Publications
120 results found
Jiang Y, Offer GJ, Jiang J, et al., 2020, Voltage hysteresis model for silicon electrodes for lithium ion batteries, including multi-step phase transformations, crystallization and amorphization, Journal of the Electrochemical Society, Vol: 167, Pages: 1-9, ISSN: 0013-4651
Silicon has been an attractive alternative to graphite as an anode material in lithium-ion batteries (LIBs). The development of better silicon electrodes and the optimization of their operating conditions for longer cycle life require a quantitative understanding of the lithiation/delithiation mechanisms of silicon and how they are linked to the electrode behaviors. Herein we present a zero-dimensional mechanistic model of silicon anodes in LIBs. The model, for the first time, quantitatively accounts for the multi-step phase transformations, crystallization and amorphization of different lithium-silicon phases during cycling while being able to capture the electrode behaviors under different lithiation depths. Based on the model, a linkage between the underlying reaction processes and electrochemical performance is established. In particular, the two sloping voltage plateaus at low lithiation depth are correlated with two electrochemical phase transformations and the emergence of the single broad plateau at high lithiation depth is correlated with the amorphization of c-Li15Si4. The model is then used to study the effects of crystallization rate and surface energy barriers, which clarifies the role of surface energy and particle size in determining the performance behaviors of silicon. The model is a necessary tool for future design and development of high-energy-density, longer-life silicon-based LIBs.
Zinong T, Bing Z, Jun J, et al., 2020, A study on the hot roll bonding of aluminum alloys, Procedia Manufacturing, Vol: 50, Pages: 56-62, ISSN: 2351-9789
Joining of aluminum alloys through plastic deformation and short-time diffusion bonding has significant applications in a wide range of engineering sectors. It is also important in the elimination of voids and defects in casting ingots through hot rolling processes. An aluminum alloy, AA1060, was selected for the roll-bonding experimental research at vacuum conditions. Parameters including grain size and orientation, dislocation distribution, oxide film breakage, interface welding ratio were studied and analyzed at different hot rolling conditions. Analytical techniques were also developed to analyze the deformation, stress and strain states of the material at hot rolling conditions. The results show that under the condition of 580℃ and 60% deformation, the welding interfaces disappears, and the alloy matrix structure is not overburned.
Zhang K, Shao Z, Jiang J, 2020, Effects of twin-twin interactions and deformation bands on the nucleation of recrystallization in AZ31 magnesium alloy, Materials & Design, Vol: 194, ISSN: 0264-1275
Investigating recrystallization is essential to optimize the microstructure including texture weakening and grain refinement in the rolling of magnesium alloys, thus to improve the mechanical properties of magnesium sheets for industrial applications. This research has gained an in-depth understanding of the effects of deformation bands and twin-twin interactions on recrystallization, which will potentially lead to improved manufacturing processes and mechanical properties of magnesium alloys. To study their individual effects, the recrystallization mechanisms of the room-temperature (RT)-rolled and liquid-nitrogen-temperature (LNT)-rolled samples during the annealing process were analysed with the quasi-in-situ electron backscatter diffraction method, respectively. It is found that recrystallization mainly occurred in deformation bands in the RT-rolled sample, which enhanced the initially formed texture, due to oriented and inhomogeneous grain growth. However, it is of great interest to see that the recrystallized sites were mainly located around the (101 ̅2)-(011 ̅2) twin-twin interactions with high kernel average misorientation (KAM) values in the LNT-rolled samples, resulting in rather weaker texture, finer grain size and more homogeneous microstructure, because of the randomized orientations of recrystallized grains and uniform grain growth, while almost no recrystallization was observed around the single tension twin variant.
