ハイエントロピー合金 (High entropy alloys) Wikipedia 英語版の簡約

以下、Wikipedia の High Entropy Alloysの簡単な和訳です (Wikipedia ハイエントロピー合金)。





High-entropy alloys (HEAs) are a class of multi-component alloys composed of 5 or more principal constituent elements each with a concentration between 5 and 35 atomic %.[1].
ハイエントロピー合金は、5元系あるいはそれよりも多成分系からなり、かつそれぞれの構成元素の組成が5~35 at.%となる合金の一つである [1]。

The defining feature of HEAs over other complex alloys is that they consist entirely or primarily of a simple solid solution phase, thus having a high entropy of mixing.

HEAs have been described in the literature that have better strength-to-weight ratios, fracture toughness, tensile strength, high-temperature strength, thermal stability,[3] or corrosion resistance than similar conventional alloys.[1][4]
ハイエントロピー合金は、一般的な合金と比べ、高い比強度、破壊靭性、高温強度、熱的安定性 [3]、耐食性などを示す[1][4]。


1. 開発の初期 (Early development)

Although HEAs were described as early as 1996,[5] significant research interest did not develop until after independent 2004 papers by Jien-Wei Yeh and Brian Cantor, with Yeh's first paper on the topic published 2 months sooner.
ハイエントロピー合金の概念については1996年以前に提出されていたが [5]、Jien-Wei Yeh教授 と Brian Cantor教授によって2004年にそれぞれ独立に提出された論文以前には大きな関心を集めてこなかった。

Yeh also coined the term "high-entropy alloy" when he attributed the high configurational entropy as the mechanism stabilizing the solid solution phase.[6]
Yeh教授は、さらに、固溶体を安定化するメカニズムとして高い配置のエントロピーが寄与する"high-entropy alloy"という言葉をつくりだした。[6]

Cantor, not knowing of Yeh's work, did not describe his alloy as a "high-entropy alloy", but the base alloy he developed, equiatomic FeCrMnNiCo, has been the subject of considerable work in the field.[7]
Cantor教授は、Yeh教授の研究を知らず、Yeh教授の合金を"high-entropy alloy"とは呼ばなかった。しかし、Cantor教授が開発した等原子組成 FeCrMnNiCo合金は、この分野における極めて重要な研究であった。[7]

Before the classification of high entropy alloys and multi-component systems, nuclear science had highlighted a system that can now be classified a high entropy alloy: within nuclear fuels Mo-Pd-Rh-Ru-Tc particles form at grain boundaries and at fission gas bubbles.[8]

Understanding the behavior of these '5 metal particles' was of specific interest to the medical industry as Tc-99m is an important medical imaging isotope.
'5 金属粒子'の挙動を理解することは、医療用イメージングアイソトープとして重要であるTc-99mとして医療の分野では重要であった。


2 定義 (Definition)

There is no universally agreed-upon definition of a HEA.

Yeh originally defined HEAs as alloys containing at least 5 elements with concentrations between 5 and 35 atomic percent.[6]

Later research however, suggested that this definition could be expanded.

Otto et al. suggested that only alloys that form a solid solution with no intermetallic phases should be considered true high-entropy alloys, as the formation of ordered phases decreases the entropy of the system.[9]

Some authors have described 4-component alloys as high-entropy alloys [10], while others have suggested that alloys meeting the other requirements of HEAs, but with only 2~4 elements[11] or a mixing entropy between R and 1.5R[12] should be considered "medium-entropy" alloys.[13]


3 合金設計 (Alloy design)

In conventional alloy design, one primary element such as iron, copper, or aluminum is chosen for its properties.

Then, small amounts of additional elements are added to improve or add properties.

Even among binary alloy systems, there are few common cases of both elements being used in nearly-equal proportions such as Pb-Sn solders.

Therefore, much is known from experimental results about phases near the edges of binary phase diagrams and the corners of ternary phase diagrams and much less is known about phases near the centers.

