38 results found
Zheng J-H, Dong Y, Zheng K, et al., 2019, Experimental investigation of novel fast–ageing treatments for AA6082 in supersaturated solid solution state, Journal of Alloys and Compounds, Vol: 810, Pages: 151934-151934, ISSN: 0925-8388
Zhang K, Zheng J, Shao Z, et al., Experimental investigation of the viscoplastic behaviours and microstructure evolutions of AZ31B and Elektron 717 Mg-alloys, Materials and Design, ISSN: 0264-1275
An insight into the thermo-mechanical behaviours of AZ31B and Elektron 717 magnesium alloys under the hot stamping conditions was established. High-temperature tensile tests (i.e. 350–450 °C) at a strain rate of 0.1 to 5/s were conducted to examine the material viscoplastic behaviours. Additionally, microstructure characterizations were performed, using the electron backscatter diffraction (EBSD), on the deformed samples to capture the underlying deformation mechanisms. Dynamic recrystallization (DRX) and texture formation were observed during the deformation at high temperature in both alloys and are the primary factors that affect the viscoplastic behaviours. The yield stress of both alloys reduced with increasing temperatures and reducing strain rates. More importantly, the ductility of the samples increased with both the temperatures and the strain rates. The higher ductility at higher strain rates was primarily attributed to finer grains and the slightly weakened textures, enabling a more uniform deformation. A maximum ductility of ~2 was observed in AZ31B under 450 °C at 1/s while ~0.9 in Elektron 717 under the identical condition. The addition of rare earth elements in Elektron 717 may suppress the active DRX. The recrystallization type was identified as discontinuous DRX. The research findings deliver understandings on the viscoplastic behaviours and the deformation mechanisms of AZ31B and Elektron 717 under the hot stamping conditions and provide scientific guidance for feasibility study on applying hot stamping technique to Mg-alloy for forming complex geometry components.
Wu M, Murphy J, Jiang J, et al., 2019, Microstructural evolution of mechanically deformed polycrystalline silicon for kerfless photovoltaics, physica status solidi (a), Vol: 216, ISSN: 1862-6300
Silicon wafers for photovoltaics could be produced without kerf loss by rolling, provided sufficient control of defects such as dislocations can be achieved. A study using mainly high resolution electron backscatter diffraction (HR‐EBSD) of the microstructural evolution of Siemens polycrystalline silicon feedstock during a series of processes designed to mimic high temperature rolling is reported here. The starting material is heavily textured and annealing at 1400 °C results in 90% recrystallization and a reduction in average geometrically necessary dislocation (GND) density from >1014 to 1013 m−2. Subsequent compression at 1150 °C – analogous to rolling – produce sub‐grain boundaries seen as continuous curved high GND content linear features spanning grain interiors. Post‐deformation annealing at 1400 °C facilitates a secondary recrystallization process, resulting in large grains typically of 100 μm diameter. HR‐EBSD gives the final average GND density in as 3.2 × 1012 m−2. This value is considerably higher than the dislocation density of 5 × 1010 m−2 from etch pit counting, so the discrepancy is investigated by direct comparison of GND maps and etch pit patterns. The GND map from HR‐EBSD gives erroneously high values at the method's noise floor (≈1012 m−2) in regions with low dislocation densities.
