Imperial College London

ProfessorFinnGiuliani

Faculty of EngineeringDepartment of Materials

Professor in Structural Ceramics
 
 
 
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Contact

 

+44 (0)20 7594 1249f.giuliani

 
 
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Location

 

RSM LM04DRoyal School of MinesSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

113 results found

Wang S, Gavalda-Diaz O, Lyons J, Giuliani Fet al., 2024, Shear and delamination behaviour of basal planes in Zr3AlC2 MAX phase studied by micromechanical testing, Scripta Materialia, Vol: 240, ISSN: 1359-6462

The mechanical properties of layered, hexagonal-structured MAX phases often show the combined merits of metals and ceramics, making them promising material candidates for safety critical applications. While their unique mechanical performance largely arises from the crystal structure, the effect of chemistry on the properties of these materials remains unclear. To study this, here we employed two in situ electron microscope small-scale testing approaches to examine the micromechanical properties of Zr3AlC2, and compared the results with the properties of Ti3SiC2: we used micropillar compression tests to measure basal slip strength, and double cantilever beam splitting tests to evaluate fracture energy for basal plane delamination. We observed distinct and systematic differences in these measured properties between Zr3AlC2 and Ti3SiC2, where Zr3AlC2 appeared to be stronger but more brittle at the microscale, and discussed the implications of the results in the selection, design, and engineering of MAX phases for targeted engineering applications.

Journal article

Wang S, Shen Z, Omirkhan A, Gavalda-Diaz O, Ryan MP, Giuliani Fet al., 2023, Determining the fundamental failure modes in Ni-rich lithium ion battery cathodes, Journal of the European Ceramic Society, Vol: 43, Pages: 7553-7560, ISSN: 0955-2219

Challenges associated with in-service mechanical degradation of Li-ion battery cathodes has prompted a transition from polycrystalline to single crystal cathode materials. Whilst for single crystal materials, dislocation-assisted crack formation is assumed to be the dominating failure mechanism throughout battery life, there is little direct information about their mechanical behaviour, and mechanistic understanding remains elusive. Here, we demonstrated, using in situ micromechanical testing, direct measurement of local mechanical properties within LiNi0.8Mn0.1Co0.1O2 single crystalline domains. We elucidated the dislocation slip systems, their critical stresses, and how slip facilitate cracking. We then compared single crystal and polycrystal deformation behaviour. Our findings answer two fundamental questions critical to understanding cathode degradation: What dislocation slip systems operate in Ni-rich cathode materials? And how does slip cause fracture? This knowledge unlocks our ability to develop tools for lifetime prediction and failure risk assessment, as well as in designing novel cathode materials with increased toughness in-service.

Journal article

Douglas JO, Conroy M, Giuliani F, Gault Bet al., 2023, In-situ sputtering from the micromanipulator to enable cryogenic preparation of specimens for atom probe tomography by focused-ion beam, Microscopy and Microanalysis, Vol: 29, Pages: 1009-1017, ISSN: 1083-0375

Workflows have been developed in the past decade to enable atom probe tomography analysis at cryogenic temperatures. The inability to control the local deposition of the metallic precursor from the gas-injection system (GIS) at cryogenic temperatures makes the preparation of site-specific specimens by using lift-out extremely challenging in the focused-ion beam. Schreiber et al. exploited redeposition to weld the lifted-out sample to a support. Here, we build on their approach to attach the region-of-interest and additionally strengthen the interface with locally sputtered metal from the micromanipulator. Following standard focused-ion beam annular milling, we demonstrate atom probe analysis of Si in both laser pulsing and voltage mode, with comparable analytical performance as a presharpened microtip coupon. Our welding approach is versatile, as various metals could be used for sputtering, and allows similar flexibility as the GIS in principle.

Journal article

Douglas JO, Conroy M, Giuliani F, Gault Bet al., 2023, In situ sputtering from the micromanipulator to enable cryogenic preparation of specimens for atom probe tomography by focused-ion beam, Microscopy and Microanalysis, Vol: 29, Pages: 1009-1017, ISSN: 1083-0375

Workflows have been developed in the past decade to enable atom probe tomography analysis at cryogenic temperatures. The inability to control the local deposition of the metallic precursor from the gas-injection system (GIS) at cryogenic temperatures makes the preparation of site-specific specimens by using lift-out extremely challenging in the focused-ion beam. Schreiber et al. exploited redeposition to weld the lifted-out sample to a support. Here, we build on their approach to attach the region-of-interest and additionally strengthen the interface with locally sputtered metal from the micromanipulator. Following standard focused-ion beam annular milling, we demonstrate atom probe analysis of Si in both laser pulsing and voltage mode, with comparable analytical performance as a presharpened microtip coupon. Our welding approach is versatile, as various metals could be used for sputtering, and allows similar flexibility as the GIS in principle.

