New Era of High–Entropy Alloys

Jien-Wei Yeh Professor

Department of Materials Science and Engineering National Tsing Hua University

Hsinchu, Taiwan

高熵合金新紀元

Outline

New alloy concept: high-entropy alloys

Four core effects of high-entropy alloys New developments on high-entropy alloys Summary

New Alloy Concept

●“High-entropy alloys” have been explored since 1995 by my group at Tsing Hua Univ., Taiwan.

● High-entropy alloys have at least 5 major metallic

elements (n ≧ 5), each having an atomic percentage between 5 % and 35 %.

Equimole: AlCoCrCuFeNi Nonequimole: AlCo0.5CrCuFe1.5Ni1.2

Minor element addition: AlCo0.5CrCuFe1.5Ni1.2B0.1C0.15

Birth of High-Entropy Alloys

Ashby M.F Materials Selection in Mechanical Design, fourth edition, Butterworth-Heinemann, Elsevier, Oxford, UK, 2011, pp. 1-13

Why use the name of high-entropy? Consider an equimolar alloy with n elements at

its liquid state or regular solid solution state, Configurational entropy △Sconf = Rln(n) Mixing entropy △Smix= △Sconf + △Svibration + △Sdipole + △Selec ～ △Sconf

n 1 2 3 4 5 6 7 8 9 10 11 12 13

ΔSconf 0 0.69R 1.1R 1.39R 1.61R 1.79R 1.95R 2.08R 2.2R 2.3R 2.4R 2.49R 2.57R

Richards’ rule for metals’ fusion: Sliquid - Ssolid = △S = R

low medium high entropy 1.0R 1.5R

Most alloys has low entropy, and some concentrated alloys has medium entropy.

System Alloy ∆Sconf. at liquid state Low-alloy steel 4340 0.22R low

Stainless steel 304 0.96R low

316 1.15R medium

High speed steel M2 0.73R low Mg alloy AZ91D 0.35R low

Al alloy 2024 0.29R low 7075 0.43R low

Cu alloy 7-3 brass 0.61R low

Ni-base superalloy Inconel 718 1.31R medium Hastelloy X 1.37R medium

Co-base superalloy Stellite 6 1.13R medium

BMG Cu47Zr11Ti34Ni8 1.17R medium

Zr53Ti5Cu16Ni10Al16 1.30R medium

HEA System Examples AlCoCrCuFeNi AlCoCrCuFeNiMox

AlCoCrCuNiTiYx

AlCoCrCuFeNiTi, AlCoCrCuFeNiMn, AlCoCrCuFeNiTiV AlCoCrFeNi AlCoCrFeNiTix

AlCoCrFeMoNi AlCrFeMnNi AlTiNiMnBx

AlCuCrFeNi, CuCrFeMoNi, CuCrFeMnNi, CuCrFeNiZr Alx(TiVCrMnFeCoNiCu)100-x

CoCrCuFeNiTix

CoCrFeNiTi CrCuFeMnNi WNbMoTaV

Alloys world

R: gas constant

1. Thermodynamics – high entropy 2. Kinetics – sluggish diffusion 3. Structure – severe-lattice-distortion 4. Properties – cocktail effect

Core Effects of High-Entropy Alloys

-- Thermodynamics --

High Entropy Effect Enhance the formation of multi-element solid solutions

Entropy effect is often ignored in the phase prediction of conventional alloys

Because conventional alloys are based on one major elements, their phases would have quite small mixing entropies. Thus

△Gmix ~ △Hmix

The equilibrium phases mainly result from the competition between the mixing enthalpies of competing phases. However, the mixing entropies of solid solution phases are much higher in HEAs and should be considered in predicting equlibrium phases.

△Gmix = △Hmix - T△Smix

Cu-Sn Phase Diagram

Ni-Sn-Zn Phase Diagram

Experimental isothermal section of the Ni-Sn-Zn system at 873 K (axes units: at%). Numbers from 1 to 6 represent the ternary phases; light grey areas indicate the binary solid solution extension; shaded areas represent the two phase field; estimated phase field boundaries and liquidus are shown by dashed lines.

