• Mahendra Kumar

      Articles written in Bulletin of Materials Science

    • Chemical segregation of solute elements in ultrasonically gas atomized aluminium alloy powders

      R D K Misra A B Pandey V V Rama Rao Mahendra Kumar

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      A new experimental approach to the evaluation of chemical segregation of solute elements in ultrasonically gas atomized aluminium-alloy powders using X-ray spectral data of scanning electron microprobe analyser is described. The experimentally obtained chemical segregation data is compared with the conventional method of quantitative analysis and with theoretical predictions as determined from Scheil’s approach to the evaluation of elemental segregation during the solidification process. A comparison of experimental and theoretical predictions confirms the validity of the experimental approach in the estimation of solute segregation levels and also suggests that the solidification conditions considered for estimation of microchemical segregation can appropriately be applied to ultrasonically gas atomized powders.

    • Effect of HIPing on conductivity and impedance measurements of DyBi5Fe2Ti3O18 ceramics

      N V Prasad G Prasad Mahendra Kumar S V Suryanarayana T Bhimasankaram G S Kumar

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      X-ray diffraction, a.c. impedance and conductivity (a.c. and d.c.) have been used to characterize DyBi5Fe2Ti3O18. Samples were prepared by solid state double sintering method. A few samples were also subjected to hot isostatic pressing (HIP) at 800°C for 2 h at 100 MPa pressure. The data on XRD, impedance and conductivity of two sets of samples are compared to understand study of effect of HIPing on the properties of DyBi5Fe2Ti3O18

    • Effect of external energy on atomic, crystalline and powder characteristics of antimony and bismuth powders

      Vikram V Dabhade Rama Mohan R Tallapragada Mahendra Kumar Trivedi

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      Next to atoms and molecules the powders are the smallest state of matter available in high purities and large quantities. The effect of any external energy on the shape, morphology and structure can thus be studied with relative ease. The present investigation deals with the effect of a non-contact external energy on the powders of antimony and bismuth. The characteristics of powders treated by external energy are compared with the as received powders (control). The average particle sizes, 𝑑50 and 𝑑99, the sizes below which 99% of the particles are present showed significant increase and decrease indicating that the energy had caused deformation and fracture as if the powders have been subjected to high energy milling.

      To be able to understand the reasons for these changes the powders are characterized by techniques such as X-ray diffraction (XRD), surface area determination (BET), thermal analytical techniques such as DTA–DTG, DSC–TGA and SDTA and scanning electron microscopy (SEM).

      The treated powder samples exhibited remarkable changes in the powder characteristics at all structural levels starting from polycrystalline particles, through single crystal to atoms. The external energy had changed the lattice parameters of the unit cell which in turn changed the crystallite size and density. The lattice parameters are then used to compute the weight and effective nuclear charge of the atom which showed significant variation. It is speculated that the external energy is acting on the nucleus through some reversible weak interaction of larger cross section causing changes in the proton to neutron ratios. Thus the effect is felt by all the atoms, and hence the unit cell, single crystal grain and grain boundaries. The stresses generated in turn may have caused deformation or fracture of the weak interfaces such as the crystallite and grain boundaries.

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