• A K TYAGI

      Articles written in Bulletin of Materials Science

    • High voltage electron microscope irradiation and observations in metallic glasses

      R V Nandedkar A K Tyagi

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      High voltage electron microscope (hvem) has been extensively used to produce radiation damage and to study the characteristics of defects so produced in crystalline solids. To understand the defect production in metallic glasses and to evaluate the influence of such defects on physical properties like crystallisation temperature etc., high voltage microscopy and subsequentin situ heating and observation has been extremely useful technique. This paper gives a qualitative overview of such work performed in metallic glasses. In particular results obtained on a nickel based metallic glass using ahvem and an electron accelerator are presented. The advantages and limitations ofhvem irradiation are highlighted.

    • SIMS depth profiling of implanted helium in Al-Mn alloy using CsHe+ molecular ion detection

      A K Tyagi K G M Nair K Krishan

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      The use of Cs+ primary ions in conjunction with the detection of CsHe+ molecular ions is proposed for the analysis of helium in metals by secondary ion mass spectrometry (SIMS). Concentration depth profiles of helium implanted at 100keV in Al60Mn40 alloy have been measured. Helium concentrations down to about 100 ppm were measured at moderately low sputtering rate of 0·5nm/sec. The experimentally determined implantation profile of helium is compared with the theoretical profile obtained using the Monte Carlo Code TRIM.

    • Mass spectrometry based evolved gas analysis system for thermal decomposition studies

      S Dash M Kamruddin A K Tyagi

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      An experimental facility for evolved gas analysis by mass spectrometry (EGA-MS) has been built in-house and extensively used to study the temperature programmed decomposition (TPD) of a number of inorganic solids. Fractional extent of reactionα acquired from real time multiple ion detection trend analysis mass spectra of gases released from thermally impressed specimen has been used to obtain functional transformf(α) of non-isothermal solid state kinetic rate expressions. The corresponding model integral functionsg(α) based on mechanisms like random nucleation, diffusion and interface motion have been used to establish kinetics control regimes for specific decomposition sequences. From ln[g(α)/T2] vs 1/T plots Arrhenius parameters like activation energy and pre-exponential factor could be determined. Signature of the rate controlling mechanism governing the gas release behaviour was found in the crystallographic transformation brought about by the temperature programme. This paper describes the scope and capabilities of our EGA-MS facility with typical results on temperature programmed decomposition of CuSO4·5H2O and AlNH4(SO4)2·12H2O.

    • Leaching studies on borosilicate glasses for the immobilization of high-level radioactive waste in the pellet form subjected to aggressive test conditions

      VIDYA S THORAT R K MISHRA V SUDARSAN AMAR KUMAR A K TYAGI C P KAUSHIK

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      Pellet leaching and associated thermal and structural changes of sodium borosilicate (NBS) glass, used for the immobilization of high-level radioactive liquid waste, subjected to aggressive test conditions have been compared with international simple glass (ISG) subjected to the same leaching conditions. The crystalline phase getting separated out from NBS glass is found to be different for pellets and powder leaching experiments and this has been explained based on the difference in the extent of leaching occurring with glass samples in the two experiments. Based on Fourier transform infrared studies, it is inferred that, unlike in the ISG sample, Si–O–Si/B structural units become more ordered with the leaching in the case of NBS glass, and this is attributed to the partial network destruction occurring with NBS glass and crystallization of the SiO₂ phase from the glass matrix. Both the NBS glass and ISG sample show L-centre emission and the emission intensity remained unaffected with leaching, confirming that the local environment around non-bridging oxygen atoms in the NBS glass and ISG sample are unaffected and leaching occurs through network dissolution.

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