Excellent ductility and serration feature of metastable CoCrFeNi high-entropy alloy at extremely low temperatures

Abstract

Seldom could metals and alloys maintain excellent properties in cryogenic condition, such as the ductility, owing to the restrained dislocation motion. However, a face-centered-cubic (FCC) CoCrFeNi highentropy alloy (HEA) with great ductility is investigated under the cryogenic environment. The tensile strength of this alloy can reach a maximum at 1,251±10 MPa, and the strain to failure can stay at as large as 62% at the liquid helium temperature. We ascribe the high strength and ductility to the low stacking fault energy at extremely low temperatures, which facilitates the activation of deformation twinning. Moreover, the FCC→HCP (hexagonal close-packed) transition and serration lead to the sudden decline of ductility below 77 K. The dynamical modeling and analysis of serrations at 4.2 and 20 K verify the unstable state due to the FCC→HCP transition. The deformation twinning together with phase transformation at liquid helium temperature produces an adequate strain-hardening rate that sustains the stable plastic flow at high stresses, resulting in the serration feature.

A Review of the Serrated-Flow Phenomenon and Its Role in the Deformation Behavior of High-Entropy Alloys

“…Transmission electron microscopy (TEM) characterization revealed that L1 2 nanoparticles were present in the matrix for the samples tested at 500 and 600 • C. In another study [193], it has been reported that, in CoCrFeNiMnV x HEAs, sigma-phase particles and dendrite boundaries can provide a barrier to dislocation motion, resulting in serrations. Twinning, which occurs at cryogenic temperatures, is another mechanism that can lead to the serrated flow in HEAs by hindering dislocation motion at twin boundaries [8,9,122,123,159]. In Reference [159], it was found that, in the CoCrFeNi HEA, twinning at 4.2 K was accompanied by a phase transformation from face-centered cubic (FCC) to hexagonal close-packed (HCP) structures.…”

High Strength and Deformation Mechanisms of Al0.3CoCrFeNi High-Entropy Alloy Thin Films Fabricated by Magnetron Sputtering

“…This novel concept is an important breakthrough of the past 25 years [ 7 , 8 ], as it is completely different from the traditional alloy design concepts in which one major component was selected, and other minor components were added to improve their related physical and chemical performances. It is worth mentioning that HEAs not only have simple phase structures, but also possess many excellent mechanical and physical properties, such as high tensile strength [ 9 , 10 , 11 ], good ductility at ambient and cryogenic temperatures [ 12 , 13 ], superior wear and fatigue resistance [ 14 ], and strong radiation tolerance [ 15 , 16 ]. These unique features qualify HEAs as potential engineering materials to meet the demanding requirements for complex and harsh environment applications, particularly in the turbine, aerospace, and nuclear industries [ 17 , 18 , 19 , 20 , 21 ].…”

Tailored microstructures and strengthening mechanisms in an additively manufactured dual-phase high-entropy alloy via selective laser melting

“…High-entropy alloys (HEAs) have attracted significant scientific interests owing to their special mechanical performances and application prospects [1,2]. Among HEAs, face-centered cubic (FCC) HEAs have superior ductility and have been extensively studied [3][4][5]. However, many FCC single-phase HEAs generally show insufficient strength for engineering applications [6].…”