Publications
203 results found
Bagot PAJ, Radecka A, Magyar AP, et 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.
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.
Joseph S, Bantounas, Lindley, et al., 2018, Slip transfer and deformation structures resulting from the low cycle fatigue of near-alpha titanium alloy Ti-6242Si, International Journal of Plasticity, Vol: 100, Pages: 90-103, ISSN: 0749-6419
Near-alpha titanium alloy Ti6242Si, widely used in aero-engine compressor discs, was subjected to low cycle fatigue loading at room temperature. Fracture initiated by facet formation, followed by striated fatigue crack growth prior to final failure. Deformation occurred primarily by planar slip, localized into slip bands in the primary alpha. Within soft-oriented grains in a microtextured region, pile-up of a slip band within one grain resulted in the direct transfer of slip into an adjacent similarly oriented grain. In contrast, pile up of dislocations in a soft grain with a ‘hard’ oriented neighbour resulted in the activation of few non-connected dislocations in the hard grain, with <a>-type dislocations being activated and the observation of cross-slip. Whilst a high density of dislocations was present from precipitation of secondary alpha in the retained beta ligaments, a little dislocation interaction was observed between the transformed beta and the primary alpha grains.
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, 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, 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.
Chapman TP, Dye D, Rugg D, 2017, Hydrogen in Ti and Zr alloys: industrial perspective, failure modes and mechanistic understanding, Philosophical transactions. Physical sciences and engineering, Vol: 375, ISSN: 0962-8428
Titanium is widely used in demanding applications, such as in aerospace. Its strength-to-weight ratio andcorrosion resistance make it well suited to highly stressed rotating components. Zirconium has a no lesscritical application where its low neutron capture cross section and good corrosion resistance in hot water andsteam make it well suited to reactor core use, including fuel cladding and structures. The similar metallurgicalbehaviour of these alloy systems make it alluring to compare and contrast their behaviour. This is rarelyundertaken, mostly because the industrial and academic communities studying these alloys have littleoverlap. The similarities with respect to hydrogen are remarkable, albeit potentially unsurprising, and so thispaper aims to provide an overview of the role hydrogen has to play through the material life cycle. Thisincludes the relationship between alloy design and manufacturing process windows, the role of hydrogen indegradation and failure mechanisms and some of the underpinning metallurgy. The potential role ofsome advanced experimental and modelling techniques will also be explored to give a tentative view ofpotential for advances in this field in the next decade or so.
Coakley J, Ma D, Frost M, et al., 2017, Lattice strain evolution and load partitioning during creep of a Ni-base superalloy single crystal with rafted gamma prime microstructure, Acta Materialia, Vol: 135, Pages: 77-87, ISSN: 1359-6454
In-situ neutron diffraction measurements were performed on monocrystalline samples of the Ni-based superalloy CMSX-4 during N-type γ′ raft formation under the tensile creep conditions of 1150 °C/100 MPa, and subsequently on a rafted sample under the low temperature/high stress creep conditions of 715 °C/825 MPa. During 1150 °C/100 MPa creep, the γ′ volume fraction decreased from ∼70% to ∼50%, the lattice parameter misfit was partly relieved, and the load was transferred from the creeping γ matrix to the γ′ precipitates. On cooling back to room temperature, a fine distribution of γ′ precipitates formed in the γ channels, and these precipitates were present in the 715 °C/825 MPa creep regime. Under low temperature/high stress creep, the alloy with rafted γ′ microstructure exhibited superior creep strength to the cuboidal γ′ microstructure produced following a standard heat-treatment. A lengthy creep incubation period was observed, believed to be associated with {111} dislocations hindering propagation of {111} dislocations. Following the creep incubation period, extensive macroscopic creep strain accumulated during primary creep as the γ phase yielded. Finally, the diffraction data suggest a loss of precipitate/matrix coherency in the (0k0) interfaces as creep strain accumulated.