873 K

Possible combinations and compounds between 80 metal elements

Mackay’s Statistical Distribution of Inorganic Crystal Structures

1. There is a sharp limit to complexity. 2. Many of the structures with large number of elements contain solid solutions, different elements occupying the same sites.

Mackay, “On Complexity”, Crystallography Reports, Vol. 46, 2001.

N≧5, high entropy effect

Enhancing the formation of solid solutions.

~1090 structures

~19000 structures

Extended Crystal Structures of Solid Solutions

Conventional crystal structure can be extended for multi-principal-element solid solutions.

J. W. Yeh, et al. Metall. Mater. Trans., Vol. 35, 2004.

-- Kinetics --

Sluggish Diffusion Effect 1. Lower diffusion rate 2. Lower phase transformation rate

Fine Precipitation in Cast Alloys

Many high-entropy cast alloys have nano or submicron precipitates in the matrix.

TEM microstructures of as-cast equimolar AlCoCrCuFeNi alloy

Kinetics Explanation The formation of new phases require cooperative diffusion

of many different kinds of atoms to accomplish the partitioning (re-distribution) of composition.

But the vacancy concentration is limited, 1. A vacancy is competed by multi-elements during diffusion. 2. The slowest-diffusion element determines the overall rate.

New phase

Vacancy

Assume the same atom size for simplicity

When heating the assembly, the HEA samples will be compressed to weld with each other and form diffusion couple due to the smaller CTE of Mo (5 ppm/K).

Design for the HEA Diffusion Couple

Mo Mo

Alumina

HEA1 HEA2

Mo

Argon-filled quartz tube

Use single FCC system of CoCrFeMnNi. Only two of the five elements had concentration

difference across the interface.

21

Composition of Diffusion Couples

Couple Alloy Composition (at.%)

Co Cr Fe Mn Ni

Cr−Mn 1 22 29 22 5 22

2 22 17 22 17 22

Fe−Co 3 33 23 11 11 22

4 11 23 33 11 22

Fe−Ni 5 23 24 30 11 12

6 23 24 12 11 30

Calculated Diffusivities

Diffusion coefficient: Mn > Cr > Fe > Co > Ni

0 exp QD DRT

= −

Evidence for Sluggish Diffusion

Low-entropy

Medium-entropy

high-entropy

HEAs have the highest Q/Tm!

Diffusion coefficients of Cr, Mn, Fe, Co, and Ni in different matrices

Ref. K.Y. Tsai, M.H. Tsai, J.W. Yeh, “Sluggish diffusion in Co–Cr–Fe–Mn–Ni high-entropy alloys”, Acta Materialia, 61(2013), pp. 4887-4897.

Advantages of Sluggish Diffusion

Slow down phase transformation Easy to get supersaturated state and fine precipitates Raise recrystallization temperature Slow down grain growth Slow down particle coarsening improve creep resistance

These advantages might benefit microstructure and property control!

-- Structure --

Severe-Lattice-Distortion Effect

Severe-Lattice-Distortion Effect Lattice distortion affects properties and

reduces the thermal vibration effect: Hardness and strength Temp. coefficient Electrical conductivity Temp. coefficient Thermal conductivity Temp. coefficient XRD peak intensity Temp. coefficient

e-

phonon

X-ray

Mechanical properties from Ni to NiCoFeCrMn

As N increases, YS, UTS, and EL significantly increase. This suggests solution hardening in FCC HEAs is superior than many hardening mechanisms..

Both strength and ductility of CoCrFeMnNi increase as temperature decreases

B. Gludovatz, A. Hohenwarter, D. Catoor, E.H. Chang, E. P. George, R.O. Ritchie, “A fracture-resistant high-entropy alloy for cryogenic applications”, Science, 345(2014), pp. 1153-1158.

Low stacking fault energy enhances nanotwinning deformation with decreasing temperature, which results in continuous steady strain hardening.

