Extra electron reflections in concentrated alloys do not necessitate short-range order

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Extra electron reﬂections in concentrated alloys do not necessitate short-range order

Flynn Walsh,1, 2 Mingwei Zhang,1, 3, 4 Robert O. Ritchie,1, 4 Andrew M. Minor,1, 3, 4 and Mark Asta1, 4, 5

1Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA

2Graduate Group in Applied Science & Technology, University of California, Berkeley, CA, USA

3National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA, USA

4Department of Materials Science & Engineering, University of California, Berkeley, CA, USA

5Corresponding author: mdasta@berkeley.edu

In many concentrated alloys of current interest, the observation of diﬀuse superlattice intensities

by transmission electron microscopy has been attributed to chemical short-range order. We brieﬂy

review these ﬁndings and comment on the plausibility of widespread interpretations, noting the

absence of expected peaks, conﬂicts with theoretical predictions, and the possibility of alternative

explanations.

The nature of chemical short-range order (SRO) in

face-centered cubic (fcc) alloys containing several 3d

principal elements, such as “medium-entropy” VCoNi

and CrCoNi, has been intensively investigated in recent

years [1–18]. While there may be little evidence that SRO

can be controlled to tailor the bulk mechanical properties

of these materials [5,6,13,17], it has been argued that

an essentially ubiquitous degree of ˚

A-scale order could

nonetheless play an important role in a wide range of

properties [1,7]. For example, many alloys containing Co

are predicted to form hexagonal close-packed lattices at

ambient conditions [19,20], but quenched-in SRO could

account for the persistent metastability of the fcc phase

[1,7,11].

This view appears to be corroborated by recent trans-

mission electron microscopy (TEM) purporting the pres-

ence of local order in a variety of samples subject to mini-

mal thermal processing beyond high-temperature homog-

enization. For example, the observation of SRO in VCoNi

was proposed [8] on the basis of diﬀuse intensities at

2{311}superlattice sites in reciprocal space while imag-

ing in the [¯

112] zone axis (ZA), which indicates the crys-

tallographic direction of electron incidence. An equiva-

lent electron diﬀraction pattern is shown in Fig. 1. Addi-

tional 1

3{422}intensities were later reported in the [¯

111]

ZA [9]. Similar observations have been at various points

attribtued to SRO in a Cr-Ni-based alloy [21], Mn-Fe-

based alloys [22–25], CrCoNi [10,13], CrMnFeCoNi [15],

and VFeCoNi [18]. The same features have also been

reported without the assumption of SRO [5,17,26–28].

Some of these reﬂections are consistent with the par-

tial formation of a CuPt-type (L11) concentration wave

involving the compositional enrichment and depletion of

alternating {111}(and simultaneously {311}) planes, as

illustrated in Fig. 2a. Diﬀuse intensities in VCoNi and

CrCoNi have been interpreted to reﬂect modulations of

V or Cr concentrations in this manner [8–10], largely

on the basis of electronic structure calculations indicat-

ing repulsive interactions between V-V and Cr-Cr neigh-

bors, although we note that CuPt-type ordering has the

same nearest-neighbor pair frequencies as a random al-

loy. Some eﬀorts have been made to support this theory

with atomic-scale composition mapping [8,10,18], but,

5 nm-1

FIG. 1: Electron diﬀraction of CrCoNi in the

[¯

112] ZA. This pattern, which is based on experimental

data from a previous study [13], is representative of

literature results for alloys discussed in the text. Diﬀuse

intensities at 1

2{311}superlattice positions are marked

with arrows, but there are no peaks at 1

2{111}sites, as

highlighted by the dotted circles.

in contrast to the diﬀuse intensities themselves, these

measurements are noisy and susceptible to local ﬂuctua-

tions, making it diﬃcult to draw statistical conclusions.

Regardless of chemical speciﬁcs, the presence of su-

perlattice reﬂections should not be regarded as incontro-

vertible evidence for ordering. In fact, widespread inter-

pretations of SRO are questionable on several accounts,

such as the absence of additional expected peaks. By

the symmetry of the fcc lattice, a CuPt-type decoration

arXiv:2210.01277v2 [cond-mat.mtrl-sci] 21 Aug 2023

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摘要：

Extraelectronreflectionsinconcentratedalloysdonotnecessitateshort-rangeorderFlynnWalsh,1,2MingweiZhang,1,3,4RobertO.Ritchie,1,4AndrewM.Minor,1,3,4andMarkAsta1,4,51MaterialsSciencesDivision,LawrenceBerkeleyNationalLaboratory,Berkeley,CA,USA2GraduateGroupinAppliedScience&Technology,UniversityofCalifor...

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Extra electron reflections in concentrated alloys do not necessitate short-range order

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