1 Influence of chemistry and structure on interfacial segregation in NbMoTaW with high -throughput atomistic simulations

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Influence of chemistry and structure on interfacial segregation in NbMoTaW

with high-throughput atomistic simulations

Ian Geiger 1, Jian Luo 2, Enrique J. Lavernia 3, Penghui Cao 4, Diran Apelian 3, Timothy J. Rupert

1,3,4,*

1 Material and Manufacturing Technology, University of California, Irvine, CA, USA

2 Department of NanoEngineering, University of California, San Diego, La Jolla, CA 92093, USA

3 Department of Materials Science and Engineering, University of California, Irvine, CA, USA

4 Department of Mechanical and Aerospace Engineering, University of California, Irvine, CA,

USA

* trupert@uci.edu

ABSTRACT

Refractory multi-principal element alloys exhibiting promising mechanical properties such as

excellent strength retention at elevated temperatures have been attracting increasing attention.

Although their inherent chemical complexity is considered a defining feature, a challenge arises

in predicting local chemical ordering, particularly in grain boundary regions with enhanced

structural disorder. In this study, we use atomistic simulations of a large group of bicrystal models

to sample a wide variety of interfacial sites (grain boundary) in NbMoTaW and explore emergent

trends in interfacial segregation and the underlying structural and chemical driving factors.

Sampling hundreds of bicrystals along the [001] symmetric tilt axis and analyzing more than one

hundred and thirty thousand grain boundary sites with a variety of local atomic environments, we

uncover segregation trends in NbMoTaW. While Nb is the dominant segregant, more notable are

the segregation patterns that deviate from expected behavior and mark situations where local

structural and chemical driving forces lead to interesting segregation events. For example,

incomplete depletion of Ta in low-angle boundaries results from chemical pinning due to favorable

local compositional environments associated with chemical short-range ordering. Finally,

machine learning models capturing and comparing the structural and chemical features of

interfacial sites are developed to weigh their relative importance and contributions to segregation

tendency, revealing a significant increase in predictive capability when including local chemical

information. Overall, this work, highlighting the complex interplay between local grain boundary

structure and chemical short-range ordering, suggest tunable segregation and chemical ordering

by tailoring grain boundary structure in multi-principal element alloys.

Keywords: Atomistic simulations, Grain boundary segregation, Multi-principal element alloy,

Refractory complex concentrated alloy, Interface structure, Chemical short-range order

I. INTRODUCTION

The rapid emergence of multi-principal element alloys (MPEAs) since 2004 marks a

significant shift from centuries of conventional alloy design.1 Whereas traditional alloys are

composed of a base metal with relatively small amounts of alloying elements, MPEAs are multi-

component metal alloys in equiatomic or near equiatomic proportions.2–4 This paradigm shift

uncovers new opportunities in compositional space with nearly limitless possibilities for elemental

combinations;5 several MPEAs have already demonstrated promising properties including

excellent high temperature strength,6–8 improved fracture toughness at cryogenic temperatures,9

and outstanding oxidation and wear resistance.10,11 Early studies aimed to correlate such properties

to the stabilization of a single-phase random solid solution by high configurational entropy,2 yet

more recent work suggests that a diversity of microstructural landscapes is more the rule than

exception, with different combinations of phases and microstructures promoted by altering the

composition, temperature, and/or processing route. For example, iterative changes in the

elemental concentrations of non-equiatomic FeMnCoCr MPEAs led to significant improvements

in strength and ductility by altering deformation mechanisms from single-phase, dislocation-

mediated plasticity to dual-phase, transformation-induced plasticity.12–14 With potentially new or

amplified mechanisms driving these enhanced properties, improved understanding and prediction

of microstructure is necessary to enable continued advancements of MPEAs.