-- Properties --

Cocktail Effect

Rule of mixture

Excess properties due to mutual interactions

Come from the basic features and mutual interactions among all constituent elements in a solution phase.

Cocktail Effect

-5 0 5 10 15 20 25 30 35 40

100

200

300

400

500

600

700

BCC phaseFCC + BCC phasesFCC phase

Hard

ness

(Hv)

Al content (at.%)

Ex. AlXCoCrCuFeNi alloys

Importance of the Four Core Effects

Four core effects are much more pronounced in HEAs as compared with traditional alloys.

It would become easier to understand HEAs

through these four core effects. Four core effects are useful guidelines for

alloy design of HEAs.

New Developments on High-Entropy Alloys and Ceramics

Spray-deposition HEAs Elevated-temperature HEAs Highly-workable HEAs Carbides and cermets with HEA binders High-entropy nitride, carbide, and oxide

coatings

For protective coatings on heat exchangers and high-temperature components

1. Spray-Deposition HEAs

304 substrate HV150

AlCoCrFeNiSiTi HV800

2. Refractory HEAs

Senkov, Wilks, Scott, and Miracle, Intermetallics, 2011, Vol. 19, pp. 698-706.

Developed by US Air Force Research Laboratory

Nb25Mo25Ta25W25 and V25Nb25Mo25Ta25W25

At 1600℃, YS can be around 460 MPa.

3. Highly-Workable HEA AlCrFeMnNi When cold-rolled by a four-high rolling mill, the rolling

extension is up to 4900%. For 304 stainless steel with a similar hardness of HV160, the

rolling extension is only 1543%. Possible applications : 3C housing, golf head.

70μm-thick foil

4. Carbides and Cermets with HEA Binders

For dies, molds, and cutting tools

SEM image: TiC + 20% CoCrFeNiTi

Based on 12 strong binary nitrides Binary nitrides Crystal structure Lattice constant (nm) Hardness (GPa)

TiN FCC (B1 NaCl) 0.4249 19.9 ZrN FCC 0.4577 15.0 HfN FCC 0.4392 16.3 VN FCC 0.4136 15.0

NbN FCC 0.4392 18.3

TaN Hexagonal a = 0.519

c = 0.291 24.0 FCC 0.4330

CrN FCC 0.4149 11.0 Mo2N Cubic 0.4169 13.8 W2N Cubic 0.4126 24

AlN Hexagonal a = 0.3114 c = 0.3896 17.7

Si3N4 Hexagonal a = 0.7608 c = 0.29107 16-18

BN Hexagonal a = 0.2504

c = 0.3615 0.08-0.09

Cubic 0.661 29.9-43.1

5. High-Entropy Nitrides

HE Nitrides Hardness (GPa)

Modulus (GPa)

(AlCrMoTaTiZr)N 40 379

(AlCrTaTiZr)N 36 360

(AlCrMoSiTi)N 35 325

(AlCrSiTiV)N 31 300

(AlBCrSiTi)N 25 260

(AlCrNbSiTiV)N 42 350

(AlCrSiTaTiZr)N 34 343

(AlMoNbSiTaTiVZr)N 37 350

HE Nitride Coating Examples All HE nitrides have simple FCC structure. The FCC structure is stable even after 1100 ℃-5h vacuum annealing.

Strengthening mechanisms:

1. Strong bonding

2. Nanograin structure

3. Solid solution hardening

4. Residual stress

AlN+CrN+TaN+TiN+ZrN (AlCrTaTiZr)N

Si

HEA nitride

Cu

Cu (300 nm)

Si Substrate

HEA Barrier (10 nm)

Superior HEA Diffusion Barriers

Barrier Material Thickness (nm) Performance

TaN 50 750 °C 60 min

ZrN 100 800 °C 60 min

Ta36Si14N50 120 850 °C 30 min

Ti34Si23N43 120 850 °C 30 min

TiB2 60 600 °C 30 min

Ta 50 550 °C 60 min

(AlCrTaTiZr)N 10 900 °C 30 min

Shou-Yi Chang and Dao-Sheng Chen, Applied Physics Letters, 2009, 94, 231909.

