Kalpakkam, India
Kalpakkam, India

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Ghosh C.,Physical Metallurgy Group | Ramachandran D.,Physical Metallurgy Group | Balakrishnan G.,PERI Institute of Technology | Kuppusami P.,Sathyabama University | Mohandas E.,Physical Metallurgy Group
Bulletin of Materials Science | Year: 2015

Phase stability of nanostructured thin films can be significantly different from the stability of the same materials in bulk form because of the increased contribution from surface and interface effects. Zirconia (ZrO2), stabilized in tetragonal and cubic phases, is a technologically important material and is used for most high temperature applications. In literature, zirconia can be found to be stabilized in its high temperature phases down to room temperature via two routes, doping with divalent or trivalent cations and crystallite size controls. Apart from these, in the alumina/zirconia thin-film multilayer system, a constraining effect on the zirconia layers provides another route to stabilization of the tetragonal zirconia phase at room temperature. However, in such nanostructured geometries, at high temperatures, the small diffusion lengths involved can influence the phase stability. The present work deals with the high-resolution transmission electron microscope (HRTEM) studies of pulsed laser ablated alumina-zirconia thin-film multilayers in the as deposited state and annealed up to 1473 K at 2 × 10-5 mbar. Conventional techniques such as X-ray diffraction lack the ability to detect localized phase changes at nanometre length scales and also for the low volume fraction of newly formed phases. Cross-sectional HRTEM techniques have been successful in detecting and characterizing these interactions. © Indian Academy of Sciences.

Jain A.,Physical Metallurgy Group | Ghosh C.,Indira Gandhi Center for Atomic Research | Ravindran T.R.,Physical Metallurgy Group | Anthonysamy S.,Physical Metallurgy Group | And 3 more authors.
Bulletin of Materials Science | Year: 2013

Structural characterization of electrodeposited boron was carried out by using transmission electron microscopy and Raman spectroscopy. Electron diffraction and phase contrast imaging were carried out by using transmission electron microscopy. Phase identification was done based on the analysis of electron diffraction patterns and the power spectrum calculated from the lattice images from thin regions of the sample. Raman spectroscopic examination was carried out to study the nature of bonding and the allotropic form of boron obtained after electrodeposition. The results obtained from transmission electron microscopy showed the presence of nanocrystallites embedded in an amorphous mass of boron. Raman microscopic studies showed that amorphous boron could be converted to its crystalline form at high temperatures. © Indian Academy of Sciences.

Hajra R.N.,Physical Metallurgy Group | Raju S.,Physical Metallurgy Group | Rai A.K.,Physical Metallurgy Group | Tripathi H.,Physical Metallurgy Group | And 2 more authors.
Advanced Materials Research | Year: 2013

The influence of magnetic interactions on high temperature thermodynamic stability of Fe-Cr binary system has been analysed in the light of accurate isothermal calorimetry measurements (400-1473 K) on Fe-20wt.%Cr alloy. The onset of two successive principal transformations namely, (i) α'(Fe-rich bcc)+α'(Cr rich bcc)→α(HT bcc) at 702±10 K; and (ii) αferro→αpara at 925 ±10 K, with their associated enthalpy effects (ΔoHmag = 2 kJ mol -1; Cp mag = 20 J mol-1 K-1) have been clearly delineated by the measured enthalpy variation with temperature. A precise quantification of magnetic contribution to high temperature thermodynamic stability has been attempted using physically based modelling approach. © (2013) Trans Tech Publications, Switzerland.

Rai A.K.,Physical Metallurgy Group | Raju S.,Physical Metallurgy Group | Vijayalakshmi M.,Physical Metallurgy Group
Journal of Nuclear Materials | Year: 2013

A differential scanning calorimetry study of high temperature phase equilibria and phase transformation kinetics in Fe100- xUx binary alloys, with x varying from 0 to 95 mass% U is undertaken. Accurate measurement of transformation temperatures pertaining to: (i) α-Fe → γ-Fe → δ-Fe polymorphic phase change, (ii) UFe2 + γ-Fe → L and U6Fe + UFe2 → L transformations and (iii) melting has been made as a function of uranium concentration. The measured transformation temperatures are used to construct the binary Fe-U phase diagram, which showed general agreement with the latest assessment. The L → UFe2 + γ-Fe eutectic reaction is found to occur at 1357 ± 5 K, with the eutectic composition of 47 mass%. The heat of transformation associated with this invariant reaction is estimated to be 19,969 ± 1736 J mol of atoms-1. The L → U6Fe + UFe2 reaction occurs at 89 mass%, and at 1001 ± 5 K, with a heat of transformation 20,250 ± 2113 J mol of atoms-1. The heat of melting of stoichiometric UFe2 is estimated to be 20,983 ± 2098 J mol of atoms-1, which is higher than the currently assessed value by 30%. A non-faceted microstructural morphology is found to accompany the eutectic solidification process of all the alloy compositions. © 2012 Elsevier B.V. All rights reserved.

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