X-Genes66.67%

XCL

European Journal of Cell Biology

Fungal lectin, XCL, is internalized via clathrin-dependent endocytosis and facilitates uptake of other molecules

Author links open overlay panelFrédéricFrancisaLaurentPaquereaub

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https://doi.org/10.1078/0171-9335-00338Get rights and content

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The lectin isolated from Xerocomus chrysenteron (XCL) displays a toxic activity towards insects. In order to assess its possible mode of action and to gather useful data for its potential use in insect-resistant transgenic plants, we investigated the effects of XCL at the cellular level. Immunofluorescence microscopy studies revealed that XCL is rapidly internalized into small endocytic vesicles that further coalesce in the perinuclear region. We show that XCL is endocytosed by the clathrin-dependent pathway, and is delivered to late endosome/lysosome compartments. The internalization of XCL seems to be general since it occurs in different cell types such as insect (SF9) or mammalian (NIH-3T3 and Hela) cell lines. In the presence of XCL, the uptake of GFP and BSA is greatly enhanced, demonstrating that XCL facilitates endocytosis. Thus, XCL could serve as a delivery agent to facilitate the endocytosis of proteins that do not enter the cell alone.

A newly defined family of fungal lectins displays no significant sequence similarity to any protein in the databases. These proteins, made of about 140 amino acid residues, have sequence identities ranging from 38% to 65% and share binding specificity to N-acetyl galactosamine. One member of this family, the lectin XCL from Xerocomus chrysenteron, induces drastic changes in the actin cytoskeleton after sugar binding at the cell surface and internalization, and has potent insecticidal activity. The crystal structure of XCL to 1.4 Å resolution reveals the architecture of this new lectin family. The fold of the protein is not related to any of the several lectin folds documented so far. Unexpectedly, the structure similarity is significant with actinoporins, a family of pore-forming toxins. The specific structural features and sequence signatures in each protein family suggest a potential sugar binding site in XCL and a possible evolutionary relationship between these proteins. Finally, the tetrameric assembly of XCL reveals a complex network of protomer–protomer interfaces and generates a large, hydrated cavity of 1000 Å3, which may become accessible to larger solutes after a small conformational change of the protein.

European Journal of Cell Biology

Volume 82, Issue 10, October 2003, Pages 515-522

Fungal lectin, XCL, is internalized via clathrin-dependent endocytosis and facilitates uptake of other molecules

Author links open overlay panelFrédéricFrancisaLaurentPaquereaub

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https://doi.org/10.1078/0171-9335-00338Get rights and content

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Structural properties, electronic band structure, real and imaginary parts of complex dielectric function of alkali chloride XCl (K, Rb and Li) compounds were investigated under various pressures using first principles calculations. Moreover, Gibbs free energies were also calculated at those pressures. Calculated results of the Gibbs free energy show that LiCl does not show any structural phase transition. However, structural phase transitions of KCl and RbCl occur from NaCl (B1) to CsCl (B2) at 4.5 and 1.7 GPa pressures, respectively. The electronic band gaps under pressure were also calculated. The calculated physical properties of these compounds are compared with the previous theoretical and experimental results and a good agreement was observed.

Journal of Solid State Chemistry

Volume 99, Issue 2, August 1992, Pages 219-225

Neue Oxochloroniobate der Seltenen Erden

Author links open overlay panelM.H.Thomas*R.Gruehn

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https://doi.org/10.1016/0022-4596(92)90309-JGet rights and content

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The new compounds Nd3NbO4Cl6 and Ln3NbO5XCl3 have been prepared. Nd3NbO4Cl6 was prepared by reacting NdCl3, NdOCl, and Nb2O5 (3:3:1) in evacuated, sealed silica ampoules (850°C, 48 hr). It is isostructural with Pr3NbO4Cl6 and crystallizes in the hexagonal space group P63m (Nr. 176) with cell dimensions a = 12.724(2)Å, c = 3,932(1)Å(powder data). New representatives of a group of isostructural compounds of formula Ln3MO5XCl3 (Ln =LaNd or Th, U; M =Nb, Ta, W, U6+; X =OH, F, O) could be obtained by firing stoichiometric quantities of oxides, oxyhalides, and chlorides in silica ampoules. They also crystallize in the hexagonal space group P63m. The cell dimensions were obtained with a Guinier camera.

