Publications
32 results found
Tjandra J, Ranjan A, Ackerman AK, et al., 2023, Effect of NaCl and Na2SO4 on low temperature corrosion of vapour- and pack-aluminide coated single crystal turbine blade alloys CMSX-4 and RR3010, Metallurgical and Materials Transactions A, Vol: 54, Pages: 3286-3299, ISSN: 1073-5623
The current work presents a systematic study of two alloy compositions (RR3010 and CMSX-4) and two types of coatings: inward grown (pack) and outward grown (vapour) deposited aluminides, exposed to 98Na2SO4–2NaCl mixture. Grit blasting was used on some of the samples, prior to coating, to mimic in-service procedures and remove oxides from the surface prior to coating. Two-point bend tests were then performed on the coated samples, with and without applied salt at 550 °C for 100 hours. Samples were pre-strained at 0.6 pct strain to deliberately pre-crack the coating and then strained at 0.3 pct for the heat treatment. Exposure to 98Na2SO4–2NaCl under applied stress of vapour-aluminide coated samples of both alloys, revealed significant coating damage in the form of secondary cracks in the intermetallic-rich inter-diffusion zone, although only CMSX-4 exhibited cracks propagating further into the bulk alloy while RR3010 proved more resistant. The pack-aluminide coating proved more protective for both alloys, with cracks propagating only into the coating and never into the underlying alloy. In addition, grit blasting proved beneficial in reducing spallation and cracking for both types of coating. The findings were used to propose a mechanism based on thermodynamic reactions, to explain the crack width changes through the formation of volatile AlCl3 in the cracks.
Pedrazzini S, Pek ME, Ackerman AK, et al., 2023, Effect of substrate bed temperature on solute segregation and mechanical properties in Ti–6Al–4V produced by laser powder bed fusion, Metallurgical and Materials Transactions A, Vol: 54, Pages: 3069-3085, ISSN: 1073-5623
Titanium alloys are particularly sensitive to temperature during additive manufacturing processes, due to their dual phase microstructure and sensitivity to oxygen uptake. In this paper, laser powder bed fusion (LPBF) was used in conjunction with a heated substrate bed at 100 °C, 570 °C and 770 °C to produce specimens of Ti–6Al–4V, to investigate the change in mechanical properties and segregation of alloying elements. An initial increase in ductility was observed when increasing the temperature from 100 °C to 570 °C, followed by a significant loss in ductility when samples were produced at 770 °C. A suite of multi-scale characterisation techniques revealed that the as-printed microstructure was drastically different across the range of temperatures. At 100 °C, α + α′ phases were identified. Deformation twinning was extensively observed in the a phase, with Al and V segregating at the twin interfaces. At 570 °C (the most ductile sample), α′, α and nano-particles of β were observed, with networks of entangled dislocations showing V segregation. At 770 °C, no martensitic α′ was identified. The microstructure was an α + β microstructure and an increased volume fraction of tangled dislocations with localised V segregation. Thermodynamic modelling based on the Gibbs-free energy of formation showed that the increased V concentration at dislocations was insufficient to locally nucleate β phase. However, b-phase nucleation at grain boundaries (not dislocations) caused pinning of grain boundaries, impeding slip and leading to a reduction in ductility. It is likely that the increased O-content within specimens printed at increased temperatures also played a key role in high-temperature embrittlement. Building operations are therefore best performed below sub-transus temperatures, to encourage the growth of strengthening phases via so
Kei SHS, van Haaften WM, Ben Britton T, et al., 2023, A Review of the Factors That Can Increase the Risk of Sulfide Stress Cracking in Thermomechanical Controlled Processed Pipeline Steels, ADVANCED ENGINEERING MATERIALS, ISSN: 1438-1656
Tjandra J, Alabort E, Barba D, et al., 2023, Corrosion, fatigue and wear of additively manufactured Ti alloys for orthopaedic implants, MATERIALS SCIENCE AND TECHNOLOGY, ISSN: 0267-0836
Kim S-H, Shin K, Zhou X, et al., 2023, Atom probe analysis of BaTiO3 enabled by metallic shielding, Scripta Materialia, Vol: 229, Pages: 1-5, ISSN: 1359-6462
Atom probe tomography has been raising in prominence as a microscopy and microanalysis technique to gain sub-nanoscale information from technologically-relevant materials. However, the analysis of some functional ceramics, particularly perovskites, has remained challenging with extremely low yield and success rate. This seems particularly problematic for materials with high piezoelectric activity, which may be difficult to express at the low temperatures necessary for satisfactory atom probe analysis. Here, we demonstrate the analysis of commercial BaTiO3 particles embedded in a metallic matrix. Density-functional theory shows that a metallic coating prevents charge penetration of the electrostatic field, and thereby suppresses the associated volume change linked to the piezoelectric effect.
