Department of Metallurgical & Materials Engineering

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IIT Jodhpur

The well known surface enhanced Raman scattering (SERS) needs a lot of experimental optimization for its proper implementation. In this report, we demonstrate the efficient SERS using gold nanoparticles (AuNPs) on quartz plate. The AuNPs were prepared by depositing direct current sputtered Au thin film followed by suitable annealing. The parameters varied for getting best SERS effect were 1) Numerical Aperture of Raman objective lens and 2) Sputtering duration of Au film. It was found that AuNPs formed from the Au layer deposited for 40s and Raman objective lens of magnification 50X are the best combination for obtaining efficient SERS effect. © 2018 Author(s).

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AIP Conference Proceedings

Experimental optimization during SERS application

In order to improve the surface biocompatibility, a composite coating of hydroxyapatite and calcium hydrogen phosphate was synthesized over 316 L grade of stainless steel through pulsed electro deposition. It was observed that relative phase composition and corresponding crystallinity vary with the deposition current density. Thus a detailed study with the help of simulated body fluid and MG63 cell line was carried out in order to compare their respective biocompatibility in terms osteoconduction and cytocompatibility. A detailed electrochemical behavior study was carried out to assess the corrosion behavior under contact with body fluid. Cell proliferation studies indicate that the coating deposited with a current density of 10 mA/cm2, which is having the highest weight percentage and crystallinity of hydroxyapatite phase, is the most preferable one for continuous, faster and interconnected cell growth. Also cell cytoskeletons spread in bigger areas around their nucleus with a strong interconnection between each other. The bio-mineralization study shows that on immersion of these coatings in simulated body fluid, coating obtained through 10 mA/cm2, trends to build up hydroxyapatite on its surface with 200 nm pores at initial 1–3 days. After 14 days of immersion, hydroxyapatite deposition trends to form a spherical shape of around 2 micron in radius which can accommodate larger surface area for tissue growth vis-à-vis better bonding, as compared to other coating surfaces. The polarization resistance of 10 mA/cm2 coating is almost 10 times higher than the others and thus indicates its slower rate of corrosion under contact with simulated body fluid. © 2017 Elsevier B.V.

Surfaces and Interfaces

Comparison of Osteoconduction, cytocompatibility and corrosion protection performance of hydroxyapatite-calcium hydrogen phosphate composite coating synthesized in-situ through pulsed electro-deposition with varying amount of phase and crystallinity

Conventional high strength alloys lack ductility due to lower work hardening and early onset of damage nucleation. To overcome this deficiency, high entropy alloys (HEAs) enjoy the benefit of metastability of phases to tune the deformation mechanisms for engineering strength and ductility. Inspired by this, here we present friction stir processed Fe40Mn20Co20Cr15Si5 HEA with ultrafine face centered cubic (f.c.c.) γ grains embedded in a refined hexagonal closed packed (h.c.p) ε matrix. Transformation of γ grains and twinning in ε matrix triggered uniform strain partitioning among these phases and sustained work hardening during deformation thereby reversing the conventional strength-ductility trade-off. © 2018 Elsevier Ltd.

Scripta Materialia

Reversed strength-ductility relationship in microstructurally flexible high entropy alloy

Metal-ion batteries are widely used energy storage devices. Commercial cathode materials show high voltages but low capacities, limiting energy densities. Although phosphorene has been prepared and delivers a high capacity, the low voltage is problematic for cathode. The structural, electronic, and electrochemical properties of F-decorated phosphorene for metal-ion batteries are investigated using first-principles calculations. The F atoms are converted during lithiation/sodiation/potassiation. Importantly, F-decorated phosphorene delivers a capacity of 536 mAh/g and voltages of 3.46, 3.09, and 2.95 V for lithiation, sodiation, and potassiation, respectively, which leads to high energy densities of 1856, 1657, and 1582 mWh/g. Graphene coating improves stability of F-decorated phosphorene. © 2018 Elsevier Ltd

Materials Today Energy

Phosphorene as cathode for metal-ion batteries: Importance of F decoration

The heterogeneity of both deformation and twinning behaviors through the thickness direction in E-form Mg alloy sheets was investigated via conventional Erichsen testing at room temperature (RT). The microtexture heterogeneity through the thickness direction of the E-form Mg alloy sheets deformed by different punch strokes (PSs) is discussed in terms of deformation twinning and de-twinning. The evolution of the microtexture, twin and KAM (kernel average misorientation) of E-form Mg alloy sheets deformed by different PSs was analyzed via electron back-scattered diffraction (EBSD) technique. A crystal plasticity finite element method (CPFEM) based on a random mapping (RM) scheme was used to simulate the heterogeneities of the strain/stress states of E-form Mg alloy sheets through the thickness direction during an Erichsen test. The evolution of the strain/stress that developed in E-form Mg alloy sheets during Erichsen testing was analyzed for different regions through the thickness direction under different PSs. EBSD analysis revealed that tension (TTWs), compression (CTWs) and double (DTWs) twins were the main deformation mechanisms in the upper portions. However, in the lower portions, a change in the sign of strain/stress components during Erichsen testing resulted in a significant formation of TTWs in the early stages and in a de-twinning of TTWs in the later stages. © 2018

Materials Science and Engineering A

Heterogeneity in deformation and twinning behaviors through the thickness direction in E-form Mg alloy sheets during an Erichsen test

