Effect of Strain Rate on the Tensile Behavior of CoCrFeNi and CoCrFeMnNi High-Entropy Alloys

Abstract

High entropy alloys (HEAs) are a new class of alloys with the potential to be used in critical load bearing applications instead of conventional alloys. The HEAs studied in this research were CoCrFeNi and CoCrFeMnNi. Both were single-phase face-centered cubic materials. The focus of this study was on the tensile behavior of the two materials at quasi-static and dynamic strain-rates (10 −4 to 10 3 s −1) and the underlying microstructural phenomena driving the behaviors. Electron back-scatter diffraction was performed on both HEAs to study the microstructure before mechanical testing. To study the effect of strain rate, tensile experiments were performed at quasi-static strain rates on hydraulic MTS load frames and dynamic strain-rates on a Split-Hopkinson Pressure Bar. HEAs stressstrain curves, modulus of elasticity, yield strength, ultimate strength, strain-rate sensitivity and work hardening rates were calculated with the data from the tensile experiments. Transmission electron microscopy was performed post-mortem to study the plastic deformation mechanisms activated at different strain rates. The dominant deformation mechanism changed from dislocation slip at quasi-static strain-rates to the addition of deformation nano-twins at dynamic strain-rates. Ultimate strength and ductility both improved with the increase of strain-rate, which can be attributed to the activation of deformation nano-twins in HEAs. CoCrFeNi and CoCrFeMnNi both have low stacking fault energies which encouraged twinning at high strains to accommodate plastic deformation. The strain-rate sensitivity component increased with increasing strain-rate, beginning with negligible strain-rate sensitivity in the quasi-static range to high strain-rate sensitivity in the dynamic range. CoCrFeMnNi showed greater strain-rate sensitivity. CoCrFeNi, with the less configurational entropy, had higher iii mechanical properties and strain hardening rates at different strain-rates compared to CoCrFeMnNi.

Tensile deformation behavior of twist grain boundaries in CoCrFeMnNi high entropy alloy bicrystals

“…Numerous recent studies on 3d transition metal HEAs, such as CoCrFeMnNi 1 , 16 – 18 , CoCrFeNi 16 , 19 , CoFeMnNi 16 , CoCrMnNi 16 , and Cr Fe Mn Ni 20 , have explored the impact of temperature 21 , 22 , grain size 23 – 25 , alloy composition 26 – 28 , strain rate 29 , strain levels 30 , and phase transformations 31 , 32 on the mechanical behavior of these alloys. The equiatomic CoCrFeMnNi, or so-called Cantor, alloy is a face-centered cubic (FCC) single-phase alloy that has been regarded as a model system to many single-phase HEAs.…”

Strain‐Rate Sensitive Deformation Behavior under Tension and Compression of Al0.3CrFeCoNiMo0.2

“…[33] In addition to the good strength-ductility-balance, many HEAs are characterized by an improved strain hardening behavior and an increased strain rate sensitivity (SRS). [29,[34][35][36][37][38][39][40][41] It was found that increasing the number of different alloying elements in FCC HEAs leads to a higher SRS when maintaining the single-phase character of the material. [35,38,42] However, if the content of alloying elements exceeds a certain value, the SRS decreases as a consequence of the formation of multiple phases, as reported, for example, for Al in CrFeCoNi.…”

“…The strain-rate sensitive deformation behavior of HEAs/CCAs has been derived from either compression tests [38,40,[43][44][45][46] or tensile tests. [11,34,36,39,41] Some studies compare the quasi-static tensile strength with the strength determined from dynamic compression tests. [29,35] A possible weakness of this approach is that a direct comparison of data gained from tensile and compression tests implies load-path independent material DOI: 10.1002/adem.202100921…”

Tensile Response of As-Cast CoCrFeNi and CoCrFeMnNi High-Entropy Alloys

“…A higher strain rate and grain refinement were found to have positive effects on the tensile properties of HEAs. While no significant effects of quasi-static strain rates on tensile properties were reported in either fine-grained CoCrFeNi or CoCrFeMnNi HEA [13], higher quasi-static strain-rate-promoted stacking faults enhanced the UTS and elongation in coarse-grained CoCrFeNi HEA [14]. The dominant microstructures of dislocation slips at quasi-static strain rates together with deformation twins at dynamic strain rates determine tensile deformation behaviors in both fine-grained CoCrFeNi and CoCrFeMnNi HEAs [13], whereas deformation-induced planar slip at lower strain together with induced stacking faults at higher strain were found in fine-grained CoCrFeNi HEA [14].…”

“…High-entropy alloys (HEAs), an intriguing new class of equiatomic solid-solution alloys, have attracted interest for their promising engineering applications as structural materials [1][2][3][4]. Comprehensive studies have been devoted to achieving excellent combinations of high strength and good ductility and determining the microstructural-evolution-driven deformation responses in single-phase, face-centered-cubic (FCC) HEAs under uniaxial loading [5][6][7][8][9][10][11][12][13][14]. The desired mechanical properties of a good strength-ductility balance in the HEAs may be tailored via tuning their elemental compositions [15,16].…”