Rodenkirchen C, Ackerman AK, Mignanelli PM, et al., 2023, Effect of Alloying on the Microstructure, Phase Stability, Hardness, and Partitioning Behavior of a New Dual-Superlattice Nickel-Based Superalloy, METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE, Vol: 54, Pages: 1902-1923, ISSN: 1073-5623
Pek ME, Ackerman AK, Appleton M, et al., 2023, Development of a novel, impurity-scavenging, corrosion-resistant coating for Ni-based superalloy CMSX-4, Oxidation of Metals, Vol: 99, Pages: 3-13, ISSN: 0030-770X
Sulfur, a common impurity arising from atmospheric and environmental contamination, is highly corrosive and detrimental to the lifespan of nickel superalloys in jet engines. However, sulfur-scavenging coatings have yet to be explored. Our study presents the successful development of a stable, uniform, impurity-scavenging Ni-Mn coating on Ni-based superalloy CMSX-4, through electroplating. The coating was characterised via combined scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy. An optimal coating thickness of > 600 nm was deposited. The coated alloy was exposed to corrosive salt mixture 98% Na2SO4–2% NaCl at 550 °C for 100 h, mimicking engine exposure conditions, thereby proving that the coating successfully trapped sulfur and prevented its diffusion into an underlying alloy. This work presents a promising development for the prevention of sulfur-induced corrosion in industrial setting such as gas turbine engine, where the effects of sulfur diffusion into the bulk alloy could lead to premature failure.
Gasparrini C, Badocco D, Di Pace L, et al., 2022, Water Chemistry in Fusion Cooling Systems: Borated Water for DTT Vacuum Vessel, IEEE TRANSACTIONS ON PLASMA SCIENCE, Vol: 50, Pages: 4287-4291, ISSN: 0093-3813
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Rodenkirchen C, Appleton M, Ryan MP, et al., 2022, A review on atom probe and correlative microscopy studies of corrosion in nickel-based superalloys, Materials Research Society (MRS) Bulletin, Vol: 47, Pages: 706-717, ISSN: 0883-7694
This article discusses challenges faced in the development of new Ni-based superalloys for applications in the hottest sections of turbine engines and the use of atom probe tomography and correlative microscopy for characterization of these complex alloys with regards to microstructural and compositional design. The two strengthening phases γ and γ′ are introduced and the precipitation of topologically close-packed phases and their potential detrimental effects on superalloy properties are reviewed. Mechanisms of environmental degradation, namely oxidation and hot corrosion, are elucidated and recent research studies on a new phenomenon of hot corrosion at relatively low temperatures below 600°C are discussed. The effect of individual alloying elements on superalloy properties is reviewed, with a focus on Mo and W. The use of atom probe in correlation with state-of-the-art microscopy, spectroscopy and diffraction techniques to study and understand oxidation and corrosion of Ni-based superalloys, including crack tip investigations, is presented.
