Deformation behavior of nanocrystalline and ultrafine-grained CoCrCuFeNi high-entropy alloys
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NANOCRYSTALLINE HIGH ENTROPY MATERIALS: PROCESSING CHALLENGES AND PROPERTIES
Deformation behavior of nanocrystalline and ultrafinegrained CoCrCuFeNi high-entropy alloys Seungjin Nam1, Jun Yeon Hwang2, Jonggyu Jeon3, Jihye Park4, Donghyun Bae3 J. Kim5, Jae-Hun Kim1,a), Hyunjoo Choi1,b)
, Moon
1
School of Materials Science and Engineering, Kookmin University, Seoul 02707, Republic of Korea Institute of Advanced Composite Materials, Korea Institute of Science and Technology, Jeonbuk 55324, Republic of Korea 3 Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Republic of Korea 4 High Temperature Energy Materials Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea 5 Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, USA a) Address all correspondence to these authors. e-mail: [email protected] b) e-mail: [email protected] 2
Received: 22 August 2018; accepted: 23 November 2018
Nanocrystalline (NC) and ultrafine-grained (UFG) CoCrCuFeNi high-entropy alloy (HEA) with grain size ranging between 59 and 386 nm was produced via powder metallurgy and heat treatment. The as-sintered HEA exhibited two face-centered cubic (FCC) phases (CoCrFeNi-rich and Cu-rich phases) and a small grain size (59 nm), whereas the alloy after heat treatment at 1000 °C exhibited a CoCuFeNi-rich phase with FCC structure and relatively larger grain size (386 nm). Moreover, the yield strength decreased from 1930 to 883 MPa, and plastic strain to failure increased by 8–32%. In terms of microstructural evolution, grain boundary strengthening coupled with lattice distortion was the dominant strengthening mechanism for NC HEAs. Furthermore, the coefficient for boundary strengthening was higher in the HEAs than in the corresponding pure elemental metals with FCC structure, possibly because of significant lattice distortion. The UFG HEAs exhibited high strength and good ductility because of the activation of dislocation.
Introduction High-entropy alloys (HEAs), which are multicomponent alloys composed of multiple elements, have widely been investigated for their outstanding mechanical properties and thermal/electrical stabilities [1]. Although HEAs generally exhibit great strength that mainly arises from the significant lattice distortion induced by atomic size differences among alloying elements, they also exhibit a wide range of strengths depending on their composition [2, 3]. For CoCrCuFeNi-based alloys, a number of studies have been performed regarding the compositional effect on microstructural evolution (i.e., phase transformation and/or second phase precipitation) and other resulting properties [4, 5, 6]. In casting materials, CoCrCuFeNi HEAs exhibit yield strength of 230 MPa, which increases to 700 MPa upon the addition of Ti [4]. The addition of elements with large atomic size (e.g., Al, Si, Ti) can significantly increase the degree of solid solution hardening [5, 7]. The Vickers hardness of AlxCoCrCuF
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