Department of Metallurgical & Materials Engineering

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Full potential linearized augmented plane wave (FP-LAPW) method based on the density functional theory (DFT) is used to investigate the structural, electronic and magnetic properties of Fe and Ni (3d transition metal) substituted Rock-salt wide band gap insulator Mg1-xMxO (M = Fe, Ni). We have performed spin polarized calculations throughout this work with generalized gradient approximation (GGA) type exchange correlation functional. Additionally, the electronic structures and density of states are computed using modified Becke-Johnson (mBJ) potential based approximation with the inclusion of coulomb energy (U = 7 eV). Based on the Vegard's law and structural optimization, the lattice parameter and bulk modulus are found to be in good agreement with experimental values. Moreover, the analysis of electronic band structures reveals an insulating character for Ni substituted MgO while semiconducting and half-metallic character for Fe substituted case. It has been found that the p-d super-exchange interaction provides a ferromagnetic character due to the 3d transition metal impurities and oxygen atom. The observed p-d hybridization at the top of the valence band edge in this investigations could be useful for magneto-optic and spintronic applications. PACS: 61.66.Dk; 71.15.Mb; 71.15.Nc; 71.20.Nr; 71.22.+ i. © 2018 Elsevier B.V.

World University root node

Journal of Magnetism and Magnetic Materials

Composition dependent tuning of electronic and magnetic properties in transition metal substituted Rock-salt MgO

Metal halide compounds with photovoltaic properties prepared from solution have received increased attention for utilization in solar cells. In this work, low-toxicity cesium bismuth iodides are synthesized from solution, and their photovoltaic and optical properties as well as electronic and crystal structures are investigated. The X-ray diffraction patterns reveal that a CsI/BiI3 precursor ratio of 1.5:1 can convert pure rhombohedral BiI3 to pure hexagonal Cs3Bi2I9, but any ratio intermediate of this stoichiometry and pure BiI3 yields a mixture containing the two crystalline phases Cs3Bi2I9 and BiI3, with their relative fraction depending on the CsI/BiI3 ratio. Solar cells from the series of compounds are characterized, showing the highest efficiency for the compounds with a mixture of the two structures. The energies of the valence band edge were estimated using hard and soft X-ray photoelectron spectroscopy for more bulk and surface electronic properties, respectively. On the basis of these measurements, together with UV-vis-near-IR spectrophotometry, measuring the band gap, and Kelvin probe measurements for estimating the work function, an approximate energy diagram has been compiled clarifying the relationship between the positions of the valence and conduction band edges and the Fermi level. © 2019 American Chemical Society.

Inorganic chemistry

Cesium Bismuth Iodide Solar Cells from Systematic Molar Ratio Variation of CsI and BiI3

The tensile and compressive flow properties, microstructure evolution and fracture behavior at room temperature were investigated in a Ti–6Al–4V alloy, caliber rolled at 750 °C. While pseudo-elastic behavior is exhibited in compression test, the same is not exhibited in the tension test. Pseudo-elasticity is ascribed to the formation of twins and martensite, which also cause a deep and elongated dimple fracture, unlike the conventional dimple fracture in tension. Electron back scatter diffraction studies reveal texture evolution in the compression test but not in the tension test. The tension-compression flow asymmetry and fracture behavior are thus attributed to the difference in microstructure and texture evolution in the two modes of deformation. © 2019 Elsevier B.V.

Materials Science and Engineering A

An evidence of pseudo-elasticity in a caliber rolled Ti–6Al–4V alloy and its effect on tension-compression flow asymmetry

