Department of Metallurgical & Materials Engineering

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The present study deals with the effect of Ultrasonic Treatment (UT) on the synthesis, characterization, and mechanical properties of Al-4.4 wt% Cu alloy reinforced with 2 vol % of Graphite (Grp) particles. Microstructural analysis of composite confirms that the UT enhances the wettability, de-agglomeration, and uniform distribution of Grp particles in the matrix. The micron-sized Grp has been flaked off to submicron particles with increase in aspect ratio from ∼4.7 to ∼6.5 due to ultrasonic cavitation effect. In comparison with monolithic Al-4.4 wt % Cu alloy an overall grain size reduction of 74% has been observed in composite with UT. Grp particles’ reinforcement together with grain refinement has improved the mechanical properties viz. hardness, compressive strength and tensile strength of the synthesized composite with UT. The strengthening mechanisms of the Grp particles in the synthesized composites have been analyzed and discussed. © 2020 Elsevier B.V.

World University root node

Journal of Alloys and Compounds

Ultrasonic assisted synthesis of Al–Cu/2 vol%Grp composite and its characterization

A new family of 2D materials, Janus MTeSe (M=Nb, Mo, or W) pristine and defective monolayers have been investigated in this work as promising catalysts for hydrogen evolution reaction (HER) based on first-principles calculations. It has been observed that these Janus monolayers are dynamically and thermodynamically stable. Hybrid exchange-correlation functional (HSE06) based electronic structures reveal Janus NbTeSe is a polarized semiconductor with an indirect bandgap of 1.478 eV with excellent optical absorption capability near infra-red region. While MoTeSe and WTeSe monolayers are direct bandgap semiconductors with a suitable bandgap of 1.859 and 1.898 eV. The carrier effective masses and mobilities in MTeSe monolayer are also calculated. Subsequently, the catalytic activity of pristine as well as defective MTeSe for HER has been identified from the reaction coordinate based on the adsorption free energy ((Formula presented.)). It is noticed that the Nb based Janus layer has comparatively weak HER activity than its peers, group VIB transition metals, Mo, W based Janus layer. The Coulomb attraction between the hydrogen and the monolayer decreases with the increase of the inner atomic radius from Nb, Mo to W, which is one of the structural frailties of 2D Janus NbSeTe as an active photocatalyst. We have further analyzed electronic structures and charge density distributions of pristine and defective MTeSe with/without H adatom to unveil the reason of the catalytic inferiority for Nb based Janus layer over W and Mo based systems. This comparative study of Janus MTeSe monolayers with HSE06 would provide a deep understanding of Janus based HER catalyst. © 2020 The Authors. Published by Wiley-VCH GmbH

ChemCatChem

Structural insight of the frailty of 2D Janus NbSeTe as an active photocatalyst

Quinones have a capacity for high energy storage and exhibit facile and reversible electrochemistry in several widely different electrolytes. They are, therefore, one of the most popular compounds currently used in organic materials based electrical energy storage. Quinone electrochemistry is, however, strongly affected by the composition of the electrolyte. This report summarizes our systematic investigation of the redox chemistry of a series of quinones with electron-withdrawing and electron-donating substituents in aqueous solution and in acetonitrile (MeCN) with tetrabutylammonium (TBA+)-, Li+-, and H+-based electrolytes. As a general trend, proton cycling, TBA+ cycling, and Li+ cycling resulted in the highest, the lowest, and intermediate redox potentials, respectively. We attribute this trend to stabilization of the reduced state, namely benzene-1,4-bis(olate) (Q2-), by the different counterions. Density functional theory (DFT) calculations showed that, in the fully reduced state, two Li+ counterions accommodated 35% of the injected electron charges while proton counterions accommodated 69% of the injected charge, thus significantly stabilizing the reduced state. However, with the bulky TBA+ as the cycling ion, this stabilization was not possible and the reduction potential was decreased. In addition, we showed that stabilization of the counterion also affected the Coulombic interaction between the successively injected charges, resulting in the well-known disproportionation of the semiquinone radical intermediate state with proton cycling, while Li+ and TBA+ cycling generally resulted in two consecutive redox reactions. Finally, we showed that the electrolyte strongly influences the effects of substitution with electron-donating and electron-withdrawing substituents. A strong relationship between the redox potential and the electron-withdrawing power of the substituent was observed in the MeCN solution. However, this relationship was completely lost in aqueous solution. The reason for the loss of the relationship was addressed using a DFT explicit-solvent model and is discussed. © 2020 American Chemical Society.

