Electronic and thermodynamic criteria for the occurrence of high entropy alloys in metallic systems

Poletti, Marco Gabriele; Battezzati, Livio

doi:10.1016/j.actamat.2014.04.033

Cited by 341 publications

(138 citation statements)

References 60 publications

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“…The upper limit corresponds to the largest value of ΔH f for which the alloy does not phase separate due to the immiscibility of any pair of elements. The upper limit of the enthalpy range (37 meV) is chosen to include all known singe-phase alloys and it is consistent with the thermodynamic model presented by Poletti and Battezzati [16]. It is worth noting that T ann could be replaced by some critical temperature (T crit ), below which diffusion is sufficiently slow that the enthalpic driving force is insufficient to result in phase decomposition on a realizable time scale.…”

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confidence: 96%

“…Although there are several proposals regarding the stability of HEAs, much of the existing work uses semiempirical approaches based, for example, on HumeRothery rules, thus focusing on the differences of the atomic sizes (δ), electronegativities (Δχ), and electron-toatom ratio (e=a) [12][13][14][15][16][17]. Some approaches utilize calculation of phase diagrams methods [18], while others consider δ, the enthalpy of mixing (ΔH mix ), and the ideal entropy of mixing of the alloys to develop criteria for the phase stability [1,12].…”

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confidence: 99%

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Criteria for Predicting the Formation of Single-Phase High-Entropy Alloys

et al. 2015

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High-entropy alloys constitute a new class of materials whose very existence poses fundamental questions regarding the physical principles underlying their unusual phase stability. Originally thought to be stabilized by the large entropy of mixing associated with their large number of components (five or more), these alloys have attracted attention for their potential applications. Yet, no model capable of robustly predicting which combinations of elements will form a single phase currently exists. Here, we propose a model that, through the use of high-throughput computation of the enthalpies of formation of binary compounds, predicts specific combinations of elements most likely to form single-phase, highentropy alloys. The model correctly identifies all known single-phase alloys while rejecting similar elemental combinations that are known to form an alloy comprising multiple phases. In addition, we predict numerous potential single-phase alloy compositions and provide three tables with the ten most likely five-, six-, and seven-component single-phase alloys to guide experimental searches. The term high-entropy alloy (HEA) has come to signify nontraditional alloy systems composed of five or more elements at, or near, equiatomic ratio that form random, single-phase solid solutions on simple underlying facecentered-cubic (fcc) and body-centered-cubic (bcc) lattices [1][2][3][4][5][6][7][8][9][10][11]. HEAs stand in sharp contrast to traditional metal alloys that are typically based on one or two primary elements and where addition of further alloying elements often leads to the formation of new phases. Clearly, the existence of HEAs poses important questions regarding the driving mechanism for their unexpected stability and how to identify the specific combinations of elements that are most likely to form a single-phase HEA.Although there are several proposals regarding the stability of HEAs, much of the existing work uses semiempirical approaches based, for example, on HumeRothery rules, thus focusing on the differences of the atomic sizes (δ), electronegativities (Δχ), and electron-toatom ratio (e=a) [12][13][14][15][16][17]. Some approaches utilize calculation of phase diagrams methods [18], while others consider δ, the enthalpy of mixing (ΔH mix ), and the ideal entropy of mixing of the alloys to develop criteria for the phase stability [1,12]. For example, in the work of Guo et al. [14], the use of δ and ΔH mix as independent variables clearly separates solid-solution phases from amorphous phases but does not necessarily isolate intermetallic compounds from either one of these phases. In addition, in the work of Otto et al. [11], there are atomic substitutions to the solid-solution CrMnFeCoNi alloy that are specifically chosen to follow the Hume-Rothery rules in respect of δ, Δχ, and crystal structure yet do not form a single-phase solid solution. Useful as many of these attempts to encapsulate features of the underlying bonding mechanisms have been, it is clear that a model that can robustly predict, out ...

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Criteria for Predicting the Formation of Single-Phase High-Entropy Alloys

et al. 2015

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“…A common approach has been to utilise the empirical rules of Hume-Rothery 144,145 and/or accessible thermodynamic quantities to form parametric criteria for the stability of HEA solid solutions, which are fitted to experimental results. 152,159,163,173,[203][204][205]207,[252][253][254][255][256][257][258][259][260][261][262][263][264][265][266][267] These phase-selection rules for as-cast HEAs were recently reviewed by Wang et al 258 and Guo, 208 and are discussed briefly here.…”

Section: Phase Prediction and Alloy Selection In Heasmentioning

confidence: 99%

High-entropy alloys: a critical assessment of their founding principles and future prospects

