Sundararajan G

Ni-W/SiC nanocomposite coatings were systematically deposited at varying pulse parameters and subsequently characterized using SEM, XRD, TEM for surface morphology, composition and SiC particle/Ni-W matrix interface phase boundary. In addition, mechanical properties of nanocomposite were also evaluated using nanoindentation. Uniform distribution of submicron SiC particles in nanocrystalline Ni-W alloy coatings was obtained using pulsed electrodeposition. The results indicated increase in SiC content of nanocrystalline Ni-W matrix with increase in pulse frequency and decrease in duty cycle. The incorporation of SiC was attributed to pulse current effect at cathode-solution interface leading to changes in pulsating diffusion layer thickness. A simplified mechanism of composite electrodeposition is proposed. The hardness and modulus of nanocomposite varied with SiC content. The variation in mechanical properties was rationalized based on rule of mixture and inverse Hall-Petch effect.

In the present work, nano oxide dispersion strengthened (ODS) steel was prepared by high energy ball milling of the elemental powders, followed by canning and upset forging. The chemical composition of the material used in this study is Fe-17.8Cr-2.33W-0.23Ti-0.35Y2O3. Isothermal compression tests were carried out on upset forged ODS-18Cr steel samples over a range of temperatures (1273 to 1423 K) and strain rates (10-2 s-1 to 10 s-1) utilizing Gleeble-3800 machine. Transmission electron microscopy and electron back scattered diffraction were carried out on the deformed samples to evaluate the grain size and to identify dynamic recrystallized grains. The true stress- true strain data obtained from compression tests at different strain rates were also utilized to estimate and rationalize the activation energy for deformation and the stress exponent. The obtained deformation mechanism parameters are greater than those for the non-ODS steels, which is attributed to the threshold stress generated due to dislocation/particle interaction. Sellars-Tegart equation was used to predict the peak flow stress and the predicted results were comparable to the experimental results.

The objective of the present work is to investigate the ultra-high temperature (1273–1473 K) plastic deformation behavior of a nano oxide dispersion strengthened Fe-18Cr ferritic steel (n-ODS-18Cr steel) over a range of strain rates (10−2 to 10 s−1). The flow (true) stress-true strain behavior of this steel, reported for the first time, has been utilized to obtain the flow stress as a function of temperature and strain rate. From the above data, the strain rate sensitivity parameter (m) and also the activation volume have been obtained as a function of temperature (1273–1473 K). It is then demonstrated that the flow stress values obtained in the present steel over the temperature range 1273–1473 K is due to a single dominant strengthening mechanism, i.e. Orowan dispersion strengthening. Hence, the predictions of the theoretical models which assume Orowan strengthening as the dominant mechanism can be compared with the experimental flow stress data. Such a comparison indicates that the model due to Rosler and Arzt predicts the experimental data very well while the prediction of the model due to Brandes, Kovarik, Miller, Daehn and Mills is satisfactory. It is also shown that the experimentally obtained activation volume is consistent with the dislocation detachment model.

Various physical and chemical properties of surface and subsurface regions of Al can be improved by the formation of transition metal intermetallic phases (AlxTMy) via coating of the transition metal (TM). The lower equilibrium solid solubility of TM in Al (<1 at.%) is a steep barrier to the formation of solid solutions using conventional alloying methods. In contrast, as demonstrated in the present work, surface engineering via a laser-aided additive manufacturing approach can effectively synthesize TM intermetallic coatings on the surface of Al. The focus of the present work included the development of process control to achieve thermodynamic and kinetic conditions necessary for desirable physical, microstructural and compositional attributes. A multiphysics finite element model was developed to predict the temperature profile, cooling rate, melt depth, dilution of W in Al matrix and corresponding micro-hardness in the coating, and the interface between the coating and the base material and the base material.

Silicon carbide (SiC) was reinforced in the pulse electrodeposited nickel-tungsten (Ni-W) coatings deposited on the steel substrate, and isothermal oxidation test was performed at 1273 K (1000 °C) for 24 hours. Addition of just 2 vol pct of SiC showed 26 pct increase in the relative oxidation resistance of Ni-W coating. The increased oxidation resistance was attributed to the phase evolution (SiO2, Cr2O3, CrSi2, Ni2SiO4, Cr7C3, Cr3C2, and Cr3Si), which suppressed the spallation of the oxide scale in Ni-W-2 vol pct SiC. The presence of Fe2O3 phase in the oxidized Ni-W coating was mainly responsible for the major multiple spallations at the interface and in the bulk, which resulted in the degradation of oxidation resistance.

