Nanomechanical behavior and structural stability of a nanocrystalline CoCrFeNiMn high-entropy alloy processed by high-pr
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Junyang He and Zhaoping Lu State Key Laboratory for Advance Metals and Materials, University of Science and Technology Beijing, Beijing 10083, People’s Republic of China
Jin-Yoo Suh High Temperature Energy Materials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea
Megumi Kawasakia) Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, South Korea
Terence G. Langdon Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089-1453, USA; and Faculty of Engineering and the Environment, Materials Research Group, University of Southampton, Southampton SO17 1BJ, UK
Jae-il Jangb) Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, South Korea (Received 27 May 2015; accepted 24 July 2015)
A CoCrFeNiMn high-entropy alloy (HEA), in the form of a face-centered cubic (fcc) solid solution, was processed by high-pressure torsion (HPT) to produce a nanocrystalline (nc) HEA. Significant grain refinement was achieved from the very early stage of HPT through 1/4 turn and an nc structure with an average grain size of ;40 nm was successfully attained after 2 turns. The feasibility of significant microstructural changes was attributed to the occurrence of accelerated atomic diffusivity under the torsional stress during HPT. Nanoindentation experiments showed that the hardness increased significantly in the nc HEA during HPT processing and this was associated with additional grain refinement. The estimated values of the strain-rate sensitivity were maintained reasonably constant from the as-cast condition to the nc alloy after HPT through 2 turns, thereby demonstrating a preservation of plasticity in the HEA. In addition, a calculation of the activation volume suggested that the grain boundaries play an important role in the plastic deformation of the nc HEA where the flow mechanism is consistent with other nc metals. Transmission electron microscopy showed that, unlike conventional fcc nc metals, the nc HEA exhibits excellent microstructural stability under severe stress conditions.
I. INTRODUCTION
Contributing Editor: Yang-T. Cheng Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2015.239
solid solutions with crystal structures of body-centered cubic (bcc) or face-centered cubic (fcc), rather than intermetallic compounds or complex phases due to the very high configurational entropy.1–4 The interesting nature of HEAs, including their simple structure, severe lattice distortion, and sluggish diffusion,3,4 leads to many promising mechanical properties, such as high strength, excellent resistance to high-temperature softening and creep, high fatigue strength, and good tribological properties.5–7 Since the properties of HEAs are not determined by a single principal element as in conventional alloys, major emphasis has been placed over the last decade in finding new compositions of HEAs d
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