Tong VS, Joseph S, Ackerman A, et al., 2017, Using transmission Kikuchi diffraction to characterise α variants in an α+β titanium alloy, Journal of Microscopy, Vol: 267, Pages: 318-329, ISSN: 1365-2818
Two phase titanium alloys are important for high-performance engineering components, such as aeroengine discs. The microstructures of these alloys are tailored during thermomechanical processing to precisely control phase fractions, morphology and crystallographic orientations. In bimodal two phase (α + β) Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) alloys there are often three microstructural lengthscales to consider: large (∼10 μm) equiaxed primary α; >200 nm thick plate α with a basketweave morphology; and very fine scaled (<50 nm plate thickness) secondary α that grows between the larger α plates surrounded by retained β. In this work, we utilise high spatial resolution transmission Kikuchi diffraction (TKD, also known as transmission-based electron backscatter diffraction, t-EBSD) and scanning electron microscopy (SEM)-based forward scattering electron imaging to resolve the structures and orientations of basketweave and secondary α in Ti-6242. We analyse the α variants formed within one prior β grain, and test whether existing theories of habit planes of the phase transformation are upheld. Our analysis is important in understanding both the thermomechanical processing strategy of new bimodal two-phase titanium alloys, as well as the ultimate performance of these alloys in complex loading regimes such as dwell fatigue. Our paper champions the significant increase in spatial resolution afforded using transmission techniques, combined with the ease of SEM-based analysis using conventional electron backscatter diffraction (EBSD) systems and forescatter detector (FSD) imaging, to study the nanostructure of real-world engineering alloys.
Rahman KM, Vorontsov VA, Flitcroft SM, et al., 2017, A high strength Ti-SiC metal matrix composite, Advanced Engineering Materials, Vol: 19, ISSN: 1527-2648
A SiC reinforced Ti-5Al-5Mo-5V-3Cr matrix metal matrix composite is developed. Monolithic blocks of alloy are hot rolled via pack rolling to produce foils for MMC panel fabrication. These are consolidated using hot isostatic pressing and solution treated and aged for optimum strength. The panels exhibited a strength of 2 GPa in tension and 3.5 GPa in compression, compared to the aerospace steel 300M, which has a tensile strength of 1.69 GPa. The fatigue performance of the material exceeded that of MMCs developed using Ti-21S or Ti-6Al-4V matrices. Finally, the reaction zone between the SiC and matrix is examined, revealing the presence of TiC.
Smith D, Joris OPJ, Sankaran A, et al., 2017, On the high- pressure phase stability and elastic properties of beta-titanium alloys, Journal of Physics-Condensed Matter, Vol: 29, ISSN: 1361-648X
We have studied the compressibility and stability of different β-titanium alloys at high pressure, including binary Ti–Mo, Ti–24Nb–4Zr–8Sn (Ti2448) and Ti–36Nb–2Ta–0.3O (gum metal). We observed stability of the β phase in these alloys to 40 GPa, well into the ω phase region in the P–T diagram of pure titanium. Gum metal was pressurised above 70 GPa and forms a phase with a crystal structure similar to the η phase of pure Ti. The bulk moduli determined for the different alloys range from 97 ± 3 GPa (Ti2448) to 124 ± 6 GPa (Ti–16.8Mo–0.13O).
Azeem MA, Lee PD, Phillion AB, et al., 2017, Revealing dendritic pattern formation in Ni, Fe and Co alloys using synchrotron tomography, Acta Materialia, Vol: 128, Pages: 241-248, ISSN: 1873-2453
The microstructural patterns formed during liquid to solid phase transformations control the properties of a wide range of materials. We developed a novel methodology that allows in situ quantification of the microstructures formed during solidification of high temperature advanced alloys. The patterns formed are captured in 4D (3D plus time) using a methodology which exploits three separate advances: a bespoke high temperature environment cell; the development of high X-ray contrast alloys; and a novel environmental encapsulation system. This methodology is demonstrated on Ni, Fe, and Co advanced alloy systems, revealing dendritic pattern formation. We present detailed quantification of microstructural pattern evolution in a novel high attenuation contrast Co-Hf alloy, including microstructural patterning and dendrite tip velocity. The images are quantified to provide 4D experimental data of growth and coarsening mechanisms in Co alloys, which are used for a range of applications from energy to aerospace.
