Sundararajan G

Uniaxial plain fatigue and fretting fatigue tests were carried out on detonation gun sprayed Cu-Ni-In coating on Al-Mg-Si alloy samples. The samples in three conditions were considered: uncoated, as-coated and ground after coating. Ground coated specimens exhibited superior plain fatigue and fretting fatigue lives compared with uncoated and as-coated specimens. The life enhancement has been discussed in terms of surface finish and residual compressive stresses at the surface. © 2008 Elsevier Ltd. All rights reserved.

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.

Powder metallurgical processing of the soft magnetic Fe-P alloys is well known. In recent times, the wrought metallurgical processing of these alloys is gaining importance due to the possibility of achieving a good combination of soft magnetic properties by optimising the microstructure through carefully designed heat treatment. The Fe-P alloys are mainly focussed towards realizing an alternative and cost effective alloy to conventional electrical steels. Since not much work on bulk Fe-P alloy system has been reported, we have put efforts on processing Fe-P alloy by wrought metallurgy route to bring out comprehensively the dependence of the microstructure on magnetic properties. Fe-0.4 wt.% P alloy sheets (~0.5 mm) were prepared by adopting the conventional melting, forging and rolling, and heat treated at various temperatures (500 – 1000 °C). Recovery and recrystallization behaviour was investigated using electron back scattered diffraction technique and correlated with the microstructural parameters and mechanical/magnetic properties. Transmission electron microscopy studies revealed the formation of Fe3P nanoprecipitates (2-4 nm) leading to a reduction in lattice strain as confirmed from X-Ray diffraction data analysis. Heat treatment at 1000 °C/1h resulted in the formation of fully recrystallized coarse grain structure with coercivity of 80 A/m. Upon further aging at 300 - 500 °C/0.5 h, the coercivity reduced to 43 A/m with a core loss of 347 W/kg. The attractive combination of soft magnetic properties was attributed to the coarse grain and nanoprecipitate microstructure with reduction in lattice strain. The results were also compared with commercial non-oriented Si-steels (M700-50A and M400-50A).

The performance of lithium-ion batteries depends on the morphology and distribution of components in the electrode. LiNi1-x-yMnxCoyO2 (NMC) with nano-micro hierarchical structure is a commercially important cathode material for electric vehicle application. The commonly used slurry preparation method is to dry-mix the active material with a conductive additive and then disperse it in the binder solution. The energy imparted during premixing, while helping in controlling the distribution of CB to establish conducting network, can deteriorate NMC by mechanically dismantling the aggregate structure. Herein, a comprehensive effort is put forward to understand dry-mixing by high-energy ball-milling. It is found that mixing-intensity influences the establishment of both long-range and short-range electronic conductive pathways.The sample mixed with very low intensity and high intensity exhibited a capacity of 68 mAh.g-1 and 20 mAh.g-1 respectively, while the sample with optimum mixing exhibited 100 mAh.g-1 at 5C. This study highlights the guidelines for preparing electrodes for high-energy and high-power applications. At low C-rates, the capacity depends on the integrity of the microstructure of NMC, while at high C-rate optimum short and longrange electronic conductivity is the deciding factor.

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.

Fe-P-Si alloys with two levels of phosphorous (0.11 and 0.33 wt%) were prepared through the wrought alloy route. The as-cast alloys were forged and rolled at high temperature (>1000 °C) followed by stack hot rolling up to ~0.5 mm thickness. The alloys were further solutionized at 1000 °C/1h followed by aging at 500 °C/30 min and characterized for microstructural, magnetic and electrical properties. X-ray diffraction studies revealed that the alloys are of single phase α-Fe. From electron backscattered diffraction (EBSD) analysis, it was inferred that the grains were equiaxed with no retained austenite phase in the alloys. Transmission electron microscopic (TEM) and Electron Energy Loss Spectroscopy (EELS) showed the presence of Fe3P/Fe3(P,Si) nanoprecipitates whose volume fraction increased from 1.17% to 2.08% as P content (wt.%) of the alloys increased from 0.11 to 0.33%, with a corresponding increase in precipitates size from 1.7 nm to 2.6 nm. To rationalize the influence of various microstructural parameters on the magnetic and electrical properties of the alloys, we have used the existing theoretical models. As a result, it is now possible to estimate the relative contributions of the microstructural features to the various electrical and magnetic properties. Low core loss (~186 W/kg) combined with low coercivity (47 A/m) and high saturation magnetization (2.1 T) obtained in high P content (0.33%) Fe-P-Si alloy makes it a promising material for automotive applications.

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.