Journal article

Wang S, Burdett P, Lovell E, Bettles R, Wilson N, Ryan M, Giuliani Fet al., 2023, Fracture properties of La(Fe,Mn,Si)13 magnetocaloric materials, Materials Letters, Vol: 338, Pages: 1-4, ISSN: 0167-577X

La(Fe,Mn,Si)13 alloys are a promising material family for magnetic refrigeration. Challenges associated with their structural integrity during device assembly and operation requires deep understanding of the mechanical properties. Here we developed a workflow to quantitatively study the fracture properties of La(Fe,Mn,Si)13 plates used in magnetic cooling devices. We employed microstructural characterisation, optical examination of defects, and four-point bending tests of samples with known defect sizes to evaluate their mechanical performance. We established the residual strength curve which directly links observed defects to mechanical strength. The estimated fracture toughness KC of hydrogenated La(Fe,Mn,Si)13 is approximately 4 MPa·m1/2 for the geometry employed. The established relationship between strength and crack length enables the prediction of mechanical performance through examination of defects via optical microscopy, therefore can be used industrially for directing plate selection to guarantee the mechanical stability of refrigeration devices.

Journal article

Wang S, Douglas JO, Lovell E, Wilson N, Guo L, Gault B, Ryan MP, Giuliani Fet al., 2023, Near-atomic scale chemical analysis of interfaces in a La(Fe,Mn,Si)13-based magnetocaloric material, Scripta Materialia, Vol: 224, Pages: 1-6, ISSN: 1359-6462

La(Fe,Mn,Si)13-based magnetocaloric materials are one of the most promising material families for the realisation of near-room temperature magnetic refrigeration. The functional and mechanical properties of these materials crucially depend on their chemistry, which is difficult to control at interfaces between microstructural units. Atom probe tomography was employed to reveal the local elemental distribution at the α-Fe/1:13 phase boundary and the 1:13/1:13 grain boundary. Strong Mn segregation (and Fe depletion) at the α-Fe/1:13 phase boundary suggests the potential effect of phase boundary area on the Curie temperature of the material. A local off-1:13 stoichiometry layer at the 1:13/1:13 grain boundary may adversely affect the magnetocaloric performance. Routes to mitigate the negative effects of interfaces on the functional and mechanical performance of these materials are discussed, in order to achieve durable and efficient operation of magnetic cooling devices.

Journal article

Smita Rao G, Shu R, Wang S, Boyd R, Giuliani F, le Febvrier A, Eklund Pet al., 2022, Thin film growth and mechanical properties of CrFeCoNi/TiNbZrTa multilayers, Materials & Design, Vol: 224, ISSN: 0261-3069

Multilayers of high entropy alloys (HEA) are picking up interest due to the possibility of altering material properties by tuning crystallinity, thickness, and interfaces of the layers. This study investigates the growth mechanism and mechanical properties of CrFeCoNi/TiNbZrTa multilayers grown by magnetron sputtering. Multilayers of bilayer thickness (Λ) from 5 nm to 50 nm were grown on Si(1 0 0) substrates. Images taken by transmission electron microscopy and energy-dispersive X-ray spectroscopy mapping revealed that the layers were well defined with no occurrence of elemental mixing. Multilayers with Λ < 20 nm exhibited an amorphous structure. As Λ increased, the CrFeCoNi layer displayed a higher crystallinity in comparison to the amorphous TiNbZrTa layer. The mechanical properties were influenced by the crystallinity of the layers and stresses in the film. The film with Λ = 20 nm had the highest hardness of approximately 12.5 GPa owing grain refinement of the CrFeCoNi layer. An increase of Λ ≥ 30 nm resulted in a drop in the hardness due to the increase in crystal domains of the CrFeCoNi layer. Micropillar compression induced shear in the material rather than fracture, along with elemental intermixing in the core of the deformed region of the compressed micropillar.

