Imperial College London
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DrYilunXu

Faculty of Engineering, Department of Materials

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B301Bessemer BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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27 results found

Xu Y, Lu X, Yang X, Li W, Aitken Z, Vastola G, Gao H, Zhang Y-Wet al., 2024, Temperature-dependent, multi-mechanism crystal plasticity reveals the deformation and failure behaviour of multi-principal element alloys, Journal of the Mechanics and Physics of Solids, Vol: 185, ISSN: 0022-5096

In this work, we have developed a temperature-dependent, multi-mechanism crystal plasticity (CP) model aimed at unravelling the deformation and failure resistance of Cantor alloy-like multi-principal element alloys (MPEA) under both uniaxial tensile and cyclic loading conditions. Three deformation mechanisms: dislocation slip, deformation twinning, and phase transformation are considered under a unified stress-driven, thermally activated law. In addition, the effect of short-range ordering (SRO) is introduced by accounting for the inhomogeneous distributions of material properties within individual grains. Our work yields the following key findings: (1) The rate- and temperature-sensitivity of the materials, such as the occurrence and sequence of dislocation slip, deformation twinning, and martensitic phase transformation observed in experiments can be captured through the calibrated material properties. (2) The enhancement of the mechanical response of the Cantor alloy-like MPEAs due to the SRO effect is intrinsically linked to the generation of geometrically necessary dislocations resulting from localized variations in material properties. (3) The excellent fatigue and fracture resistance exhibited by Cantor alloy-like MPEAs at low temperatures can be attributed to the homogenization of stored energy density within the microstructure. This homogenization arises from the development of deformation twinning and martensitic phase transformation. Our newly developed CP model and the key findings provide a valuable guide for the design of MPEAs to achieve superior fatigue and fracture resistance without compromising their inherent strength.

Journal article

Yu X, Xu Y, Morales-Espejel G, Dunne F, Dini Det al., 2024, On the importance of Crystal Plasticity Finite Element discretisation for the identification of crack initiation in RCF using energy-based criteria, Computational Materials Science, Vol: 232, ISSN: 0927-0256

Material microstructure plays a key role in crack initiation under rolling contact fatigue. When studying microstructure with crystal plasticity finite element method (CPFE), mesh sensitivity study is of great importance, as the surface-near region is under high uniaxial stresses. In this paper, a new structured mesh strategy is purposed and compared with the classical unstructured mesh strategy. Modelling tests on a bi-grain and a polycrystal model show the calculation of geometrically necessary dislocation (GND) density, recently proposed as a suitable fatigue damage indicator, is highly dependent on mesh morphology, when GND hotspots tend to appear near distorted elements even in homogeneous materials. With uniform mesh size and shape, structured mesh elements can provide physically more acceptable GND calculations, which is particularly important in loading scenarios with complex stresses, such as rolling contact fatigue. Computational efficiency is also improved compared to unstructured models because a smaller number of elements are required in a structured mesh model and pre-processing of the mesh is not required.

Journal article

Wan W, Xu Y, Yu X, Dunne FPEet al., 2023, Microstructurally-sensitive fatigue crack nucleation in a Zircaloy-4 alloy, Journal of the Mechanics and Physics of Solids, Vol: 180, Pages: 1-15, ISSN: 0022-5096

Microstructurally-sensitive fatigue crack nucleation and subsequently short crack growth were observed at an edge-notch in a textured Zircaloy-4 sample with the c-axis aligned perpendicular to the viewing surface. To understand the competition between the main crack and the secondary crack nucleation at the edge-notch root, a combined experimental and computational investigation was conducted to analyse various micro-scale mechanical quantities, including local strain, local stress, GND density, and stored energy density. A computational crystal plasticity finite element simulation that incorporated grain morphology, crystallographic orientation, and loading conditions was found to have good agreement with experimental results from the three-point bend fatigue test. The results showed that both the main crack and secondary crack nucleation sites had the highest magnitudes of local stored energy density, indicating that this factor was both necessary and sufficient for crack nucleation.