Roscioli G, Repka M, Pedrazzini S, et al., 2022, Chemo-mechanical effects on the cutting-induced mixed-mode II-III fracture of martensitic stainless steels: An in-situ investigation, ACTA MATERIALIA, Vol: 231, ISSN: 1359-6454
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Anzini E, Glaenzer N, Mignanelli PM, et al., 2020, The effect of manganese and silicon additions on the corrosion resistance of a polycrystalline nickel-based superalloy, Corrosion Science, Vol: 176, ISSN: 0010-938X
The service lives of nickel superalloys are often limited by environmental degradation. The present study compares oxidation, sulfidation and hot corrosion at 750 °C of three variants of a polycrystalline superalloy: a baseline alloy, a variant containing 1 wt% Mn and one containing 0.5 wt% Si. Mn reduced the oxidation rate without changing the scale morphology. The MnCr2O4 scale formed proved more protective against sulfidation and hot corrosion, but internal sulfides extended the damage depth. Si modified the oxide morphology to a continuous Cr2O3-Al2O3 dual layer. This provided improved protection, reducing the sulfidation depth by 2/3 and the hot corrosion depth by ½.
Pedrazzini S, Pek ME, Ackerman AK, et al., 2020, Influence of powder-bed temperature on the microstructure and mechanical properties of Ti-6Al-4V produced by selective laser melting, Publisher: arXiv
Advanced characterisation techniques were used on LPBF Ti-6Al-4V samplesproduced on a heated base plate. When the substrate temperature is 100{\deg}Cthe elongation is 6\%, which increases and peaks at 10\% at 570{\deg}C, thensharply decreases to zero ductility at 770{\deg}C. At 100{\deg}C, a heavilystrained and twinned microstructure, primarily composed of {\alpha}+{\alpha}',was observed and it was comparable to asbuilt microstructures obtained byconventional LPBF methods. At higher temperatures, twins are no longer presentand instead nano-scale {\beta} precipitates are observed within {\alpha}' and{\alpha}, as well as dislocation networks (570{\deg}C) and tangles(770{\deg}C). Solute segregation at crystal defects was observed in allpre-heating conditions. Al and V segregation at microtwins was observed in the100{\deg}C sample, reporting for the first time `selective' and mutuallyexclusive Al- and V-rich regions forming in adjacent twins. V segregation atdislocations was observed in the 570{\deg}C and 770{\deg}C samples, consistentwith the higher preheating temperatures. High O contents were measured in allsamples but with apparent opposing effects. At 100{\deg}C and 570{\deg}C wasestimated to be below the critical threshold for O embrittlement and locallyaids in maintaining a strength high by solid solution strengthening, whereas at770{\deg}C it was above the threshold, therefore failing in a brittle fashion.Based on these observations, the initial increase in ductility from 100{\deg}Cto 570{\deg}C is attributed to a reduction in microtwins and the dislocationnetworks acting as `soft barriers' for slip within a coarser microstructure.The lack of ductility at 770{\deg}C was attributed to local soluteredistribution causing dislocation pinning and an increase of O content in thissample.
Pedrazzini S, Galano M, Audebert F, et al., 2019, High strain rate behaviour of nano-quasicrystalline Al93Fe3Cr2Ti2 alloy and composites, Materials Science and Engineering: A, Vol: 764, ISSN: 0921-5093
We demonstrate the outstanding dynamic strength of nano-quasicrystalline Al93Fe3Cr2Ti2 at.% alloy and composites. Unlike most crystalline Al alloys, this alloy exhibits substantial strain rate sensitivity and retains ductility at high strain rates. This opens new pathways for use in safety-critical materials requiring impact resistance.
Pedrazzini S, Rowlands BS, Turk A, et al., 2019, Partitioning of Ti and kinetic growth predictions on the thermally grown chromia scale of a polycrystalline nickel-based superalloy, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, Vol: 50, Pages: 3024-3029, ISSN: 1073-5623
Titanium is commonly added to nickel superalloys but has a well-documented detrimental effect on oxidation resistance. The present work constitutes the first atomistic-scale quantitative measurements of grain boundary and bulk compositions in the oxide scale of a current generation polycrystalline nickel superalloy performed through atom probe tomography. Titanium was found to be particularly detrimental to oxide scale growth through grain boundary diffusion.