Journal article

Xu Y, Gu T, Xian J, Giuliani F, Ben Britton T, Gourlay CM, Dunne FPEet al., 2022, Multi-scale plasticity homogenization of Sn–3Ag-0.5Cu: From β-Sn micropillars to polycrystals with intermetallics, Materials Science and Engineering: A, Vol: 855, Pages: 1-15, ISSN: 0921-5093

The mechanical properties of β-Sn single crystals have been systematically investigated using a combined methodology of micropillar tests and rate-dependent crystal plasticity modelling. The slip strength and rate sensitivity of several key slip systems within β-Sn single crystals have been determined. Consistency between the numerically predicted and experimentally observed slip traces has been shown for pillars oriented to activate single and double slip. Subsequently, the temperature-dependent, intermetallic-size-governing behaviour of a polycrystal β-Sn-rich alloy SAC305 (96.5Sn–3Ag-0.5Cu wt%) is predicted through a multi-scale homogenization approach, and the predicted temperature- and rate-sensitivity reproduce independent experimental results. The integrated experimental and numerical approaches provide mechanistic understanding and fundamental material properties of microstructure-sensitive behaviour of electronic solders subject to thermomechanical loading, including thermal fatigue.

Journal article

Kim S-H, Dong K, Zhao H, El-Zoka AA, Zhou X, Woods EV, Giuliani F, Manke I, Raabe D, Gault Bet al., 2022, Understanding the degradation of a model si anode in a li-ion battery at the atomic scale., Journal of Physical Chemistry Letters, Vol: 36, Pages: 8416-8421, ISSN: 1948-7185

To advance the understanding of the degradation of the liquid electrolyte and Si electrode, and their interface, we exploit the latest developments in cryo-atom probe tomography. We evidence Si anode corrosion from the decomposition of the Li salt before charge-discharge cycles even begin. Volume shrinkage during delithiation leads to the development of nanograins from recrystallization in regions left amorphous by the lithiation. The newly created grain boundaries facilitate pulverization of nanoscale Si fragments, and one is found floating in the electrolyte. P is segregated to these grain boundaries, which confirms the decomposition of the electrolyte. As structural defects are bound to assist the nucleation of Li-rich phases in subsequent lithiations and accelerate the electrolyte's decomposition, these insights into the developed nanoscale microstructure interacting with the electrolyte contribute to understanding the self-catalyzed/accelerated degradation Si anodes and can inform new battery designs unaffected by these life-limiting factors.

Journal article

Collard B, Giuliani F, Ingenbleek G, Verbist G, Dini Det al., 2022, A fracture mechanics analysis of the micromechanical events in finite thickness fibre push-out tests, Theoretical and Applied Fracture Mechanics, Vol: 121, Pages: 103441-103441, ISSN: 0167-8442

Understanding the micromechanical events of interfacial failure in fibre reinforced composites is vital to accurately characterising micromechanical properties and, consequently, the macroscopic properties of the composite. A fracture mechanics model of the fibre push-out test is developed, with an emphasis on the effect of sample thickness and residual stresses on the mechanisms of interfacial crack advancement. The model is applied to both a SiC-SiC ceramic matrix composite and a SiC-Ti metal matrix composite. The model demonstrates that previous assumptions about the micromechanical events of interfacial cracking are consistent with the measured values of interfacial fracture energy for ceramic matrix composites. Moreover, the model can identify the range of geometries for which different micromechanical cracking mechanisms occur simultaneously in a given material system. Identifying this range is important in choosing the sample geometry for fibre push-out testing because the interaction of advancing cracks affects the measurement of interfacial fracture energy by classical models.

Journal article

Wang S, Gavalda-Diaz O, Luo T, Guo L, Lovell E, Wilson N, Gault B, Ryan MP, Giuliani Fet al., 2022, The effect of hydrogen on the multiscale mechanical behaviour of a La(Fe,Mn,Si)13-based magnetocaloric material, Journal of Alloys and Compounds, Vol: 906, Pages: 1-10, ISSN: 0925-8388

Magnetocaloric cooling offers the potential to improve the efficiency of refrigeration devices and hence cut the significant CO2 emissions associated with cooling processes. A critical issue in deployment of this technology is the mechanical degradation of the magnetocaloric material during processing and operation, leading to limited service-life. The mechanical properties of hydrogenated La(Fe,Mn,Si)13-based magnetocaloric material are studied using macroscale bending tests of polycrystalline specimens and in situ micropillar compression tests of single crystal specimens. The impact of hydrogenation on the mechanical properties are quantified. Understanding of the deformation/failure mechanisms is aided by characterization with transmission electron microscopy and atom probe tomography to reveal the arrangement of hydrogen atoms in the crystal lattice. Results indicate that the intrinsic strength of this material is ~3-6 GPa and is dependent on the crystal orientation. Single crystals under compressive load exhibit shearing along specific crystallographic planes. Hydrogen deteriorates the strength of La(Fe,Mn,Si)13 through promotion of transgranular fracture. The weakening effect of hydrogen on single crystals is anisotropic; it is significant upon shearing parallel to the {111} crystallographic planes but is negligible when the shear plane is {001}-oriented. APT analysis suggests that this is associated with the close arrangement of hydrogen atoms on {222} planes.