Journal article

Xu Y, Worsnop F, Dye D, Dunne FPEet al., 2023, Slip intermittency and dwell fatigue in titanium alloys: a discrete dislocation plasticity analysis, Journal of the Mechanics and Physics of Solids, Vol: 179, Pages: 1-13, ISSN: 0022-5096

Slip intermittency and stress oscillations in titanium alloy Ti–7Al–O that were observed using in-situ far-field high energy X-ray diffraction microscopy (ff-HEDM) are investigated using a discrete dislocation plasticity (DDP) model. The mechanistic foundation of slip intermittency and stress oscillations are shown to be dislocation escape from obstacles during stress holds, governed by a thermal activation constitutive law. The stress drop events due to <a>-basal slip are larger in magnitude than those along <a>-prism, which is a consequence of their differing rate sensitivities, previously found from micropillar testing. It is suggested that interstitial oxygen suppresses stress oscillations by inhibiting the thermal activation process. Understanding of these mechanisms is of benefit to the design and safety assessment of jet engine titanium alloys subjected to dwell fatigue.

Journal article

Xu Y, Balint D, Greiner C, Dini Det al., 2023, On the origin of plasticity-induced microstructure change under sliding contacts, Friction, Vol: 11, Pages: 473-488, ISSN: 2223-7704

Discrete dislocation plasticity (DDP) calculations are carried out to investigate the response of a single crystal contacted by a rigid sinusoidal asperity under sliding loading conditions to look for causes of microstructure change in the dislocation structure. The mechanistic driver is identified as the development of lattice rotations and stored energy in the subsurface, which can be quantitatively correlated to recent tribological experimental observations. Maps of surface slip initiation and substrate permanent deformation obtained from DDP calculations for varying contact size and normal load suggest ways of optimally tailoring the interface and microstructural material properties for various frictional loads.

Journal article

Kunzelmann B, Rycerz P, Xu Y, Arakere NK, Kadiric Aet al., 2023, Prediction of rolling contact fatigue crack propagation in bearing steels using experimental crack growth data and linear elastic fracture mechanics, INTERNATIONAL JOURNAL OF FATIGUE, Vol: 168, ISSN: 0142-1123

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 ?-Sn micropillars to polycrystals with intermetallics, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 855, ISSN: 0921-5093

Journal article

Worsnop FF, Lim RE, Bernier JV, Pagan DC, Xu Y, McAuliffe TP, Rugg D, Dye Det al., 2022, The influence of alloying on slip intermittency and the implications for dwell fatigue in titanium, Nature Communications, Vol: 13, ISSN: 2041-1723

Dwell fatigue, the reduction in fatigue life experienced by titanium alloysdue to holds at stresses as low as 60% of yield, has been implicated in severaluncontained jet engine failures. Dislocation slip has long been observed to bean intermittent, scale-bridging phenomenon, similar to that seen in earthquakesbut at the nanoscale, leading to the speculation that large stress bursts mightpromote the initial opening of a crack. Here we observe such stress bursts atthe scale of individual grains in situ, using high energy X-ray diffractionmicroscopy in Ti-7Al-O alloys. This shows that the detrimental effect ofprecipitation of ordered Ti_3Al is to increase the magnitude of rare pri<a> andbas<a> slip bursts associated with slip localisation. In contrast, the additionof trace O interstitials is beneficial, reducing the magnitude of bas<a> slipbursts and increasing the homogeneity between basal and prismatic <a> slip.This is further evidence that the formation of long paths for easy basal planeslip localisation should be avoided when engineering titanium alloys againstdwell fatigue.

Journal article

Xu Y, Xian J, Stoyanov S, Bailey C, Coyle R, Gourlay C, Dunne Fet al., 2022, A multi-scale approach to microstructure-sensitive thermal fatigue in solder joints, International Journal of Plasticity, Vol: 155, ISSN: 0749-6419

This paper presents a multi-scale modelling approach to investigate the underpinning mechanisms of microstructure-sensitive damage of single crystal Sn-3Ag-0.5Cu (wt%, SAC305) solder joints of a Ball Grid Array (BGA) board assembly subject to thermal cycling. The multi-scale scheme couples board-scale modelling at the continuum macro-scale and individual solder modelling at the crystal micro-scale. Systematic studies of tin crystal orientation and its role in fatigue damage have been compared to experimental observations. Crystallographic orientation is examined with respect to damage development, providing evidence-based optimal solder microstructural design for in-service thermomechanical fatigue.

