-Al20-V5 respectively. The α2 phase exists either as bands or as nanoscale spherical precipitates. Nitrogen also penetrates the surface, but to a lesser extent, while vanadium partitions to α2 or in distinctly separate phases. The results demonstrate that oxygen stimulates precipitation of α2, helping to explain embrittlement produced in the O-enriched layer beneath the oxide.

Journal article

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