Liang XZ, Dodge MF, Jiang J, et al., 2019, Using transmission Kikuchi diffraction in a scanning electron microscope to quantify geometrically necessary dislocation density at the nanoscale, ULTRAMICROSCOPY, Vol: 197, Pages: 39-45, ISSN: 0304-3991
Shao Z, Jiang J, Lin J, 2018, Feasibility study on direct flame impingement heating applied for the solution heat treatment, forming and cold die quenching technique, Journal of Manufacturing Processes, Vol: 36, Pages: 398-404, ISSN: 1526-6125
The solution heat treatment, forming and cold die quenching (HFQ) process has been developed and adopted for forming high strength complex-shaped components of light alloys in the automotive industry. In order to exploit and increase the competitiveness of this technology, production cycle time and manufacturing costs need to be reduced to enable high productivity and energy efficiency. This can be realised by reducing the cycle time for heating a metallic sheet to its solution heat treatment temperature during the HFQ process, and by decreasing post ageing time. Rapid heating methods are capable of providing a solution to be integrated into this novel forming technique of HFQ. This paper presents feasibility study on the adoption of the direct flame impingement (DFI) heating method that has a high potential for non-ferrous blanks to achieve higher heating rate in HFQ processes, compared to convection heating in a conventional furnace. The adaptability of DFI heating for HFQ process has been validated, in terms of capability of high heating rate, quality of surface layer examination and lap-shear strength measurement of bonded samples.
Zhao C, Stewart D, Jiang J, et al., 2018, A comparative assessment of iron and cobalt-based hard-facing alloy deformation using HR-EBSD and HR-DIC, Acta Materialia, Vol: 159, Pages: 173-186, ISSN: 1359-6454
Three iron-based alloys (Nitronic 60, Tristelle 5183 and RR2450) and a cobalt alloy (Stellite 6) are studied using bend-testing to induce progressive straining and both high resolution DIC and EBSD are utilized to provide quantitative characterization of the deformation mechanisms. The roles of austenite, ferrite and carbide/silicide phases are investigated, together with how each contributes to slip activation and localisation, GND development and hardening through to particle pull-out and fracture. The observed mechanisms are discussed in the context of galling performance.The results suggest that a distribution of fine precipitates, both intra-granular and at grain/phase boundaries, promote more homogeneous and distributed slip, and the development of distributed higher densities of GNDs. The latter promotes hardening which in turn also facilitates homogeneity of deformation and potentially better galling resistance. A uniform size of fine precipitates is also helpful; large silicides lead to particle fracture and pull-out, likely highly damaging under conditions of sliding contact and galling.
Jiang J, qinmeng L, 2018, Static recrystallization study on pure aluminium using crystal plasticity finite element and phase-field modelling, Metal Forming 2018, Publisher: Elsevier, Pages: 1800-1807, ISSN: 2351-9789
In-depth understanding of the recrystallization process in alloys is critical for generating desirable small grains and weak textured microstructure, which provides high strength and toughness for metal formed parts. The manufacturing industry has a high demand for a valid computational model to accurately predict the level of recrystallization and recrystallized grain size under different strain paths and temperatures. However, current understanding and numerical calculation have not been linked properly for a reliable, physically based model to simulate the deformation and annealing process. Our phase-field model coupled with crystal plasticity simulations, which is based on the theory of stored energy minimization, enables a reliable prediction on the microstructure evolution under different processing routes. We hope that this modelling work provides a solution for the prediction of some long standing microstructure formation problems.
Zhao L, Zhang X, Deng T, et al., 2018, Develop an effective oxygen removal method for copper powder, Advanced Powder Technology, Vol: 29, Pages: 1904-1912, ISSN: 0921-8831
At present, one of crucial limitations for the hot isostatically pressed (HIPed) Cu-3Ag-0.5Zr alloy, which is used on the combustion chamber liner of aerospace engine, is the high oxygen content, which easily results in the intergranular fracture under high temperature, pressure, liquid hydrogen and oxygen environment during operation. In this study, a novel effective oxygen control method is developed, for which vacuum degassing process is integrated with a flowing hydrogen reduction reaction at an elevated temperature before HIP. For this technique, a container is designed with two gas pipes for hydrogen inflow and outflow, so the hydrogen circulation can be established. Allowing hydrogen to react effectively with oxygen, the oxygen content of HIPed alloy is found to drop significantly from 140 ppm (raw powder) to 28 ppm, which is equivalent to the oxygen-free copper and copper alloys. As a result of the reduction, no prior particle boundaries could be observed in the low oxygen content material. Although the tensile strength of the materials with and without employing this technique does not vary significantly, the ductility of low oxygen content material has improved by about 70% at 500°C. This significant improvement of ductility is critical to ensure the safety critical PM components.