Pedrazzini S, Danaie M, Brittles GD, et al., 2019, Characterisation of radiation damage in perovskite superconductorYttrium Barium copper oxide
High temperature superconducting materials are currently utilised to generatethe magnetic fields required for the confinement of plasma in fusion reactors.The present study aims to assess the microstructural degradation resulting fromion implantation at room temperature with 2 MeV Xe+ ions to a total fluence of1 x 1016 ions/cm2 in Yttrium Barium Copper Oxide (YBCO) using high-resolutioncharacterisation techniques. Atom Probe Tomography (APT) and TransmissionElectron Microscopy (TEM) analyses were performed and correlated withalterations in superconducting behaviour measured using a Magnetic PropertiesMeasurement System (MPMS). TEM analysis was performed to depth-profile thedegree of crystallinity on irradiated samples. The maximum-damage layer wasfound 800 nm below the sample surface. Ion implantation caused thesuperconducting temperature to decrease by 10 K and the critical currentdensity to display a 10-fold reduction.
Gardner H, Pedrazzini S, Douglas JO, et al., 2019, Atom probe tomography investigations of microstructural evolution in an aged nickel superalloy for exhaust applications, Metallurgical and Materials Transactions A, Vol: 50, Pages: 1862-1872, ISSN: 1073-5623
The formation and evolution of nanoscale γ″ (Ni3(Nb, Ti, Al)) precipitates formed during thermal aging in the nickel superalloy Inconel 625 has been characterized using Atom Probe Tomography. The onset of γ″ precipitation has been found to occur after only one hour, markedly shorter than the aging time reported in the current literature. Evolution of precipitate composition and morphology during aging has been analyzed, and the potential onset of the γ″ → δ (Ni3(Nb, Mo, Cr, Fe, Ti)) transformation after long aging times is discussed.
Pedrazzini S, Galano M, Audebert F, et al., 2018, High strain rate behaviour of Nano-quasicrystalline Al93Fe3Cr2Ti2 alloy and composites, Publisher: arXiv
In the present work, we demonstrate for the first time the outstandingdynamic mechanical properties of nano-quasicrystalline Al93Fe3Cr2Ti2 at.% alloyand composites. Unlike most crystalline aluminium-based alloys, this alloy andcomposites exhibit substantial strain rate sensitivity and retain much of theirductility at high rates of strain. This opens new pathways for use insafety-critical materials where impact resistance is required.
Pedrazzini S, Rowlands BS, Turk A, et al., 2018, On the effect of Ti on oxidation behaviour of a polycrystalline nickel-based superalloy, Publisher: arXiv
Titanium is commonly added to nickel superalloys but has a well-documenteddetrimental effect on oxidation resistance. The present work constitutes thefirst atomistic-scale quantitative measurements of grain boundary and bulkcompositions in the oxide scale of a current generation polycrystalline nickelsuperalloy performed through atom probe tomography. Titanium was found to beparticularly detrimental to oxide scale growth through grain boundarydiffusion.
Exertier F, La Fontaine A, Corcoran C, et al., 2018, Atom probe tomography analysis of the reference zircon gj-1: An interlaboratory study, CHEMICAL GEOLOGY, Vol: 495, Pages: 27-35, ISSN: 0009-2541
In recent years, atom probe tomography (APT) has been increasingly used to study minerals, and in particular the mineral zircon. Zircon (ZrSiO4) is ideally suited for geochronology by utilising the U-Th-Pb isotope systems, and trace element compositions are also widely used to constrain petrogenetic processes. However, while standard geoanalytical techniques provide information at micrometer scale lengths, the unique combination of chemical/isotopic sensitivity and spatial resolution of APT allows compositional and textural measurements at the nanoscale. This interlaboratory study aims to define the reproducibility of APT data across research facilities and assess the role of different aspects of the atom probe workflow on reproducibility. This is essential to allow correct evaluation of APT results and full utilization of this emerging technique within the geoscience community. In this study, nine samples from the same homogeneous, GJ-1/87 zircon reference grain were sent to nine APT institutes in Germany, the UK, USA, Canada and Australia. After preparing the sample out of a selectioned slab, each institute conducted three different rounds of APT analyses: using (i) unconstrained analysis parameters, (ii) pre-defined analysis parameters, and (iii) interpreting and quantifying a provided dataset. Data such as the measured elemental composition, acquisition parameters, or mass spectrum peak identifications, were recorded and analyzed. We observe a significant variation in the measured composition across this interlaboratory study as well as the number of trace elements identified. These differences are thought to directly result from the user's choice of atom probe data analysis parameters. The type of instrument does not seem to be a critical factor. Consequently, comparison of absolute trace element concentrations on zircon using APT between laboratories is only valid if the same workflow has been ensured.