Journal article

Wang S, Lovell E, Guo L, Wilson N, Ryan MP, Giuliani Fet al., 2022, Environment-assisted crack nucleation in La(Fe,Mn,Si)13-based magnetocaloric materials, International Journal of Refrigeration, Vol: 135, Pages: 1-4, ISSN: 0140-7007

Cracking in La(Fe,Si)13-based magnetocaloric materials has been observed to predominantly form around La-rich (La2O3) particles and pose a threat to their long-term structural integrity. To understand the formation of these cracks, FIB cross-sectional polishing followed by SEM characterisation was employed to study local microstructural evolution after air exposure. Results suggest that volume expansion and internal degradation associated with a chemical reaction between La2O3 particles and water/moisture can lead to crack nucleation in the 1:13 phase adjacent to La-rich particles. This observation indicates that the formation of La-rich phase should be suppressed, or their size minimised during material processing to ensure the long-term structural integrity of La(Fe,Mn,Si)13 magnetocaloric materials.

Journal article

Jun T-S, Bhowmik A, Maeder X, Sernicola G, Giovannini T, Dolbnya I, Michler J, Giuliani F, Britton Bet al., 2022, In-situ diffraction based observations of slip near phase boundaries in titanium through micropillar compression, MATERIALS CHARACTERIZATION, Vol: 184, ISSN: 1044-5803

Journal article

Garcia-Giner V, Han Z, Giuliani F, Porter AEet al., 2021, Nanoscale imaging and analysis of bone pathologies, Applied Sciences-Basel, Vol: 11, Pages: 1-32, ISSN: 2076-3417

Understanding the properties of bone is of both fundamental and clinical relevance. The basis of bone’s quality and mechanical resilience lies in its nanoscale building blocks (i.e., mineral, collagen, non-collagenous proteins, and water) and their complex interactions across length scales. Although the structure–mechanical property relationship in healthy bone tissue is relatively well characterized, not much is known about the molecular-level origin of impaired mechanics and higher fracture risks in skeletal disorders such as osteoporosis or Paget’s disease. Alterations in the ultrastructure, chemistry, and nano-/micromechanics of bone tissue in such a diverse group of diseased states have only been briefly explored. Recent research is uncovering the effects of several non-collagenous bone matrix proteins, whose deficiencies or mutations are, to some extent, implicated in bone diseases, on bone matrix quality and mechanics. Herein, we review existing studies on ultrastructural imaging—with a focus on electron microscopy—and chemical, mechanical analysis of pathological bone tissues. The nanometric details offered by these reports, from studying knockout mice models to characterizing exact disease phenotypes, can provide key insights into various bone pathologies and facilitate the development of new treatments.

Journal article

Collett H, Bouville F, Giuliani F, Schofield Eet al., 2021, Structural monitoring of a large archaeological wooden structure in real time, post PEG treatment, Forests, Vol: 12, Pages: 1-13, ISSN: 1999-4907

Large archaeological wooden structures are potentially at risk of structural failure through deformation and cracking over time if they are left untreated and their structural health is not maintained. This could be in part due to, for example, the shrinkage of waterlogged wood as it dries, or time-dependent creep processes. These dimensional changes are accompanied by associated stresses. However, there are few studies analysing the movement of large wooden structures in real time as they dry, particularly after their conservation treatment. This paper follows the structural monitoring of the Mary Rose from after the conservation treatment, where it was sprayed with polyethylene glycol, through to the ship’s air-drying process and beyond to assess the effects that drying has had on the displacement of the timbers. A laser-based target system was used to collect displacement data between 2013 and 2020 and the data showed a significant slowing of displacement as the drying reached an equilibrium.

Journal article

Vreeswijk M, Kot A, Giuliani F, Humphry-Baker Set al., 2021, Abnormal WC crystal growth from liquid Co flux occurs via eta phase decomposition, International Journal of Refractory Metals and Hard Materials, Vol: 99, Pages: 1-6, ISSN: 0263-4368

The growth mechanism of large WC crystals from a liquid Co-based flux is identified. This is achieved by systematically varying the growth temperature and Co content from 1200 to 1400 °C and 70–83 at.% respectively. Crystal growth was characterised using metallography and X-ray diffraction. The WC grains were bimodally distributed, consisting of a smaller (10–20 μm) population of grains, which nucleated homogenously from the liquid, and a secondary population of abnormally large grains, several millimetres in size. The abnormal grains nucleated on the (W,Co)6C eta phase, and subsequently consumed it through a carburisation reaction. The size of abnormal grains was enhanced by adopting the eutectic composition, such that the first solid phase to form was the eta phase, whilst at the same time cooling slowly through the eta➔WC transformation temperature, at ~1300 °C. This growth mechanism could be exploited for a variety of metal carbides with similarly sluggish diffusion rates.