Galindo-Nava EI, Jing YJ, Jiang J, 2018, Predicting the hardness and solute distribution during brazing of Ti-6Al-4V with TiZrCuNi filler metals, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 712, Pages: 122-126, ISSN: 0921-5093
Jing Y, Gao X, Su D, et al., 2018, The effects of Zr level in Ti-Zr-Cu-Ni brazing fillers for brazing Ti-6Al-4V, JOURNAL OF MANUFACTURING PROCESSES, Vol: 31, Pages: 124-130, ISSN: 1526-6125
Chen B, Jiang J, Dunne FPE, 2017, Is stored energy density the primary meso-scale mechanistic driver for fatigue crack nucleation?, International Journal of Plasticity, Vol: 101, Pages: 213-229, ISSN: 0749-6419
Fatigue crack nucleation in a powder metallurgy produced nickel alloy containing a non-metallic inclusion has been investigated through integrated small-scale bend testing, quantitative characterisation (HR-DIC and HR-EBSD) and computational crystal plasticity which replicated the polycrystal morphology, texture and loading. Multiple crack nucleations occurred at the nickel matrix-inclusion interface and both nucleation and growth were found to be crystallographic with highest slip system activation driving crack direction. Local slip accumulation was found to be a necessary condition for crack nucleation, and that in addition, local stress and density of geometrically necessary dislocations are involved. Fatemi-Socie and dissipated energy were also assessed against the experimental data, showing generally good, but not complete agreement. However, the local stored energy density (of a Griffith-Stroh kind) identified all the crack nucleation sites as those giving the highest magnitudes of stored energy.
Jiang J, Hooper P, Li N, et al., 2017, An integrated method for net-shape manufacturing components combining 3D additive manufacturing and compressive forming processes, ICTP 2017, Publisher: Elsevier, Pages: 1182-1187, ISSN: 1877-7058
Published by Elsevier Ltd. Additive manufactured (AM) or 3D printed metallic components suffer poor and inconsistent mechanical properties due to the presence of a large number of micro-voids, residual stress and microstructure inhomogeneity. To overcome these problems, a new forming process has been proposed, which effectively combines AM and compressive forming. The aim of this study is to prove the feasibility of this newly proposed method by providing preliminary results. Thus, we compared the tensile performance of hot-forged additive manufactured stainless steel 316L samples to none-hot-forged additive manufactured ones. Significant improvement in mechanical properties has been found in the tensile tests as well hardness test. In addition, our EBSD characterized grain orientation maps at each stage of the process revealed the corresponding microstructure revolution which provides insights into underlying mechanistic.
Jing Y, Su D, Yue X, et al., 2017, The development of high strength brazing technique for Ti-6Al-4V using TiZrCuNi amorphous filler, Materials Characterization, Vol: 131, Pages: 526-531, ISSN: 1044-5803
The brazing joint of the Ti-6Al-4V alloy was produced with a designed brazing filler alloy and the optimized brazing temperature which is lower than the β-phase transformation of the matrix. The strength and the ductility of brazing joined Ti-6Al-4V samples were evaluated by conventional tensile tests with a DIC 2D–strain field measurement. The Widmanstätten microstructure with no voids or cracks or intermetallic compounds was found throughout the joint with a width of β-lamellar as ~ 1 μm. Due to the fine acicular α-Widmanstätten and β-lamellar, and the uniformly diffused filler elements throughout the entire joint, the strength of the joint was as much as the matrix. In addition, the hardness test results agreed well with the tensile strength tests. All fractures occurred in the matrix rather than the brazing joints. Furthermore, the maximum local tensile strain was measured as 20% in the matrix, while under the same stress, the brazing joint only reached 6.3% tensile plastic strain. Thus, the mechanical properties of the joint with the associated microstructure demonstrated that a successful brazing filler alloy has been developed for the Ti-6Al-4V alloy.
