Papers by Author: K. Anantha Padmanabhan

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Authors: K. Anantha Padmanabhan, S. Balasivanandha Prabu
Abstract: Different routes of severe plastic deformation (SPD) processing employed for the production of ultrafine grained and nanostructured materials are identified. The structural changes accompanying SPD that lead to improved mechanical properties are noted. Some uncertainties associated with the processing, which cause variations in the mechanical properties, are highlighted. Special attention is paid to equi-channel angular pressing/ extrusion, high pressure torsion, accumulative roll bonding/fold - roll processing, reciprocating extrusion – compression, cyclic close die forging, and repetitive corrugation and straightening- processes many of which can be adapted to suit standard metal working equipment, fitted with inexpensive devices and tools. The conflicting observations on the mechanical properties of ultrafine grained and nanostructured materials are discussed.
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Authors: Sergiy V. Divinski, K. Anantha Padmanabhan, Gerhard Wilde
Abstract: Systematic radiotracer diffusion studies on metals present in severely deformed, ultra-fine grained (UFG) states have revealed the existence of ultra-fast transport paths, which include the so-called “non-equilibrium” grain boundaries and other defects including excess free volume. Under certain experimental conditions percolating porosity is produced even in a ductile metal like pure copper. This result indicates the importance of the cavitation phenomena in severe plastic deformation under those conditions. It is well known that micro-cracking can take place in metals rather early, if the local maximum shear stress equals or exceeds the shear yield stress of the material. However, the growth and propagation of these cracks will be postponed till very late in the deformation process because of the intrinsic ductility of metals, the effect of the superimposed hydrostatic component of the stress system and/ or concurrent dynamic recovery/ recrystallization, when the latter two are present (which is likely to be the case, if the severe plastic deformation operation is successful). That is, the stage in which crack growth and propagation is present represents a material state in which the scope for further deformation is exhausted and fracture processes have taken over. Using these and similar ideas, the load required for equal channel angular pressing, the change in the slope of the Hall-Petch plot with decreasing grain size and the theoretical limit for the smallest grain size attainable in a metal subjected to a severe plastic deformation (SPD) process are predicted and checked against experimental results.
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Authors: Jörn Leuthold, Matthias Wegner, Sergiy V. Divinski, K. Anantha Padmanabhan, Daria Setman, Michael Zehetbauer, Gerhard Wilde
Abstract: Disks of copper samples were produced by High Pressure Torsion (HPT). Specimens for tensile creep experiments were cut from the disks and subjected to creep deformation at 348 K to obtain elongations greater than 30%. Electron backscatter diffraction (EBSD) was used to analyze the texture after HPT deformation and after additional tensile elongation.
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Authors: K. Anantha Padmanabhan, S. Sankaran, V. Subramanya Sarma, Satyam Suwas, Olaf Engler, Simon Miller-Jupp
Abstract: Even anisotropic superplastic flow, which is a result of an elongated grain shape and texture, can lead to extreme elongations to fracture (superplasticity). Therefore, to identify the mechanisms of deformation present during superplastic flow alone, the effects of the microstructure should be eliminated first. Using an Al 5083 alloy, in which an equi-axed microstructure is present from the beginning, it is shown that grain boundary sliding, accompanied by grain rotations, is the rate controlling mechanism.
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