Journal article

Gavalda-Diaz O, Manno R, Melro A, Allegri G, Hallett SR, Vandeperre L, Saiz E, Giuliani Fet al., 2021, Mode I and Mode II interfacial fracture energy of SiC/BN/SiC CMCs, Acta Materialia, Vol: 215, Pages: 1-11, ISSN: 1359-6454

Quantifying the mixed mode fracture toughness of interfaces in ceramic matrix composites (CMCs) is crucial for understanding their failure. In this work we use in situ micromechanical testing in the scanning electron microscope to achieve stable interfacial crack propagation in Mode I (Double Cantilever Beam) and Mode II (Push out) and measure the corresponding fracture resistances. We use this approach to measure the interfacial fracture resistance in SiC/BN/SiC CMCs and compare it to the fracture energy of the fibres. During in-situ testing, fracture paths can be observed while data is acquired simultaneously. We clearly observe debonding at the BN-fibre interface (i.e. inside adhesive debonding). The critical energy release rate of the BN-fibre interface for Mode I and II (GIc ≈ 2.1 ± 1.0 J/m2 and GIIc ≈ 1.2 ± 0.5 J/m2) are equivalent and is lower than that measured for the fibre using microscopic DCB tests (GIc ≈ 6.0 ± 2.0 J/m2). These results explain the generalized fibre debonding and pull out observed in the fracture of these CMCs. By enabling direct observation of crack paths and quantifying the corresponding fracture energies, we highlight possible routes for the optimisation and modelling of the new generation of CMC interphases.

Journal article

De Meyere RMG, Song K, Gale L, Harris S, Edmonds IM, Marrow TJ, Saiz E, Giuliani F, Armstrong DEJ, Gavalda-Diaz Oet al., 2021, A novel trench fibre push-out method to evaluate interfacial failure in long fibre composites, Journal of Materials Research, Vol: 36, Pages: 2305-2314, ISSN: 0884-2914

Traditional fibre push-outs for the evaluation of interfacial properties in long fibre ceramic matrix composites present their limitations—solutions for which are addressed in this work by introducing the novel trench push-out test. The trench push-out makes use of a FIB milling system and an SEM in-situ nanoindenter to probe a fibre pushed into a trench underneath, allowing in-situ observations to be directly correlated with micromechanical events. SiCf/BN/SiC composites—candidate material for turbine engines—were used as model materials in this work. Different fibre types (Hi-Nicalon and Tyranno type SA3) were coated with BN interphases, presenting mean interfacial shear stresses of 14 ± 7 MPa and 20 ± 2 MPa, respectively, during fibre sliding. The micromechanical technique enabled visualisation of how defects in the interphase (voids, inclusions & milled notches) or in the fibre (surface asperities, non-uniform coatings) affected the variability of interfacial property measurement.

Journal article

Gavalda-Diaz O, Lyons J, Wang S, Emmanuel M, Marquardt K, Saiz E, Giuliani Fet al., 2021, Basal plane delamination energy measurement in a Ti3SiC2 MAX phase, JOM, Vol: 73, Pages: 1582-1588, ISSN: 1047-4838

The {0001} basal plane delamination dominating the crack-wake bridging in MAX phases at a bulk scale has been investigated by studying the small-scale fracture of a Ti3SiC2. In situ micro-double cantilever beam (DCB) tests in a scanning electron microscope were used to grow a stable crack along the basal plane, measure the fracture energy, and study the crack propagation mechanism at the nanoscale. The results show that the fracture energy (10–50 J/m2) depends on small misorientations angles (e.g., 5°) of the basal plane to the stress field. This induces permanent deformation which can be observed once the DCB has been unloaded. The nanoscale study of the crack shows that the plasticity at the crack tip is small, but a number of pairs of dislocations are forming at each side of the crack. Hence, this study helps to explain the enhanced fracture energy values and possible sources of energy dissipation in basal plane delamination, which is the one of the main toughening mechanisms in the bulk fracture of MAX phases.