Pruncu CI, Hopper C, Hooper PA, et al., 2020, Study of the effects of hot forging on the additively manufactured stainless steel preforms, Journal of Manufacturing Processes, Vol: 57, Pages: 668-676, ISSN: 1526-6125
The production of wrought stainless steel components in power generators can involve a combination of many manufacturing processes. These are expensive in tooling costs and number of operations, as in the Hot Forging (HF) of stainless steel turbine blades. Additive Manufacturing (AM) techniques provide a valuable opportunity to produce near-net-shaped preforms, thus avoiding the material wastage and high tooling costs associated with the intermediate stages of HF processes. This study focuses on the proposed hybrid AM and HF method, in which AM is used to produce near-net–shape preforms which are subsequently formed into net-shaped parts by HF. The HF process is used to significantly reduce the material defects introduced by, and intrinsic to, AM processes. To understand the mechanical and microstructure changes during various AM and HF conditions, single-phase 316 L stainless steel was used as the test material. Samples were produced by AM using a laser powder-bed fusion AM machine. Two different AM build directions were used to produce samples, so, as to allow evaluation of the anisotropic properties induced by AM. These samples subsequently underwent a HF process, in which various processing conditions of plastic strain and forging temperature were applied, to study the general effects of thermal plasticity on the AM microstructure. Tensile testing, optical microscopy (OM), scanning electron microscope (SEM) together with electron backscatter diffraction (EBSD) techniques were used to characterise the evolution of mechanical properties, porosity and grain size. The HF technique was found to remove defects from the AM material, resulting in enhanced mechanical strength, ductility, and isotropy. The technique therefore offers a potential alternative to conventional forging while retaining the required level of material performance.
McGilvery C, Jiang J, Rounthwaite N, et 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.
Zhang K, Zheng J-H, Huang Y, et al., 2020, Evolution of twinning and shear bands in magnesium alloys during rolling at room and cryogenic temperature, Materials and Design, Vol: 193, Pages: 1-11, ISSN: 0264-1275
Twinning and shear bands are two main deformation structures in magnesium alloys at low temperatures, however, the relationship between these two deformation structures is still under debate. To clarify their relationship and behaviours at low temperatures, rolling tests to various thickness reductions at room temperature (RT) and liquid nitrogen temperature (LNT) were conducted for AZ31 magnesium alloys. The evolutions of shear bands and twinning, and their interactions with geometrically necessary dislocation (GND), were observed during the RT- and LNT-rolling process. Abundant shear bands, evolving from {101 ̅1}-{101 ̅2} double twins (DTWs), were observed in the RT-rolled samples, while a high quantity of twins, including {101 ̅2} tension twins (TTWs), twin-twin interactions and twinning sequence, were observed in the LNT-rolled samples. More importantly, a rarely observed twinning sequence behaviour, namely primary TTW-TTW interactions→ secondary TTW-TTW interactions, creating a 45° <202 ̅1 ̅> misorientation peak, was studied. Abundant GNDs accumulated around these twin-twin interactions, twinning sequence, DTWs and shear bands, while the GND density was low around TTWs. This research delivers a systematic investigation into the deformation structures in Mg alloys during the rolling process from RT to cryogenic temperature and provides insights into the newly discovered twinning sequence and twin-twin interactions.
Yasmeen T, Zhao B, Zheng J-H, et al., 2020, The study of flow behavior and governing mechanisms of a titanium alloy during superplastic forming, Materials Science and Engineering: A, Vol: 788, Pages: 1-19, ISSN: 0921-5093
TA15 (Ti–6Al–2Zr–1Mo–1V) is a near-α titanium alloy and has wide applications in the aerospace industry because of its high strength to mass ratio, good weldability, and superior creep resistance at high temperatures up to 550 °C, compared to other titanium alloys. This study investigates the flow behavior and microstructural evolution as functions of temperatures and strain rates during deformations under the superplastic conditions at 880 °C/0.01s−1, 900 °C/0.01s−1, 880 °C/0.001s−1, and 920 °C/0.0005s−1. Results showed that this alloy exhibit excellent superplastic behavior for all selected temperatures and strain rates. The maximum tensile elongation of 1450% is achieved at 880 °C with a strain rate of 0.001s−1. Flow softening is observed under deformation conditions of 880 °C/0.01s−1 and 900 °C/0.01s−1, while strain hardening is observed at deformation conditions of 880 °C/0.001s−1 and 920 °C/0.0005s−1. These complex flow behaviors are rationalized by characterizing the underlying microstructures on the interrupted tensile samples using electron backscatter diffraction (EBSD) and backscattered electrons (BSE). The geometrically necessary dislocations (GNDs) density, which is caused by lattice rotation and misorientations and plays a vital role in the plastic constitutive behaviors, was for the first time, systematically revealed. Together with other key microstructures, i.e. grain sizes, texture, phase fractions, the results show that the dominant deformation mode changes at initial, intermediate, and final stages of the deformation. The probable deformation mechanisms, such as grain boundary sliding (GBS) under different deformation conditions, are discussed in terms of grain morphology, GNDs, and texture evolution. Also, it is observed that the β-phase transformation is accelerated during deformation and contributes to the enhancemen