Journal article

Bergsmo A, Xu Y, Poole B, Dunne FPEet al., 2022, Twin boundary fatigue crack nucleation in a polycrystalline Nickel superalloy containing non-metallic inclusions, JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, Vol: 160, ISSN: 0022-5096

Journal article

Li K-S, Cheng L-Y, Xu Y, Wang R-Z, Zhang Y, Zhang X-C, Tu S-T, Miura Het al., 2022, A dual-scale modelling approach for creep-fatigue crack initiation life prediction of holed structure in a nickel-based superalloy, INTERNATIONAL JOURNAL OF FATIGUE, Vol: 154, ISSN: 0142-1123

Journal article

Xu Y, Balint DS, Dini D, 2021, On the Origin of Plastic Deformation and Surface Evolution in Nano-Fretting: A Discrete Dislocation Plasticity Analysis, MATERIALS, Vol: 14

Journal article

Xu Y, 2021, A non-local methodology for geometrically necessary dislocations and application to crack tips, INTERNATIONAL JOURNAL OF PLASTICITY, Vol: 140, ISSN: 0749-6419

Journal article

Xu Y, Ruebeling F, Balint D, Greiner C, Dini Det al., 2021, On the origin of microstructural discontinuities in sliding contacts: A discrete dislocation plasticity analysis, INTERNATIONAL JOURNAL OF PLASTICITY, Vol: 138, ISSN: 0749-6419

Journal article

Xu Y, Gu T, Xian J, Giuliani F, Britton TB, Gourlay CM, Dunne FPEet 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

Journal article

Ruebeling F, Xu Y, Richter G, Dini D, Gumbsch P, Greiner Cet al., 2021, Normal Load and Counter Body Size Influence the Initiation of Microstructural Discontinuities in Copper during Sliding, ACS APPLIED MATERIALS & INTERFACES, Vol: 13, Pages: 4750-4760, ISSN: 1944-8244

Journal article

Xu Y, Wan W, Dunne FPE, 2021, Microstructural fracture mechanics: Stored energy density at fatigue cracks, JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, Vol: 146, ISSN: 0022-5096

Journal article

Xu Y, Joseph S, Karamched P, Fox K, Rugg D, Dunne FPE, Dye Det al., 2020, Predicting dwell fatigue life in titanium alloys using modelling and experiment, NATURE COMMUNICATIONS, Vol: 11, ISSN: 2041-1723

Journal article

Xu Y, Fox K, Rugg D, Dunne FPEet al., 2020, Cyclic plasticity and thermomechanical alleviation in titanium alloys, INTERNATIONAL JOURNAL OF PLASTICITY, Vol: 134, ISSN: 0749-6419

Journal article

Lu X, Dunne FPE, Xu Y, 2020, A crystal plasticity investigation of slip system interaction, GND density and stored energy in non-proportional fatigue in Nickel-based superalloy, INTERNATIONAL JOURNAL OF FATIGUE, Vol: 139, ISSN: 0142-1123

Journal article

Xu Y, Dini D, 2020, Capturing the hardness of coating systems across the scales, Surface and Coatings Technology, Vol: 394, ISSN: 0257-8972

A two-dimensional multi-scale modelling approach that concurrently couples discrete dislocation plasticity and crystal plasticity finite element has been applied to study the hardness variation of coating systems across different scales, covering nano- to micro-indentation. The difference in indentation size sensitivity between film and substrate gives rise to three regimes of hardness, one typically dictated by the intrinsic coating indentation size effect, which is regulated by dislocations activity, and the other two linked to the continuum response of the coating and the substrate. We propose a new hardness formula that incorporates physics-based indentation size effects of thin films into established continuum hardness transition formulae. This formula is shown to substantially improve the hardness prediction of coating systems, particularly when relative indentation depth is at the nanometre scale.

Journal article

Prastiti NG, Xu Y, Balint DS, Dunne FPEet al., 2020, Discrete dislocation, crystal plasticity and experimental studies of fatigue crack nucleation in single-crystal nickel, International Journal of Plasticity, Vol: 126, Pages: 1-14, ISSN: 0749-6419

Dislocation configurational energy and stored energy densities are determined in discrete dislocation and crystal plasticity modelling respectively and assessed with respect to experiments on single crystal nickel fatigue crack nucleation. Direct comparisons between the three techniques are provided for two crystal orientation fatigue tests. These provide confirmation that both quantities correctly identify the sites of fatigue crack nucleation and that stored energy density is a reasonable approximation to the more rigorous dislocation configurational energy. GND density is shown to be important in locating crack nucleation sites because of its role in the local configurational energy density.