Telematics and Informatics

Volume 8, Issue 4, 1991, Pages 283-295

Transformation reborn: A new generation expert system for planning HST operations*

Author links open overlay panelAndrewGerb1

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https://doi.org/10.1016/S0736-5853(05)80054-1Get rights and content

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The Transformation expert system (TRANS) converts proposals for astronomical observations with the Hubble Space Telescope (HST) into detailed observing plans. It encodes expert knowledge to solve problems faced in planning and commanding HST observations to enable their processing by the Science Operations Ground System (SOGS). Among these problems are determining an acceptable order of executing observations, grouping of observations to enhance efficiency and schedulability, inserting extra observations when necessary, and providing parameters for commanding HST instruments. TRANS is currently an operational system and plays a critical role in the HST ground system. It was originally designed using forward-chaining provided by the OPS5 expert system language, but has been reimplemented using a procedural knowledge base. This reimplementation was forced by the explosion in the amount of OPS5 code required to specify the increasingly complicated situations requiring expert-level intervention by the TRANS knowledge base. This problem was compounded by the difficulty of avoiding unintended interaction between rules. To support the TRANS knowledge base, XCL, a small but powerful extension to Common Lisp was implemented. XCL allows a compact syntax for specifying assignments and references to object attributes. XCL also allows the capability to iterate over objects and perform keyed lookup. The reimplementation of TRANS has greatly diminished the effort needed to maintain and enhance it. As a result of this, its functions have been expanded to include warnings about observations that are difficult or impossible to schedule or command, providing data to aid SPIKE, an intelligent planning system used for HST long-term scheduling, and providing information to the Guide Star Selection System (GSSS) to aid in determination of the long-range availability of guide stars.

Combustion and Flame

Volume 92, Issue 4, March 1993, Pages 419-439

Numerical simulation of stoichiometric premixed flames burning CH3Cl / CH4 / air mixtures at atmospheric pressure with a full and short reaction mechanism and comparison of the flame speeds with experimental results

Author links open overlay panelK.Y.LeeI.K.Puri

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https://doi.org/10.1016/0010-2180(93)90153-TGet rights and content

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Numerical simulations of freely moving premixed flames burning mixtures of methane and methyl chloride in air are conducted at atmospheric pressure in order to understand the effect of hydrocarbon bound chlorine on hydrocarbon-air flames. A chemical kinetic mechanism is employed, the adopted scheme involving 38 gas-phase species and 358 elementary reaction steps due to 179 forward reactions. A simplification procedure is followed in order to reduce the reaction set to a smaller one, in order to make the simulations less time intensive. A 63-reaction set containing 25 species is determined after the application of detailed sensitivity analyses, and found to accurately predict the salient features of the simulated flames. The flame speeds predicted by both the full and short mechanism are found to be in good agreement with those deduced from experiments on symmetrical stretched planar counterflow premixed flames. Further study is required if the short mechanism is to be extended to other systems involving XCl compounds, such as HCl.

Ceramics International

Available online 2 December 2020

In Press, Corrected ProofWhat are Corrected Proof articles?

Charge storage in binder-free 2D-hexagonal CoMoO4 nanosheets as a redox active material for pseudocapacitors

Author links open overlay panelShahidHussaina1GuanjunQiaoa

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https://doi.org/10.1016/j.ceramint.2020.11.237Get rights and content

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Binder-free CoMoO4 hexagonal nanosheets have been directly grown on the surface of conductive carbon fabric cloth (CoMoO4@CFC) as a hybrid electrode material for pseudocapacitors (PCs) with outstanding electrochemical properties. The as-prepared CoMoO4@CFC sample was structurally and morphologically characterized using various techniques. Microstructure analysis reveals that the hexagonal like 2D structure possesses mesoporous characteristics with abundant electroactive sites as a charge storage host. The CoMoO4@CFC was evaluated as a positive electrode material for pseudocapacitors, which revealed a maximum specific capacitance of 1210 F/g at 2.5 A/g with exceptional rate capability and outstanding cycling stability of 91% after 10,000 charge/discharge cycles. The 2D mesoporous hexagonal-like structure provides improved electrolyte movement during charging/discharging process and additional active sites for redox reactions. In addition, the charge storage quantification of diffusion and capacitive charge mechanism was determined by employing Power's law, and accordingly, the CoMoO4@CFC electrode was attributed to a high capacitive charge value of (80% at rate 2.5 mV/s). Thus, this work specifies simple and cost-effective method to fabricate pseudocapacitors electrode materials with high energy density and improved cyclic life for energy storage devices.

Combustion and Flame

Volume 92, Issue 4, March 1993, Pages 419-439

Author links open overlay panelK.Y.LeeI.K.Puri

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https://doi.org/10.1016/0010-2180(93)90153-TGet rights and content

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Volume 65, Issue 11, November 2004, Pages 1871-1878

Structural and electronic properties of matlockite MFX (MSr, Ba, Pb; XCl, Br, I) compounds

Author links open overlay panelF. Elhaj HassanaA.Mokhtarib

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https://doi.org/10.1016/j.jpcs.2004.07.002Get rights and content

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First-principles calculations have been used to study the structural and electronic properties of technologically important matlockite compounds MFX (MBa, Sr, Pb; XCl, Br, I) using a full potential linearized augmented plane-wave method within density functional theory. We used the local density approximation and the generalized gradient approximation, as well as the Engel–Vosko's GGA formalism to find the band gap and the partial density of states at equilibrium volume. We also optimized internal parameters by relaxing the atomic positions in the force directions. The calculated total energy allowed us to investigate several structural properties in particular the equilibrium lattice constants a and c, c/a ratio, bulk modulus, pressure derivative of the bulk modulus, cohesive energy, interatomic distances, interlayer distances along c axis and the angles between different atomic bonds. We calculated the valence charge density at the equilibrium volume for BaFCl and PbFCl and concluded that the bonding nature in these compounds is mainly ionic. Results are discussed and compared with experimental and other theoretical data.

