Department of Metallurgical & Materials Engineering

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High temperature deformation behaviour of 2507 super duplex stainless steel was investigated by conducting isothermal hot compression tests. The dominant restoration processes in ferrite and austenite phases present in the material were found to be distinct. The possible causes for these differences are discussed. Based on the dynamic materials model, processing map was developed to identify the optimum processing parameters. The microstructural mechanisms operating in the material were identified. A unified strain-compensated constitutive equation was established to describe the high temperature deformation behaviour of the material under the identified processing conditions. Standard statistical parameter such as correlation coefficient has been used to validate the established equation. © 2017, ASM International.

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Journal of Materials Engineering and Performance

On the High Temperature Deformation Behaviour of 2507 Super Duplex Stainless Steel

The microstructure, mechanical properties, and corrosion behavior of 2507 super duplex stainless steel after multipass friction stir processing (FSP) were examined. A significant refinement in grain size of both ferrite and austenite was observed in stir zone resulting in improved yield and tensile strength. Electrochemical impedance spectroscopy and anodic polarization studies in 3.5 wt.% NaCl solution showed nobler corrosion characteristics with increasing number of FSP passes. This was evident from the decrease in corrosion current density, decrease in passive current density, and increase in polarization resistance. Also, the decrease in density of defects, based on Mott-Schottky analysis, further confirms the improvement in corrosion resistance of 2507 super duplex stainless steel after multipass FSP. © 2016, ASM International.

Effect of Multipass Friction Stir Processing on Mechanical and Corrosion Behavior of 2507 Super Duplex Stainless Steel

Sensitization of austenitic stainless steels is well-known in the temperature range of 400–800 °C. This effect can be lowered by the introduction of low-Σ coincident site lattice (CSL) boundaries in the material. In present work, 304 stainless steel was thermo-mechanically processed to obtain microstructures containing varied fractions of low-Σ CSL boundaries. Processing consisted of multi-step cross rolling at −41 °C followed by a short heat treatment at 1000 °C. In order to understand the effect of grain boundary character distribution (GBCD) on sensitization behavior, firstly, heating condition corresponding to the maximum degree of sensitization (DOS) was found by comparing double loop electro-chemical potentiodynamic reactivation (DL-EPR) plots for all the samples, which were given sensitization heat treatment at 600 °C for various time. Later, a relation between the GBCD and DOS was established by conducting DL-EPR test on all the processed samples. Low-temperature cross rolling was found to be effective in keeping the fraction of low angle grain boundaries (LAGBs) low and ensuring equi-axed grain structure. The sample, which was given low strain followed by short heat treatment, showed highest fraction of CSL boundaries among all other conditions. Grain size was found to decrease with increase in pre-strain. The results indicated that sensitization was inversely related to the fraction of CSL boundaries and grain size, while it was directly related to the fraction of random high angle grain boundaries (HAGBs). It was also found that sensitization was weakly related to the fraction of LAGBs indicating that these boundaries did not show “special” behavior, as they do in many other grain boundary related phenomenon. © 2016 Elsevier B.V.

Journal of Materials Processing Technology

Low temperature cross-rolling to modify grain boundary character distribution and its effect on sensitization of SS304

Grain boundary character distribution plays an important role in determining the functional and mechanical properties of polycrystalline materials. The aim of this work was to achieve improved coincident site lattice (CSL) fraction without increasing low angle grain boundary (LAGB) proportion. We utilized single-step thermo-mechanical processing route involving rolling followed by short heat treatment and compared the effect of rolling temperature. Our results indicated that rolling at elevated temperature led to significant increase in the fraction of special boundaries while keeping the fraction of LAGB very low, as desired. We conducted thermal stability of our sample-conditions at elevated temperatures for various lengths of time and found the microstructure of the samples to be stable up to 1000 °C. This study showed that even commercially suitable process (single step processing with short heat treatment duration) could lead to microstructure with considerable increase in CSL boundaries fraction, improved hardness values and good thermal stability. © 2016, The Indian Institute of Metals - IIM.

Transactions of the Indian Institute of Metals

Microstructure and Property Evolution for Hot-Rolled and Cold-Rolled Austenitic Stainless Steel 316L

Zinc oxide-based superhydrophobic surfaces were fabricated on aluminium oxide-seeded glass substrates via sonochemical approach by varying the parameter, the sonication time duration. The fabricated structures have nanowall-like morphology with an average long axis length and thickness of ∼300 and ∼40 nm, respectively. The surface roughness created by surface-modified ZnO nanowalls and the air pockets trappedwithin the dense nanowalls, transformed the hydrophobic glass substrates into superhydrophobic surfaces with water contact angle of 156° during 20 min of sonication. An independent analysis was carried out to study the growth of ZnO nanowalls over glass substrates in the absence of the aluminium oxide seed layer and sonication process. The results suggested that the synergistic effect of the aluminium oxide seed layer and sonochemical process can enable the formation of ZnO nanowall structures favourable for superhydrophobic property. A possible growth mechanism of ZnO nanowalls formation during sonication process has been discussed in detail. © Indian Academy of Sciences.