Coakley J, Song B-S, Dye D, et al., 2017, Isothermal omega kinetics in beta-titanium alloys, Philosophical Magazine Letters, Vol: 97, Pages: 83-91, ISSN: 0950-0839
Small angle X-ray and neutron scattering (SAXS/SANS) provide a non-localised quantitative analysis of nanoscaleωprecipitates in Ti-7Mo at.% and Gum metal, a Ti-Nb based alloy. The nucleation and coarsening kinetics are more rapidin the Ti-Nb alloy than the Ti-Mo alloy, presumably as Nb diffuses more rapidly than Mo in Ti. Therefore Mo-basedβ-Ti alloys are more stable duringωageing than Nb-basedβ-Ti alloys. To the authors’ knowledge, this represents thefirst publication quantifyingωsize, volume fraction and element partitioning in Gum metal.
Azeem MA, Jones NG, Raghunathan SL, et al., 2016, Functional stability of a ferromagnetic polycrystalline Ni2MnGa high temperature shape memory alloy, Scripta Materialia, Vol: 130, Pages: 274-277, ISSN: 1359-6462
Electrocaloric Ni2MnGa is of interest for solid state refrigeration applications, as well as a high temperature thermal shape memoryalloy. Here, polycrystalline Ni54Mn25Ga21is examined using in situ synchrotron X-ray diraction. The initial martensite (Mf) andaustenite (Af) finish temperatures were found to be 232 C and 298 C respectively.Mfwas observed to decline by 8 C/cycleandAfincreased by 1 C/cycle. Both below and surprisingly, above the Curie temperature, the application of an e.m.f. wasfound to aect the lattice parameters measured. A change in the thermal expansion of the two phases was found around the Curietemperature.
Ismail FB, Vorontsov VA, Lindley TC, et al., 2016, Alloying effects on oxidation mechanisms in polycrystalline Co-Ni base superalloys, Corrosion Science, Vol: 116, Pages: 44-52, ISSN: 0010-938X
Oxidation mechanisms in polycrystalline Co-Ni-Cr-Al-W-Ta alloys were investigated using 16O/18O isotopic tracer anal-ysis in the focused ion-beam secondary ion mass spectrometer (FIB-SIMS). It was found that Al additions favour theformation of a continuous alumina-rich layer, that Cr indirectly improved oxidation resistance and that increasing theCo fraction resulted in poorer oxidation performance. In the alloy containing 15 at.% Cr and 10 at.% Al, the outer scaleformed after 200 h oxidation at 800 ⁰C comprised oxides less than 1 µm thick. It is concluded that protective oxide scalescan be formed in Co-Ni base superalloys.
Coakley J, Isheim D, Radecka A, et al., 2016, Microstructural evolution in a superelastic metastable beta-Ti alloy, Scripta Materialia, Vol: 128, Pages: 87-90, ISSN: 1359-6462
The microstructural evolution of Ti-24Nb-4Zr-8Sn wt.% during low-temperature ageing is examined by atom-probe tomography(APT) and X-ray diffraction (XRD). This ageing is deleterious to the desirable mechanical properties, suchas ultra-low elastic modulus and superelasticity. Initially, the cold-rolled alloy possesses a martensitic α00-precipitate/β-matrix microstructure. On ageing, Ti-rich/solute-lean precipitates grow in linear arrangements, which are likely associatedwith dislocations. Additionally, the composition and number density of Nb-rich domains (which are associatedwith superelasticity) are quantified for the first time. The domains are unstable, but decrease in number density duringageing, causing the deterioration in mechanical properties.
coakley J, radecka A, Dye D, et al., 2016, Isothermal omega formation and evolution in the Beta-Ti alloy Ti-5Al-5Mo-5V-3Cr, Philosophical Magazine Letters, Vol: 96, Pages: 416-424, ISSN: 0950-0839
phase of Ti-5Al-5Mo-5V-3Cr wt.% is formed within a heat treatment at and identified by atom probe tomography as Ti-rich/solute lean precipitates. The composition and size remain essentially constant during ageing, although the volume fraction increases to 9.5% after ageing for 8 h. This is consistent with an ongoing transformation process of athermal to isothermal . The / interface becomes enriched with oxygen. This may be of significance as oxygen strongly stabilizes the phase, and the / interface has previously been suggested as the nucleation site for subsequent formation.