Ren X, Ren H, Shang Y, et al., 2020, Microstructure evolution and mechanical properties of Ti2AlNb/TiAl brazed joint using newly-developed Ti-Ni-Nb-Zr filler alloy, PROGRESS IN NATURAL SCIENCE-MATERIALS INTERNATIONAL, Vol: 30, Pages: 410-416, ISSN: 1002-0071
Bhaduri D, Penchev P, Dimov S, et al., 2020, On the surface integrity of additive manufactured and post-processed AlSi10Mg parts, 5th CIRP CSI 2020, Publisher: Elsevier BV, Pages: 339-344, ISSN: 2212-8271
The research centres on the evaluation of surface integrity of AlSi10Mg parts produced via laser-based powder bed fusion (LPBF) process, followed by vibratory surface finishing. The alloy is chosen for its applications in lightweight components used in electronic packaging, automotive and aerospace sectors. Initial experiments involve optimisation of key LPBF process parameters by analysing the surface roughness and density data of the built parts. A Taguchi L18 orthogonal array is used for the optmisation trials with variations in the laser power (P), beam scanning speed (v), hatch spacing (H) and island size (I). Latter experimental phase deals with microhardness and microstructure assessment of heat treated LPBF specimens that are produced using the optimised LPBF parameters, i.e. P: 250 W, v: 1500 mm/s, H: 75 µm and I: 2 mm. Microhardnesses of the annealed samples reduce by ~12% with respect to the as-built parts and the values remain almost unchanged from the annealed state following solution treatment and ageing. The fish-scale like melt-pools observed on the unheat treated samples begin to fade off in the annealed specimens and completely disappear after solution treatment and ageing, with silicon particles dispersed all over the aluminium matrix. The final experimental phase involves vibratory surface finishing of the as-built LPBF parts using a vibrating ceramic media mixed with different acid and amine based liquid compounds for 1-6 hours, followed by vibrating in a maize based media for another 1-6 hours. The process aids in reducing the parts’ roughness, Sa by ~35-70%, however the effect is more prominent when using the ceramic media.
Zhou X, Shao Z, Pruncu CI, et al., 2020, A study on central crack formation in cross wedge rolling, Journal of Materials Processing Technology, Vol: 279, ISSN: 0924-0136
Cross wedge rolling (CWR) is an innovative roll forming process, used widely in the transportation industry. It has high production efficiency, consistent quality and efficient material usage. However, the continual occurrence of crack formation in the centre of the workpiece is a critical problem excluding the CWR technique from more safety-critical applications, in particular, aerospace components. The mechanisms of central fracture formation are still unclear because of a combination of complicated stress and strain states at various stages of CWR. Thus, the aim of this study is to understand the stress/strain distribution and evolution during the CWR process and identify the key variables which determine central crack formation. A comprehensive investigation was then conducted to simulate 27 experimental cases. The stress and strain distributions in the workpiece were evaluated by finite element analysis. Various damage models from literature were applied and compared. A new fracture criterion was proposed, which was able to successfully determine the central crack formation in all 27 experimental cases. This criterion can be applied in CWR tool and process design, and the enhanced understanding may enable the adoption of CWR by the aerospace industry.
Jing Y, Xiong H, Shang Y, et al., 2020, Design TiZrCuNi filler materials for vacuum brazing TA15 alloy, Journal of Manufacturing Processes, Vol: 53, Pages: 328-335, ISSN: 1526-6125
TA15 alloy with near α microstructure offers excellent high temperature mechanical properties for future gas turbine blades. To developing for the thin-wall and complex structure of this material, brazing is preferred. However, a new brazing material and process are required. In this study, a new theoretical modeling approach for designing Ti-based filler material for TA15 is presented by simulation on the bond strength in the crystal unit for the brazing filler material (BFM). The BFM chemical composition was determined as Ti-(17∼19)Zr-15Cu-15Ni wt.% to balance its strength, the toughness and relative lower melting temperature for TA15. All the fracture happened in the matrix rather than the brazing joints. It means that the strength of the brazed TA15 joint at room temperature was at least as strong as that of the matrix. Furthermore, under its service temperature (600℃), the strength of the brazing joint maintained almost 70 % of strength. The high joint strength and the ductile connection were attributed to the formed Widmanstätten structure and the sufficient diffusion of Cu and Ni solutes from the BFM across the interface. No intermetallics compound was formed in the joint. This model is established to optimizing a Ti-based filler material for achieving the excellent properties of brazed joints.