Friction stir processing (FSP) of a new rare earth-free Mg-5Al-3.5Ca-1Mn (AXM541) alloy resulted in significant microstructural refinement and mechanical properties comparable to existing AXM series Mg alloys. Severe refinement of the Al-Ca (C36) phase during FSP led to uniform dispersion throughout the microstructure. The synergistic effect of high heat input and the presence of these 1–4 µm size particles resulted in dynamic recrystallization via particle stimulated nucleation (PSN), with an average grain size of 4.5 µm after FSP. Further, improved mechanical properties of the AXM541 alloy along the processing direction produced a tensile yield strength (TYS) of 322 ± 14 MPa and a total elongation of 16 ± 3%. The increase in strength was also attributed to the dispersion strengthening effect associated with Al-Ca and Al-Mn (D810) particles. Moreover, the AXM541 alloy showed a better mechanical response compared with other AXM alloys irrespective of high Ca/Al ratio, due to the effective refinement and redistribution of Al-Ca phases resulting from FSP. However, strain hardening ability and TYS of the AXM541 alloy could be further improved if the Mn/Al ratio could be tailored, as Al-Mn particles do not break down during FSP. © 2018 Elsevier B.V.

Fulltext

Friction stir processing of newly-designed Mg-5Al-3.5Ca-1Mn (AXM541) alloy: Microstructure evolution and mechanical properties

We have envisaged the hydrogen evolution and oxygen evolution reactions (HER and OER) on two-dimensional (2D) noble metal free borophene monolayer based on first-principles electronic structure calculations. We have investigated the effect of Ti functionalization on borophene monolayer from the perspective of HER and OER activities enhancement. We have probed the activities based on the reaction coordinate, which is conceptually related to the adsorption free energies of the intermediates of HER and OER, as well as from the vibrational frequency analysis with the corresponding charge transfer mechanism between the surface and the adsorbate. Ti-functionalized borophene has emerged as a promising material for HER and OER mechanisms. We believe that our probing method, based on reaction coordinate coupled with vibrational analysis that has been validated by the charge transfer mechanism, would certainly become as a robust prediction route for HER and OER mechanisms in coming days. © Copyright 2018 American Chemical Society.

ACS Applied Energy Materials

Scrupulous probing of bifunctional catalytic activity of borophene monolayer: mapping reaction coordinate with charge transfer

Precipitation of δ-hydride in α-Zr matrix deleteriously affects the matrix toughness. Hence, the formation of these hydrides and their equilibrium shape and orientation is studied. The precipitation of hydrides is associated with a volume change and associated chemical free energy. Hence, a multi-physics finite element model is developed to determine the optimal shape and orientation of the hydrides by minimizing accommodation free energy. The model is applied at various temperatures viz., 25, 100, 200, 300 and 400 °C. Three cases of fully elastic, elastic perfectly-plastic and elastic work-hardening plastic systems are studied and compared. The model takes into account the temperature variation of stiffness tensor, yield strength, hardening coefficients, volumetric change in the precipitation process and volume change in matrix due to hydrogen depletion. It is found that a flat hydride with its geometric orientation such that its major axis coincide the basal plane of the parent matrix is the most optimal shape and orientation. This is irrespective of whether the system is modeled as fully elastic, elastic perfectly-plastic or elastic work-hardening plastic. The results are discussed based on microscopic experiments and from experimental observations from the literature. © 2017 Elsevier B.V.

Journal of Alloys and Compounds

Optimum shape and orientation of δ-hydride precipitate in α-zirconium matrix for different temperatures

Ultrafine grained austenitic stainless steel, with two different types of grain size distributions, was studied for tensile deformation behavior. Tensile deformed specimens were analyzed by electron backscatter diffraction using scanning electron microscope. It was found from this study that uni-modal grained stainless steel (SS) having a larger fraction of submicron grains exhibited early fracture, which is attributed to the development of extensive strain localization. On the other hand, the microstructure of SS having bimodal grain size distribution showed a good combination of strength and ductility. EBSD analysis of the deformed region of these two samples revealed the presence of a distinct transition zone between undeformed or less deformed and extensively sheared regions. Multiple micro shear bands were found to be associated with the transition zone of unimodal type microstructure. The micro shear bands seen in the transition zone of unimodal SS led to the development of strain-induced martensite (SIM), which, in turn, is helpful in delaying the strain localization. However, in bimodal grained SS, the larger fraction of micron size grains undergoes a shape change, with a rotation towards [112] orientation, which results in the formation of a larger fraction of SIM having [112] orientation. The propensity for development of high SIM was found to prevent strain localization in bimodal grained SS. © 2018 Elsevier B.V.

Effects of concurrent strain induced martensite formation on tensile and texture properties of 304L stainless steel of varying grain size distribution

Extensive studies on experimental constitutive relationship for superplastic deformation of both the ideal equiaxed microstructure and the non-equiaxed microstructure were taken up over a period of more than four decades in the group. While our understanding of microstructure-superplastic flow relationship has grown significantly, there remains a gap between the theoretical modelling, based on steady-state, and the realistic behavior investigated for several materials. The observations made here are also supported by numerous publications by other investigators in the literature, with growing development of not only metals but ceramics, composites, intermetallic compounds and nano-crystalline materials aimed to push superplastic behavior to higher strain rate and lower temperature condition. Here, an attempt is made to list the range of microstructure changes that occur and as such the need to consider the same while evaluating the parameters of constitutive relationship. Some suggestions are provided to minimize the gap between the ideal and real behavior of superplastic flow based on the types of stress-strain curves reported in the superplastic literature and their effect on the nature of stress-strain rate relation. It includes the consideration of effective stress for superplastic deformation, and to incorporate the effects of strain and concurrent microstructure in the experimental constitutive relationship. © 2018, Institute for Metals Superplasticity Problems of Russian Academy of Sciences. All rights reserved.