Grain boundaries are an ubiquitous microstructural feature that have been extensively

studied in traditional alloys to better understand how structure and composition at interfaces affect

both microstructural evolution and subsequent material properties.15,16 In multi-component

systems, the addition of even small amounts of dopants can significantly alter properties through

grain boundary segregation, as shown in Al-Co,17 Pt-Au,18 and Mg-Gd19 binary alloys. In Bi-

doped Cu or Ni, for example, the formation of Bi bilayers at the grain boundaries drastically lowers

the cohesive strength of the interfaces and transforms the generally ductile Cu or Ni into a brittle

material.20–23 Interfacial segregation is similarly observed in MPEAs, but is even more

complicated as the inherent chemical complexity in elemental bonding often translates to complex

grain boundary compositions. This behavior has been probed both experimentally24–26 and

predicted computationally,27,28 again showing significant influence on mechanical properties. For

example, using atom probe tomography, Ming et al.26 showed that grain boundary decohesion via

Ni, Cr, and Mn nanoclustering drives the loss of ductility in a CrMnFeCoNi MPEA. The

substantial influence of interfacial state on material behavior underscores the need to better

understand interfacial segregation in MPEAs as a critical step in improving material design.

The development of theoretical frameworks has been a principal focus of grain boundary

engineering over the last century and has successfully uncovered the thermodynamic driving

forces for segregation in simpler alloys.29–31 Despite some progress, open questions remain about

how such driving forces are expressed in chemically complex alloys. In many dilute alloys, atomic

size mismatch can contribute to low bulk solubility, inducing segregation and simultaneously

relaxing bulk and boundary stresses.21,32 However, recent work from He et al.33 showed that larger

Bi and Pb atoms can segregate to compressed grain boundary sites at Mg coherent twin boundaries,

thus indicating that chemical bonding can dominate over structural considerations even for low

solute concentrations. Beyond dilute alloys and two-component systems, chemical interactions

likely exhibit greater influence on segregation tendencies, requiring a thorough analysis. For

MPEAs, the foremost chemical interactions that should be considered are often expressed in the

bulk as elemental clustering (enthalpic favorability between two like elements) and chemical short-

range ordering (CSRO) (enthalpic favorability between two unlike elements). These ordering

tendencies have been observed to be prominent in several promising MPEAs.34–36 Antillon et al.36

performed Monte Carlo (MC)/molecular dynamics (MD) simulations of face-centered cubic

CoFeNiTi to highlight different clustering and ordering tendencies at 700 K, 1100 K, and 1500 K,

ultimately showing that chemical effects in the bulk are a strong function of temperature. Wynblatt

and Chatain37 simulated a CoNiCrFeMn alloy to decipher the role of chemical and structural

driving forces at both surfaces and grain boundaries by adapting a two-component model that

relates the enthalpy of segregation to: (1) interfacial energy of a solute atom, (2) elastic strain

minimization, and (3) chemical interactions. These authors showed that excess Cr at the interface

is the result of a strong affinity of Cr for itself, despite Mn having lower interfacial energy and

greater reduction in elastic strain in high volume defect sites. Similar temperature-dependent

ordering tendencies have been the focus of several studies on refractory, body-centered cubic

(BCC) MPEAs that exhibit promising strength retention at elevated temperatures.7,38,39

Furthermore, correlations between CSRO and dislocation propagation suggest that dislocation

mobility, and therefore plastic deformation, is heavily influenced by an intrinsically rocky energy

landscape.40 Few studies to date have aimed to decipher how strong CSRO affects equilibrium

grain boundary compositions, a critical component of microstructural stability in these refractory

MPEAs.

In this work, high-throughput atomistic simulations are used to study local and global

trends in segregation behavior of a BCC NbMoTaW refractory MPEA that exhibits a strong

tendency for CSRO.27,39,40 To probe a variety of structures and grain boundary sites, a dataset of

243 equilibrium and metastable symmetric tilt bicrystal models are generated and relaxed

chemically via hybrid MC/molecular statics (MS) simulations. Our results show that Nb

segregates most heavily, while Ta and W deplete almost completely, with Mo showing more subtle

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1InfluenceofchemistryandstructureoninterfacialsegregationinNbMoTaWwithhigh-throughputatomisticsimulationsIanGeiger1,JianLuo2,EnriqueJ.Lavernia3,PenghuiCao4,DiranApelian3,TimothyJ.Rupert1,3,4,*1MaterialandManufacturingTechnology,UniversityofCalifornia,Irvine,CA,USA2DepartmentofNanoEngineering,Univers...

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1 Influence of chemistry and structure on interfacial segregation in NbMoTaW with high -throughput atomistic simulations

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