Outstanding properties of HEA Film For CIGS solar cell, HEA film as back electrode could provide

thermal stable amorphous structure, diffusion barrier, higher reflectivity and conductivity.

Efficiency is improved by 9％ as compared with Mo back electrode.

41 Glass / Stainless Steel / Polymer Substrate

CIGS (1.5~2μm)

Mo (0.5~1.5μm) Back electrode

Absorption layer

CdS(0.05μm) Buffer layer

i-ZnO(0.05μm) Pure ZnO TCO (ZnO:Al)(0.5~1.5μm) TCO of ZnO:Al

Top electrode Ni / Al

Sputtering method

•Vacuum method •Non-Vacuum method

Chemical bath deposition

Sputtering method

(AlCrTaTiZr)-Si-N coatings on carbide inserts

Film thickness is 1 μm. (AlCrTaTiZr)-Si-N has better flank-wear

resistance than that of TiN and TiAlN commercial coatings.

High cutting performance of multi-layer HEA/HEN coating

WC-Co I-TiAlN I1-200N I-TiN

Workpiece: 304 stainless steel

Multi-functional HEA coatings Hard, abrasive, dent-resistant, anti-corrosion, anti-fingerprint, anti-bacteria, anti-static, and colorful liquid metal

A

Materials World (based on mixing entropy)

Growing Interest Worldwide in HE Materials

At least 300 individual research groups have started to explore this new field.

1038 papers have been published from 2004 to 2015.

First HEA paper

Anyone who does HEA research knows MSE of NTHU, Taiwan!!

MSE, NTHU

47

International Symposiums Symposium of BMGs and HEAs IUMRS-ICA Conference, Qingdao, China, Sep. 25-28, 2010. IUMRS-ICA Conference, Taipei, Taiwan, Sep. 19-22, 2011. IUMRS-ICA Conference, Busan, Korea, Aug. 26-31, 2012. IUMRS-ICAM Conference, Qingdao, China, Sep. 22-28, 2013.

Symposium of HEAs MS&T-2012 Meeting, Pittsburgh, PA, USA, Oct. 7-11, 2012 - TMS-2013 Annual Meeting, San Antonio, Texas, USA, March

3-7, 2013 ---- HEAs(I) TMS-2014 Annual Meeting, San Diego, CA, USA, Feb. 16-20,

2014 ---- HEAs(II) TMS-2015 Annual Meeting, Orlando Florida, USA, March 15-

19, 2015 ---- HEAs(III) TMS-2016 Annual Meeting, Nashville, Tennessee, Feb. 14 –

18, 2016 ---- HEAs(IV)

Workshop on HEAs in India 2015

International Conference on High-Entropy Materials

(ICHEM 2016) November 6th – 9th 2016,

National Tsing Hua University, Hsinchu, Taiwan Chairs: J.W. Yeh, P.K. Liaw, O.N. Senkov

Topics of interest: Materials design Physical metallurgy Processing development Structure characterization Mechanical, physical, and chemical properties Surface and structural stability Simulation and modeling Applications

