• P Ramachandrarao

      Articles written in Bulletin of Materials Science

    • Production and characterisation of amorphous alloys at varanasi

      P Ramachandrarao S Ranganathan T R Anantharaman

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      Amorphous alloys, more commonly referred to as metallic glasses, represent a striking advance in inorganic materials technology of recent times. While the probable atomic arrangements in noncrystalline alloys have aroused scientific curiosity, their unusual mechanical strength, attractive magnetic properties and remarkable corrosion resistance have excited technological interest. This report describes the progress of research at Varanasi on the following aspects: adaptation, innovation and development of techniques for rapid solidification, study and refinement of structural models, calculation of thermodynamic quantities, evaluation of strength and corrosion resistance and studies of glass to crystal transition.

    • Processing of ceramics and glasses in space

      P Ramachandrarao

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      Possible experiments in space on ceramics, composites and inorganic glasses are listed. Advantages in processing these materials under microgravity conditions, anticipated effects and likely problems are discussed. Theoretical conclusions and experimental results to date are reviewed. It is suggested that experiments on metallic glasses in space could prove to be rewarding.

    • Diffusion kinetics and some aspects of solidification under microgravity environment

      P Ramachandrarao

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      The paper surveys the available literature on the direct influence of microgravity on diffusion in liquid metals, rate of solidification, growth of dendrites, undercooling of liquid metals and alloys and monotectic solidification. Agreement between theoretical predictions and experimental observations is discussed critically and areas requiring further study are highlighted.

    • Undercooling studies on dispersed bismuth droplets

      Sanjay Chaubey Vakil Singh P Ramachandrarao

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      In the zinc-bismuth system, a monotectic reaction occurs at 689 K and 0·6 at.% Bi composition. Rapid solidification of the as-cast monotectic alloy led to a micromorphology in which bismuth was uniformly and bimodally distributed as small droplets in the zinc matrix. Statistical analysis of the electron micrographs obtained from different transparent regions of the foils revealed that the size of most of the droplets was about 6 nm. These droplets undercooled by 132 K. An analysis of the nucleation rate measurements shows that the activation energy barrier to nucleation is of the order of 39·8 kcal/mol at the maximum undercooling.

    • Viscous and thermodynamic behaviour of glass-forming organic liquids

      K S Dubey P Ramachandrarao

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      The free energy difference (ΔG) between an undercooled liquid and its corresponding equilibrium solid has been evaluated on the basis of a method involving Taylor series expansion of ΔG around its value at the equilibrium melting temperature. The resultant expression is shown to be capable of correctly estimating ΔG at temperatures as low as the glass transition temperature. The method is then enlarged to obtain the configurational entropy and used in conjunction with the Adam and Gibbs model to derive a novel expression for the viscosity of undercooled liquids. Most commonly used expressions for the temperature dependence of viscosity are shown to be approximations of the equation obtained in this study.

    • Foreword

      P Rama Rao K J Rao P Ramachandrarao S Ranganathan

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    • Rapid solidification of steels

      P Ramachandrarao

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      Solidification at very high rates of cooling results in considerable refinement in the microstructure of alloys. It enables the formation of extremely fine grains, extends solid solubilities and leads to the nucleation of many metastable crystalline phases. We discuss briefly the methods of rapid solidification and their impact on microstructure and structure of alloys. Each of the aspects of structure development is illustrated with examples from steels.

    • Synthesis of high-energy-density Pr2Fe14−xCoxB,x⩽3, magnets for practical applications

      S Haldar S Ram P Ramachandrarao H D Banerjee

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      Stable magnetic powders, of 1–2µm particle size, of partially Co-substituted, Pr2Fe14−xCoxB,x⩽3, alloys together with 2–4 at% excess Pr were prepared by rapidly quenching the associated melts into thin ribbons and then mechanical attriting the ribbons in the refined particle sizes. The saturation magnetizationMs, remanent magnetizationJr, intrinsic coercivityHci and Curie temperatureTc were studied in characterizing the powders for fabricating into sintered or polymer bonded magnets. It is found that the smallx=0·4–0·8 substitution of the Co on Fe sites in this series sensitively leads to an increase in the value ofHci, by as much as 40%, with the optimum value of 21 kOe atx ∼ 0·55, together with an improvement in theTc from 292°C to 325°C, without significantly diluting theMs∼150 emu/g andJr∼8·0 kG values. The Co-substituted Pr2Fe14B alloy particles are better stable and corrosion resistant in ambient atmosphere. The results are discussed with the microstructure and comparison with the data for Nd2Fe14B powders processed under the same conditions.

    • Formation of Nd2Fe14B hydride by milling of anhydride particles in toluene in a closed reactor

      S Ram H D Banerjee S Haldar P Ramachandrarao

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      When milling micrometer thin Nd2Fe14B platelets, of an average 1–2 mm diameter, in toluene in a closed reactor, part of the toluene decomposes at the surface of the platelets and yields nascent hydrogen and carbon/low hydrocarbons. The hydrogen diffuses into the Nd2Fe14B platelets and the carbon forms a thin surface passivation layer of the platelets, forming the stable Nd2Fe14BHx,x ≤ 5, hydride at room temperature. On heating in a calorimeter, the hydrogen desorbs off the sample with a well-defined endotherm between 370 and 425 K. An N2 gas atmosphere, if used during the heating, facilitates the H-desorption process with the modified kinetic parameters. For example, the enthalpy of the H-desorption ΔH and the related activation energyEa have the measured values ΔH = 153 J/g andEa = 58·2 kJ/mol in argon and ΔH = 256 J/g andEa = 41·6 kJ/mol in N2. It is argued that N2 gas has a fast reaction with the H atoms desorbing off the thin sample platelets and forms NH3 gas with an instantaneous decrease of the total external gas pressure at the sample. This supports the fast desorption of H atoms in the sample with the modified desorption kinetics in N2 gas.

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