Chen B, Jiang J, Dunne F, 2017, Microstructurally-sensitive fatigue crack nucleation in Ni-based single and oligo crystals, Journal of the Mechanics and Physics of Solids, Vol: 106, Pages: 15-33, ISSN: 1873-4782
An integrated experimental, characterisation and computational crystal plasticity study of cyclic plastic beam loading has been carried out for nickel single crystal (CMSX4) and oligocrystal (MAR002) alloys in order to assess quantitatively the mechanistic drivers for fatigue crack nucleation.The experimentally validated modelling provides knowledge of key microstructural quantities (accumulated slip, stress and GND density) at experimentally observed fatigue crack nucleation sites and it is shown that while each of these quantities is potentially important in crack nucleation, none of them in its own right is sufficient to be predictive. However, the local (elastic) stored energy density, measured over a length scale determined by the density of SSDs and GNDs, has been shown to predict crack nucleation sites in the single and oligocrystals tests. In addition, once primary nucleated cracks develop and are represented in the crystal model using XFEM, the stored energy correctly identifies where secondary fatigue cracks are observed to nucleate in experiments. This (Griffith-Stroh type) quantity also correctly differentiates and explains intergranular and transgranular fatigue crack nucleation.
Rounthwaite N, McGilvery CM, Jiang J, et al., 2017, A chemical and morphological study of diesel injector nozzle deposits - insights into their formation and growth mechanisms, SAE 2017 World Congress and Exhibition, Publisher: SAE International, Pages: 106-114, ISSN: 1946-3960
Modern diesel passenger car technology continues to develop rapidly in response to demanding emissions, performance, refinement, cost and fuel efficiency requirements. This has included the implementation of high pressure common rail fuel systems employing high precision injectors with complex injection strategies, higher hydraulic efficiency injector nozzles and in some cases <100µm nozzle hole diameters. With the trend towards lower diameter diesel injector nozzle holes and reduced cleaning through cavitation with higher hydraulic efficiency nozzles, it is increasingly important to focus on understanding the mechanism of diesel injector nozzle deposit formation and growth. In this study such deposits were analysed by cross-sectioning the diesel injector along the length of the nozzle hole enabling in-depth analysis of deposit morphology and composition change from the inlet to the outlet, using state-of-the-art electron microscopy techniques. Deposits produced in the injector nozzles of the industry standard fouling test (CEC F-98-08 DW10B bench engine) were compared with those formed in a vehicle driven on a chassis dynamometer, using a drive cycle more representative of real world vehicle conditions, to explore the effects of differing drive cycles and engine technologies. Fouling in all tests was accelerated with the addition of 1ppm zinc neodecanoate, as specified in the CEC DW10B test. This in-depth characterisation revealed a complex multi-layered system of deposits inside the diesel injector nozzle. Through analysing these layers the mechanisms enabling the initial deposit formation and growth can be postulated.
Jiang J, Dunne F, Britton T, 2017, Toward predictive understanding of fatigue crack nucleation in Ni-based Superalloys, JOM, Vol: 69, Pages: 863-871, ISSN: 1047-4838
Predicting when and where materials fail is a holy grail for structural materials engineering. Development of a predictive capability in this domain will optimize the employment of existing materials, as well as rapidly enhance the uptake of new materials, especially in high-risk, high-value applications, such as aeroengines. In this article, we review and outline recent efforts within our research groups that focus on utilizing full-field measurement and calculation of micromechanical deformation in Ni-based superalloys. In paticular, we employ high spatial resolution digital image correlation (HR-DIC) to measure surface strains and a high-angular resolution electron backscatter diffraction technique (HR-EBSD) to measure elastic distortion, and we combine these with crystal plasticity finite element (CPFE) modeling. We target our studies within a system of samples that includes single, oligo, and polycrystals where the boundary conditions, microstructure, and loading configuration are precisely controlled. Coupling of experiment and simulation in this manner enables enhanced understanding of crystal plasticity, as demonstrated with case studies in deformation compatibility; spatial distributions of slip evolution; deformation patterning around microstructural defects; and ultimately development of predictive capability that probes the location of microstructurally sensitive fatigue cracks. We believe that these studies present a careful calibration and validation of our experimental and simulation-based approaches and pave the way toward new understanding of crack formation in engineering alloys.
