Diffuse Electron Diffraction Intensities in Concentrated Solid Solutions Do Not Necessarily Come from Short-range Order

The observation of diffuse streaking and superlattice intensities by energy-filtered transmission electron microscopy selected area electron diffraction (EFTEM-SAED) and high-resolution STEM (HRSTEM) techniques has been attributed to the presence of chemical short-range order (SRO) in many concentrated solid solutions (Figure 1), most notably in FCC single-phase high and medium entropy alloys (HEA/MEA) [1–5]. Recently, alternative interpretations have been offered on the existence of nanoscale planar defects, higher order Laue Zone effects, and atomic surface steps causing these intensities [6]. In this study, we examine the plausibility of these interpretations by performing EFTEM-SAED, EF-STEM, HRS/TEM, and multislice diffraction pattern simulations on quenched and slow-cooled CrCoNi MEA. Our theoretical predictions reveal the absence of additional peaks that are expected from SRO, and we discover a large density of stacking faults (SF) and small stacking fault tetrahedra (SFT) in quenched samples (Figure 2) that contribute to the same diffuse intensities as in current experimental observations. However, the diffuse intensities persist in the slow-cooled samples where SFTs were not observed [7]. Experimental EFTEM-SAED diffraction patterns of CrCoNi in the [110], [111], and [112] zone axes, showing the presence of diffuse streaking, 1/3[422] diffuse superlattice peaks, and 1/2[311] superlattice peaks, respectively. The figures were taken from Refs. [1, 2, 4]. STEM-BF image of SFs and small SFTs present in quenched CrCoNi.