We have surgically treated 771 patients for thoracic and thoracoabdominal aortic aneurysms since 1983. Our primary effort has been to develop experimentally validated strategies to reduce paraplegia, renal failure, and mortality in these high-risk patients. This approach has led to a spinal cord protection protocol that has reduced paraplegia risk by 80% (observed/expected ratio = 0.19) with the use of cerebral spinal fluid drainage, moderate hypothermia (31°C–33°C), endorphin receptor antagonist (naloxone), and thiopental burst suppression while optimizing mean arterial pressure (>90 mm Hg) and cardiac index. The elective mortality rate is 2.80% (17% for acute patients), and with rapid renal cooling for renal protection, only 0.88% required permanent dialysis. These results were achieved without the use of assisted circulation. We have reattached intercostal arteries since 2005 using preoperative magnetic resonance angiographic localization, but it remains unclear whether intercostal reimplantation reduces paraplegia risk, as we had initially proposed. We strongly believe that a consistent anesthetic and postoperative care protocol uniformly built and applied around these principles greatly enhances our surgical outcomes. We also show that improved outcomes with assisted circulation and hypothermic arrest in treatment of thoracoabdominal aortic disease follow similar principles of spinal cord and end-organ protection.This paper integrates elements from the theory of agency, the theory of property rights and the theory of finance to develop a theory of the ownership structure of the firm. We define the concept of agency costs, show its relationship to the 'separation and control' issue, investigate the nature of the agency costs generated by the existence of debt and outside equity, demonstrate who bears these costs and why, and investigate the Pareto optimality of their existence. We also provide a new definition of the firm, and show how our analysis of the factors influencing the creation and issuance of debt and equity claims is a special case of the supply side of the completeness of markets problem.

The directors of such [joint-stock] companies, however, being the managers rather of other people's money than of their own, it cannot well be expected, that they should watch over it with the same anxious vigilance with which the partners in a private copartnery frequently watch over their own. Like the stewards of a rich man, they are apt to consider attention to small matters as not for their master's honour, and very easily give themselves a dispensation from having it. Negligence and profusion, therefore, must always prevail, more or less, in the management of the affairs of such a company.

This study reports a detailed investigation of the deformation behavior and microstructural evolution of a novel ultra-low carbon Cr-Mo alloyed dual-phase steel rebar aimed for marine applications. The rebar matrix consists of the lamellar ferrite/bainite dual phases with the lamellar interfaces along the rolling direction. The soft ferrite phase is composed of larger grains with a low dislocation density, while the hard bainite phase is composed of finer grains with a much higher dislocation density. By comparing the strain hardening behavior from different sites at the rebar with varying ferrite and bainite volume fractions, it shows that the hetero-deformation induced (HDI) hardening is strong and dominates the overall work hardening behavior in the early stage of plastic deformation by prevailing over the conventional sum-up of contribution of each phase alone. In this stage, the plastic deformation of ferrite was constrained by the disproportionally-strained, neighboring bainite, creating the accumulation of geometrically necessary dislocations (GNDs) at the phase interface and the long-range HDI stress. The results also reinforce the understanding of deformation behavior of dual-phase steels, especially around the role of HDI stress and hardening.

Keywords

Alloyed steel rebar

Dual-Phase steel

Heterogeneous materials

Mechanical performance

Hetero-Deformation induced hardening

EBSD characterization

1. Introduction

Steel rebar is the main reinforcing material in concrete building structures for load-bearing applications [1], [2], [3], [4]. Further development of reinforced concrete structure (RCS) for high-rise, long-span, aseismic and other multi-functional applications requires steel rebar materials with more advanced and comprehensive mechanical properties, including but not limited to high strength, sufficient toughness and adequate plasticity [5], [6]. Among those, the marine RCSs demand not only high strength but also excellent corrosion resistance to the chloride-rich environment as the chloride ions induce severe corrosion, leading to the degradation and the accelerated failure of RCS components [7], [8], [9], [10], [11], [12], [13]. Alloying is the direct and most effective way to improve the corrosion resistance of the steel rebar matrix [14], [15], [16], [17], [18], [19]. The main corrosion-resistant elements for steels include chromium (Cr), nickel (Ni), molybdenum (Mo). In addition, reducing the carbon content, i.e., diminishing the cathodic carbide, also benefits the corrosion resistance of a steel rebar [20], [21].

