Through-Focal HAADF-STEM Analysis of Dislocation Cores in a High-Entropy Alloy

Abstract

High-entropy alloys (HEAs) are a new class of multi-component alloys that exhibit surprising characteristics, [1] including very large strain hardening rates, large fracture toughness at room temperature [2], and a strong temperature dependence of yield strength at or below room temperature. These properties are closely linked to nano-twinning and dislocation-mediated plasticity, yet little experimental work has explored dislocation dissociation, stacking fault energy, or core structures in these alloys [3]. In this study, an HEA, containing 5 elements (Cr, Co, Mn, Fe, and Ni) with equiatomic composition was deformed to a 5% plastic strain at room temperature [4]. Post-mortem 3mm disks were electro-polished using a solution consisting of 21% Perchloric acid and 79% Acetic acid and analyzed using a probe-corrected Titan 3 80-300kV along a [110] zone axis. Highly planar deformation was first observed by Otto et al. [5] and was active for this study as well. This planar deformation, involving dislocation arrays on {111} slip systems, may imply the existence of short-range order, low stacking fault energy (SFE), and/or supplementary displacements in the wake of dislocations. Smith et al. [6] previously demonstrated that high and medium angle annular dark field scanning transmission electron microscopy (HAADF/MAADF-STEM) could effectively be used to determine the misalignment of a dislocation through foil thickness. This misalignment created a contrast " plume " when imaged in a MAADF condition. Recently, Smith et al. revealed the presence of a broad distribution of stacking fault widths, suggesting the concept of a " local " stacking fault energy in HEAs which affect the the dislocation dissociation and may play a role in how these dislocations glide [7]. To further explore this misalignment and how it relates to the dislocation core structure, through-focal HAADF-STEM imaging was employed. Acquisition of a through-focal STEM series was shown to enable detection of the crystal rotation in association with the " Eshelby twist " around screw dislocations [8]. This technique has been employed presently to create a 3D analysis of dislocation cores in the Cantor alloy as shown in Figure 1(a) and 1(b). Changing defocus allowed different depths along a dislocation line to be imaged, allowing for a three dimensional analysis of the whole dislocation core. The field of focus (z) was calculated using [9]: í µí± =