Pickering

Jones

2016

International Materials Reviews

703

299

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High-entropy alloys (HEAs) are a relatively new class of materials that have gained considerable attention from the metallurgical research community over recent years. They are characterised by their unconventional compositions, in that they are not based around a single major component, but rather comprise multiple principal alloying elements. Four core effects have been proposed in HEAs: (1) the entropic stabilisation of solid solutions, (2) the severe distortion of their lattices, (3) sluggish diffusion kinetics and (4) that properties are derived from a cocktail effect. By assessing these claims on the basis of existing experimental evidence in the literature, as well as classical metallurgical understanding, it is concluded that the significance of these effects may not be as great as initially believed. The effect of entropic stabilisation does not appear to be overarching, insufficient evidence exists to establish the strain in the lattices of HEAs, and rapid precipitation observed in some HEAs suggests their diffusion kinetics are not necessarily anomalously slow in comparison to conventional alloys. The meaning and influence of the cocktail effect is also a matter for debate. Nevertheless, it is clear that HEAs represent a stimulating opportunity for the metallurgical research community. The complex nature of their compositions means that the discovery of alloys with unusual and attractive properties is inevitable. It is suggested that future activity regarding these alloys seeks to establish the nature of their physical metallurgy, and develop them for practical applications. Their use as structural materials is one of the most promising and exciting opportunities. To realise this ambition, methods to rapidly predict phase equilibria and select suitable HEA compositions are needed, and this constitutes a significant challenge. However, while this obstacle might be considerable, the rewards associated with its conquest are even more substantial. Similarly, the challenges associated with comprehending the behaviour of alloys with complex compositions are great, but the potential to enhance our fundamental metallurgical understanding is more remarkable. Consequently, HEAs represent one of the most stimulating and promising research fields in materials science at present.

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“…An atomistic simulation cell consisting of a BCC lattice with lattice constant 2.86A was created, with periodic boundary conditions along three orthogonal directions x= [1][2][3][4][5][6][7][8][9][10], y= [110] and z= [001], with dimensions of ~300A in each direction (~ 2 million atoms). BCC lattice sites were randomly occupied by Co, Fe, Ni, and Ti atoms to achieve an average composition Co16.67Fe36.67Ni16.67Ti30.…”

Section: Computational Model Of the Bcc Co1667fe3667ni1667ti30 Ranmentioning

confidence: 99%

“…The number of phases in a HEA system can be significantly smaller than the maximum number of phases present at equilibrium as predicted by the well-known Gibbs phase rule [1]. While many alloys that meet the definition of an HEA [2,3] contain multiple phases, including intermetallics [4,5], there are many single-phase systems [6]. These single-phase systems, both FCC and BCC, generally have unusually high yield strength and/or high ductility.…”

Section: Introductionmentioning

confidence: 99%

Atomistic simulations of dislocations in a model BCC multicomponent concentrated solid solution alloy

et al. 2017

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Molecular statics and molecular dynamics simulations are presented for the structure and glide motion of a/2<111> dislocations in a randomly-distributed model-BCC Co16.67Fe36.67Ni16.67Ti30 alloy. Core structure variations along an individual dislocation line are found for a/2<111> screw and edge dislocations. One reason for the core structure variations is the local variation in composition along the dislocation line. Calculated unstable stacking fault energies on the (110) plane as a function of composition vary significantly, consistent with this assessment. Molecular dynamics simulations of the critical glide stress as a function of temperature show significant strengthening, and much shallower temperature dependence of the strengthening, as compared to pure BCC Fe as well as a reference mean-field BCC alloy material of the same lattice and elastic constants of the target alloy. Interpretation of the strength versus temperature in terms of an effective kink-pair activation model shows the random alloy to have a much larger activation energy than the mean-field alloy or BCC Fe. This is interpreted as due to the core structure variations along the dislocation line that are often unfavorable for glide in the direction of the load. The configuration of the gliding dislocation is wavy, and significant debris is left behind, demonstrating the role of local composition and core structure in creating kink pinning (super jogs) and/or deflection of the glide plane of the dislocation. This is a pre-print of the following article: Rao, S.I.; Varvenne, C.; Woodward, C.; Parthasarathy, T.A.; Miracle, D.; Curtin, W.A. Acta Materialia 2017, 125, 311--320.. The formal publication is available at http://dx

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Electronic and thermodynamic criteria for the occurrence of high entropy alloys in metallic systems

Cited by 341 publications

References 60 publications

Criteria for Predicting the Formation of Single-Phase High-Entropy Alloys

Criteria for Predicting the Formation of Single-Phase High-Entropy Alloys

High-entropy alloys: a critical assessment of their founding principles and future prospects

Atomistic simulations of dislocations in a model BCC multicomponent concentrated solid solution alloy

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