Dunne FPE, Guan Y, Britton TB, et al., 2016, Crystal Plasticity Modelling and HR-DIC Measurement of Slip Activation and Strain Localisation in Single and Oligo-crystal Ni Alloys under Fatigue, International Journal of Plasticity, Vol: 88, Pages: 70-88, ISSN: 0749-6419
Single crystal (CMSX4) and oligocrystal (MAR002) nickel have been studied using three-point beambending under conditions of cyclic loading. SEM images have enabled identification of slip activation,and high resolution digital image correlation has been utilized to quantify the developing strain fieldsand the strain localization in both single and oligocrystals in fatigue. The single and oligocrystalmicrostructures have been replicated within crystal plasticity finite element models and the fatigueloading analysed such that grain-by-grain comparisons of slip may be carried out. Single and multipleslip activation, slip localisation and microstructure-sensitive stress evolution have been examined.Single crystal bend fatigue gives rise to non-symmetric slip fields and localisation depending oncrystallographic orientation. Modelling correctly captures slip activation and the developing nonsymmetricslip fields. Oligocrystal slip is markedly heterogeneous, with grain misorientations drivingstrong variations, also reasonably captured by the model. Microstructure behaviour is found to varyspatially and include elastic-plastic hysteresis which is stable, and which undergoes mean stressrelaxation so that plastic shakedown occurs. Remarkable variations occur between locations eitherside of grain boundaries, providing appropriate opportunities for fatigue crack nucleation.
Peng Z, Tian G, Jiang J, et al., 2016, Mechanistic behaviour and modelling of creep in powder metallurgy FGH96 nickel superalloy, Materials Science and Engineering A, Vol: 676, Pages: 441-449, ISSN: 0921-5093
The creep properties of a nickel-based superalloy at 700 °C and 690 MPa resulting from differing aging heat treatments have been investigated. The heat treatments gave rise to significantly different tertiary γ′ precipitate distributions which in turn influence the propensity for precipitate shearing. The creep life was found to decrease with an increase of volume fraction of tertiary γ′. It is shown that the γ′ precipitates undergo shearing by matrix dislocations resulting in residual stacking faults which have been identified from TEM studies.A physically-based crystal slip model for creep deformation in FGH96 has been developed. A critical γ′ precipitate size is found to exist above which precipitate shearing occurs by strongly-coupled dislocations pairs. The critical size for FGH96 superalloy is less than 15 nm, which is smaller than most of the γ′ precipitates in the aged treated samples. A bimodal precipitate hardening model has been presented from which the slip strength resulting from differing heat treatments may be determined. Creep strain rate is found to decrease with increasing slip strength. The crystal slip model successfully captures the effect of heat treatment (in terms of γ′ precipitate distributions) on resulting creep behaviour in alloy FGH96.