Zhou X, Shao Z, Tian F, et al., 2020, Microstructural effects on central crack formation in hot cross-wedge-rolled high-strength steel parts, Journal of Materials Science, Vol: 55, Pages: 9608-9622, ISSN: 0022-2461
Central cracking in cross-wedge-rolled workpieces results in high wastage and economic loss. Recent cross-wedge rolling tests on two batches of steel showed that one batch formed central cracks, while the other was crack-free. The batches were both nominally of the same chemical composition and thermomechanical treatment history. In addition, both batches had passed all the standard quality assessments set for conventional forging processes. It was suspected that the different cracking behaviours were due to differences in microstructure between the two as-received steel billets, and the material in cross-wedge rolling (CWR) was more sensitive to the initial microstructure compared with other forging processes due to its specific loading condition including ostensibly compression and large plastic strain. Nevertheless, no previous study of this important problem could be identified. The aim of this study is, therefore, to identify the key microstructural features determining the central crack formation behaviour in CWR. The hot workability of the as-received billets was studied under uniaxial tensile conditions using a Gleeble 3800 test machine. Scanning electron microscope with energy-dispersive X-ray spectroscopy and electron backscatter diffraction was applied to characterise, quantitatively analyse, and compare the chemical composition, phase, grain, and inclusions in these two billets, both at room temperature and also at the CWR temperature (1080 °C). Non-metallic inclusions (oxides, sulphides, and silicates) in the billets were determined to be the main cause of the reported central cracking problem. The ductility of the steels at both room and elevated temperatures deteriorated markedly in the presence of the large volumes of inclusions. Grain boundary embrittlement occurred at the CWR temperature due to the aggregation of inclusions along the grain boundaries. It is suggested that a standard on specifying the inclusion quantity and size in CWR billets b
Shao Z, Lee J, Wang J, et al., 2020, A study of various heating effects on the microstructure and mechanical properties of AA6082 using EBSD and CPFE, Journal of Alloys and Compounds, Vol: 5 nov 2019, Pages: 1-13, ISSN: 0925-8388
The solution heat treatment (SHT) process resolving hardening precipitates in high strength aluminium alloys is a critical step for high-efficient forming processes, such as Hot Form Quench (HFQ®). SHT largely determines the overall cycle time of a forming process. However, effects of heating process parameters, such as the heating rate and soaking time, on the microstructure and the associated mechanical properties of aluminium alloy 6082, one of the most commonly used aluminium alloys, for HFQ applications have not been systematically investigated. The aim of this study is to explore and understand the relationships among heat treatment conditions, grain microstructure and associated mechanical properties for AA6082. A series of uniaxial tensile tests conducted under various SHT conditions revealed significant variation on mechanical behaviour characterised by stress-strain curves. To correlate these stress-strain relationship with underlying microstructure, the grain and orientation distribution of each heat-treated sample were characterised by the electron backscatter diffraction (EBSD) technique. Due to the presence of a large number of microscopic variables, such as grain size, morphology, texture, grain boundary and etc., the crystal plasticity finite element (CPFE) modelling was employed to identify the key microscopic factors which determine the differences in the observed strength and ductility for all samples. A new CPFE model integrated with local strain criterion was proposed and validated to correlate the ductility and the strength with the material microstructure. This rigorous investigation provides more insights on how microstructure (grain size and texture) affects the mechanical behaviour for AA6082, which enables to enlarge the capability of HFQ for industrial applications.
Luan Q, Lee J, Zheng J, et al., 2020, Combining microstructural characterization with crystal plasticity and phase-field modelling for the study of static recrystallisation in pure aluminium, Computational Materials Science, Vol: 173, ISSN: 0927-0256
An in-depth understanding of the recrystallization process in alloys is critical to manufacturing metal parts with superior properties. However, the development of recrystallization model under various processing conditions is still in its early research stage and becoming an urgent demand for both the manufacturing industry and scientific research. In this work, a validated numerical model that is capable of predicting the recrystallized grain structure, incubation time for the grain nucleation and texture evolution, was developed using a Kobayashi, Warren and Carter (KWC) phase-field model coupled with crystal plasticity finite element (CPFE) analysis. Through characterising the microstructural evolution of static recrystallization (SRX) by quasi-in-situ Electron Backscatter Diffraction (EBSD) mapping, insights into dislocation density, grain nucleation position, grain growth rate and recrystallized grain orientation were established and compared with the computational model. This model enables a reliable and accurate prediction of recrystallized grain morphology and texture.