In higher-order (4+ components) systems that cannot be easily represented on a 2-dimensional phase diagram, virtually nothing is known. [7]


3.1 相の形成 (Phase formation)

Gibbs' phase rule, F=C-P+2, can be used to determine an upper bound on the number of phases that will form in an equilibrium system.
ギブスの相律によれば、 F = C-P+2 (F は自由度、C は成分の数、P は相の数)であり、これを用いて、平衡状態における相の数の上限を決定することができる。

In his 2004 paper, Cantor created a 20-component alloy containing 5 at% of Mn, Cr, Fe, Co, Ni, Cu, Ag, W, Mo, Nb, Al, Cd, Sn, Pb, Bi, Zn, Ge, Si, Sb, and Mg.
2004年、Cantor教授は、Mn, Cr, Fe, Co, Ni, Cu, Ag, W, Mo, Nb, Al, Cd, Sn, Pb, Bi, Zn, Ge, Si, Sb, および Mgが5 at%となる20成分合金を作製した。

At constant pressure, the phase rule would allow for up to 21 phases at equilibrium, but far fewer actually formed.

The predominant phase was a face-centered cubic solid solution phase, containing mainly Fe, Ni, Cr, Co, and Mn.
合金の主相はe, Ni, Cr, Co, および Mnを含むFcc固溶体相であった。

From that result, the FeCrMnNiCo alloy, which forms only a solid solution phase, was developed.[7]

The Hume-Rothery rules have historically been applied to determine whether a mixture will form a solid solution.

Research into high-entropy alloys has found that in multi-component systems, these rules tend to be relaxed slightly.

In particular, the rule that solvent and solute elements must have the same crystal structure does not seem to apply, as Fe, Ni, Cr, Co, and Mn have 4 different crystal structures as pure elements (and when the elements are present in equal concentrations, there can be no meaningful distinction between "solvent" and "solute" elements).[9]
特に、溶質と溶媒元素の結晶構造が同じであるというルールは適用できないようである。例えば、Fe, Ni, Cr, Co, および Mnでは、純物質の結晶構造は4つの異なるタイプである。(さらに、等原子組成の場合、「溶質」と「溶媒」の定義は意味をなさない。)[9]


3.2 熱力学的メカニズム (Thermodynamic mechanisms)

The multi-component alloys Yeh developed also consisted mostly or entirely of solid solution phases, contrary to what had been expected from earlier work in multi-component systems, primarily in the field of metallic glasses.[6][14]

Yeh attributed this result to the high configurational, or mixing, entropy of a random solid solution containing numerous elements.

Because ΔG=ΔH-TΔS and the phase with the lowest Gibbs free energy of formation (ΔG) will be the phase formed at equilibrium, increasing ΔS (entropy) will increase the likelihood of a phase being stable.
なぜなら、次の理由からである。ギブスの自由エネルギー (ΔG) はΔG=ΔH-TΔS で与えられ、ΔGが最小となる相が平衡状態で形成される。ΔS(エントロピー)が増加は、このこのランダムな固溶体相が安定化すると考えられる。

The mixing entropy for a random ideal solid solution can be calculated by:


where R is the ideal gas constant, N is the number of components, and ci is the atomic fraction of component i.

From this it can be seen that alloys in which the components are present in equal proportions will have the highest entropy, and adding additional elements will increase the entropy.

A 5 component, equiatomic alloy will have a mixing entropy of 1.61R.[6][15]

However entropy alone is not sufficient to stabilize the solid solution phase in every system.

The enthalpy of mixing (ΔH), must also be taken into account.
混合のエンタルピー (ΔH)も考慮に入れなければならない。

This can be calculated using:
混合のエンタルピー (ΔH)は次式で計算される:




is the binary enthalpy of mixing for A and B.[16]

Zhang et al. found, empirically, that in order to form a complete solid solution, ΔHmix should be between -10 and 5 kJ/mol.[15]
Zhangらは、経験的に、固溶体相単相が形成されるためには、ΔHmix の値が-10 から 5 kJ/molの値である傾向を見出した。

In addition, Otto et al. found that if the alloy contains any pair of elements that tend to form ordered compounds in their binary system, a multi-component alloy containing them is also likely to form ordered compounds.[9]

Both of the thermodynamic parameters can be combined into a single, unitless parameter Ω:


here Tm is the average melting point of the elements in the alloy.

Ω should be greater than or equal to 1.1 to promote solid solution development.[4]


3.3 速度論的メカニズム (Kinetic mechanisms)

The atomic radii of the components must also be similar in order to form a solid solution.
Zhang et al. proposed a parameter δ representing the difference in atomic radii:
Zhangらは、原子半径の違いを表すパラメーター δ を提案した。


where ri is the atomic radius of element i.