Journal of Physical Chemistry C

Effect of Cycling Ion and Solvent on the Redox Chemistry of Substituted Quinones and Solvent-Induced Breakdown of the Correlation between Redox Potential and Electron-Withdrawing Power of Substituents

An electronic counter is an integral component in an analog to digital (A/D) or digital to analog signal conversion circuit. The number of flip-flops (n) in these devices decides the quality of the conversion as the output is proportional to 2n. Since each flip-flop is a combination of transistors, and each transistor occupies some space, there is a limitation in the quality of conversion. The smallest 4-bit asynchronous counter is built by using a self-assembled redox-active organic molecule 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), where each DDQ molecule is acting as an individual flip-flop which is the building block of a counter. These molecules reduce the size of the counter by two orders. The output of the counter is obtained by tracking the translational motion of a group of molecules which are oscillating about the average value concerning the applied bias. Sixteen new spatial locations are obtained corresponding to sixteen distinct analog inputs (bias voltage) to the flip-flops. So, unconventional conversion of an analog signal to ultra-digital (UD) signal is possible by using an n-bit counter consisting of as many numbers of flip-flops and thus can become a system for the conversion of A/UD signal. © 2020 Wiley-VCH GmbH

Department of Physics

Advanced Materials Interfaces

Voltage Induced Molecular Motors Constitute the Smallest Self-Assembled Molecular Electronic Counter

Exciting a metallic glass with a small oscillatory perturbation can yield significant insights into the atomistic mechanisms governing relaxation phenomenon. Dynamic mechanical analysis (DMA) is a versatile tool to study the structural relaxation of amorphous materials that exhibit prominent peaks in loss modulus as a function of temperature. In this work, room temperature dynamic indentation was performed on Zr67Cu33 MG ribbon at 1 and 2 mN loads. Small oscillatory loads of 50 and 75 μN have been superimposed on the applied loads during a frequency sweep experiment (10–160 Hz). The results indicate that the loss modulus is independent of the frequency for this sample in the tested range. When the as cast MG ribbon was characterized through high resolution transmission electron microscopy (HRTEM), it revealed the presence of nanocrystals with a diverse crystallographic nature. The frequency independent dynamic response in this study is ascribed to the damping of oscillatory load by the nanocrystals. © 2020 Elsevier B.V.

Room temperature dynamic indentation response of partially crystallized Zr–Cu metallic glass

The impact toughness of high frequency electric resistance welded (ERW) line pipe depends on the steel chemical composition, welding procedure, and post-welding heat treatment. Among several microstructural factors that may influence the impact toughness of high frequency electric resistance welded bond line, the crystallographic texture factor is often assumed, but not sufficiently studied. The evolution of texture during high frequency electric resistance welding and simulated post-welding heat treatments (PWHT) of API X70 pipeline steel was characterized using X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). Results of a texture factor calculated using the θ/2θ XRD scans indicated that following the post-weld normalizing treatment, the texture factor of {100} planes for the weld bond line appeared to have increased with the increase in post-weld normalizing temperature. The texture factor results obtained from θ/2θ XRD scans were further verified by using the EBSD on the CVN tested samples in the regions near the fracture path. The XRD texture factor was also used to correlate the evolution of crystallographic texture and Charpy toughness for the PWHT samples. Based on the observations from both XRD and EBSD, the {100} crystallographic orientation of planes in the notched direction correlates well with the cleavage fracture planes of the Charpy impact tested samples. Therefore, the post-weld heat treatment should be designed to minimize any preferred crystallographic texture with {100} planes in the bond line, to promote ductile fracture and improved bond line impact toughness at low temperatures. Copyright © 2020 ASME

Proceedings of the Biennial International Pipeline Conference, IPC

Characterization of crystallographic texture of erw welded and heat-treated API X70 pipeline steel

