A comparative study on the evolution of microstructure and hardness during monotonic and cyclic high pressure torsion of
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NANOCRYSTALLINE HIGH ENTROPY MATERIALS: PROCESSING CHALLENGES AND PROPERTIES
A comparative study on the evolution of microstructure and hardness during monotonic and cyclic high pressure torsion of CoCuFeMnNi high entropy alloy Reshma Sonkusare1, Nimish Khandelwal2, Pradipta Ghosh3, Krishanu Biswas1, Nilesh Prakash Gurao1,a) 1
Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India; and Department of Metallurgical and Materials Engineering, Malaviya National Institute of Technology Jaipur, Jaipur 302017, India 3 Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Leoben 8700, Austria a) Address all correspondence to this author. e-mail: [email protected] 2
Received: 31 August 2018; accepted: 21 November 2018
Discs of CoCuFeMnNi face centered cubic high entropy alloy were subjected to monotonic and cyclic high pressure torsion (HPT) in a single step and multiple steps of 5° forward and reverse cycle for 100° and 360° twist, respectively, under 5 GPa pressure at room temperature. It was observed that the 100° cyclic HPT sample shows the highest hardness at the periphery comparable to 360° monotonic HPT sample, while the cyclic 360° HPT sample shows the lowest hardness throughout the sample. High hardness of 100° cyclic HPT sample can be attributed to finer grain size and unstable dislocation substructure by continuous change in strain path from initial compression to forward–reverse torsion, while stable dislocation structure corresponding to shear contributes to increase in hardness from 100° to 360° for monotonic HPT sample. The unstable dislocation substructure promotes grain boundary migration–enabled grain growth leading to low hardness throughout the 360° cyclic HPT sample.
Introduction The production of bulk submicron and nanocrystalline metallic materials by severe plastic deformation (SPD) techniques, like equal-channel angular pressing (ECAP), accumulative roll bonding, multiaxial forging (MAF), and high pressure torsion (HPT), offers significant property enhancement for conventional metals and alloys [1, 2, 3]. Most SPD techniques, like ECAP and MAF, exploit change in strain path to achieve optimum grain refinement. In ECAP, various routes, like A, B, C and D, along with their variants are designed on the basis of rotation of sample from one pass to another to achieve different extent of grain refinement [4, 5], while strain path change is pivotal to MAF and tri-axial forging [6, 7]. However, the role of strain path change in HPT, which is by far the most potent processing technique to produce bulk nanocrystalline metallic materials, is in its infancy [8, 9] and needs to be investigated in details. Preliminary investigation on iron, nickel, aluminum, and aluminum–magnesium alloys have indicated the important role of geometrically necessary dislocations (GNDs) during cyclic HPT [10, 11, 12, 13, 14]. It is
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