Formation of a solid solution phase requires a δ≤6.6%, but some alloys with 4%<δ≤6.6% do form intermetallics.[4][15]
固溶体の形成にはδ≤6.6%が必要である。しかし、いくつか合金では、 4%<δ≤6.6%の場合に金属間化合物が形成する。[4][15]


3.4 その他の特徴 (Other properties)

For those alloys that do form solid solutions, an additional empirical parameter has been proposed to predict the crystal structure that will form.

If the average valence electron concentration (VEC) of the alloy is ≥8, the alloy will form a face-centered cubic (fcc) lattice.
合金の平均価電子密度(VEC)が ≧ 8 の合金では、fcc構造が形成される。

If the average VEC is <6.87, it will form a body-centered cubic (bcc) lattice.
価電子密度(VEC)が < 6.87 の合金では、bcc構造が形成される。

For values in between, it will form a mixture of fcc and bcc.[17]

VEC has also been used to predict the formation of σ-phase intermetallics (which are generally brittle and undesirable) in chromium and vanadium-containing HEAs.[18]


Empirical parameters and design guidelines for forming solid solution HEAs

∆Hmix> -10 かつ < 5 kJ/mol
Ω≥ 1.1
δ≤ 6.6%
VEC≥ 8 for fcc, <6.87 for bcc

4 作製 (Synthesis)

High-entropy alloys are mostly produced using distinct methods that depend on the initial phase - starting either from a liquid, solid, or gas state.

Most HEAs have been produced using liquid-phase methods include arc melting, induction melting, and Bridgman solidification.[4]

Solid-state processing is generally done by mechanical alloying using a high-energy ball mill.

This method produces powders that can then be processed using conventional powder metallurgy methods or spark plasma sintering.

This method allows for alloys to be produced that would be difficult or impossible to produce using casting, such as AlLiMgScTi, in which the melting points of the constituent elements has a range of nearly 1500 °C.[4][19][20]

Gas-phase processing includes processes such as sputtering or molecular beam epitaxy (MBE), which can be used to carefully control different elemental compositions to get high-entropy metallic[3] or ceramic films.[4]

Other HEAs have been produced by thermal spray, laser cladding, and electrodeposition.[4][21]


5 モデリングとシミュレーション (Modeling and simulation)

The atomic-scale complexity presents additional challenges to computational modelling of high-entropy alloys.

Thermodynamic modelling using the CALPHAD method requires extrapolating from binary and ternary systems.[22]

Most commercial thermodynamic databases are designed for, and may only be valid for, alloys consisting primarily of a single element.

Thus, they require experimental verification or additional ab initio calculations such as density functional theory (DFT).[23]

However, DFT modeling of complex, random alloys has its own challenges, as the method requires defining a fixed-size cell, which can introduce non-random periodicity.

This is commonly overcome using the method of "special quasirandom structures," designed to most closely approximate the radial distribution function of a random system,[24] combined with the Vienna Ab-initio Simulation Package.
この問題は、一般に "special quasirandom structures (SQS)" 法を用いて解決される。SQSは、ランダムな系における動径分布関数を最も正確に近似するようにデザインされた方法であり、Vienna Ab-initio Simulation Package(VASP)と連動している。

Using this method, it has been shown that results of a 4-component equiatomic alloy begins to converge with a cell as small as 24 atoms.[25][26]

The exact muffin-tin orbital method with the coherent potential approximation has also been employed to model HEAs.[25][27]

Other techniques include the 'multiple randomly populated supercell' approach, which better describes the random population of a true solid solution (although is far more computationally demanding).[28]
真の固溶体におけるランダムな分布をより記述可能な、'multiple randomly populated supercell'を含む他の手法も試みられた。

This method has also been used to model glassy/amorphous (including bulk metallic glasses) systems without a crystal lattice.[29][30]

Further, modeling techniques are being used to suggest new HEAs for targeted applications.

The use of modeling techniques in this 'combinatorial explosion' is necessary for targeted and rapid HEA discovery and application.
この'combinatorial explosion'は、ハイエントロピー合金の迅速な開発や応用に必要である。

Simulations have highlighted the preference for local ordering in some high entropy alloys.

And, when the enthalpies of formation are combined with terms for configurational entropy, transition temperatures between order and disorder can be estimated.[31] - allowing one to understand when effects like age hardening and degradation of an alloy's mechanical properties may be an issue.