Jiang J, Yang J, Zhang T, et al., 2016, Microstructurally sensitive crack nucleation around inclusions in powder metallurgy nickel based superalloys, Acta Materialia, Vol: 117, Pages: 333-344, ISSN: 1359-6454
Nickel based superalloys are used in high strength, high value applications, such as gas turbine discs in aeroengines. In these applications the integrity of the disc is critical and therefore understanding crack initiation mechanisms is of high importance. With an increasing trend towards powder metallurgy routes for discs, sometimes unwanted non-metallic inclusions are introduced during manufacture. These inclusions vary in size from ~ 10 μm to 200 μm which is comparable to the grain size of the Nickel based superalloys. Cracks often initiate near these inclusions and the precise size, shape, location and path of these cracks are microstructurally sensitive. In this study, we focus on crack initiation at the microstructural length scale using a controlled three-point bend test, with the inclusion deliberately located within the tensile fibre of the beam. Electron backscatter diffraction (EBSD) is combined with high spatial resolution digital image correlation (HR-DIC) to explore full field plastic strain distributions, together with finite element modelling, to understand the micro-crack nucleation mechanisms. This full field information and controlled sample geometry enable us to systematically test crack nucleation criteria. We find that a combined stored energy and dislocation density provide promising results. These findings potentially facilitate more reliable and accurate lifing prediction tools to be developed and applied to engineering components.
Wan VVC, Cuddihy MA, Jiang J, et al., 2016, An HR-EBSD and computational crystal plasticity investigation of microstructural stress distributions and fatigue hotspots in polycrystalline copper, Acta Materialia, Vol: 115, Pages: 45-57, ISSN: 1873-2453
High resolution EBSD studies on a deformed copper polycrystal have been carried out to quantify the microstructural residual stress distributions, and those of stress state including triaxiality of importance in defect nucleation studies. Crystal plasticity analysis of a representative, similarly textured, model polycrystal has been carried out showing that the experimental distributions of microstructural residual stress components, effective stress, hydrostatic stress and stress triaxiality are well captured.The crystal model enables point-wise microstructural Schmid factors to be calculated both globally (ie with respect to the macroscopic remote loading) and locally from full knowledge of the grain-level stress state. Significant differences are demonstrated such that global Schmid analysis tends to overestimate slip activity and the frequency of high Schmid factors, indicating that the local microstructural heterogeneity is significant and caution is necessary in interpreting polycrystal behaviour using global Schmid factors.A stored energy criterion for fatigue crack nucleation indicates that preferential sites for fatigue crack nucleation are local to grain boundaries (as opposed to triple junctions), and that hard-soft grain interfaces where high GND densities develop are preferable.
Zhang T, Jiang J, Britton B, et al., 2016, Crack nucleation using combined crystal plasticity modelling, high-resolution digital image correlation and high-resolution electron backscatter diffraction in a superalloy containing non-metallic inclusions under fatigue, Proceedings of the Royal Society A: Mathematical, Physical & Engineering Sciences, Vol: 472, ISSN: 1471-2946
A crystal plasticity finite-element model, which explicitly and directly represents the complex microstructures of a non-metallic agglomerate inclusion within polycrystal nickel alloy, has been developed to study the mechanistic basis of fatigue crack nucleation. The methodology is to use the crystal plasticity model in conjunction with direct measurement at the microscale using high (angular) resolution-electron backscatter diffraction (HR-EBSD) and high (spatial) resolution-digital image correlation (HR-DIC) strain measurement techniques. Experimentally, this sample has been subjected to heat treatment leading to the establishment of residual (elastic) strains local to the agglomerate and subsequently loaded under conditions of low cyclic fatigue. The full thermal and mechanical loading history was reproduced within the model. HR-EBSD and HR-DIC elastic and total strain measurements demonstrate qualitative and quantitative agreement with crystal plasticity results. Crack nucleation by interfacial decohesion at the nickel matrix/agglomerate inclusion boundaries is observed experimentally, and systematic modelling studies enable the mechanistic basis of the nucleation to be established. A number of fatigue crack nucleation indicators are also assessed against the experimental results. Decohesion was found to be driven by interface tensile normal stress alone, and the interfacial strength was determined to be in the range of 1270–1480 MPa.