Constrained groove pressing (CGP) is a severe plastic deformation (SPD) technique which results in enhanced strength of materials, however, it is also known that this technique leads to inhomogeneity and anisotropy in materials' properties owing to the geometry of CGP die used. Microstructural, as well as strain inhomogeneity, can be alleviated by inducing discontinuous recrystallization, but this may also lead to decrease in strength. In this work, a Cu-Zn alloy was processed through CGP for a single pass and then heat treated at 700 °C for short durations of 1, 2, 4, and 10 min. The aim of the work is to examine the influence of short heat-treatment (SHT) on microstructural evolution and mechanical behavior of single pass CGP processed Cu-Zn alloy. It was found that SHT at 700 °C induces recrystallization and leads to homogenization of microstructure and materials’ properties. Tensile test showed that constrained groove pressed sample and the sample heat-treated for 1 min fractured before attaining the necking zone, which can be directly related to the presence of voids or micro-cracks while samples heat treated beyond 1 min showed the necking. It was found that recrystallization leads to healing of internal voids or micro-cracks, which results in high ductility for samples given SHT beyond 1 min. Strain hardening behavior of various sample conditions was also examined, and it was found that heat-treatment of 2 min and longer duration completely transforms the hardening behavior of severely deformed samples. This implies that a minimum fraction of recrystallization is required for healing of voids/micro-cracks to alleviate the problem of premature fracture. We further analyzed the thermal stability of partially recrystallized samples and found these to be stable up to 300 °C. This study shows that post-process heat-treatment can be used as a design tool for severely deformed materials to obtain tailored microstructures with desired properties. © 2019 Elsevier B.V.

Materials Chemistry and Physics

Influence of short heat-treatment on microstructural and mechanical inhomogeneity of constrained groove pressed Cu-Zn alloy

Adsorption property of graphene oxide (GO) towards cationic dyes such as methylene blue (MB) and rhodamine B (RhB) has been investigated experimentally. GO structure used in our study is decorated with experimentally observed oxygen functional groups (OFGs). Fast and effective adsorption of MB with GO in single and binary solute solution has been observed experimentally. In support of our experimental data and to understand the interaction of adsorbent (GO) and dye, density functional theory (DFT) calculation has been used for calculating individual binding energy of GO, GO/dye in aqueous medium. Further, study on dye interaction with GO reveals that adsorption was facilitated by presence of oxygen functional group through electrostatic interactions. © 2019 Elsevier B.V.

Inducing dye-selectivity in graphene oxide for cationic dye separation applications

A Cu based functionally graded coating (FGC) has been deposited on an annealed Cu substrate by galvanostatic pulse reverse electrodeposition (PRED) route. The objective is to develop a hard surface with highly ductile and conductive interior. The cathodic current density (CCD) has been increased stepwise (from 50 to 200 mA/cm2) to synthesize Cu FGC on an annealed Cu substrate. It has three layers of Cu coating (20 μm each) with a gradual reduction in crystallite size along the thickness. Two layers of Cu-SiC nanocomposite coating with an increment in the amount of incorporated SiC nanoparticles (from 2 to 7 vol%) are electrodeposited on Cu FGC. This is done by introducing bath agitation (350 and 450 rpm) during deposition at CCD of 200 mA/cm2, which has resulted in Cu, Cu-SiC FGC with five layers (12 μm each). SiC nanoparticles are used to impart hardness to the coating through dispersion strengthening. The Cu, Cu-SiC FGC possesses higher hardness (∼3.8 GPa), lower residual compressive stress (∼291 MPa), and lower surface roughness (∼0.9 μm) as compared to electrodeposited single layer Cu-SiC nanocomposite coating. With such properties Cu, Cu-SiC FGC on annealed Cu substrate can serve as a novel prospective electrical contact material. © 2018 Elsevier B.V.

Applied Surface Science

Synthesis and characterization of novel Cu, Cu-SiC functionally graded coating by pulse reverse electrodeposition

Microstructural evolution during thermomechanical processing of a Fe-Mn-Co-Cr-Si-Cu high entropy alloy (HEA) was studied to compare the effect of friction stir processing (FSP) with cold rolling. The difference in microstructural evolution in the two routes was evaluated. A thorough annealing study of the FSP and rolled specimens illustrated the propensity of phase, grain and twin evolution, and quantified the same. Recrystallization behavior during annealing was determined to be dependent on prior processing as well as annealing route. Single-step annealing of FSP specimens and two-step annealing of FSP and rolled specimens yielded similar microstructural evolution with increasing annealing time. However, single-step annealing of rolled specimens led to entirely different microstructural evolution. A detailed study revealed that various interdependent and competing phenomena such as phase transformation, precipitation, recovery, recrystallization and grain growth and twinning are responsible for the overall microstructural evolution. Some unique mechanisms of annealing twin formation that were observed included formation of a twin at the interface of an epsilon (hexagonal close packed) martensite plate with the gamma (face centered cubic) parent grain and at a grain boundary triple junction containing a precipitate. © 2019 Acta Materialia Inc.

