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Hyderabad, India

Satheesh Kumar S.S.,Near Net Shape Group | Raghu T.,Near Net Shape Group
Materials and Design | Year: 2011

In this study commercially pure nickel sheets are severe plastically deformed at room temperature by constrained groove pressing (CGP) technique and the effect of pass number on the room temperature mechanical behaviour is investigated. Increase in strength observed after first pass is much higher than the increase observed during subsequent passes. Mechanical behaviour of constrained groove pressed sheets indicated negligible strain hardening ability during initial passes; gain in strain hardening ability is observed during latter passes resulting in enhanced ductility. The observation of shortened uniform elongation phase during tensile testing of CGP processed sheets could be linked to the lack of strain hardenability and change in deformation mechanism. Constitutive mechanical behaviour in uniform plastic deformation regime of nickel sheets in annealed condition obeys Hollomon relation whereas severely deformed sheets obey Voce relation closely. Strain hardening characteristics of groove pressed sheets analysed by Kocks-Mecking approach revealed stage-III hardening behaviour associated with high initial hardening rate when compared to annealed sheets. The influence of pass number on dislocation density evolution is assessed by Taylor's expression. The synergistic effect of dislocation generation and recovery on the evolution of constitutive mechanical behaviour in the uniform elongation regime is described by applying single parameter based Kocks model. The kinetics of dislocation storage and dislocation annihilation in severely deformed nickel sheets during deformation corroborated with mechanical properties and dislocation density indicates the dominance of dislocation generation during earlier passes. © 2011 Elsevier Ltd. Source


Satheesh Kumar S.S.,Near Net Shape Group | Raghu T.,Near Net Shape Group
Journal of Materials Processing Technology | Year: 2013

Pure nickel sheets are subjected to severe plastic deformation by constrained groove pressing technique at room temperature thereby imparting an effective plastic strain of 3.48. The evolution of mechanical behaviour with increasing number of passes revealed intensive increase in strength properties after first pass; however marginal increase is observed subsequently. Gain in ductility which is attributed to dislocation recovery is observed after third pass along with marginal drop in strength. Microstructural evolution during groove pressing of sheets is characterized by X-ray diffraction profile analysis using Williamson-Hall method. Besides the observation of strong shear texture in constrained groove pressed sheets, improvement in strain isotropy with increased straining is revealed from Williamson-Hall plots. The sub-grain/cell size estimated by analysing the diffraction profile of deformed sheets is found to be ∼1390 nm after processing up to three passes. Low grain refinement efficiency observed in this process compared to other severe plastic deformation techniques at similar strain conditions is explained by the deformation characteristics and loading behaviour experienced by the sheets during constrained groove pressing. © 2012 Elsevier B.V. Source


Balasundar I.,Near Net Shape Group | Raghu T.,Near Net Shape Group | Kashyap B.P.,Indian Institute of Technology Bombay
Materials Science and Engineering A | Year: 2014

High temperature creep is an important property of titanium alloys used in aeroengines. Creep resistance of titanium alloys generally varies with heat treatment, temperature and cooling rate. Both the parameters affect the morphology and topology of the α (HCP) and β (BCC) phase present in the material. Various theories have been proposed in the literature to explain (i) the increase in creep strain with decreasing solution treatment temperature and (ii) the U-shaped variation of creep strain with cooling rate. Some of these theories are quite contradictory. An attempt is made here to systematically (a) evaluate and establish a direct microstructure-mechanical property correlation and (b) to explain the observed variation in the creep behaviour of a near-α titanium alloy IMI 834. The results obtained indicate that the observed U-shaped variation of creep curve is due to the counter acting nature of various microstructural features present in the material. © 2014 Elsevier B.V. Source


Satheesh Kumar S.S.,Near Net Shape Group | Raghu T.,Near Net Shape Group
Materials and Design | Year: 2014

High purity aluminium sheets (~99.9%) are subjected to intense plastic straining by constrained groove pressing method successfully up to 5. passes thereby imparting an effective plastic strain of 5.8. Transmission electron microscopy studies of constrained groove pressed sheets divulged significant grain refinement and the average grain sizes obtained after five pass is estimated to be ~0.9. μm. In addition to that, microstructural evolution of constrained groove pressed sheets is characterized by X-ray diffraction peak profile analysis employing Williamson-Hall method and the results obtained fairly concur with electron microscopy findings. The tensile behaviour evolution with increased straining indicates substantial improvement of yield strength by ~5.3. times from 17. MPa to 90. MPa during first pass corroborated to grain refinement observed. Marginal increase in strengths is noticed during second pass followed by minor drop in strengths attributed to predominance of dislocation recovery is noticed in subsequent passes. Quantitative assessment of degree of deformation homogeneity using microhardness profiles reveal relatively better strain homogeneity at higher number of passes. © 2013 Elsevier Ltd. Source


Satheesh Kumar S.S.,Near Net Shape Group | Raghu T.,Near Net Shape Group
Materials and Design | Year: 2015

Change in strain path during severe plastic deformation (SPD) of metallic materials has shown significant influence on the microstructural evolution, grain boundary characteristics and mechanical behaviour. In the present work high purity aluminium sheets are severe plastically deformed at room temperature by conventional constrained groove pressing (CGP) technique and cross-CGP technique up to 2,4, and 6 passes thereby imparting total effective plastic strain of 2.32, 4.64 and 6.96 respectively. Change in strain path is imposed during cross-CGP by rotating the sheets by ±. 90° along thickness axis between each pass. Microstructural evolution of processed sheets studied by electron back scattered diffraction (EBSD) analysis revealed ultra-fine grains (~. 1. μm) irrespective of change in strain path. Analysis of grain boundary characteristics showed significant influence of strain path change on the evolution of relative fraction of low angle and high angle boundaries. The grain refinement mechanism during deformation processing in both conventional CGP and cross-CGP is corroborated to the evolution of misorientation distribution. Though considerable improvement in room temperature tensile characteristics is observed in both cases, cross-CGP processed Al sheets exhibited superior tensile properties. © 2015 Elsevier Ltd. Source

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