Britton TB, Jiang J, Vilata-Clemente A, et al., 2016, Tutorial: Crystal Orientations and EBSD – or which way is up?, Materials Characterization, Vol: 117, Pages: 113-126, ISSN: 1873-4189
Electron backscatter diffraction (EBSD) is an automated technique that can measure the orientation of crystals in a sample very rapidly. There are many sophisticated software packages that present measured data. Unfortunately, due to crystal symmetry and differences in the set-up of microscope and EBSD software, there may be accuracy issues when linking the crystal orientation to a particular microstructural feature. In this paper we outline a series of conventions used to describe crystal orientations and coordinate systems. These conventions have been used to successfully demonstrate that a consistent frame of reference is used in the sample, unit cell, pole figure and diffraction pattern frames of reference. We establish a coordinate system rooted in measurement of the diffraction pattern and subsequent linking to all other coordinate systems. A fundamental outcome of this analysis is to note that the beamshift coordinate system needs to be precisely defined for consistent 3D microstructure analysis. This is supported through a series of case studies examining particular features of the microscope settings and/or unambiguous crystallographic features. These case studies can be generated easily in most laboratories and represent an opportunity to demonstrate confidence in use of recorded orientation data. Finally, we include a simple software tool, written in both MATLAB® and Python, which the reader can use to compare consistency with their own microscope set-up and which may act a springboard for further offline analysis.
Wan VVC, Jiang J, MacLachlan DW, et al., 2016, Microstructure-sensitive fatigue crack nucleation in a polycrystalline Ni superalloy, International Journal of Fatigue, Vol: 90, Pages: 181-190, ISSN: 1879-3452
Large-grained polycrystalline Ni alloy RS5 has been tested in fatigue. Morphology and texture have been characterised using EBSD and utilised to construct representative 3D finite element crystal plasticity models. A stored energy criterion has been used to predict scatter in fatigue crack nucleation life and the results compared with experimental findings. Good quantitative prediction of experimental fatigue lives is obtained. The observed progressive increase in scatter with decreasing strain range is captured. The stored energies for fatigue crack nucleation determined for Ni alloy RS5 and ferritic steel and were found to be 13,300 J/m2 and 580 J/m2 respectively, showing very good consistency with the corresponding Griffith fracture energies of 48,700 J/m2 for Ni alloy and 1900 J/m2 for ferritic steel.Local microstructural variations are shown to influence corresponding grain-level stress–strain response. At the microstructural level, purely elastic, reversed plastic and ratcheting behaviour are all observed. In addition, plastic and elastic shakedown are also found to occur which depend upon features of the microstructure and the nature of the applied loading. These phenomena all influence fatigue crack nucleation.
Jiang J, Zhang T, Dunne F, et al., 2016, Deformation Compatibility in a Single Crystalline Ni Superalloy, Proceedings of the Royal Society A: Mathematical, Physical & Engineering Sciences, Vol: 472, ISSN: 1471-2946
Deformation in materials is often complex and requires rigorous understanding to predict engineering component lifetime. Experimental understanding of deformation requires utilization of advanced characterization techniques, such as high spatial resolution digital image correlation (HR-DIC) and high angular resolution electron backscatter diffraction (HR-EBSD), combined with clear interpretation of their results to understand how a material has deformed. In this study, we use HR-DIC and HR-EBSD to explore the mechanical behaviour of a single-crystal nickel alloy and to highlight opportunities to understand the complete deformations state in materials. Coupling of HR-DIC and HR-EBSD enables us to precisely focus on the extent which we can access the deformation gradient, F, in its entirety and uncouple contributions from elastic deformation gradients, slip and rigid body rotations. Our results show a clear demonstration of the capabilities of these techniques, found within our experimental toolbox, to underpin fundamental mechanistic studies of deformation in polycrystalline materials and the role of microstructure.