Journal article

Emmanuel M, Gavalda-Diaz O, Sernicola G, Msaoubi R, Persson T, Norgren S, Marquardt K, Britton TB, Giuliani Fet al., 2021, Fracture energy measurement of prismatic plane and Σ2 boundary in cemented carbide, JOM, Vol: 73, Pages: 1589-1596, ISSN: 1047-4838

The grain boundary network of WC in WC-Co is important, as cracks often travel intergranularly. This motivates the present work, where we experimentally measure the fracture energy of Σ2 twist grain boundaries between WC crystals using a double cantilever beam (DCB) opened with a wedge under displacement control in a WC-10wt%Co sample. The fracture energy of this boundary type was compared with cleaving {101 ̅0} prismatic planes in a WC single crystal. Fracture energies of 7.04 ± 0.36 Jm-2 and 3.57 ± 0.28 Jm-2 were measured for {101 ̅0} plane and Σ2 twist boundaries respectively.

Journal article

Aldegaither N, Sernicola G, Mesgarnejad A, Karma A, Balint D, Wang J, Saiz E, Shefelbine SJ, Porter AE, Giuliani Fet al., 2021, Fracture toughness of bone at the microscale, Acta Biomaterialia, Vol: 121, Pages: 475-483, ISSN: 1742-7061

Bone's hierarchical arrangement of collagen and mineral generates a confluence of toughening mechanisms acting at every length scale from the molecular to the macroscopic level. Molecular defects, disease, and age alter bone structure at different levels and diminish its fracture resistance. However, the inability to isolate and quantify the influence of specific features hampers our understanding and the development of new therapies. Here, we combine in situ micromechanical testing, transmission electron microscopy and phase-field modelling to quantify intrinsic deformation and toughening at the fibrillar level and unveil the critical role of fibril orientation on crack deflection. At this level dry bone is highly anisotropic, with fracture energies ranging between 5 and 30 J/m2 depending on the direction of crack propagation. These values are lower than previously calculated for dehydrated samples from large-scale tests. However, they still suggest a significant amount of energy dissipation. This approach provides a new tool to uncouple and quantify, from the bottom up, the roles played by the structural features and constituents of bone on fracture and how can they be affected by different pathologies. The methodology can be extended to support the rational development of new structural composites.

Journal article

Xu Y, Gu T, Xian J, Giuliani F, Britton T, Gourlay C, Dunne Fet al., 2021, Intermetallic size and morphology effects on creep rate of Sn-3Ag-0.5Cu solder, International Journal of Plasticity, Vol: 137, ISSN: 0749-6419

The creep behaviour of directionally solidified SAC305 (96.5Sn-3Ag-0.5Cu wt%) alloy has been investigated with integrated particle matrix composite (PMC) crystal plasticity modelling and quantitative experimental characterisation and test. In this manuscript, the mechanistic basis of creep rate dependence is shown to be influenced by plastic strain gradients, and the associated hardening due to geometrically necessary dislocation (GND) density. These gradients are created due to heterogenous deformation at the Sn phase and intermetallic compound (IMCs) boundaries. The size and distribution of IMCs is important, as finer and well dispersed IMCs leading to higher creep resistance and lower creep rates, and this agrees with experimental observations. This understanding has enabled the creation of a new microstructurally homogenized model which captures this mechanistic link between the GND hardening, the intermetallic size, and the corresponding creep rate. The homogenised model relates creep rates to the microstructure found within the solder alloy as they evolve in service, when ageing and coarsening kinetics are known.

Journal article

Wang S, Giuliani F, Britton T, 2020, Slip-hydride interactions in Zircaloy-4: Multiscale mechanical testing and characterisation, Acta Materialia, Vol: 200, Pages: 537-550, ISSN: 1359-6454

The interactions between δ-hydrides and plastic slip in a commercial zirconium alloy, Zircaloy-4, under load were studied using in situ secondary electron microscope (SEM) micropillar compression tests of single crystal samples and ex situ digital image correlation (DIC) macroscale tensile tests of polycrystalline samples. The hydrides decorate near basal planes in orientation, and for micropillars orientated for 〈a〉 basal slip localised shear at the hydride–matrix interface is favoured over slip in α-Zr matrix due to a lower shear stress required. In contrast, for pillars oriented for 〈a〉 prismatic slip the shear stress needed to trigger plastic slip within the hydride is slightly higher than the critical resolved shear stress (CRSS) for the 〈a〉 prismatic slip system. In this case, slip in the hydride is likely achieved through 〈110〉-type shear which is parallel to the activated 〈a〉-type shear in the parent matrix. At a longer lengthscale, these results are used to inform polycrystalline samples analysed using high spatial resolution DIC. Here localised interface shear remains to be a significant deformation path which can both cause and be caused by matrix slip on planes closely-oriented to the phase boundaries. Matrix slip on planes nearly perpendicular to the adjacent hydride–matrix interfaces can either result in plastic slip within the hydrides or get arrested at the interfaces, generating local stress concentration. Through these mechanisms, the presence of δ-hydrides leads to enhanced strain localisation in Zircaloy-4 early in the plastic regime.