Weekes HE, Lindley TC, Jones NG, et al., 2016, Hydride reorientation in Zircaloy-4 examined by in situ synchrotron X-ray diffraction, Journal of Nuclear Materials, Vol: 478, Pages: 32-41, ISSN: 1873-4820
The phenomenon of stress-reorientation has been investigated using in situ X-ray diffraction during the thermomechanicalcycling of hydrided Zircaloy-4 tensile specimens. Results have shown that loading along a sample’s transverse direction(TD) leads to a greater degree of hydride reorientation when compared to rolling direction (RD)-aligned samples. Theelastic lattice micro-strains associated with radially oriented hydrides have been revealed to be greater than those orientedcircumferentially, a consequence of strain accommodation. Evidence of hydride redistribution after cycling, to α-Zr grainsoriented in a more favourable orientation when under an applied stress, has also been observed and its behaviour hasbeen found to be highly dependent on the loading axis. Finally, thermomechanical loading across multiple cycles hasbeen shown to reduce the difference in terminal solid solubility of hydrogen during dissolution (TSSD,H) and precipitation(TSSP,H).
vorontsov VA, Barnard JS, Rahman KM, et al., 2016, Coarsening behaviour and interfacial structure of γ′ precipitates in Co-Al-W based superalloys, Acta Materialia, Vol: 120, Pages: 14-23, ISSN: 1359-6454
This work discusses the effects of alloying on the coarsening behaviour of the L12 ordered γ′ phase and the structure of the γ/γ′ interfaces in three Co-Al-W base superalloys aged at ∼90 °C below the respective solvus temperatures: Co-7Al-7W, Co-10Al-5W-2Ta and Co-7Al-7W-20Ni (at.%). The coarsening kinetics are adequately characterised by the classical Lifshitz-Slyozov-Wagner model for Ostwald ripening. Co-7Al-7W exhibited much slower coarsening than its quaternary derivatives. Alloying can be exploited to modify the coarsening kinetics either by increasing the solvus temperature by adding tantalum, or by adding nickel to shift the rate controlling mechanism towards dependence on the diffusion of aluminium rather than tungsten. Lattice resolution STEM imaging was used to measure the widths of the order-disorder (structural) and Z-contrast (compositional) gradients across the γ/γ′ interfaces. Similarly to nickel base superalloys, the compositional gradient was found to be wider than the structural. Co-7Al-7W-20Ni had much wider interface gradients than Co-7Al-7W and Co-10Al-5W-2Ta, which suggests that its γ′ phase stoichiometry is less constrained. A possible correlation between temperature and misfit normalised r vs. t1/3 coarsening rate coefficients and the structural gradient width has also been identified, whereby alloys with wider interfaces exhibit faster coarsening rates.
Swinburne TD, Glavicic MG, Rahman KM, et al., 2016, Picosecond dynamics of a shock-driven displacive phase transformation in Zr, Physical Review B, Condensed Matter, Vol: 93, ISSN: 0163-1829
High pressure solid state transformations at high strain rates are usually observed after the fact,either during static holding or after unloading, or inferred from interferometry measurements of thesample surface. The emergence of femtosecond X-ray diffraction techniques provides insight intothe dynamics of short-timescale events such as shocks. We report laser pump-probe experiments ofthe response of Zr to laser driven shocks over the first few nanoseconds of the shock event, enablingthe α → ω transition and orientation relationship to be observed in real time with picosecondresolution. A clear orientation relationship of (10¯10)α||(10¯11)ω is found, in conflict with ω → αannealing experiments in zirconium and the two α → ω pathways proposed for titanium.
Radecka A, Bagot PAJ, Martin TL, et al., 2016, The formation of ordered clusters in Ti-7Al and Ti-6Al-4V, Acta Materialia, Vol: 112, Pages: 141-149, ISSN: 1873-2453
Ti-Al alloys can suffer from a chemical decomposition on ageing around 500!C or on air cooling.At long ageing times this results in the formation of a2 (Ti3Al) precipitates. At reduced timesor elevated temperatures, diffuse electron or neutron diffraction peaks can be observed, whichhas been termed ‘short range ordering’ (SRO). Here, we present correlative transmission electronmicroscopy (TEM) and atom probe tomography (APT) results showing that the reaction proceedsthrough the formation of ordered Al-rich precipitate clusters. Notably, Al-Al clustering could beobserved well before the appearance of distinct precipitates in the TEM. In addition, the Vcontaininga phase of Ti-6Al-4V formed ordered clusters much faster than in binary Ti-7Al.This implies that the ternary addition of b stabilisers exacerbates the problem of a2 precipitateformation in commercial dual phase titanium alloys.
