Imperial College London

DrStellaPedrazzini

Faculty of EngineeringDepartment of Materials

Lecturer in Engineering Alloys and Metallurgy
 
 
 
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Contact

 

s.pedrazzini Website

 
 
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Location

 

1.18Royal School of MinesSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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20 results found

Pedrazzini S, Galano M, Audebert F, Siegkas P, Gerlach R, Tagarielli VL, Smith GDWet 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.

Journal article

Pedrazzini S, Rowlands BS, Turk A, Parr IMD, Hardy MC, Bagot PAJ, Moody MP, Galindo-Nava E, Stone HJet 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.

Journal article

Pedrazzini S, Danaie M, Brittles GD, Speller S, Grovenor CRM, Edmondson PD, Moody MP, Bagot PAJet 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.

Working paper

Gardner H, Pedrazzini S, Douglas JO, De Lille D, Moody MP, Bagot PAJet 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.

Journal article

Pedrazzini S, Galano M, Audebert F, Gerlach R, Siegkas P, Tagarielli V, Smith GDWet 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.

Working paper

Pedrazzini S, Rowlands BS, Turk A, Parr IMD, Hardy MC, Bagot PAJ, Moody MP, Galindo-Nava E, Stone HJet 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.

Working paper

Exertier F, La Fontaine A, Corcoran C, Piazolo S, Belousova E, Peng Z, Gault B, Saxey DW, Fougerouse D, Reddy SM, Pedrazzini S, Bagot PAJ, Moody MP, Langelier B, Moser DE, Botton GA, Vogel F, Thompson GB, Blanchard PT, Chiaramonti AN, Reinhard DA, Rice KP, Schreiber DK, Kruska K, Wang J, Gairney JMet 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.

Journal article

Pedrazzini S, Child DJ, Aarholt T, Ball C, Dowd M, Girling A, Cockings H, Perkins K, Hardy MC, Stone HJ, Bagot PAJet al., 2018, On the effect of environmental exposure on dwell fatigue performance of a fine-grained nickel-based superalloy, 3rd European Conference on Superalloys (Eurosuperalloys), Publisher: Springer Verlag, 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.

Conference paper

Pedrazzini S, Kiseeva ES, Escoube R, Gardner HM, Douglas JO, Radecka A, Mignanelli PM, Hughes GM, Chapman G, Edmondson PD, Stone HJ, De Lille D, Bagot PAJet 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.

Journal article

Pedrazzini S, Galano M, Audebert F, Smith GDWet 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.

Journal article

Barba D, Alabort E, Pedrazzini S, Collins DM, Wilkinson AJ, Bagot PAJ, Moody MP, Atkinson C, Jerusalem A, Reed RCet 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.

Journal article

Pedrazzini S, London AJ, Gault B, Saxey D, Speller S, Grovenor CRM, Danaie M, Moody MP, Edmondson PD, Bagot PAJet 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.

Journal article

Bagot PAJ, Silk OBW, Douglas JO, Pedrazzini S, Crudden D, Martin TL, Hardy MC, Moody MP, Reed RCet 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.

Journal article

Kontis P, Pedrazzini S, Gong Y, Bagot PN, Moody MP, Reed RCet 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.

Journal article

Barba D, Pedrazzini S, Vilalta-Clemente A, Wilkinson AJ, Moody MP, Bagot PAJ, Jerusalem A, Reed RCet 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.

Journal article

Danaie M, Kepaptsoglou D, Ramasse QM, Ophus C, Whittle KR, Lawson SM, Pedrazzini S, Young NP, Bagot PAJ, Edmondson PDet al., 2016, Characterization of Ordering in A-Site Deficient Perovskite Ca1-xLa2x/3TiO3 Using STEM/EELS, Inorganic 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.

Journal article

Pedrazzini S, Galano M, Audebert F, Collins DM, Hofmann F, Abbey B, Korsunsky AM, Lieblich M, Garcia Escorial A, Smith GDWet 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.

Journal article

Pedrazzini S, Child DJ, West G, Doak SS, Hardy MC, Moody MP, Bagot PAJet al., 2016, Oxidation behaviour of a next generation polycrystalline Mn containing Ni-based superalloy, SCRIPTA MATERIALIA, Vol: 113, Pages: 51-54, ISSN: 1359-6462

Journal article

Kontis P, Yusof HAM, Pedrazzini S, Danaie M, Moore KL, Bagot PAJ, Moody MP, Grovenor CRM, Reed RCet al., 2016, On the effect of boron on grain boundary character in a new polycrystalline superalloy, ACTA MATERIALIA, Vol: 103, Pages: 688-699, ISSN: 1359-6454

Journal article

Segersall M, Kontis P, Pedrazzini S, Bagot PAJ, Moody MP, Moverare JJ, Reed RCet al., 2015, Thermal-mechanical fatigue behaviour of a new single crystal superalloy: Effects of Si and Re alloying, ACTA MATERIALIA, Vol: 95, Pages: 456-467, ISSN: 1359-6454

Journal article

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