Materialia

Revealing the microstructural evolution in a high entropy alloy enabled with transformation, twinning and precipitation

Activation of different slip systems in hexagonal close packed (h.c.p.) metals depends primarily on the c/a ratio, which is an intrinsic property that can be altered through alloying addition. In conventional h.c.p. alloys where there is no diffusion-less phase transformation and associated transformation volume change with deformation, the c/a ratio remains constant during deformation. In the present study, c/a ratio and transformation volume change of h.c.p. epsilon martensite phase in transformative high entropy alloys (HEAs) were quantified as functions of alloy chemistry, friction stir processing and tensile deformation. The study revealed that while intrinsic c/a is dependent on alloying elements, c/a of epsilon in transformative HEAs changes with processing and deformation. This is attributed to transformation volume change induced dependence of h.c.p. lattice parameters on microstructure and stress state. Lower than ideal c/a ratio promotes non-basal pyramidal 〈c + a〉 slip and deformation twinning in epsilon phase of transformative HEAs. Also, a unique twin-bridging mechanism was observed, which provided experimental evidence supporting existing theoretical predictions; i.e., geometrical factors combined with grain orientation, c/a ratio and plastic deformation can result in characteristic twin boundary inclination at 45–50°. © 2019, The Author(s).

Fulltext

Scientific Reports

On the evolving nature of c/a ratio in a hexagonal close-packed epsilon martensite phase in transformative high entropy alloys

Nanoindentation of three metastable dual-phase high entropy alloys (HEAs) was performed to obtain their inherent elastoplastic deformation responses. Excellent combination of hardness and elastic modulus in as-cast condition confirmed that, their inherently higher strength compared to other HEAs reported in literature, can be attributed to alloy chemistry induced phase stability. Further, hardness of 8.28 GPa combined with modulus of 221.8 GPa was obtained in Fe-Mn-Co-Cr-Si-Cu HEA by annealing the as-cast material, which is the best hardness-modulus combination obtained to date in HEAs from nanoindentation. On the other hand, although Fe-Mn-Co-Cr-Si HEA showed lower hardness and modulus than Fe-Mn-Co-Cr-Si-Al and Fe-Mn-Co-Cr-Si-Cu HEAs, the former alloy exhibited the highest strain rate sensitivity, as determined from tests performed at five different strain rates. The three alloys also had subtle differences in incipient plasticity and elastoplastic behavior, while retaining similar levels of hardness; and nanoindentation response showed microstructural dependence in friction stir processed, annealed and tensile-deformed specimens. Thus, the study highlighted that while higher strength was achieved by designing a class of HEAs with similar composition, any of the individual alloys can be tuned to obtain enhanced properties. © 2019, The Author(s).

Nanoindentation behavior of high entropy alloys with transformation-induced plasticity

Microstructure evolution in E-form and AZ31 magnesium (Mg) alloys was studied via ex-situ mini-V-bending tests. Initially, the E-form and AZ31 Mg alloys had different crystallographic textures and average grain-size distributions. Direct observation of microstructural evolution during the ex-situ mini-V-bending tests was experimentally observed via electron back-scatter diffraction (EBSD) technique. The EBSD results revealed how twin bands (TBs) developed at different punch strokes (PSs) in the deformed grains made a significant contribution to the localized deformation zones in both Mg alloys under the mini-V-bending process. Eventually, the TBs and grain boundaries (GBs) in the localized deformation zones were responsible for crack initiation sites in the tension region. At lower PSs, compression (CTW) and double (DTW) twins were more prominent in E-form than in AZ31 under the mini-V-bending process. High-resolution cross-sectional t-EBSD analysis showed that surface relief allowed the grains residing on the free surface to be less affected by stress concentration while the sub-surface grains were more affected by stress concentration, which promoted the development of twinning. © 2018 Elsevier B.V.