Pedrazzini S, Child DJ, Aarholt T, et al., 2018, On the effect of environmental exposure on dwell fatigue performance of a fine-grained nickel-based superalloy, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, Vol: 49, Pages: 3908-3922, ISSN: 1073-5623
The influence of sulfur contamination on the corrosion-fatigue behavior of a polycrystalline superalloy used in aero-engines is considered. Samples tested under a variety of environmental conditions (including exposures to air, SOx gas, and salt) are characterized through a suite of high-resolution characterization methods, including transmission electron microscopy (TEM), secondary ion mass spectroscopy (nanoSIMS), and atom probe tomography (APT). The primary effect of sulfur contamination is to accelerate the crack growth rate by altering the failure mechanism. The SIMS and TEM analyses indicate Cr-Ti sulfide particle formation at grain boundaries ahead of and around oxidized cracks. The APT analysis suggests that these particles then oxidize as the crack propagates and are enveloped in chromia. The chromia is surrounded by a continuous layer of alumina within the cracks. All of the sulfur detected was confined within the particles, with no elemental segregation found at grain boundaries.
Pedrazzini S, Kiseeva ES, Escoube R, et al., 2018, In-Service Oxidation and Microstructural Evolution of a Nickel Superalloy in a Formula 1 Car Exhaust, OXIDATION OF METALS, Vol: 89, Pages: 375-394, ISSN: 0030-770X
The oxidation response and microstructural evolution of an Inconel 625 alloy exhaust manifold exposed to an automobile racing environment has been examined using a range of advanced electron microscopy-based techniques, atom probe tomography and high-sensitivity laser ablation mass spectrometry. The dynamic, corrosive gas conditions result in accelerated oxidation, with the inner exhaust surface also heavily contaminated by multiple species including Zn, P, K and Na. Nb carbides and Ti nitrides identified in stock control samples evolve into mixed (Ti, Nb)N species during exposure, decorated by smaller Mo, Si-rich precipitates. The exposed alloy component therefore reveals unique surface and subsurface features following in-service use.
Pedrazzini S, Galano M, Audebert F, et al., 2017, Elevated temperature mechanical behaviour of nanoquasicrystalline Al93Fe3Cr2Ti2 alloy and composites, Materials Science and Engineering A: Structural Materials Properties Microstructure and Processing, Vol: 705, Pages: 352-359, ISSN: 0921-5093
Rapidly solidified nano-quasicrystalline Al93Fe3Cr2Ti2 at% alloy has previously shown outstanding tensile and compressive strength and microstructural stability up to elevated temperatures. Despite this, no study had previously assessed the effect of plastic deformation at elevated temperature to simulate thermal-mechanical forging processes for the production of engineering components. The present work analysed bars consisting of a nano-quasicrystalline Al93Fe3Cr2Ti2 at% alloy matrix, with the addition of 10 and 20 vol% pure Al ductilising fibres, produced through gas atomisation and warm extrusion. The microstructure was made primarily of nanometre-sized icosahedral particles in an α-Al matrix. Compression tests were performed across a range of temperatures and strain rates. The measured yield strength at 350 °C was over 3x that of “high strength” 7075 T6 Al alloy, showing outstanding thermal stability and mechanical performance. However, the microstructure was shown by XRD to undergo a phase transformation which resulted in the decomposition of the icosahedral phase around ~500 °C into more stable intermetallic phases. Serrated flow associated with dynamic strain ageing was observed and a semi-quantitative analysis matching elemental diffusion speeds with dislocation speed at specific strain rates was performed, which tentatively identified Ti as the solute species responsible within the selected range of temperatures and strain rates.
