• G Venkataraman

      Articles written in Bulletin of Materials Science

    • Indigenous development of niobium-based superconducting materials

      D K Bose C K Gupta T S Radhakrishnan Y Hariharan G Venkataraman

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      A programme jointly launched by Bhabha Atomic Research Centre, Trombay and Reactor Research Centre, Kalpakkam to develop indigenously, niobium titanium alloys for superconducting magnets is described. Results of short sample tests on specimens with different treatment are presented. Future plans are also outlined.

    • Resistance fluctuation spectroscopy

      G Venkataraman V Balakrishnan

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    • Soft modes and structural phase transitions

      G Venkataraman

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      This paper presents a survey of soft modes and their relationship to structural phase transitions. After introducing the concept of a soft mode, the origin of softening is considered from a lattice-dynamical point. The Landau theory approach to structural transitions is then discussed, followed by a generalisation of the soft-mode concept through the use of the dynamic order-parameter susceptibility. The relationship of soft modes to broken symmetry is also examined. Experimental results for several classes of crystals are next presented, bringing out various features such as the co-operative Jahn-Tellev effect. The survey concludes with a discussion of the central peak, touching upon both the experimental results and the theoretical speculations.

    • Possible studies on adhesion in space

      G Venkataraman

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      This paper reviews possible experiments on adhesion that can be carried out in a space lab environment. After introducing the basic concepts of adhesion physics, the proposed European experiment on surface energy measurement by dynamic methods in space environment is discussed and reviewed. It is concluded that as of now more studies on adhesion in terrestrial environment are desirable than space experiments.

    • Foreword

      C N R R V Balakrishnan G Venkataraman

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    • Instabilities in yielding

      G Venkataraman

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      This paper addresses itself to instabilities observed during tensile testing, and complements the papers of Rodriguez and Ananthakrishna presented at this Meeting. The work of Cagliotiet al on the elastic to plastic transformation is first reviewed. The work of Kubinet al on the serrated (repeated) yielding observed at liquid helium temperatures is then discussed in brief. Finally, our own work relying on electronic simulation is described. We conclude with some brief remarks on a few important questions that merit attention in the future.

    • Phase transformations—a physicist’s perception

      G Venkataraman

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      Echoing a recent remark by Prof. Ramaseshan, a perspective review is made of the field of phase transformation, highlighting areas that might appeal to physicists. Prominent in this category are the study of ground state structures, the study of phase diagrams and the study of kinetic phenomena.

      Enumeration of ground state structures calls for inputs from group theory as well as reliable potentials, and physicists are therefore well equipped to make significant contributions. The theoretical analysis of phase diagrams (concerning which much experimental information already exists) raises challenging questions in equilibrium statistical mechanics. Since the problems map to Ising models of varying complexity and their generalizations, there are also very interesting connections to magnetism. Lately, computer simulation has added a new dimension, opening up fresh vistas both for theory and experiments. The study of kinetics belongs to the newly emerging area of non-equilibrium statistical mechanics. The focus being ontemporal phenomena, physicists can play a key role, particularly through the application of various types of relaxation spectroscopy. On the theoretical side too there are challenging problems, especially on account of the dominance of nonlinearities. Indeed if one is bold enough, one could even speculate on possible universalities underlying the complex microstructures invariably seen in late-stage evolution. In short, though the subject of phase transformations has thus far been nursed predominantly by metallurgists, there is enough room for physicists to make an independent entry and to make distinctive contributions.

    • Quasi crystals—an overview

      G Venkataraman

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      This paper gives a broad overview of recent developments concerning quasi crystals. After briefly recapitulating various experimental facts, the architecture of quasi periodic systems in various dimensions is discussed. The various density-wave models are then reviewed. This is followed by a discussion of the extra long wavelength models which appear in such systems. Some comments are then made about possible relationship to recent theories of glass which stress local icosahedral order. The paper concludes with an outlook for the future.

    • Simulation of serrated yielding with noise effects included

      K Neelakantan G Venkataraman

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      The phenomenon of repeated yield drops is extensively studied using computer simulation. The basic input is the Cottrell-Bilby model for dislocation drag, used in conjunction with a suitable scenario for work hardening. Stochastic effects associated with dislocation velocity and density are built in via multiplicative noise. Using the above scheme for modelling the plastic response and assuming first that the yielding is homogeneous, the machine equation is solved for successive time steps to obtain complete stress-strain curves, similar to those obtained in laboratory experiments. Real-life patterns are well simulated, including several detailed features observed in diverse experiments. The role of noise in the region of marginal stability is explored. The simulation study is next extended to the case of inhomogeneous yielding where, in addition to the model earlier proposed by Penning, we explore one which takes some account of dislocation flow across the sample. Attention is then drawn to similarities observed in the flow of sand in a sand pile. Finally, some suggestions are made for further work.

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