Luan Q, Jiang J, 2020, How would the deformation bands affect recrystallization in pure aluminium?, Publisher: Elsevier BV
Deformation bands (DBs), formed after plastic deformation, are known to have an impact on the recrystallization (RX) process. The exact mechanisms of how DBs influence the grain nucleation and grain growth remain unclear. In this paper, deformed single and multicrystal pure aluminium samples are annealed to explore the likely effects of DBs on the grain nucleation and the subsequent grain growth. Regarding the prediction of the recrystallized (RXed) texture, it is noticeable that the orientations of nucleated grains nearby DB are originated from the orientation in DB. Regarding the nucleated positions, it is demonstrated that potential nucleation sites are more likely located in DBs in comparison with the initial grain boundary. Regarding the rate of RX, the number of nucleated grains is also predicted to have a strong positive correlation with the area fraction of DBs which would consequently affect the kinetics of the grain growth in the deformed microstructure. All the above observations imply that the RX process is strongly controlled by the ensemble characteristics of DBs rather than the initial grain boundaries.
Luan Q, Xing H, Zhang J, et al., 2020, Experimental and crystal plasticity study on deformation bands in single crystal and multi-crystal pure aluminium, Acta Materialia, Vol: 183, Pages: 78-92, ISSN: 1359-6454
Deformation bands (DBs) formed in metals even in single crystals are known to give rise to the microstructural heterogeneities, thus contributing to some long-standing microstructure formation problems, such as the occurrence of recrystallization on the basis of deformed microstructure. Previous experimental transmission electron microscope (TEM) work has identified two types of DBs in the microscopic scale, i.e. kink bands and bands of secondary slips, showing the importance of understanding the slip activation for DBs. To extend the theory in mesoscale, single crystal and multi-crystal pure aluminium, as well as their corresponding crystal plasticity finite element (CPFE) models, are used in this paper to explore the effect of grain orientation, strain level and neighbouring grains on the formation of DBs. It is demonstrated that slip band intersection of primary and secondary slips is predicted to constrain the lattice sliding but facilitate the lattice rotation for the formation of DBs regarding the wall of DBs and its orientation. It is found that the impact of the above factors on the formation of DBs is caused by the slip field of primary slips. A sufficient amount of primary slips activated inside grains would be the key to the formation of distinct DBs with high area fraction and aspect ratio.
Shao Z, Jiang J, Lin J, 2020, Damage in advanced processing technologies, Mechanics of Materials in Modern Manufacturing Methods and Processing Techniques, Pages: 143-172, ISBN: 9780128182338
This chapter introduces the recent development for damage mechanics applied in advanced processing technologies of hot stamping and hot forming used for manufacturing complex-shaped components. Mechanical behavior and formability of materials are able to be predicted by using constitutive equations in continuum damage mechanics–based material models, which are calibrated by experimental data obtained from uniaxial and multiaxial tensile tests.
Guo Q, Lang L, Li K, et al., 2020, Research on the hydroforming regularity and process optimization control of complex aluminum alloy part with variable cross-section size, 18TH INTERNATIONAL CONFERENCE ON METAL FORMING 2020, Vol: 50, Pages: 332-336, ISSN: 2351-9789
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Zhang K, Zheng J, Shao Z, et al., 2019, Experimental investigation of the viscoplastic behaviours and microstructure evolutions of AZ31B and Elektron 717 Mg-alloys, Materials and Design, Vol: 184, Pages: 1-13, 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.
Yasmeen T, Shao Z, Zhao L, et al., 2019, Constitutive modelling for the simulation of the superplastic forming of TA15 titanium alloy, International Journal of Mechanical Sciences, Vol: 164, ISSN: 0020-7403
Titanium alloy, TA15, has a high strength-to-weight ratio, high weldability, and superior creep resistance at high temperatures up to 550°C. TA15 is difficult to deform, especially for forming complex-shaped large-scale web–rib components, due to its low plasticity, large inhomogeneous deformation and narrow processing window. The objective of this research is to model the superplastic mechanisms in TA15 alloy with equiaxed, fine grain structure, and applying the proposed constitutive model to investigate the maximum grid aspect ratio, that can be achieved in superplastic forming (SPF), for a TA15 sheet with an initial thickness of 1.2 mm. Thermo-mechanical tensile tests are conducted first to characterize the superplastic behaviour of the material in the temperature range of 880– 940°C and the strain-rate of 0.0005 – 0.01s−1. A set of mechanism-based unified visco-plastic constitutive equations has been proposed and calibrated based on the results of stress-strain data. A gradient-based optimization method is applied for the calibration of constitutive equations. The constitutive model is incorporated into FEA code through creep subroutine to check the validity of the proposed material model against the experimental SPF test of a multi-box die. Predicted sheet thickness and thinning in a die entry radius region at the end of forming are examined in detail. Preliminary results show a good agreement between the computational and experimental results.