Journal article

Diaz OG, Marquardt K, Harris S, Gale L, Vandeperre L, Saiz E, Giuliani Fet al., 2020, Degradation mechanisms of SiC/BN/SiC after low temperature humidity exposure, Journal of the European Ceramic Society, Vol: 40, Pages: 3863-3874, ISSN: 0955-2219

The environmental degradation of SiC/BN/SiC CMCs under low temperature water exposure is still an unexplored field. This work shows how the effect of low temperature humid environments can be detrimental for turbostratic BN interphases, leading to a drop in mechanical properties. Furthermore, initial low-temperature humid environments can induce a faster degradation during subsequent thermal exposure. In order to understand how low temperature water exposure affects the CMC and how these changes affect the material response to subsequent exposures, intermediate temperature (800 °C) exposures have been studied before and after the low temperature humidity tests. The main challenge of this work consists of understanding how different constituents of the CMC structure (e.g. fibres and interphases) are degrading and consequently affecting the overall bulk mechanical performance and failure modes of the material. For this, linking the change in morphology and chemistry of the interphases with the micromechanical properties each constituent has been crucial.

Journal article

McGilvery C, Jiang J, Rounthwaite N, Williams R, Giuliani F, Britton Tet al., 2020, Characterisation of carbonaceous deposits on diesel injector nozzles, Fuel: the science and technology of fuel and energy, Vol: 274, Pages: 1-9, ISSN: 0016-2361

Diesel injector nozzles are highly engineered components designed to optimise delivery of fuel into the combustion chamber of modern engines. These components contain narrow channels to enhance spray formation and penetration, hence mixing and combustion. Over time, these injectors can become clogged due to fouling by carbonaceous deposits which may affect the long-term performance of a diesel engine. In this paper we explore the chemical composition and structure of deposits formed within the nozzle at the nanometre scale using electron microscopy. We focus on comparing deposits generated using a chassis dynamometer-based test with Zn fouled fuel with a DW10B dirty up test. We have developed and applied a method to precisely section the deposits for ‘top view’ scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analysis of the morphology and relative accumulation of deposits formed during chassis dynamometer and engine based dirty-up tests. We extend this analysis to finer length scales through lift-out of ~70 nm thick electron transparent cross section foils, including both the metal substrate and deposit, using focussed ion beam (FIB) machining. These foils are analysed using scanning transmission electron microscopy (STEM) and STEM-EDS. These thin foils reveal thin-film growth and chemical stratification of Zn, C, O and other elements in the organic deposit layers developed during growth on the steel substrate during industry standard fouling tests.

Journal article

Bhowmik A, Lee J, Adande S, Wang-Koh M, Jun T-S, Sernicola G, Ben Britton T, Rae CMF, Balint D, Giuliani Fet al., 2020, Investigating spatio-temporal deformation in single crystal Ni-based superalloys using in-situ diffraction experiments and modelling, Materialia, Vol: 9, Pages: 1-14, ISSN: 2589-1529

In this study, we perform a detailed analysis of room temperature deformation of a [100]–orientated singlecrystal Ni-based superalloy, CMSX-4 micropillar, using a combinatorial and complimentary characterisation approach of micro-Laue diffraction coupled with post-deformation microscopy and crystal plasticity modelling.Time-resolved micro-Laue data indicated that deformation was initiated by activation of multiple slip (after 5%engineering strain) which led to the generation of a plastic strain accumulation accompanied by a two-foldincrease in the dislocation density within the micropillar. Subsequent to that, slip occurred primarily on two systems (11̄1)[101] and (111)[1̄01] with the highest Schmid factor in the single crystal micropillar thereby resultingin little accumulation of unpaired GNDs during a major part of the loading cycle, upto 20% strain in this case.Finite element crystal plasticity modelling also showed good agreement with the experimental analyses, wherebysignificant strains were found to develop in the above slip systems with a localisation near the centre of themicropillar. Post-deformation transmission electron microscopy study confirmed that deformation was mediatedthrough a/2<110> dislocations on {111} planes in the 𝛾-phase, while high stress levels led to shearing of the 𝛾′precipitates by a/2<110> partials bounding an anti-phase boundary free to glide on the {111} planes. Duringthe deformation of the single crystal micropillar, independent rotations of the 𝛾 and 𝛾′ phases were quantified byspatially resolved post-deformation micro-Laue patterns. The degree of lattice rotation in the 𝛾-phase was higherthan that in the 𝛾′-phase.