Based on these design principles, a novel corrosion-resistant alloyed steel was developed for marine RCSs recently [22]. Compositionally, the steel rebar consists of ultra-low carbon (0.02 wt.%), 10 wt.% Cr, 1.2 wt.% Mo, 1.49 wt.% manganese (Mn) and 0.49 wt.% silicon (Si). Previous studies revealed the excellent corrosion resistance of this steel rebar in the marine chloride environment, holding a great potential of prolonging the service life of the RCS components [23], [24]. Meanwhile, despite the absence of carbides, which was perceived critical to strengthening the steel [25], [26], [27], its mechanical properties appear encouraging by meeting the ISO standard of 400 MPa grade steel rebar [28] in early preliminary evaluations. However, the detailed deformation mechanism and the attendant microstructure evolution is not fully understood yet. In contrast to the conventional high-strength steel rebar where ferrite and pearlite are the primary microconstituents [29], [30], the new steel is comprised of ferrite and carbon-free bainite due to the ultra-low carbon content [23]. One scientific interest is how does these ferrite/bainite dual-phase microstructure accommodate the deformation.

The dual-phase microstructure can be viewed as one category of heterogeneous materials, which garnered extensive interests recently due to their potential of overcoming the strength-ductility trade-off by offering extra strain hardening during deformation [31], [32]. The hetero-deformation induced (HDI) stress and hardening was proposed to contribute to the outperformance of the heterogeneous materials compared to their conventional homogeneous counterparts [33], [34], [35]. In light of this general finding, understanding the role of HDI stress and hardening and its evolution may offer the key insights into the deformation behavior of this novel dual-phase steel rebar. Experimental exploration of this theme would be a central theme of the present study, which was rarely conducted in early investigations.

In this paper, we will combine microscopy characterization and mechanical testing to investigate the microstructure and the deformation behavior of this novel steel rebar in detail. The emphasis of the present work is on the role of the dual-phase microstructure and its induced HDI stress and hardening in affecting the mechanical performance of the rebar. Our observation and analysis will shed critical insights into understanding and designing novel dual-phase steels for advanced applications.

2. Materials and methods

2.1. Thermo-mechanical processing of the material

The ultra-low carbon Cr-Mo alloyed steel rebar with a diameter of 16 mm was provided by Shagang Group, China. The melting processing includes iron molting via electric-arc furnace, top-blown converter steel making and square billet continuous casting. The casting square billets were then hot rolled at the speed of 13∼15 m/s. The rolling temperature began with 1000 ± 20 °C, followed by cooling at a rate of 2 K/s. The billets were finally cooled by a cooling bed [22]. During this process, the alloyed steel underwent two phase transformations, from austenite to both ferrite and bainite at the designate cooling rate. The alloying elements also affect the transformation in their own manners. Cr is widely viewed to promote the formation of ferrite and will eliminate the austenite if it reaches 13 wt. % in composition [36], [37]. The high Cr content and the ultra-low carbon content significantly suppress the austenite phase and increase the ferrite phase in the final room-temperature microstructure [38]. Mn and Mo will decrease the bainite transition temperature (Bs) and increase the volume fraction of the bainite phase [39], [40]. Tuning the alloying contents thus becomes an effective approach to achieve the desired dual-phase microstructure. In the present study, the chemical composition of the alloyed steel rebar was detected by a photoelectric emission spectrograph (Shimadzu PDA-700, Japan). The rebar contains 0.02 wt.% C, 0.49 wt.% Si, 1.49 wt.% Mn, 0.01 wt.% P, 0.01 wt.% S, 0.05 wt.% V, 10.06 wt.% Cr, 1.26 wt.% Mo and balanced by Fe.

2.2. Microstructure characterization

The microstructure of the alloyed steel rebar was characterized via a scanning electron microscope (SEM, Sigma 500, ZEISS, Germany). Samples were cut from the rebar along the rolling direction via the electrical discharge machining, mechanically polished to a mirror finish, and chemically etched with an etchant for about 15∼20 s to reveal the grain boundaries and phase morphologies. The etchant consists of anhydrous ferric chloride (10 g), hydrochloric acid (30 mL), and deionized water (120 mL). An electron probe microanalyzer (EPMA, Shimadzu 1610, Japan) was used to characterize the alloy element distribution of the rebar matrix. The sample preparation of EMPA analysis is the same as that used for SEM observation. Electron backscattered diffraction (EBSD) analysis of the alloyed rebar matrix was conducted via an Oxford EBSD detector equipped on a ZEISS Auriga crossbeam microscope. The EBSD scans were performed at 20 kV with a step size of 500 nm to reveal the detailed dual-phase microstructural. The EBSD samples were mechanically polished and further electrochemically polished at room temperature. The applied electrochemical-polishing voltage, current density and duration were 4 V, 0.1 A∙cm−2, and 40 s, respectively. The electrolytic polishing solution was composed of sulfuric acid, phosphoric acid, nitric acid, and ethanol with a volume ratio of 3:4:1:8. The substructures, such as intragranular dislocation configuration and grain/subgrain boundaries, were further characterized by a transmission electron microscope (TEM, FEI Tecnai G2 F20). The TEM foils were mechanically polished to ∼20 μm and perforated by ion milling (Gatan 695C) at -30 ℃.

