Department of Metallurgical & Materials Engineering

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Herein, porous 1D n-p type ultra-long ZnO@Bi2O3 heterojunction nanorods have been synthesized by a solvothermal method and their complex charge transport characteristics pertaining to NO2 gas sensing properties have been investigated. The porous structure of the ZnO@Bi2O3 heterojunction nanorods assisted in achieving superior sensing properties compared to pristine ZnO nanorods. Temperature-dependent in situ electrical studies of the porous heterojunction nanorods explored the unique electron transport properties under different environments, which revealed the accumulation/depletion of electrons and charge carrier recombination leading to band bending at the metal oxide heterojunctions. The formation of electron depletion layers at n-ZnO/p-Bi2O3 interfaces is believed to increase the adsorption of oxidizing gas, resulting in a fast response time (10-12 s) and 10 times higher sensitivity than that of the ZnO nanorod-based sensor towards 500 ppb NO2. To study the structure-property correlation of the ultra-long ZnO@Bi2O3 heterojunction nanorods-based sensor, a crystallographic model supported by transmission electron microscopy analysis was adopted to understand the NO2 gas adsorption properties on the surface. The crystallographic model helps to visualize the dangling bonds and the ratio of metal to oxygen ions present at the exposed crystal planes. The results suggest that porous, ultralong n-p type ZnO@Bi2O3 heterojunction nanorods could be a promising candidate for a high performance NO2 sensor for real time applications. This journal is © 2020 the Owner Societies.

World University root node

Physical Chemistry Chemical Physics

Porous, n-p type ultra-long, ZnO@Bi2O3 heterojunction nanorods - Based NO2 gas sensor: New insights towards charge transport characteristics

Al-4.4Cu/TiB2 composites were fabricated with and without post-in-situ reaction ultrasonic melt treatment. The structural and mechanical behaviors of the composites in both the as-cast (F) and T6—peak-aged conditions were analyzed and compared with the base alloy Al-4.4Cu. The microstructural result reveals that the ultrasonic-assisted processing enhanced the dispersion of nano-sized TiB2 particles. The ultrasonic treatment-assisted fabrication has improved the yield strength of Al-4.4Cu/2TiB2 composite about ~ 2 times over the monolithic Al-4.4Cu alloy in both the as-cast and peak-aged condition while retaining &gt; 90 pct ductility of the matrix alloy. The various strengthening mechanisms operating in the materials, namely, base alloy, micro- and nanocomposite were discussed and the theoretical yield strength was estimated using appropriate equations. The theoretical yield strength estimates were found to correlate well with the experimental observations. © 2019, The Minerals, Metals & Materials Society and ASM International.

Metallurgical and Materials Transactions B: Process Metallurgy and Materials Processing Science

Structural and Mechanical Behavior of Al-4.4Cu/2TiB2 In-Situ Nanocomposites Fabricated by Post-In-Situ Reaction Ultrasonic Processing

In the present work, attempts have been made to design and develop low-carbon, low-alloy, high strength carbide-free bainitic steels for application in heavy-haul rail tracks. The study starts with an analytical approach for understanding the effects of different alloying elements on the thermodynamic and kinetic aspects of bainite transformation and the resultant microstructure and mechanical properties. Based on the analysis, two steel compositions were designed for further investigation. The alloys were prepared by melting and casting and processed by hot-rolling and air cooling, which is the usual route for the industrial production of rail sections. Microstructures of the developed steels primarily consisted of plates of carbide-free bainitic ferrite, interspersed with fine films of retained austenite. These continuously cooled steels offer ultra-high strength levels with very good ductility which are far superior to existing pearlitic grade rail steels. Detailed characterization and quantification of microstructural constituents have been carried out and they are correlated with the tensile proprieties using semi-empirical formulations available in literature. © 2019 Elsevier B.V.