Special Issues on High-Entropy Alloys

1. High-Entropy Alloys, Annales de Chimie Science des Matériaux, Vol. 31, 2006.

2. High-Entropy Alloys, Entropy, Vol. 15, 2013.

3. High-Entropy alloys, Journal of Metals, Vol. 65, 2013.

4. Progress in High-Entropy alloys, Journal of Metals, Vol. 67, 2015.

5. High-Entropy alloys, Materials Science and Technology, Vol.31, 2015.

6. High-Entropy alloy Coatings, Coatings, Feb. 2016.

7. High-Entropy Alloys, Entropy, Vol. 18, 2016.

8. High-Entropy Alloys, Metals, Dec. 2016.

9. Concentrated Solid Solution Alloys, Current Opinion in Solid State & Materials Science, 2016

High-Entropy Alloys By B.S. Murty, Jien-Wei Yeh, S. Ranganathan This book provides a complete review of the current state of the art in the field of high entropy alloys (HEA). The conventional approach to alloy design is to select one principal element and add elements to it in minor quantities in order to improve the properties. In 2004, Professor J.W. Yeh and his group first reported a new approach to alloy design, which involved mixing elements in equiatomic or near-equiatomic proportions, to form multi-component alloys with no single principal element. These alloys are expected to have high configurational entropy and hence were termed as "high entropy alloys." HEAs have a broad range of structures and properties, and may find applications in structural, electrical, magnetic, high-temperature, wear-resistant, corrosion-resistant, and oxidation-resistant components. Due to their unique properties, high entropy alloys have attracted considerable attention from both academics and technologists. This book presents the fundamental knowledge present in the field, the spectrum of various alloy systems and their characteristics studied to date, current key focus areas, and the future scope of the field in terms of research and technological applications.

Published in June, 2014

• Introduces and explains the source of high-entropy alloys, their advantages and weakness, and advances in understanding this class of materials including their industrial applications • Explains all aspects of high-entropy alloys including formation rules, processing, physical metallurgy, mechanical behavior, functional properties, prediction of the structure using various ab initio methods, thermodynamics and elasticity, liquid structure and solidification, CALPHAD modeling, and future prospects • Details computational procedures for high-entropy alloys and the latest computational development of high-entropy alloys

High-Entropy Alloys Fundamentals and Applications

Editors: Gao, M.C., Yeh, J.W., Liaw, P.K., Zhang, Y.

XiaoZhi Lim, ”多元金屬合成的更強更韌更延合金”， 自然，第533卷，第7603期，5月19日， 2016年， 第306-307頁

Summary High-entropy alloys have four core effects: high

entropy, sluggish diffusion, severe-lattice-distortion and cocktail effects and provide a wide spectrum of properties.

The concept and core effects of high-entropy

alloys could be applied to other high-entropy materials.

High-entropy materials have promising

applications such as elevated-temperature components, carbide tools, cermet tools, thermal spray coatings, and functional coatings.

Thank you for your attention!

開拓人煙未至的新世界

沒有參考書及論文 取名稱，做定義 提出核心效應及基本原理 擴展到高熵陶瓷及高熵高分子材料 開馬路，打先鋒，做示範 （1）鑄造成型 （2）加工成型及熱處理 （3）粉末冶金成型 （4）鍍膜 （5）各種結構及特性探討 （6）發展應用

高熵合金的應用

傳統材料應用在嚴苛的地方，若性能不足，壽命不長，正是高熵合金展現身手的機會。近12年的發展，已看到了解決的機會，指日可待。例如：

1.高速切削且無切削液 （1）車刀、銑刀：耐磨、耐溫，提高數倍壽命 （2）超硬鍍膜：再提高耐溫、耐磨，提高數倍壽命 2.噴射引擎葉片：耐溫 > 極限1150℃，原效率增10％以上 3.現今核能廠燃料管由400℃提至900℃，以免福島氫爆重演 4.下世代核能反應爐直接可耐900℃耐輻照損傷的材料 5.大溫度範圍極低電阻溫度係數的薄膜電阻，-60～＋200℃ 6.室溫或更高溫的超導材料，目前高壓下-120℃ 7.抗沾黏鍍膜，不沾鍋不沾模 8.塑膠射出機的螺桿的耐溫耐磨耐蝕鍍層 9.採礦、鑽油井等耐溫耐磨耐蝕的高熵合金及超硬高熵合金 10.手機美觀耐磨多功能的保護層

金屬的調合學 更強、更韌、更延：冶金學家以新配方正在創造新一代性能顯著的合金

XIAOZHI LIM 撰

高熵合金可用於噴射引擎高溫段的葉片

XiaoZhi Lim, ”多元金屬合成的更強更韌更延合金”， 自然，第533卷，第7603期，5月19日， 2016年， 第306-307頁

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