2.3. Mechanical property evaluation

Berkovich nano-indentation (Micro Materials Nano Test, UK) tests were carried out to probe the mechanical responses of different microconstituents in the rebar matrix. The same sample preparation procedure for SEM was adopted to make indentation specimens. A maximum load of 50 mN was used for the indentation. The indentation morphologies were observed by SEM (Zeiss Sigma 500).

Quasi-static uniaxial tension testing was conducted on dog-bone shaped samples from different sites of the rebar. Samples with a dimension of 10 (gauge length) × 2 (thickness) × 2.5 (width) mm3 were cut from the rebar an electronic tensile testing machine (Water Bai LFM 20 K N, Switzerland), with their longitudinal axes parallel to the rolling direction and mechanically polished before testing. The strain rate for the tests was 3.33 × 10−4 s-1. More than 3 parallel samples are used for testing to ensure the repeatability of the mechanical property. Ex-situ SEM/EBSD characterization at different strain levels (0%, 5% and 10%) was performed to reveal the microstructure evolution and to understand the underlying deformation mechanism. To enable a direct and consistent comparison, we collected each set of microscopy data from the same area of each sample by carefully tracing the local features.

Cyclic loading-unloading-reloading (LUR) tension testing was conducted to evaluate the evolution of HDI stress during plastic deformation [41]. The LUR samples have the same dimension for the regular tensile samples.

3. Results and discussion

3.1. Microstructure characteristics of the ferrite/bainite dual phases

Fig. 1a is an SEM micrograph of the rebar matrix microstructure viewed along the transverse direction, demonstrating the lamellar dual phases with elongated grains. As provided in the inset with higher magnification, the bainite phase is plate- or lath-like in morphology, whereas the other microcontinent with a slightly lower aspect-ratio is ferrite, according to early reports on dual-phase steels [42], [43]. In addition, as shown in Fig. 1a, the ferrite grains are relatively larger in grain size, but the two phases always appear together all over the microstructure. To further investigate the dislocation density and other substructure information, TEM observation was conducted, and a representative micrograph is provided in Fig. 1b. The bainite grains include more dislocations and subgrain boundaries inside, whereas the adjacent ferrite grains are cleaner as well as coarser. This general observation agrees well with other published data on the ferrite/bainite dual-phase carbon steel [44], [45]. In addition, there are no appreciable precipitated particles in the microstructure despite the considerable alloying content.

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Fig. 1. SEM (a) and TEM (b) observation of microstructure in the alloyed steel rebar.

EPMA was conducted to illustrate the distribution of alloying elements in the rebar matrix. As shown in Fig. 2, with the assistance of the morphological features under secondary electron imaging, the phase nature, bainite or ferrite, in the region of interest can be identified. The elemental maps reveal that the ferrite phase contains more Cr and Mo but less Mn than the bainite phase. The alloying distribution is, though, uniform within each phase. Together with the absence of precipitated particles under TEM observation, we conjecture that the most alloying contents are present in the form of solid solutes, primarily contributing to solution strengthening. Given the substitutional nature, these solutes are perceived not as effective as the interstitial solutes (such as C and N) in solution strengthening [46].

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Fig. 2. EPMA maps of the alloying element distributions of the alloyed steel rebar matrix.

3.2. Respective nano-indentation behavior of the ferrite and bainite phases

Fig. 3 presents the representative load-displacement curves and the post-indentation morphologies of both phases. The primary goal of nano-indentation testing, though, is to further corroborate the dual-phase structure, the soft ferrite and hard bainite phases. To achieve this goal, we conducted more than 10 indents to reveal the statistics. Given the same maximum load, the indentation depth on the bainite region is shallower (640 nm) compared to that on the ferrite region (860 nm). The imprint size is also smaller on the bainite region. More than ten indentation tests were performed on each phase of interest and revealed the same trend. The average Young's modulus (Er) of the bainite and ferrite phase measures 250 ± 5 GPa and 210 ± 4 GPa, respectively. The average micro-hardness of the bainite and ferrite phase are 4.6 ± 0.4 GPa and 2.6 ± 0.3 GPa, respectively. Thus, the mechanical properties are indeed different in these two phases. It should be noted that when indenting the bainite region that is very small and narrow, part of some indents inevitably have a minor coverage for the ferrite grain nearby. However, this minor coverage does not affect the overall statistics to corroborate the hard and soft dual-phase structure. The bainite phase appears relatively harder primarily because it contains more intragranular dislocations. Overall, the soft ferrite grains and the harder bainite grains comprise the heterogeneous microstructure of the rebar matrix.