6 特徴と実用化の可能性 (Properties and potential uses)


6.1 機械的性質 (Mechanical)

The crystal structure of HEAs has been found to be the dominant factor in determining the mechanical properties.

Bcc HEAs typically have high yield strength and low ductility and vice versa for fcc HEAs.

Some alloys have been particularly noted for their exceptional mechanical properties.

A refractory alloy, VNbMoTaW maintains a high yield strength (>600 MPa (87 ksi)) even at a temperature of 1,400 °C (2,550 °F), significantly outperforming conventional superalloys such as Inconel 718.
耐熱合金である、VNbMoTaWハイエントロピー合金は、1400℃においても極めて高い降伏強度 (>600 MPa (87 ksi))を示し、この特性はInconel 718といった一般的な超合金を上回る特性を示す。

However, room temperature ductility is poor, less is known about other important high temperature properties such as creep resistance, and the density of the alloy is higher than conventional nickel-based superalloys.[4]

CoCrFeMnNi has been found to have exceptional low-temperature mechanical properties and high fracture toughness, with both ductility and yield strength increasing as the test temperature was reduced from room temperature to 77 K (?321.1 °F).

CoCrFeMnNiハイエントロピー合金は、卓越した低温度での機械的性質と高い破壊靭性を示すことが見いだされている。低温度特性として、延性と降伏強度は、室温から77 K (?321.1 °F)と温度低下とともに増加する。

This was attributed to the onset of nanoscale twin boundary formation, an additional deformation mechanism that was not in effect at higher temperatures.

As such, it may have applications as a structural material in low-temperature applications or, because of its high toughness, as an energy-absorbing material.[32]

However, later research showed that lower-entropy alloys with fewer elements or non-equiatomic compositions may have higher strength[33] or higher toughness.[34]

No ductile to brittle transition was observed in the bcc AlCoCrFeNi alloy in tests as low as 77 K.[4]

Al0.5CoCrCuFeNi was found to have a high fatigue life and endurance limit, possibly exceeding some conventional steel and titanium alloys.

But there was significant variability in the results, suggesting the material is very sensitve to defects introduced during manufacturing such as aluminum oxide particles and microcracks.[35]

A single-phase nanocrystalline Al20Li20Mg10Sc20Ti30 alloy was developed with a density of 2.67 gcm-3 and microhardness of 4.9 ~ 5.8 GPa, which would give it an estimated strength-to-weight ratio comparable to ceramic materials such as silicon carbide,[19] though the high cost of scandium limits the possible uses.[36]
単相ナノ結晶Al20Li20Mg10Sc20Ti30合金は、密度が2.67 gcm-3でマイクロ硬さが4.9 ~ 5.8 GPaであり、Scを用いることによる高コストがその利用を制限するものの、シリコンカーバイドのようなセラミックス材料と同程度の比強度を持つ可能性がある。[36]

Rather than bulk HEAs, small-scale HEA samples (e.g. NbTaMoW micro-pillars) exhibit extraordinarily high yield strengths of 4-10 GPa -one order of magnitude higher than that of its bulk form- and their ductility is considerably improved.
バルクのハイエントロピー合金にl比べ、小さなスケールのハイエントロピー合金試料(すなわち NbTaMoW マイクロピラー)も4-10 GPaという極めて高い降伏強度を示す。この値は、バルク形状の値より一桁高く、またその延性も改善される。

Additionally, such HEA films show substantially enhanced stability for high-temperature, long-duration conditions (at 1,100 ?°C for 3 days).

Small-scale HEAs combining these properties represent a new class of materials in small-dimension devices potentially for high-stress and high-temperature applications.[3][37]


6.2 電気的および磁気的性質 (Electrical and magnetic)

CoCrCuFeNi is an fcc alloy that was found to be paramagnetic.

But upon adding titanium, it forms a complex microstructure consisting of fcc solid solution, amorphous regions and nanoparticles of Laves phase, resulting in superparamagnetic behavior.[38]

High magnetic coercivity has been measured in a BiFeCoNiMn alloy [21]


6.3 その他 (Other)

The high concentrations of multiple elements leads to slow diffusion.

The activation energy for diffusion was found to be higher for several elements in CoCrFeMnNi than in pure metals and stainless steels, leading to lower diffusion coefficients.[39]


7 参考文献 (References)

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これは、Wikipedia の High Entropy Alloyshttps://en.wikipedia.org/wiki/High_entropy_alloys)の簡単な和訳です。

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