Barba D, Alabort E, Pedrazzini S, et al., 2017, On the microtwinning mechanism in a single crystal superalloy, Acta Materialia, Vol: 135, Pages: 314-329, ISSN: 1359-6454
The contribution of a microtwinning mechanism to the creep deformation behaviour of single crystal superalloy MD2 is studied. Microtwinning is prevalent for uniaxial loading along at for the stress range 625 to MPa and for 625 MPa. Using quantitative stereology, the twin fraction and twin thickness are estimated; this allows the accumulated creep strain to be recovered, in turn supporting the role of the microtwinning mode in conferring deformation. Atom probe tomography confirms the segregation of Cr and Co at the twin/parent interface, consistent with the lowering of the stacking fault energy needed to support twin lengthening and thickening. A model for diffusion-controlled growth of twins is proposed and it is used to recover the measured creep strain rate. The work provides the basis for a thermo-mechanical constitutive model of deformation consistent with the microtwinning mechanism.
Pedrazzini S, London AJ, Gault B, et al., 2017, Nanoscale stoichiometric analysis of a high-temperature superconductor by atom probe tomography, Microscopy and Microanalysis, Vol: 23, Pages: 414-424, ISSN: 1431-9276
The functional properties of the high-temperature superconductor Y1Ba2Cu3O7−δ (Y-123) are closely correlated to the exact stoichiometry and oxygen content. Exceeding the critical value of 1 oxygen vacancy for every five unit cells (δ>0.2, which translates to a 1.5 at% deviation from the nominal oxygen stoichiometry of Y7.7Ba15.3Cu23O54−δ ) is sufficient to alter the superconducting properties. Stoichiometry at the nanometer scale, particularly of oxygen and other lighter elements, is extremely difficult to quantify in complex functional ceramics by most currently available analytical techniques. The present study is an analysis and optimization of the experimental conditions required to quantify the local nanoscale stoichiometry of single crystal yttrium barium copper oxide (YBCO) samples in three dimensions by atom probe tomography (APT). APT analysis required systematic exploration of a wide range of data acquisition and processing conditions to calibrate the measurements. Laser pulse energy, ion identification, and the choice of range widths were all found to influence composition measurements. The final composition obtained from melt-grown crystals with optimized superconducting properties was Y7.9Ba10.4Cu24.4O57.2.
Bagot PAJ, Silk OBW, Douglas JO, et al., 2017, An Atom Probe Tomography study of site preference and partitioning in a nickel-based superalloy, Acta Materialia, Vol: 125, Pages: 156-165, ISSN: 1359-6454
Atom Probe Tomography (APT) has been utilised for an in-depth examination of the commercial polycrystalline Ni-based superalloy RR1000, assessing compositions of the primary, secondary and tertiary γ′ phases. Clear differences in the phase chemistries are noted, particularly for the tertiary γ′ to which much of the alloy strength is attributed. Trace amounts of Hf are found to segregate strongly to the primary and secondary γ′ phases, but also exhibit an extended diffusion profile across the γ-γ′ interface up to 80 nm wide. Ti, Al and Mo demonstrate similar, yet not as pronounced diffusion profiles, indicating assumed phase chemistries may not be representative of those regions adjacent to the γ-γ′ interface. Within γ′, unique element site-occupancy preferences for this alloy were identified. Finally, the grain boundary chemistry across a γ-γ interface and that of an intragranular boride were analysed, identifying the latter as a mixed M5B3 boride rich in Mo and Cr. These demonstrate further the depth of information on Ni-alloys accessible by APT, while the overall implications of results in comparison with other in-service/model alloys are also discussed.
Barba D, Pedrazzini S, Vilalta-Clemente A, et al., 2017, On the composition of microtwins in a single crystal nickel-based superalloy, Scripta Materialia, Vol: 127, Pages: 37-40, ISSN: 1359-6462
Correlative analysis is performed using atom probe tomography and high resolution transmission Kikuchi diffraction techniques on microtwins of ∼10 nm thickness, in a ⟨011⟩-orientated single crystal superalloy crept at 800 °C and 650 MPa. Composition profiles across the microtwins and microtwin-parent boundaries are presented. Enrichment of microtwin-parent interfaces by approximately 2 at.% Cr and 1 at.% Co within the γ′-phase is found; no compositional variations of other elements – Ta, Nb, Mo, W – are detected. Our results provide unique insights into the mechanism of microtwin formation and the likely influence of alloy composition on deformation kinetics.