Journal article

Wang S, Kalácska S, Maeder X, Michler J, Giuliani F, Britton Tet al., 2019, The effect of δ-hydride on the micromechanical deformation of Zircaloy-4 studied by in situ high angular resolution electron backscatter diffraction, Scripta Materialia, Vol: 173, Pages: 101-105, ISSN: 1359-6462

Zircaloy-4(Zr-1.5%Sn-0.2%Fe-0.1%Cr wt%)is usedas nuclear fuel cladding materials and hydride embrittlementis amajor failure mechanism. To explore the effect of δ-hydrideon plastic deformation and performance of Zircaloy-4, in situhigh angular resolution electron backscatter diffraction(HR-EBSD)was used to quantify stress andgeometrically necessarydislocation(GND)density during bending tests of hydride-free and hydride-containingsingle crystalZircaloy-4 microcantilevers. Results suggest that while the stress applied was accommodated by plastic slip in the hydride-free cantilever,the hydride-containing cantilever showedprecipitation-induced GND pile-up at hydride-matrix interfacepre-deformation, andconsiderable locally-increasing GNDdensity under tensile stressupon plastic deformation.

Journal article

Giuliani F, Ciurea C, Bhakhri V, Werchota M, Vandeperre LJ, Mayrhofer PHet al., 2019, Deformation behaviour of TiN and Ti–Al–N coatings at 295 to 573 K, Thin Solid Films, Vol: 688, ISSN: 0040-6090

Temperature-dependent nanoindentation testing was employed to investigate the deformation behaviour of magnetron sputtered (100) TiN and Ti1-xAlxN (x = 0.34, 0.52, 0.62) coatings in the temperature range from 295 to 573 K. The maximum temperature is sufficiently below the deposition temperature of 773 K to guarantee for stable microstructure and stress state during testing. The TiN coating displayed the same hardness as bulk single crystal (SC) TiNbulk. The addition of aluminium to TiN (to form single-phase face centred cubic structured Ti1-xAlxN coatings) increased the room temperature hardness due to increased bond strength, lattice strain and higher activation energy for the dislocation slip. For coatings with a low aluminium content, Ti0.66Al0.34N, the decrease in hardness with temperature was similar to the TiN coating and SC-TiNbulk. In contrast, the hardness of coatings with moderate, Ti0.48Al0.52N, and high, Ti0.38Al0.62N, aluminium contents varied little up to 573 K. Thus, the Ti1-xAlxN matrix is mechanically more stable at elevated temperatures than its TiN relative, by providing a lower decrease in lattice resistance to the dislocation flow with increasing temperature. The findings suggest that the addition of Al to TiN (to form Ti1-xAlxN solid solutions) not only improves the hardness but also leads to stable hardness with temperature, and emphasizes the importance of bonding states and chemical fluctuations, next to structure and morphology of the coatings that develop with changing the chemistry.

Journal article

Boughton O, Ma S, Cai X, Yan L, Peralta L, Laugier P, Marrow J, Giuliani F, Hansen U, Abel R, Grimal Q, Cobb Jet al., 2019, Computed tomography porosity and spherical indentation for determining cortical bone millimetre-scale mechanical properties, Scientific Reports, Vol: 9, ISSN: 2045-2322

The cortex of the femoral neck is a key structural element of the human body, yet there is not a reliable metric for predicting the mechanical properties of the bone in this critical region. This study explored the use of a range of non-destructive metrics to measure femoral neck cortical bone stiffness at the millimetre length scale. A range of testing methods and imaging techniques were assessed for their ability to measure or predict the mechanical properties of cortical bone samples obtained from the femoral neck of hip replacement patients. Techniques that can potentially be applied in vivo to measure bone stiffness, including computed tomography (CT), bulk wave ultrasound (BWUS) and indentation, were compared against in vitro techniques, including compression testing, density measurements and resonant ultrasound spectroscopy. Porosity, as measured by micro-CT, correlated with femoral neck cortical bone’s elastic modulus and ultimate compressive strength at the millimetre length scale. Large-tip spherical indentation also correlated with bone mechanical properties at this length scale but to a lesser extent. As the elastic mechanical properties of cortical bone correlated with porosity, we would recommend further development of technologies that can safely measure cortical porosity in vivo.Introduction

Journal article

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