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Fig. 3. Nano-indentation testing of the ferrite and bainite phase of the alloyed steel rebar: (a) load-displacement curves; (b) imprint morphologies.

3.3. Site-specific dual-phase microstructure in the rebar

As the cooling processing usually induced a thermal field in the rebar, its microstructure can be slightly different from the edge to the core in terms of the phase composition [47], [48]. EBSD analysis was conducted to investigate such potential various microstructures in three different sites of the longitudinal section of the rebar, as sketched in Fig. 4. Fig. 5 presents the EBSD band-contrast (Fig. 5a), grain boundary (Fig. 5b), and inverse pole figure mapping (Fig. 5c) of those different positions. By comparing the morphological features in Fig. 5a and Fig. 1a, ferrite and the bainite phases can be identified (some have been marked in Fig. 5). Two observations are worth to noting. First, the grain size of the bainite phase is relatively finer than that of the ferrite phase, consistent with early observation under SEM. There are also more subgrain boundary structures in the bainite phase, as depicted by the low-angle GBs in Fig. 5b, where black lines denote the high-angle GBs larger than 15° and the gray ones denote the low-angle GBs (greater than 2° but less than 15°). Large ferrite grains that are free of subgrain boundaries are common in the microstructure. Second, the fraction of bainite phase decreased from the near edge to the core. To further illustrate this trend, grain size statistics were analyzed using HKL Channel5 software. Since the ferrite and bainite phases do not vary in crystal structure (Fig.5c), the phase nature of each grain was determined manually according to their morphologies. Fig. 6 summarizes the grain size distributions and the area fractions of each phase in the different sites in Fig. 5. The average grain size of the bainite phase is smaller compared to the ferrite phase in each site. The bainite ratio gradually decreases from the edge (45% for position 1) to the core (16% for position 3) of the rebar. The lower fraction of bainite phase in the core should be related to the relatively low cooling rate during processing. The different fractions of the bainite phase will affect the mechanical performance but also offer a unique opportunity to examine the role of the HDI hardening as detailed later.

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Fig. 4. Schematic illustration of the different sites of the longitudinal section of the rebar for EBSD analysis and tensile tests.

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Fig. 5. EBSD results of the rebar microstructure at different position: (a) Band contrast mapping; (b) Grain boundary mapping; (c) Inverse pole figure mapping.

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Fig. 6. Grain size distributions of ferrite/bainite phases at different locations with various phase ratios in the alloyed steel rebar.

3.4. Uniaxial tensile behavior of samples with various dual-phase microstructure

To reflect the potential site-to-site difference, uniaxial tensile testing was carried out using samples cut from the three locations on the longitudinal section in Fig. 4. The quasi-static uniaxial tensile engineering stress-strain curves are provided in Fig. 7a. The corresponding strength and elongation are summarized in Table 1. First, all the samples have the yield tensile stress (YTS) larger than 400 MPa, testifying the competence of the novel steel rebar as a 400 MPa grade high-strength rebar according to ISO standards [28]. More importantly, it is found that the strength increases and the elongation decreases gradually from the core to the outer edge of the rebar. The gradual alteration of the mechanical properties should be associated with the various dual-phase microstructure of the alloyed steel rebar.

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Fig. 7. Engineering tensile stress-strain curves (a) of the samples obtained at different positions of the alloyed rebar, the corresponding strain hardening rates (b) and the modified C-J curve (c).

Table 1. Mechanical properties from the quasi-static uniaxial tension tests.

SamplesYTS (MPa)UTS (MPa)Eu (%)Ef (%)Position 1480 ± 8660 ± 1310.9 ± 0.212.5 ± 0.3Position 2450 ± 6628 ± 1011.3 ± 0.314.7 ± 0.4Position 3405 ± 5560 ± 813.5 ± 0.516.9 ± 0.6