In the present study the sliding wear behavior of pulse electrodeposited nanocrystalline Ni coatings as a function of grain size including bulk annealed Ni has been systematically studied using pin-on-disc configuration against the WC-Co counter body. The sliding wear has been analyzed with respect to wear rate, coefficient of friction, subsurface deformation and composition of wear debris. The result indicates that the sliding wear rate and coefficient of friction of Ni decreases with decreasing grain size. The subsurface beneath the worn pin surface is composed of a near surface shear region and beneath it a region of bulk plastic deformation. The ratio of the depth of the shear region to the depth of bulk deformed region decreases with decreasing grain size indicating a greater localization of near surface deformation with decreasing grain size. © 2012 Elsevier B.V.

Nanoparticle agglomerates of passivated Fe (n-Fe) and Fe0.96Cu0.04 (n-Fe0.96Cu0.04), synthesized through the levitational gas condensation (LGC) process, were compacted and sintered using the conventional powder metallurgy method. The n-Fe0.96Cu0.04 agglomerates produced lower green density than n-Fe, and when compacted under pressure beyond 200 MPa, they underwent lateral cracking during ejection attributed to the presence of a passive oxide layer. Sintering under dynamic hydrogen atmosphere can produce a higher density of compact in n-Fe0.96Cu0.04 in comparison to n-Fe. Both the results of dilatometry and thermogravimetric (TG) measurements of the samples under flowing hydrogen revealed enhancement of the sintering process as soon as the reduction of oxide layers could be accomplished. The shrinkage rate of n-Fe0.96Cu0.04 reached a value three times higher than n-Fe at a low temperature of 723 K (450 °C) during heating. This enhanced shrinkage rate was the manifestation of accumulation of Cu at the surface of the particles. The formation of a thin-surface melted layer enriched with copper during heating to isothermal holding facilitated as a medium of transport for diffusion of the elements. The compacts produced by sintering at 773 K (500 °C), with relative density 82 pct, were found to be unstable and oxidized instantly when exposed to ambient atmosphere. The stable compacts of density more than 92 pct with 300- to 450-nm grain size could only be produced when sintering was carried out at 973 K (700 °C) and beyond. The 0.22 wt pct residual oxygen obtained in the sintered compact is similar to what is used for conventional ferrous powder metallurgy products.

In the ongoing pursuit toward a high-performance lithium-ion battery (LIB), an understanding of the solid electrolyte interface (SEI) layer is important to enhance the performance and lifetime of LIB. Despite many years of dedicated research on the study of the SEI layer, the well-known mosaic model of the SEI layer has not yet been fully established experimentally. Herein, we report a comprehensive experimental evidence of the formation and growth process of the mosaic structure of the SEI layer by using a specially designed cell. Sequential in situ and ex situ characterizations provide experimental evidence for the mosaic structure of the SEI layer. Our experimental characterizations open up a promising approach to investigate the electrode-electrolyte interface comprehensively in advanced battery systems.

Controlling the grain size in electrodeposited coatings for the prevention of corrosion is highly important. To understand the relationship with grain size and electrochemical performance many experiments need to be undertaken to vary the grain size of the deposit. In the present work the (crystallite) grain size of electrodeposited Ni coatings formed in the presence of metalloids such as boron (B), sulphur (S) and phosphorus (P) was estimated from analysing mass transfer at the cathode-electrolyte interface. A mathematical model has been proposed which indicates that the grain size of the deposit is directly proportional to current efficiency and the deposition rate while being inversely proportional to the current density and metalloid (B, S, P) content in the coatings. A simple relationship is developed which is in agreement with experimental data and data that is reported in the literature. The development of such a model should significantly decrease the amount of experimentation required to achieve the desired grain size in such systems (Ni-B, Ni-S, Ni-P coatings) obtained by electrodeposition.

Carbon encapsulation of electrode materials is way to improve lifetime of lithium ion batteries by minimizing direct contact with electrolyte. However achieving uniform carbon encapsulation on electrochemically active metal oxides, specifically on layered compounds, is an extremely challenging task because of the contrastive-ambient requirement for the formation of carbon coating and the oxide. We demonstrate a novel in-situ method for uniform encapsulation of carbon on LiNi1/3Mn1/3Co1/3O2 (LNMCO) using ethylene glycol (EG), which is intercalated in metal hydroxide interlayers pillaring the structure. Heat treating EG-pillared Ni1/3Co1/3Mn1/3(OH)2 with lithium hydroxide under air ambient results in an uniform carbon coating during the growth and crystallization of LiNi1/3Mn1/3Co1/3O2 particles. The trapping of carbon precursors in between the collapsed hydroxide layers minimizes the reaction of carbon precursor with oxygen even when heat-treated in air. This in-situ carbon encapsulation mechanism is revealed using detailed analyses carried out by Raman spectroscopy, TEM, EDS mapping and thermal analysis coupled with mass spectroscopy. Superior cyclic stability of C-LiNi1/3Mn1/3Co1/3O2 with a capacity retention of 82% (75%) after 150 (300) cycles of charging/discharging is demonstrated with an optimum carbon thickness in contrast to 42% capacity retention in uncoated LNMCO samples after 100 cycles.