Journal article

Gu T, Xu Y, Gourlay CM, Britton TBet al., 2020, In-situ electron backscatter diffraction of thermal cycling in a single grain Cu/Sn-3Ag-0.5Cu/Cu solder joint, Scripta Materialia, Vol: 175, Pages: 55-60, ISSN: 1359-6462

The heterogeneous evolution of microstructure in a single grain Cu/SAC305/Cu solder joint is investigated using in-situ thermal cycling combined with electron backscatter diffraction (EBSD). Local deformation due to thermal expansion mismatch results in heterogeneous lattice rotation within the joint, localised towards the corners. This deformation is induced by the constraint and the coefficient of thermal expansion (CTE) mismatch between the β-Sn, Cu6Sn5 and Cu at interfaces. The formation of subgrains with continuous increase in misorientation is revealed during deformation, implying the accumulation of plastic slip at the strain-localised regions and the activation of slip systems (110)[11]/2 and (0)[111]/2.

Journal article

Xu Y, Balint D, Dini D, 2019, A new hardness formula incorporating the effect of source density on indentation response: a discrete dislocation plasticity analysis, Surface and Coatings Technology, Vol: 374, Pages: 763-773, ISSN: 0257-8972

Planar discrete dislocation plasticity (DDP) calculations that simulate thin single crystal films bonded to a rigid substrate indented by a rigid wedge are performed for different values of film thickness and dislocation source density. As in prior studies, an indentation size effect (ISE) is observed when indentation depth is sufficiently small relative to the film thickness. Thedependence of the ISE on dislocation source density is quantified in this study, and a modified form of the scaling law for the dependence of hardness on indentation depth, first derived by Nix and Gao, is proposed, which is valid over the entire range of indentation depths and correlates the length scale parameter with the average dislocation source spacing. Nanoindentation experimental data from the literature are fitted using this formula, which further verifies the proposed scaling of indentation pressure on dislocation source density.

Journal article

Xu Y, Balint DB, Dini DD, 2016, A method of coupling discrete dislocation plasticity to the crystal plasticity finite element method, Modelling and Simulation in Materials Science and Engineering, Vol: 24, ISSN: 1361-651X

A method of concurrent coupling of planar discrete dislocation plasticity (DDP) and a crystal plasticityfinite element (CPFE) method was devised for simulating plastic deformation in large polycrystals withdiscrete dislocation resolution in a single grain or cluster of grains for computational efficiency;computation time using the coupling method can be reduced by an order of magnitude compared toDDP. The method is based on an iterative scheme initiated by a sub-model calculation, which ensuresdisplacement and traction compatibility at all nodes at the interface between the DDP and CPFEdomains. The proposed coupling approach is demonstrated using two plane strain problems: (i)uniaxial tension of a bi-crystal film and (ii) indentation of a thin film on a substrate. The latter was alsoused to demonstrate that the rigid substrate assumption used in earlier discrete dislocation plasticitystudies is inadequate for indentation depths that are large compared to the film thickness, i.e. theeffect of the plastic substrate modelled using CPFE becomes important. The coupling method can beused to study a wider range of indentation depths than previously possible using DDP alone, withoutsacrificing the indentation size effect regime captured by DDP. The method is general and can beapplied to any problem where finer resolution of dislocation mediated plasticity is required to studythe mechanical response of polycrystalline materials, e.g. to capture size effects locally within a largerelastic/plastic boundary value problem.

Journal article

Xu Y, Balint DS, Dini D, 2013, Multi-scale modeling of indentation and contact fatigue: A coupled CPFE/DD approach, Pages: 3416-3419

Conference paper

Wang CE, Cui DL, Yan ZY, Xu YLet al., 2011, Finite element triangle mesh generation in planar area, Jisuanji Jicheng Zhizao Xitong/Computer Integrated Manufacturing Systems, CIMS, Vol: 17, Pages: 256-260, ISSN: 1006-5911

To improve efficiency and quality of finite element method in complicated assembly design, based on studies of several current Computer Aided Engineering (CAE) platforms, a triangle mesh generation system was established. In this system, isomeric mesh data from heterogeneous Ansys and Patran platform could be imported. Size control methods of mesh element were implemented from several aspects to generate size field information. Hybrid application of Advancing Front Technique (AFT) /Delauany/mapping algorithms for multi-area problem was realized. This system was applied in coupled thermo-solid analysis on axisymmetric components of aero-engine. Application results showed that the proposed method was effective.

Journal article

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