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, ISSN: 0925-8388
Developing a fast-ageing treatment can significantly reduce the current processing time (180 °C × 9 h) of high strength AA6082 automotive components. In this study, a fast ageing treatment in supersaturated solid solution state was developed, such that the mechanical properties can be rapidly achieved after the paint bake (PB) treatment through introducing a pre-ageing (PA) treatment. The determined fast ageing method considered effects of temperature & time, heating rate and subsequent PB on the ageing response. Tensile tests and TEM observations of typical conditions were undertaken to examine evolved strength and precipitate distribution. Results showed that 210 °C was the optimum pre-ageing temperature as uniformly sized and distributed small precipitates were obtained. The final strength of about 280 MPa, that is 95% of the nominal strength for T6 temper, can be obtained within 15 min soaking for fast heating, and nearly this value for slow heating. More prolific nucleation occurred during slow heating, resulting in more finely distributed precipitates and a higher strengthening. It was observed that PB further increased the strength of over-aged alloy pre-aged at a high temperature of 240 °C. The subsequent PB enabled further nucleation of small clusters and growth of the pre-ageing-induced precipitates which were smaller than 20 nm. This resulted in an improvement in the material strength potentially to satisfy the safety requirements in automotive industry.
Birosca S, Liu G, Ding R, et al., 2019, The dislocation behaviour and GND development in a nickel based superalloy during creep, International Journal of Plasticity, Vol: 118, Pages: 252-268, ISSN: 0749-6419
In the current study, dislocation activity and storage during creep deformation in a nickel based superalloy (Waspaloy) were investigated, focussing on the storage of geometrically necessary (GND) and statistically stored (SSD) dislocations. Two methods of GND density calculation were used, namely, EBSD Hough Transformation and HR-EBSD Cross Correlation based methods. The storage of dislocations, including SSDs, was investigated by means of TEM imaging. Here, the concept of GND accumulation in soft and hard grains and the effect of neighbouring grain orientation on total dislocation density was examined. Furthermore, the influence of applied stress (below and above the yield stress of Waspaloy) during creep on deformation micro-mechanism and dislocation density was studied. It was demonstrated that soft grains provided pure shear conditions on at least two octahedral (111) slip systems for easy dislocation movement. This allowed dislocations to reach the grain boundary without significant geometrically necessary dislocation accumulation in the centre of the grain. Hence, the majority of the soft grains appeared to have minimum GND density in the centre of the grain with high GND accumulation in the vicinity of the grain boundaries. However, the values and width of accumulated GND depended on the surrounding grain orientations. Furthermore, it was shown that the hard grains were not favourably oriented for octahedral slip system activation leading to a grain rotation in order to activate any of the available slip systems. Eventually, (i) the hard grain resistance to deformation and (ii) neighbouring grain resistance for the hard grain reorientation caused high GND density on a number of octahedral (111) slip systems. The results also showed that during creep below the yield stress of Waspaloy (500 MPa/700 °C), the GND accumulation was relatively low due to the insufficient macroscopic stress level. However, the regions near grain boundaries showed high GND densit
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
It is challenging to quantify the geometrically necessary dislocation (GND) density at the nanoscale using conventional electron backscatter diffraction due to its limited spatial resolution. To overcome this problem, in this study, the transmission Kikuchi diffraction (TKD) technique is used to measure lattice orientation and to calculate the corresponding nanoscale GND density. Using the TKD method, a variation of GND density from 6 × 1014 to 1016 m−2 has been measured in a welded super duplex stainless steel sample. The distribution of dislocation density is shown to be in good agreement with transmission electron microscope (TEM) result. Compared with dislocation measurements obtained by TEM, the TKD–GND method is revealed to be a relatively accurate, fast and accessible method.
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.
Li J, Jian H, Chen Y, et al., 2018, Studies of Graphdiyne-ZnO Nanocomposite Material and Application in Polymer Solar Cells, SOLAR RRL, Vol: 2, ISSN: 2367-198X
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- Citations: 14
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.
Chen B, Jiang J, Dunne FPE, 2018, 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.
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