Increasing the fraction of special boundaries such as Σ3-related coincident site lattice (CSL) boundaries has been a promising route for improvement in mechanical as well as functional properties. Although various routes of thermomechanical processing have been applied for increasing the fraction of annealing twin boundaries, a proper understanding of the mechanism would allow optimization of the processing parameters to attain a maximum fraction of twin boundaries. Present study throws some light onto the evolution of twin boundaries during annealing of deformed low stacking fault energy (SFE) FCC materials. Cold-rolled Cu[sbnd]Zn alloy and 316 L stainless steel were given heat-treatment for various lengths of time in order to study the variation in GBCD during recrystallization and grain growth. We analyse the results by distributing twins of various deviations and then observing their evolution with recrystallization. Based on our data, we propose that higher deviation twins devolve into lower deviation twins and, very small angle boundaries. During grain growth, twin fraction was found to remain constant throughout the annealing times. This is in contrast to the observations in the literature about the decrease in twin density during grain growth due to consumption of twins by growing grains. However, fraction of smaller deviation twins indicates that grain growth actually takes place in two stages which can be clearly delineated using GBCD variation. The first stage is marked by a drop in twin density, proposed to be caused by the internal stress at the boundaries between growing grains during the grain growth stage and not with the consumption of twins by adjacent growing grains as suggested earlier (Jin et al., 2014 [1]). While, the second stage showed further increment in twin fraction continuously up to larger annealing times, which we believe is again due to dissociation of very small angle boundaries. © 2019 Elsevier Inc.

Materials Characterization

New insights into the evolution of twin boundaries during recrystallization and grain growth of low-SFE FCC alloys

The isothermal bainite transformation in low-carbon low-alloy steel under direct and step-quenching conditions have been investigated in the present work. Varying quantities of prior bainite and prior martensite were formed separately by quenching above and below Ms temperature to understand their effects on the extent and kinetics of subsequent bainite transformation in Bs − Ms regime. Formation of prior bainite at higher temperature has been found to stabilise part of remaining austenite against subsequent bainite transformation at lower temperature showing thermal stabilisation of austenite. Hence, the total amount of bainite in two-step treatment is less than that obtained in single-step isothermal bainite transformation at the latter temperature, indicating the inapplicability of ‘additivity rule’ for continuously cooled bainite. A decrease in the bainitic ferrite plate volume due to higher strength of parent austenite and higher nucleation rate, together with limited number of bainite nucleation sites at the latter temperature are believed to be responsible for the thermal stabilisation of austenite. Prior martensite also stabilises part of remaining austenite against further bainite transformation through fragmentation of austenite grains and increased defect density in austenite. The kinetics of bainite transformation in each of the above-mentioned conditions have been analysed quantitatively using an established model and the results are correlated with the microstructures. © 2020, © 2020 Informa UK Limited, trading as Taylor & Francis Group.

Philosophical Magazine

Understanding the effect of prior bainite/martensite on the formation of carbide-free bainite

Inconsistencies exist in literature regarding the effect of prior austenite grain size (PAGS) on the extent and kinetics of bainite transformation. Attempts have been made in the present work to address these issues in a low-alloy carbide-free bainitic steel using dilatometry, over a range of PAGS. The bainite transformation kinetics in the above-mentioned conditions have been analysed quantitatively using established kinetic model to extract information related to the transformation mechanisms in such conditions. Greater obstruction from grain boundaries in fine-grained austenite restricts sheaves of bainite to develop completely and thereby reduces the volume fraction of bainite in comparison with coarse-grained austenite. Initial nucleation rate of bainite transformation increases with decreasing PAGS due to an increase in the nucleation site density. However, the maximum nucleation rate decreases consistently with decreasing PAGS due to gradual reduction in the autocatalytic factor. © 2020 Informa UK Limited, trading as Taylor & Francis Group.

Effect of prior austenite grain size on the formation of carbide-free bainite in low-alloy steel

In the present work, we studied the twinning-detwinning (TDT) behavior of E-form Mg alloy (EFMA) sheets during cyclic loading-unloading via in-plane compression and in-situ tensile tests, respectively. An in-plane compression test was conducted to induce pre-twinning in order to tailor the initial texture of an EFMA sheet. Detwinning kinetics of EFMA was linked to both crystallographic orientations and the morphology of twin bands. EBSD results indicated that most of the pre-twins were removed at lower tensile strains (0.04-0.06) during the in-situ tensile test. The thickness direction strains were effectively accommodated via the detwinning of twin bands in the pre-twinned EFMA sheet under lower tensile strains. Under higher tensile strains (0.08-0.12), however, the deformations were accommodated via the dislocation accumulation in localized deformation regions, which resulted in relatively higher average KAM (kernel average misorientation) and GOS (grain orientation spread) values at the grain and twin boundaries (TBs) during in-situ tensile testing. Resolved shear stress (RSS) analysis and a crystal plasticity finite element method (CPFEM) were used to investigate the detwinning behavior during in-situ tensile testing. The rate of detwinning showed a linear relationship with both the average RSS for twin bands and the average accumulated detwinning fraction when multiplied by the real average twin width. © 2019 Elsevier Ltd.