Materials Science and Engineering: A

Development of continuously cooled low-carbon, low-alloy, high strength carbide-free bainitic rail steels

IN Patent C22C38/

Ultra-high strength and wear resistant carbide-free bainitic rail steel and method of manufacturing the same

The selective laser melting (SLM) is a popular additive manufacturing (AM) technique used for the fabrication of metal parts. In the present study, two 316L stainless steel specimens (SLM-I and SLM-II) with different microstructures were fabricated with different levels of energy density by changing the laser power and scanning speed, which are the main SLM process conditions. The deformation and fracture behavior of miniature SLM specimens under uniaxial tension were experimentally measured via optical microscopy (OM), field emission scanning microscopy (FE-SEM), and an electron back-scattered diffraction (EBSD) technique. In order to analyze the deformation heterogeneities under uniaxial tension, the inverse pole figure (IPF) map, kernel average misorientation (KAM) map, Taylor factor (TF) map, grain boundaries (GBs), E3 twin boundaries (TBs), and melt pool boundaries (MPBs) developed in deformed SLM specimens were analyzed at different strain levels. The effect of microstructural factors on the deformation heterogeneities of SLM specimens was explained by the evolution of KAM, GBs, MPBs, and E3 TBs. The initial microstructures of the SLM specimens significantly influenced the generation and propagation of cracks under uniaxial tension. (C) 2020 The Author(s). Published by Elsevier B.V.

Fulltext

JOURNAL OF MATERIALS RESEARCH AND TECHNOLOGY-JMR\&T

Effect of initial microstructure on the deformation heterogeneities of 316L stainless steels fabricated by selective laser melting processing

Herein, the symmetry of the experimentally observed soft phonons in the body-centered cubic β-phase (Im (Formula presented.) m) of titanium is analyzed. The harmonic phonon dispersion relations are calculated using the first-principles calculations. Using the group-theoretical methods, the symmetry of the calculated unstable phonons is determined. The symmetries of the unstable phonons observed at wave vectors (Formula presented.) (N) and (Formula presented.) ((Formula presented.)) are the same as the symmetries of the (Formula presented.) and (Formula presented.) irreducible representations, respectively. Transformations of the β-phase due to the atomic motion of unstable phonons and the subsequent structure relaxation are discussed. One possible way to explain the transformation of the β-phase to the hexagonal close-packed α-phase ((Formula presented.) /mmc) is through an orthorhombic structure (either Cmcm or Pnnm). The atomic motion of an unstable (Formula presented.) phonon results in the orthorhombic structure and following structure relaxation transforms the orthorhombic structure to the α-phase. Similarly, the transformation of the β-phase to another hexagonal close-packed ω-phase (P6/mmm) can be considered to be happening through a trigonal structure (either P (Formula presented.) m1 or P3m1). The atomic motion of an unstable (Formula presented.) phonon forms the trigonal structure and subsequent structure relaxation transforms the trigonal structure to the ω-phase. The space group of the intermediate phase is a common subgroup of the space groups of the initial β-phase and the final α/ω-phase. Therefore, the β–α/ω transformation can be described as an unstable phonon-induced reconstructive transformation of the second type. There is no activation energy barrier along each of the four energy-minimizing paths, and the transformation strains are accommodated. © 2020 Wiley-VCH GmbH

Physica Status Solidi (B) Basic Research

Reconstructive Phase Transformations in Body-Centered Cubic Titanium

Deformation mechanisms of a friction stir processed Fe40Mn20Co20Cr15Si5 transformative high entropy alloy using three different process parameters were studied to explain the microstructural dependence of tensile response in these specimens. The relative strain hardening contribution due to transformation and twinning effects was different for the three different microstructural conditions and thus resulted in different magnitude of work hardening. Crystal plasticity simulations confirmed that synergistic activity of face centered cubic and hexagonal close packed slip and twin mechanisms resulted in sustained work hardening and enhanced uniform elongation. The non-uniform ductility and extent of void nucleation and growth in these specimens were limited. Nevertheless, fractography and X-ray microscopy of fractured tensile specimens verified that microstructural flexibility induced different propensities for void growth and ductile fracture mode. Thus, alloy design induced phase stability, adaptive phase evolution due to friction stir processing, extent and kinetics of martensitic transformation and related work hardening capability of the individual phases combined together to determine the overall tensile response. The crystallographic orientation dependence of deformation induced phase transformation was also thoroughly studied. This study included quantitative determination of the crystallographic orientation where resolved shear stress on leading and trailing partial dislocations favored separation of the partials to cause transformation at a critical value of applied stress. © 2019