Jiang J, Yang J, Zhang T, et al., 2015, On the mechanistic basis of fatigue crack nucleation in Ni superalloy containing inclusions using high resolution electron backscatter diffraction, Acta Materialia, Vol: 97, Pages: 367-379, ISSN: 1873-2453
A series of interrupted three-point bend low-cycle fatigue tests were carried out on a powder metallurgy FHG96 nickel superalloy sample containing non-metallic inclusions. High resolution electron backscatter diffraction (HR-EBSD) was used to characterise the distribution and evolution of geometrically necessary dislocation (GND) density, residual stress and total dislocation density near a non-metallic inclusion. A systematic study of room temperature cyclic deformation is presented in which slip localisation, cyclic hardening, ratcheting and stabilisation occur, through to crack formation and microstructurally-sensitive propagation. Particular focus is brought to bear at the inclusion–matrix interface. Complex inhomogeneous deformation structures were directly observed from the first few loading cycles, and these structures were found not to vary significantly with increasing number of cycles. A clear link was observed between crack nucleation site and microstructurally-sensitive growth path and the spatially-resolved sites of extreme values of residual stress and GND density.
Zhang T, Jiang J, Shollock BA, et al., 2015, Slip localization and fatigue crack nucleation near a non-metallic inclusion in polycrystalline nickel-based superalloy, Materials Science and Engineering A: Structural Materials: Properties, Microstructure and Processing, Vol: 641, Pages: 328-339, ISSN: 1873-4936
Fatigue crack nucleation at a non-metallic agglomerate inclusion has been studied by high spatial resolution Digital Image Correlation (HR-DIC) and high angular resolution Electron Backscatter Diffraction (HR-EBSD). Spatial and temporal characterization and correlation of deformation with underlying microstructures has been performed, with distributions of plastic strain measured from HR-DIC; and residual stress and density of geometrically necessary dislocations (GND) measured from HR-EBSD. Initial residual stress and GND fields, as a consequence of differing thermal expansivities in the metallic and oxide phases, localized around the agglomerate have been quantified using HR-EBSD. The localization of the pre-existing stress and dislocation states appear to lead to early onset of plasticity upon subsequent mechanical loading. Heterogeneous distributions of plastic strain have been observed in the course of the fatigue test by HR-DIC. Crack nucleation via agglomerate/nickel interface decohesion and particle fracture has been demonstrated and this is correlated with the elevation in strain and dislocation density. The measurements of residual stress, strain, and dislocation density provide key information for the mechanisms of fatigue cracking and the development of damage nucleation criteria in these material systems.
Jiang J, Ben Britton T, Wilkinson AJ, 2015, The orientation and strain dependence of dislocation structure evolution in monotonically deformed polycrystalline copper, INTERNATIONAL JOURNAL OF PLASTICITY, Vol: 69, Pages: 102-117, ISSN: 0749-6419
Jiang J, Britton TB, Wilkinson AJ, 2015, Evolution of intragranular stresses and dislocation densities during cyclic deformation of polycrystalline copper, Acta Materialia, Vol: 94, Pages: 193-204, ISSN: 1873-2453
Tong V, Jiang J, Wilkinson AJ, et al., 2015, The effect of pattern overlap on the accuracy of high resolution electron backscatter diffraction measurements, Ultramicroscopy, Vol: 155, Pages: 62-73, ISSN: 0304-3991
High resolution, cross-correlation-based, electron backscatter diffraction (EBSD) measures the variation of elastic strains and lattice rotations from a reference state. Regions near grain boundaries are often of interest but overlap of patterns from the two grains could reduce accuracy of the cross-correlation analysis. To explore this concern, patterns from the interior of two grains have been mixed to simulate the interaction volume crossing a grain boundary so that the effect on the accuracy of the cross correlation results can be tested. It was found that the accuracy of HR-EBSD strain measurements performed in a FEG-SEM on zirconium remains good until the incident beam is less than 18 nm from a grain boundary. A simulated microstructure was used to measure how often pattern overlap occurs at any given EBSD step size, and a simple relation was found linking the probability of overlap with step size.
Wilkinson AJ, Tarleton E, Vilalta-Clemente A, et al., 2014, Measurement of probability distributions for internal stresses in dislocated crystals, APPLIED PHYSICS LETTERS, Vol: 105, ISSN: 0003-6951
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