Kontis P, Pedrazzini S, Gong Y, et al., 2017, The effect of boron on oxide scale formation in a new polycrystalline superalloy, Scripta Materialia, Vol: 127, Pages: 156-159, ISSN: 1359-6462
Boron addition to a new polycrystalline nickel-based superalloy promotes the formation of a hitherto unreported aluminoborate phase, in the scale produced by oxidation in air at 900 °C for ~ 10,000 h. Atom probe tomography provides unambiguous confirmation of this. The ramifications of its formation are discussed.
Danaie M, Kepaptsoglou D, Ramasse QM, et al., 2016, Characterization of ordering in A-site deficient perovskite Ca1-xLa2x/3TiO3 using STEM/EELS, Inorganic Chemistry: including bioinorganic chemistry, Vol: 55, Pages: 9937-9948, ISSN: 0020-1669
The vacancy ordering behavior of an A-site deficient perovskite system, Ca1–xLa2x/3TiO3, was studied using atomic resolution scanning transmission electron microscopy (STEM) in conjunction with electron energy-loss spectroscopy (EELS), with the aim of determining the role of A-site composition changes. At low La content (x = 0.2), adopting Pbnm symmetry, there was no indication of long-range ordering. Domains, with clear boundaries, were observed in bright-field (BF) imaging, but were not immediately visible in the corresponding high-angle annular dark-field (HAADF) image. These boundaries, with the aid of displacement maps from A-site cations in the HAADF signal, are shown to be tilt boundaries. At the La-rich end of the composition (x = 0.9), adopting Cmmm symmetry, long-range ordering of vacancies and La3+ ions was observed, with alternating La-rich and La-poor layers on (001)p planes, creating a double perovskite lattice along the c axis. These highly ordered domains can be found isolated within a random distribution of vacancies/La3+, or within a large population, encompassing a large volume. In regions with a high number density of double perovskite domains, these highly ordered domains were separated by twin boundaries, with 90° or 180° lattice rotations across boundaries. The occurrence and characteristics of these ordered structures are discussed and compared with similar perovskite systems.
Pedrazzini S, Galano M, Audebert F, et al., 2016, Strengthening mechanisms in an Al-Fe-Cr-Ti nano-quasicrystalline alloy and composites, Materials Science and Engineering A: Structural Materials Properties Microstructure and Processing, Vol: 672, Pages: 175-183, ISSN: 0921-5093
We report a study of the structure-processing-property relationships in a high strength Al93Fe3Cr2Ti2 nano-quasicrystalline alloy and composites containing 10 and 20 vol% ductilising pure Al fibres. The superimposed contributions of several different strengthening mechanisms have been modelled analytically using data obtained from systematic characterisation of the monolithic alloy bar. An observed yield strength of 544 MPa has been substantiated from a combination of solid solution strengthening, work hardening, precipitation hardening and Hall-Petch grain size dependent effects. These materials have been shown by other authors in previous published work to be highly sensitive to the size distribution of particles in the powder from which they are made, and the subsequent thermomechanical processing conditions. The processing condition employed in this study provided micron-sized grains with a strong [111] preferential orientation along the extrusion direction and a bimodal size distribution of the icosahedral nano-quasicrystalline precipitates. Both were deemed to be a significant contributor to the high yield strength observed. The addition of pure Al fibres was found to decrease the yield strength linearly with increasing Al content, and to augment the ductility of the composites.
Pedrazzini S, Child DJ, West G, et al., 2016, Oxidation behaviour of a next generation polycrystalline Mn containing Ni-based superalloy, SCRIPTA MATERIALIA, Vol: 113, Pages: 51-54, ISSN: 1359-6462
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