Papers by Author: K. Anantha Padmanabhan

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Authors: M. Ravithul Basariya, K. Anantha Padmanabhan
Abstract: A viewpoint that suggests that grain/ interphase boundary sliding (GBS) that develops to a mesoscopic scale (“cooperative boundary sliding”) controls optimal superplastic (SP) deformation is able to explain superplasticity in metals and alloys, ceramics, intermetallics, composites and bulk metallic glasses of grain sizes ranging from a few microns down to a few nanometers. It is extended here to describe grain-size-sensitive (GSS) flow in minerals, rocks and ice within narrow experimental ranges. In this approach the accommodation processes of grain boundary diffusion, dislocation emission from sliding boundaries and/ or grain rotation accompanying boundary sliding are present over extremely short distances and are assumed to be faster than GBS. Analysis shows that GSS creep in geological and glacial materials can be accounted for in terms of four “universal”, mesoscopic scale constants of average values, = 0.197, = 0.415 J.m-2, = 8.9 and = 0.166, where is the average shear strain associated with a basic boundary sliding event at the level of the atomistics, is the specific grain boundary energy (assumed to be isotropic), is the number of boundaries that align to form a mesoscopic boundary glide plane and “” is a constant that obeys the condition 0<a<0.5, whose magnitude depends on grain shape and size distribution in the material. It is demonstrated that with the help of these four constants and the Frost-Ashby equations for estimating the shear modulus, it is possible to predict steady state GSS creep flow in any geological or glacial material, including those whose mechanical response was not used to obtain the “universal” constants. Whether these observations are evidence for “superplasticity” in these materials can be known only if the findings are reproduced in tensile deformation also.
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Authors: Anna Misochenko, Jeevanandham Vijaya Tilak Kumar, Sudha Jayaprakasam, K. Anantha Padmanabhan, Vladimir Stolyarov
Abstract: Influence of grain size on the martensitic transformation and mechanical properties of shape memory alloy Тi49.3Ni50.7 was studied. The features of the mechanical response of coarse-grained and nanostructured alloys were identified. The microstructure investigations involved the use of TEM and SEM.
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Authors: V. Babu, S. Balasivanandha Prabu, K. Anantha Padmanabhan
Abstract: Aluminium (AA6063) alloy was subjected at 200oC to cyclic expansion extrusion (CEE) - a severe plastic deformation (SPD) process - up to 10 passes. The extrusion die angle is 22.5o. Microstructure examination revealed evidence for grain refinement, which had reduced from 20μm±5.8 (average) in the parent material to 4.8 μm±1.4 (average) after 10 passes (i.e. ~76% of reduction). The microhardness and tensile strength had increased, while the elongation decreased with the number of passes. The specimen after 4 passes had the highest hardness of 591 HV (about 40% improvement) and the highest tensile strength value of 153 MPa which is ~23% more than that of the parent material. The hardness and tensile strength values decrease marginally on further processing, although throughout they are higher than that of the parent material. With increasing of number of passes, grain refinement and the fraction of high-angle boundaries continued to increase, which in the end was ~ 40%.
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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: K. Anantha Padmanabhan, S. Balasivanandha Prabu, A. Arsath Abbas Ali
Abstract: “Power law’’ representation is used to describe minimum creep rate and “steady state” superplastic deformation. In creep isothermal log stress – log strain rate relationship is linear for so long as the rate controlling mechanism remains unchanged. During optimal superplastic flow the slope of this curve changes even when there is no change in the rate controlling mechanism, i.e. the stress exponent, n, at a constant temperature and grain size is a function of strain rate. For a constant rate controlling mechanism, in both the phenomena, n decreases with increasing temperature. Grain size has no effect on creep, but its effect is significant in superplasticity. Therefore, analyzing creep and superplasticity data by treating n for any given mechanism as a constant independent of stress and temperature is questionable. In this analysis stress is normalized with respect to a reference stress, rather than the shear modulus. The microstructure dependence comes through the Buckingham Pi theorem. When contribution from microstructure terms to isothermal strain rate is constant, Laurent’s theorem helps generate a set of values for n. It is shown that the simplest solution, viz. n is independent of stress, but is a linear function of temperature, describes steady state creep. (The case n is independent of both stress and temperature follows as a special case.) The second simplest solution, viz. n is a linear function of both temperature and stress corresponds to steady state superplasticity. Using the equations, the values of n, activation energies for the rate controlling processes and strain rates at different temperatures and stresses could be estimated for both creep and superplasticity. The analysis is validated using experimental results concerning many systems. iiThis lecture is dedicated to the sacred memory of late Prof. Oleg D. Sherby.
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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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