• K Krishan

      Articles written in Pramana – Journal of Physics

    • Evaluation of irradiation-induced creep rate: application to the vacancy dislocation loop contribution

      K Krishan V Balakrishnan S K Ray

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      Irradiation (as in a nuclear reactor) drastically affects the defect structure and its time evolution in a material, and induces new creep mechanisms in it. We present a formalism to evaluate the contribution to creep owing to such mechanisms. Beginning with the phenomenological constitutive relation for the strain appropriate to a given mechanism, we put in simple statistical considerations to derive an expression for the corresponding creep rate. This formal expression is in terms of the defect production rate and a non-equilibrium probability distribution function involving the pertinent properties of the defect type concerned. A convenient approximation scheme for practical calculations is employed, that also makes contact with standard rate theory and provides a proper interpretation for the variables occurring there. As an illustration, we evaluate the contribution to irradiation-induced creep from the orientation-dependent shrinkage of vacancy dislocation loops in an applied stress field. The circumstances inducing transient and non-transient creep are clarified and a numerical estimate is given for the latter component.

    • Effect of irradiation at low doses and incubation of voids within the rate theory approach

      K Krishan R V Nandedkar

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      The evolution of defects in a material under irradiation is studied at low doses (∼5 dpa or less) using rate equations. It is shown that as a function of temperature at a critical valueTc a transition occurs in the behaviour of the solutions of the rate equations. BelowTc the voids show incubation effects. An expression is derived for the critical dislocation density at which the void growth starts. This is related to the trapped vacancy fraction ε in vacancy dislocation loops. AboveTc the incubation effects are shown to be related to the gas production rate which becomes the rate controlling parameter in determining the evolution of the defects. A gas-bubble to void transition occurs at a critical void radius and expressions are derived for the critical void size and dose at which the transition appears. It is shown that closely related to this is the incubation dose for interstitial loops. Finally, these features are corroborated by actual numerical integration of the rate equations.

    • Lattice static properties of vacancy clusters and interstitials in hcp magnesium: Computer simulation studies

      H K Sahu S Srinivasan K Krishan

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      Computer simulation studies have been made to investigate the static properties of mono-, di- and tri-vacancy clusters and of self-interstitials in hcp magnesium in different configurations. Three interatomic potentials have been chosen for which the results have been compared. A crystallite containing about 1500 atoms and a model with the interatomic interaction extending upto the fourth neighbour distance have been used. Relaxation field, defect relaxation and formation energies, strength dipole tensors and relative changes in volume in the above defects have been computed and our final results compared with those of earlier workers. The formation energies of the defects are highly sensitive to the choice of the potential whose detailed structure guides the nature of relaxation and the dipole tensors. Calculations have been done for octahedral, tetrahedral and dumb-bell interstitials of which the last is found to be the most stable.

    • Radiation-induced structural changes in alloys

      K Krishan C V Sundaram

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      Development of alloys for reliable performance in extreme radiation environments is vital for the viability of advanced nuclear reactor systems. Over the past decade, there has been a considerable growth in our understanding of the basic processes of radiation damage, the nature of the induced defects, their interaction and migration, and the influence of these on the mechanical behaviour of metals. This understanding has however come mainly from studies in pure metals and dilute alloys, and there are difficulties when applying these concepts to concentrated alloys, particularly of technological interest. The present article, which attempts to bridge this gap, discusses recent research developments and some of the emerging new concepts as applicable to alloy systems. Interstitialcy transport; percolation effects in defect migration; short range and long range ordering and restructuring of alloys; defects and damage behaviour of metallic glasses; synergetic processes and phase instabilities; and finally, swelling, irradiation creep and ductility behaviour of alloy systems are the topics discussed.

    • Solid state effects during deuterium implantation into copper and titanium

      H K Sahu M C Valsakumar B Panigrahi K G M Nair K Krishan

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      Results of neutron counting experiments during deuterium implantation into titanium and copper are reported. Models for neutron yield have been developed by taking into account different solid state effects like energy degradation of incident ions, energy dependent d-d fusion cross section and diffusion of implanted deuterium possibly influenced by surface desorption and formation of metal deuterides. The asymptotic time dependence of the neutron yield during implantation has been compared with the experimental results. Using these results, solid state processes that might occur during deuterium implantation into these metals are inferred.

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