Martin TL, Radecka A, Sun L, et al., 2016, Insights into microstructural interfaces in aerospace alloys characterised by atom probe tomography, Materials Science and Technology, Vol: 32, Pages: 232-241, ISSN: 1316-2012
Atom probe tomography (APT) is becoming increasingly applied to understand the relationship between the structure and composition of new alloys at the micro- and nanoscale and their physical properties. Here, we use APT datasets from two modern aerospace alloys to highlight the detailed information available from APT analysis, along with potential pitfalls that can affect data interpretation. The interface between two phases in a Ti–6Al–4V alloy is used to illustrate the importance of parameter choice when using proximity histograms or concentration profiles to characterise interfacial chemistry. The higher number density of precipitates and large number of constituent elements in a maraging steel (F1E) present additional challenges such as peak overlaps that vary across the dataset, along with inhomogeneous interface chemistries.
Bhowmik A, Dolbnya I, Britton TB, et al., 2016, Using coupled micropillar compression and micro-Laue diffraction to investigate deformation mechanisms in a complex metallic alloy Al13Co4, Applied Physics Letters, Vol: 108, ISSN: 0003-6951
In this study, we have used in-situ micro-Laue diffraction combined with micropillar compression of focused ion beam milled Al13Co4 complex metallic alloy to investigate the evolution of deformation in Al13Co4. Streaking of the Laue spots shows that the onset of plastic flow occurs at stresses as low as 0.8 GPa, although macroscopic yield only becomes apparent at 2 GPa. The measured misorientations, obtained from peak splitting, enable the geometrically necessary dislocation density to be estimated as 1.1 1013 m-2.
Radecka A, Coakley J, Vorontsov VA, et al., 2016, Precipitation of the ordered α2 phase in a near-α titanium alloy, Scripta Materialia, Vol: 117, Pages: 81-85, ISSN: 1359-6462
Precipitate evolution in a near-α alloy was studied using transmission electron microscopy (TEM) and correlative atom probetomography (APT) after ageing at 550–700 ◦C for times up to 28 days. It is found that precipitation occurs much faster and ismore prolific in samples heat treated at higher temperatures. Particles were spherical after ageing at 550 ◦C, while after ageing at700 ◦C they become ellipsoids with the major axis lying close to the [0001] direction. At longer ageing times, the α2 precipitateswere found to contain greater amounts of Sn+Si, indicating that Sn and Si are stronger Ti3(Al,Sn,Si) formers than Al.
Tympel PO, Lindley TC, Saunders EA, et al., 2016, Influence of complex LCF and dwell load regimes on fatigue of Ti–6Al–4V, Acta Materialia, Vol: 103, Pages: 77-88, ISSN: 1359-6454
Real components are usually subjected to variable amplitude fatigue, and yet the deformation micromechanisms that occur due to such load changes have barely been the subject of study. Here, unidirectionally rolled equiaxed Ti–6Al–4V plate was subjected to mixed dwell and variable amplitude low cycle fatigue (LCF), with the finding that overloads near the yield stress were found to retard subsequent fatigue crack growth, whilst elastic underloads were found to accelerate subsequent growth. Dwell intervals were found to be especially damaging, to a far greater extent than either dwell or LCF alone. Dwell facets were found to initiate subsurface and to be smoother than LCF facets, but were otherwise similar in orientation (∼30° to the loading axis) and crystallographic plane, 2–13° from (0002). However, no alteration of the slip bands underlying striations was observed at the point of load changes using TEM. In failure investigation, striation counting is an important tool; the loading changes used were not found to affect the number of striations formed. Dislocation networks were found between similarly oriented grains in the as-received material, which disintegrated under dwell loading and at high stresses.