Fig. 7b presents the strain hardening rates of the three samples. According to the references [49], [50], [51], the strain-hardening rates of the tensile curves do not reach zero, and the calculated true stress after the point of instability (UTS) is not valid. Herein, the strain hardening rates data we used for plots were prior to the necking points. As clearly see in the inset figure of the Fig. 7b, the least strain-hardening were around 600−1000 MPa of the three samples obtained at different positions. Generally, the tensile deformation of a ferrite/bainite steel should be divided into three stages [52], [53]. First, both phases experience uniform elastic deformation. In the second stage, the ferrite experiences plastic deformation but constrained by the bainite that is undergoing elastic deformation. In the third stage, both phases experience plastic deformation simultaneously until the necking and fracture. Here, the strain hardening consideration in Fig. 7b primarily stems from the second and the third stages, i.e., plastic deformation. Strain hardening during this stage can be considered from two parts. One is from the individual behavior of each phase, whose total contribution follows the rule of the mixture based on the respective volume fraction of each phase. The other originates from the interaction between hard and soft phases when heterogeneous deformation emerges, also known as HDI hardening. Conventionally, the overall strain hardening rate is perceived governed solely by the first part by assuming the mutual interaction between them is immaterial. More ferrite phase would lead to higher hardening rates. Interestingly, Fig.7b reveals a different trend, especially in the early plastic stage. The sample from position 1, which has the lowest ferrite fraction, has the highest strain hardening rate. Therefore, the constraint between the two phases in the second stage offers significant extra hardening. While the higher hardening rate during this stage was observed in early studies of dual-phase steel [54], the critical role of the constraint/interaction between the soft and hard phases only received more appreciation in recent investigations of heterogeneous materials and their HDI hardening [32], [33]. In the present study, it is found that the HDI hardening appears so pronounced that it even prevails over the contribution only from the phase volume fractions, dominating the overall work hardening behavior in early plastic straining below 2-4% (Fig. 7b). Two facts from other studies may further validate this observation when HDI hardening is taken into accounts. First, recent reports suggested that a lower volume fraction of the soft phase may offer much stronger HDI hardening because the hard phase with a higher fraction enables a full constraint for soft phase from all surroundings [31]. Position 1 has the lowest volume fraction of the soft phase among all three samples. Second, the HDI hardening was reported more influential in the early stage of deformation, while the generic dislocation forest hardening became more prominent later [55]. Taken as a whole, the fact that a higher strain hardening emerges from a sample with a lower volume fraction of soft phase suggests that HDI hardening is vital in determining the mechanical performance of the dual-phase steel rebar. It plays a larger role in the early stage of plastic straining.

To reveal the deformation stage of dual-phase steels, modified Crussard-Jaoul (C–J) analysis [56] as a phenomenological tool has been often employed by a log-log plot of strain hardening (dσ/dε) and true stress (σ). We performed a similar analysis to further corroborate the correlation between deformation stages and the role of HDI hardening. The modified C–J analysis on samples from position 1 is presented in Fig. 7c, where three distinctive stages can be readily recognized according to the slope values. Results from the other two positions suggest similar patterns. Consistent with many other analyses in dual-phase steels [57], [58], [59], these three stages correspond to I) the deformation in the soft phase (ferrite) alone; II) the mutually constrained deformation of the hard (bainite) and soft (ferrite) phase bounded by their interfaces, where the plastic flow is heterogeneous; III) the less heterogeneous or almost concurrent deformation of both phases. It is noteworthy that the transition level of strain hardening from stage II to III is about 2500 MPa, which coincides with to the turning point for the strain hardening comparison among three samples (inset in Fig. 7b). Above this level, the strain hardening at Position 1 is greater. The match between the deformation stage transition in modified C–J analysis and the inflection point in strain hardening comparison lends further support to the decisive role of HDI hardening during the mutually constrained deformation (Stage II) for the dual-phase steel.

3.5. Ex-situ characterization of the microstructural evolution of the alloyed steel rebar during tensile tests

To link the microstructure evolution with the deformation behavior during tension, especially the role of HDI hardening, we further conducted ex-situ EBSD characterization during tensile tests. Since samples from all positions are subjected to heterogeneous deformation, we thus only select one of them for this purpose (here position 2 in Fig.4). The SEM morphologies of the same site on the sample but subjected to different strains are provided in Fig. 8. The surface morphology evolution lends further support to the claim of the two-stage plastic deformation of the material. Compared to the unstrained state, profuse slip traces were observed on the ferrite phase at the overall 5% strain (marked in Figs. 8c and d) [60], [61]. Note that the surrounding bainite phase does not exhibit apparent slip traces yet, suggesting the ferrite bears larger plastic strain while the bainite is still likely undergoing elastic deformation. At 10% strain, slip traces are found all over the sample surface. They appear uniform, and it becomes challenging to identify the phase nature of the grains under observation. Clearly, both phases are plastically deformed at the same time. This observation validates the different deformation stages of the dual-phase rebar.

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Fig. 8. SEM morphology of the same site in the sample subjected to different tensile strains: original state with (0% strain) low magnification (a) and high magnification (b); tension for 5% strain with low magnification (c) and high magnification (d); tension for 10 % strain with low magnification (e) and high magnification (f).

Fig. 9 presents the GB mappings and local misorientation mappings obtained from another sample at the tensile strains of 0%, 5% and 10%. Here, the high-angle and low-angle GBs are marked as black lines and red lines, respectively (Fig. 9a-c). Again, the ferrite grains were pre-identified manually according to initial SEM morphologies. Consistent with the previous slip trace observation, large ferrite grains are apparently plastically deformed at the overall 5% strain (a grain example is marked by blue arrows in Fig. 9a-c). The local misorientation calculated by kernel average misorientation (KAM) method can be linked to the geometrically necessary dislocation (GND) density evolution [62], [63] during tension and thus further illustrate the site-specific deformation information. Statistics of the local misorientation data in each phase is provided in Fig. 10 to facilitate the understanding of their respective GND density evolution. Clearly, the ferrite phase starts with a relatively pristine microstructure and is subject to an appreciable increase of misorientation during tension (average misorientation from 0.75° at zero strain to 1.67° at 10% strain). In contrast, for the bainite phase, the increase of misorientation is relatively smaller (from 2.07° at zero strain to only 2.57° at 10% strain) though its absolute value is always higher due to the initial large dislocation density. The larger increase of misorientation indicates the more GND accumulation in the ferrite phase, which was believed crucial to determine the HDI hardening [33], [64], especially in the early stage of deformation [52].