Acta Materialia

Deformation mechanisms and ductile fracture characteristics of a friction stir processed transformative high entropy alloy

Use of laser powder bed fusion (LPBF) additive manufacturing (AM) in structural applications requires development of a) damage-tolerant alloys (alloys that exhibit high strength and ductility despite the presence of pores and/or microcracks), and b) corrosion resistant alloys. High entropy alloys (HEAs) offer abundant alloy design space that can be used to tune deformation mechanisms and address both these challenges effectively. In line with that, transformation induced plasticity (TRIP) assisted Fe38.5Mn20Co20Cr15Si5Cu1.5 HEA (Cu-HEA) was printed with LPBF-AM. Despite the presence of 1.5 vol. % of microcracks and pores, as-built Cu-HEA exhibits tensile strength of ∼1235 MPa (highest among as-built HEAs) and ductility of ∼17.2 %, thus displaying damage-tolerant behavior. Additionally, as-built Cu-HEA exhibits a steeper polarization slope as compared to SS 17-4 PH and as-cast Cu-HEA, thus exhibiting higher passivation tendency. These findings demonstrate an effective strategy for developing damage-tolerant anticorrosive materials for LPBF-AM. © 2020

Additive Manufacturing

Damage-tolerant, corrosion-resistant high entropy alloy with high strength and ductility by laser powder bed fusion additive manufacturing

Cyclic voltammetry (CV) for Pd electrodeposition on Au from PdCl2+HCl solution between +1.100 and –0.400 V vs Ag/AgCl (3M NaCl) was performed. The CVs exhibit nucleation loop (NL), which can be characterized by higher cathodic current density (j) in the reverse scan rather than that in the forward scan. The reasons for the appearance of NL are investigated by making use of the surface condition of electrode and overpotential (η). Since it is difficult to estimate η during CV, it was obtained by potentiostatic electrodepositions (ED) on the substrates with similar surface conditions as those within NL. These substrates were produced by a potential sweep in (i) forward (FS) and (ii) forward+reverse (RS) scans from open circuit potential up to a representative potential (+0.330 V) within NL. These FS and RS substrates possess partial and significant Pd coverage, respectively. ED at +0.330 V on these substrates shows similar j trends in the initial stages as in NL. η during ED is estimated from the equilibrium potentials and the potential drop due to solution resistance (Rs). Rs is obtained from electrochemical impedance spectroscopy at +0.330 V. The estimated η is higher on RS, supporting higher j during electrodeposition on RS and reverse scan of CV within NL. Electrochemical and morphological analyses suggest that Pd deposition on the FS is nucleation driven whereas that on the RS is growth driven with the surface condition playing an important role in the appearance of NL during CV. © 2020 Elsevier B.V.

Surfaces and Interfaces

Influence of surface condition on the current densities rendering nucleation loop during cyclic voltammetry for electrodeposition of Pd thin films

Microstructural characterisation of artificially aged and service-exposed ASTM A204 Grade C steel reveals changes in the cementite particles of pearlite and microstructure. The ageing temperatures were chosen to be high enough to replicate the service degradation but below A c1 to avoid austenitisation in the inter-critical regime. Tensile testing of artificially aged materials shows a decrease in the ultimate tensile strength (UTS) along with an increase in elongation. On the other hand, service-exposed material resulted in an increase in UTS and a decrease in elongation. The softening of pearlite due to spheroidisation of cementite and segregation of alloying elements towards grain boundaries were responsible for the lower UTS of the artificially aged samples. Whereas, the increase in UTS in the service-exposed sample was attributed to the formation of nano-sized carbides inside ferrite grains, hence strengthening the steel matrix. © 2020 Institute of Materials, Minerals and Mining.