Isheim D, Coakley J, Radecka A, et al., 2016, Atom-probe tomography: Detection efficiency and resolution of nanometer-scale precipitates in a Ti-5553 alloy, Microscopy and Microanalysis, Vol: 22, Pages: 702-703, ISSN: 1431-9276
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Chapman TP, Vorontsov VA, Sankaran A, et al., 2016, The Dislocation Mechanism of Stress Corrosion Embrittlement in Ti-6Al-2Sn-4Zr-6Mo, Metallurgical and Materials Transactions A-Physical Metallurgy and Materials Science, Vol: 47, Pages: 282-292, ISSN: 1543-1940
An observation of the dislocation mechanisms operating below a naturally initiated hot-salt stress corrosion crack is presented, suggesting how hydrogen may contribute to embrittlement. The observations are consistent with the hydrogen-enhanced localized plasticity mechanism. Dislocation activity has been investigated through post-mortem examination of thin foils prepared by focused ion beam milling, lifted directly from the fracture surface. The results are in agreement with the existing studies, suggesting that hydrogen enhances dislocation motion. It is found that the presence of hydrogen in (solid) solution results in dislocation motion on slip systems that would not normally be expected to be active. A rationale is presented regarding the interplay of dislocation density and the hydrogen diffusion length.
Coakley JC, Rahman KM, Vorontsov VA, et al., 2015, Effect of precipitation on mechanical properties in the beta-Ti alloy Ti-24Nb-4Zr-8Sn, Materials Science and Engineering A - Structural Materials Properties Microstructure and Processing, Vol: 655, Pages: 399-407, ISSN: 0921-5093
Tensile testing and cyclic tensile loading measurements were performed on heat-treated samples of annealed Ti-2448and cold-rolled Ti-2448. Quenching from above the β-transus produces an alloy that is highly superelastic, has ultra-lowelastic modulus (10−25 GPa) and exhibits hysteresis on loading-unloading cycles. On repeated cycling the strain energyabsorbed in each cycle decreases. Annealed Ti-2448 exhibits a stable hysteresis loop. Peaks from the α00 phase areobserved in X-ray diffraction (XRD) patterns, thus the material is quite lean in β-stabilising additions. The alloy isshown to be highly unstable when heat-treated. A combination of small angle X-ray scattering (SAXS), transmissionelectron microscopy (TEM) and X-ray diffraction (XRD) was employed to relate the thermally induced microstructuralevolution to the change in mechanical properties. A heat-treatment of 80 ◦C to the cold-rolled material precipitated theω phase, causing embrittlement. Increasing the ageing temperature from 80 to 300 ◦C increased the stiffness, made theelastic regime more linear, and further embrittled the alloy. The low temperature heat-treatments precipitate both ωand α00 phases. A higher temperature ageing treatment at 450 ◦C increased the yield strength to over 1GPa and causedembrittlement, indicating co-precipitation of α and ω phases.
Chapman TP, Kareh KM, Knop M, et al., 2015, Characterisation of short fatigue cracks in titanium alloy IMI 834 using X-ray microtomography, Acta Materialia, Vol: 99, Pages: 49-62, ISSN: 1359-6454
A first attempt at the three-dimensional evaluation of naturally initiated surface connected and internal fatigue cracks is presented. Fatigue crack initiation and growth in air and vacuum environments have been investigated through X-ray microtomography in air and vacuum environments at elevated temperatures (350 °C), accompanied by post-mortem electron microscopy of the fracture surfaces. In vacuum (<10⁻⁵ mbar), multiple internal and surface-connected crack initiation was observed, but only the surface-connected cracks grew. In contrast, fewer cracks formed in air, these were mostly surface-connected and all were observed to grow. In all instances the initiation features were associated with globular primary α. An improved fatigue life was found in vacuum, which was mostly a consequence of delayed initiation, but was also due to slower fatigue crack propagation. The non-propagation of internal cracks was taken to imply that even the good laboratory vacuum obtained here was insufficient to simulate the conditions obtained for an internal crack in a component. The crack shape evolved towards a semi-circular shape a/c=1 in air during fatigue crack growth, whilst the vacuum cracks remained semi-elliptical (a/c≃1.4). This was taken to imply that oxide-induced crack closure played a role in fatigue crack growth in air.
Coakley J, Vorontsov VA, Jones NG, et al., 2015, Precipitation processes in the Beta-Titanium alloy Ti-5Al-5Mo-5V-3Cr, JOURNAL OF ALLOYS AND COMPOUNDS, Vol: 646, Pages: 946-953, ISSN: 0925-8388
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