Dislocation plasticity in FeCoCrMnNi high-entropy alloy: quantitative insights from in situ transmission electron microscopy deformation

Abstract

The mechanical properties of high-entropy alloys (HEAs) are still not deeply understood. Detailed knowledge of the strengthening mechanism, especially, the atomistic origin of solid solution hardening and its interplay with dislocation plasticity is needed. Here, we report on the dislocation glide behavior of a FeCoCrNiMn face-centered cubic (FCC) single crystal studied by in situ deformation in a transmission electron microscope (TEM). The threshold shear stress for dislocation glide in a thin foil is measured from dislocation curvature as exceeding 400 MPa. Interestingly, dislocations are prevented from straightening upon unloading due to high frictional stresses. IMPACT STATEMENT The fiction stress for dislocation glide in a FeCoCrMnNi HEA is assessed by direct measurement of dislocation line curvature during in situ TEM deformation, which is higher compared to other FCC metals, explaining the outstanding yield and flow stress of the HEA.

Can Experiment Determine the Stacking Fault Energy of Metastable Alloys?

“…They showed that the friction stress is significantly larger for CrCrNi (218 MPa) and CrMnFeCoNi (125 MPa) than those for pure Al (4 MPa) and Ni (14 MPa). Particularly, Lee et al [70] measured the critical resolved shear stress acting on moving [17]) and the negative SFE (−5 mJ/m 2 ), which is consistent with our above discussions. The large lattice friction in these alloys is related to the significantly roughened Peierls potential surface due to e.g., local lattice distortions or complexed chemical/magnetic configurations [28,104,105].…”

“…The average separation distance was 4.82 nm (~5.5 nm [13]) but with a large range of variation (±3.4 nm), an order of magnitude higher than the normal variation (±0.45 nm) in pure fcc metals [29]. No evidence of alloying element segregation or ordering was found in the alloy [70], but the large variations in dislocation separations were still ascribed to the local chemical inhomogeneity. It is however not clear how the local composition inhomogeneity or SROs, if exist, in the length scales of~1 nm or less [26,62,69] affect the behaviors of dislocations with significantly longer lengths and larger separation widths in the processes of faulting or twinning.…”

“…Locally, each d may be affected by the local chemical environment, but the obtained experimental SFE will be an averaged value anyway. Furthermore, recent experiments showed that the element distribution in the homogenized CrCoNi MEA is in fact highly random and homogeneous [62,69,70]. The metastability of the random CrCoNi MEA, as well CrMnFeCoNi HEA, is evidenced by the high pressure experiments showing that the fcc → hcp phase transformation occurs during pressing at ambient temperature [107][108][109].…”

“…Li et al [28] further argued that the wide variety of local chemical ordering produces a wide range of generalized stacking fault energies, therefore, increasing the ruggedness of the energy landscape for dislocation activities and influencing the selection of dislocation pathways in slip, faulting and twinning. Being aware of the difficulties of experimental methods in characterizing SROs, recent experiments have found that in the homogenized CrCoNi specimens, SRO (or local chemical variation) is however very weak, if not absent [62,69,70]; only by aging the homogenized CrCoNi MEA at 1000°C for 120 h followed by furnace cooling, Zhang et al [62] reported observation of SRO domains of~1 nm and that the measured SFE based on partial separation distance increases from 8.18 ± 1.43 mJ/m 2 for the water-quenched sample to 23.33 ± 4.31 mJ/m 2 for the annealed one. Therefore, the discrepancy in the theoretical and experimental SFEs is not solved in CrCoNi MEA, neither in other HEAs.…”

High-temperature low cycle fatigue behavior of an equiatomic CoCrFeMnNi high-entropy alloy

“…The detailed grain size effect on the HEA fatigue life still needs further investigations. Furthermore, CoCrFeMnNi relatively low SFE and severely distorted lattice [47] could also retard massive cross-slip of dislocations, which could inhibit strain localization.…”