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Fig. 9. Ex-situ EBSD characterization of the steel rebar during tension. (a)∼(c) are the grain-boundary mappings of the rebar after tensing for strain of 0%, 5% and 10 %; (d)∼(f) are the corresponding local misorientation mappings.

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Fig. 10. Statistics of local misorientation data in (a) ferrite phase and (b) bainite phase after different tension strains using KAM method.

The GNDs are perceived effective to promote HDI hardening if they accumulate near the soft/hard interfaces [33], as sketched in Fig. 11a (here the presented form of accumulation is pile-up). Further TEM examination is carried out to disclose the dislocation substructure near the ferrite/bainite interfaces. Fig. 11b is a representative TEM micrograph taken from the ferrite/baintie interface from a sample strained to 10%. Compared to the original state (see the TEM image in Fig. 1b), there is a significant accumulation of dislocations found in the ferrite grain near the ferrite/bainite interface. Those dislocations should be caused by the incompatible deformation during the tension and thus serve as GNDs to accommodate the induced local strain gradient, contributing a critical component in the measured GND density in Fig. 10. The GNDs near such interfaces will lead to strong HDI stress and hardening. While the presented TEM data is taken from the 10%-strain sample, similar dislocation arrangement is also expected at lower strains when the incompatible elastic/plastic deformation initiates between the bainite and ferrite phase.

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Fig. 11. (a) Sketched diagram of the GND distribution at the ferrite/bainite interfaces in the microstructure; (b) a representative TEM observation at the ferrite/bainite interface from the 10 %-strained sample.

The exact amount of HDI stress and hardening can be quantified using the loading-unloading-reloading (LUR) tension tests. Fig. 12 shows the LUR stress-strain curves and the derived HDI stress using the analysis proposed by Yang et al. [41]. The dotted line is the monotonic loading curve as a benchmark. Importantly, it can be reckoned from Fig.12b that the HDI stress takes more than half of the total stress, suggesting its predominant role in determining the mechanical performance of the studied dual-phase steel. The HDI hardening, which can be approximated by the slope of the curve in Fig. 12b, increases rapidly first and gradually saturates. Hence, HDI hardening is relatively stronger at early deformation, which is consistent with our early analysis and agrees well with published studies [55]. This does not come as a surprise since the mutual interaction between soft/hard phases is expected to be strongest in the early stage of plastic deformation. The larger role of HDI hardening during this stage also further validates the counterintuitive overall hardening rate ranking of different samples in Fig. 7b in the early stage. In the later stage, both phases are plastically deformed congruently, mitigating the heterogeneous deformation and slowing down the increase of HDI stress.

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Fig. 12. Loading-unloading-reloading (LUR) test results. LUR curve of the sample from position 2 (a). The dotted line is the monotonic loading curve for the sample to serve as a benchmark. The inset presents the detailed data in one LUR loop. The derived HDI stress evolution as a function of strain (b).

4. Summary

Microstructure and deformation behavior of an ultra-low carbon Cr-Mo alloyed steel rebar were detailed investigated through mechanical testing and microstructure characterization. In general, the alloyed rebar exhibits a typical ferrite/bainite dual-phase microstructure where the ferrite grains are relatively larger and softer, and the bainite grains are smaller and harder. Due to the various cooling rate at different locations, the dual-phase microstructure varies from the outer region to the core in terms of the ferrite/bainite volume fractions. Our tests and analysis show that the dual-phase induced HDI hardening plays a critical role in determining the mechanical performance of the steel rebar. HDI hardening appears to prevail in the overall hardening behavior in the early plastic deformation stage due to the profuse accumulation of GNDs at the ferrite/bainite interfaces in the microstructure. Our observation and analysis offer critical insights into understanding and designing novel dual-phase steels for advanced applications.

Conflicts of interest

The authors declare no conflicts of interest.

Data availability

Data will be made available on request.

Acknowledgements

The authors acknowledge the financial support from the Fundamental Research Funds for the Central Universities of Hohai university (B200202122 & 2019B76814), Natural Science Foundation of China (51878246 & 51979099), Six Talent Peaks Project in Jiangsu Province (2016-XCL-196), Science and Technology Support Program funded project of Suqian City (Industrial H201817), Applied Fundamental Research Foundation of Nantong City (JC2018110).