Atomic segregation at twin boundaries in a Mg-Ag alloy

Segregation of solute atoms at twin boundaries (TBs) plays a critical role in mechanical properties and thermal stability of magnesium alloys. Here, segregation structures at {10 (1) over bar1}, {10 (1) over bar2} and {10 (1) over bar3} TBs are characterized in a Mg-Ag alloy by means of the atomic resolution high-angle annular dark-field technique based on scanning transmission electron microscopy. Of particular finding is the unique complex segregation at {10 (1) over bar3} TBs, where Ag atoms occupy both substitutional and interstitial sites. By contrast, Ag atoms only substitutionally segregate at {10 (1) over bar1} and {10 (1) over bar2} TBs. Calculation simulation of segregation energy and three-dimensional structure of TBs helps understanding of hybrid segregation. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Structure motif of chemical short-range order in a medium-entropy alloy

The chemical short-range orders (CSRO) are the sub-nanosized entities inherent in high-/medium-entropy alloys (H/MEA). To discern CSRO poses the challenge with pending issues as to under what zone axis in electron diffraction, signature scattering specific to CSRO will appear and what the structure motif of CSRO is. Here, we show extra diffuse scattering by CSRO from [111] and [112] directions in a VCoNi MEA and accordingly, construct the L1(1)-type structure motif, and derive the spacial shape of flat cuboid of CSRO in virtue of diffractions under varying zone axis. Moreover, we demonstrate the methods to identify CSRO in lattice images. [GRAPHICS] IMPACT STATEMENT The structure motif is constructed as L1(1) -type for chemical short-range order in a medium-entropy alloy in terms of electron diffractions under the multiple zone axis

Chemical short-range order in Fe50Mn30Co10Cr10 high-entropy alloy

Chemical short-range order (CSRO) is generally possible in concentrated solid solutions and currently of considerable interest for multi-principal element alloys. However, a convincing demonstration of CSRO has been challenging and achieved thus far only for ternary medium-entropy alloys such as VCoNi. Here, we report definitive proof of CSRO in a quaternary face-centered-cubic Fe50Mn30Co10Cr10 high-entropy alloy, acquired from systematic electron microscopy experiments. The evidence includes extra diffuse disks in nano-beam electron diffraction patterns, images in state-of-the-art aberration-corrected scanning transmission electron microscope, as well as compositional profiles across neighboring atomic planes/columns in atomic-resolution chemical maps. The CSRO regions are found to occupy an areal fraction of 20% and have dimensions on a sub-nanometer scale. This length scale, as well as the diffraction features of the CSRO, are different from those of intermetallic compound precipitates; as such, the CSRO is not a growing stage of a nucleated second phase, the precipitation of which has been dealt with previously in classical alloys. We further conducted a spatial correlation analysis of the concentrations in atomic columns in the chemical map, enabling us to uncover a general tendency toward nearest-neighbor chemical ordering, specifically, preference for unlike species (such as Fe-Mn) and avoidance for like-species (such as Fe-Fe). The persistence of this trend, the same as that found in VCoNi MEA, recently, is somewhat intriguing for a high-entropy alloy in which all the constituent elements are similar in atomic size and have rather a small enthalpy of mixing. (C) 2021 The Author(s). Published by Elsevier Ltd

Atomic-scale evidence of chemical short-range order in CrCoNi medium-entropy alloy

High (or medium)-entropy alloys (H/MEAs) are complex concentrated solid solutions that may develop chemical short-range order (CSRO). In this regard, CrCoNi, the prototypical face-centered-cubic MEA, has recently kindled a debate in the H/MEA community, as it is uncertain if CSRO can possibly form in such a multi-principal-element solution, where no equilibrium or metastable intermetallic compounds have ever been seen or predicted. To answer this challenging question, here we present firm experimental evidence for the CSRO from electron diffraction as well as atomic-resolution chemical mapping, under an appropri-ate zone axis. We also develop a methodology to reliably determine the locations of atomic columns from the line scan profiles in the chemical maps, as well as a quantitative covariance-based correlation analysis of the column chemical compositions to reveal the spatial correlations between various atomic pairs. The detailed chemical information affirms the tendency for like-pair avoidance and unlike-pair preference, specifies the preferred atomic packing and plane stacking by the three constituent species, and suggests a proposed atomic configuration that constitutes the CSRO motif. The fraction of CSRO regions is mod-erately lowered after either plastic deformation or high-temperature heating. A comparison is also made with previous attempts to identify CSROs in H/MEAs. (c) 2021 The Author(s). Published by Elsevier Ltd on behalf of Acta Materialia Inc. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/