Fulltext

Bulletin of Materials Science

Ultrasonic-assisted fabrication of superhydrophobic ZnO nanowall films

Superhydrophobic surfaces based on aluminium oxide coatings had been developed on glass substrates via solution based approach for solar panel cover glass applications. The fabricated surface was amorphous with an interconnected porous network of nanoflakes. The static contact angle of the prepared coatings was 161° and exhibited superior self–cleaning behaviour at a tilting angle less than 10°. The 300 nm thick coating with refractive index value gradually decreased from air (n −1), through the fabricated coating (n −1.35) towards the glass substrate (n – 1.56) enabled it to acquire a high average transmittance of 95% with anti-reflecting property. The photovoltaic performance of commercially available solar cells covered with uncoated glass substrate and aluminium oxide coated glass substrate was measured under various conditions (such as – fabricated, artificially contaminated and self – cleaned conditions) and the results were compared. The uncoated glass substrate and aluminium oxide coated superhydrophobic glass substrate recovered the efficiency of saw dust contaminated solar panel by 67% and 91%, respectively, thereby enabling the fabricated superhydrophobic glass substrate to be effectively useful for self–cleaning cover glass applications. © 2017 Elsevier B.V.

Solar Energy Materials and Solar Cells

Transparent alumina based superhydrophobic self–cleaning coatings for solar cell cover glass applications

In this study, the foaming behavior of metal matrix composites (MMCs) containing micron- and nano-sized TiB2 particles was compared. Foaming was achieved by adding blowing agent into the MMC melt at 670 °C. Macrostructural and microstructural characterization of the foams were performed by using X-ray tomography and scanning electron microscopy. Quasi-static compression tests were conducted to analyze the mechanical behavior of the foams. Foams stabilized by nanoparticles showed higher expansion than those stabilized by micron-sized particles indicating a better stabilization by nanoparticles. The foams containing nanoparticles also exhibited better mechanical properties. This is mainly due to the presence of nanoparticles in the microstructure. © 2017 Elsevier B.V.

Materials Science and Engineering A

Influence of nano and micro particles on the expansion and mechanical properties of aluminum foams

Large area MoS2 and WS2 van der Waals epitaxial thin films with control over number of layers including monolayer is grown by pulsed laser deposition utilizing slower growth kinetics. The films grown on c-plane sapphire show stiffening of A1g and E12g phonon modes with decreasing number of layers for both MoS2 and WS2. The observed stiffening translate into the compressive strain of 0.52% &amp; 0.53% with accompanying increase in fundamental direct band gap to 1.74 and 1.68 eV for monolayer MoS2 and WS2, respectively. The strain decays with the number of layers. HRTEM imaging directly reveals the nature of atomic registry of van der Waals layers with the substrate and the associated compressive strain. The results demonstrate a practical route to stabilize and engineer strain for this class of material over large area device fabrication. © 2017 Elsevier B.V.

Journal of Crystal Growth

Substrate induced tuning of compressive strain and phonon modes in large area MoS2 and WS2 van der Waals epitaxial thin films

We investigate the effect of sputtered gold nanoclusters (AuNCs) on the Raman spectra of graphene. The two prominent Raman peaks of graphene – 2D and G – show drastically different behaviour with AuNC introduction. The Raman 2D peak intensity decreases with gold coverage and finally is quenched for moderate gold nanoparticle coverage, while the G peak shows a AuNC size-dependent enhancement in peak intensity. The quenching of the 2D peak is attributed to the hybridization of graphene's 2pz orbital with the 5d orbitals of Au. The G peak enhancement is due to the surface-enhanced Raman scattering induced by the surface plasmons created in the AuNC by the incident laser beam (He–Ne laser, wavelength = 632.8 nm). The existence of surface plasmons is independently confirmed by UV-visible spectrophotometry measurements. Copyright {\textcopyright} 2017 John Wiley & Sons, Ltd.

Journal of Raman Spectroscopy

Au concentration-dependent quenching of Raman 2D peak in graphene

Through-thickness microstructure development and residual stress evolution were explored in a part-pilgered Zircaloy-4 tube. Clear gradients in microstructure and residual stress were experimentally established at different locations. Such locations were considered along the axial length and also across the wall thickness. These were naturally subjected to different von Mises effective strains and were reflected on strain/microstructural gradients. The deformation gradients were also simulated with a three-dimensional elastoplastic finite element model. The model used both isotropic and anisotropic yielding. Though deformation gradients were similar with both yielding criteria, the anisotropic yielding provided a better match with experimental residual stress gradients. © 2017, The Minerals, Metals & Materials Society and ASM International.