Wei-Hong WU, Chih-Chao YANG, and Jien-Wei YEH, INDUSTRIAL DEVELOPMENT OF HIGH-ENTROPY ALLOYS, Annales De Chimie Science des Materiaux, 31(2006), pp. 737-747.

INDUSTRIAL DEVELOPMENT OF HIGH-ENTROPY ALLOYS

Wei-Hong WUa, Chih-Chao YANGa, and Jien-Wei YEHb

aNano-Powder and Thin Film Technology Center, Industrial Technology Research Institute, Tainan 70955, Taiwan bDepartment of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan Abstract- High-entropy alloys (HE alloys) are composed of at least five major metal elements, as opposed to traditional alloy systems that are typically based on only one or two major elements. Dependent upon their composition and/or processing route, HE alloys have been found to possess a wide range of microstructures and properties. Some of them exhibit promising properties such as high hardness, wear resistance, chemical inertness and high-temperature softening resistance. By exploiting their merits, properly-designed HE alloys are developed with an aim to replace traditional alloys in specific applications. This paper describes the current status of several industrial applications of HE alloys.

Rsum Dveloppements industriels dalliages haute entropie. Les alliages haute entropie (AHE) sont composs dau moins cinq lments mtalliques majeurs, par opposition aux systmes dalliages classiques qui sont typiquement bass sur seulement un ou deux lments majeurs. En fonction de leur composition et du procd dlaboration, il sest avr que les AHE pouvaient avoir un large spectre de microstructures et de proprits. Certains _______________________________________________________________________ Reprints: C.C. Yang, Nano-Powder and Thin Film Technology Center, Industrial Technology Research Institute, Tainan 70955, Taiwan dentre eux prsentent des proprits prometteuses telles que duret leve, rsistance lusure, rsistance aux agents chiques et rsistance ladoucissement haute temprature. En exploitant

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ces avantages, des AHE convenablement conus sont en cours de dveloppement en vue de remplacer des alliages classiques dans des applications spcifiques. Cet article dcrit ltat actuel de plusieurs applications industrielles dAHE.

1. INTRODUCTION

There are about thirty traditional alloy systems used in practical applications. They are typically based on one principal element, as in the iron-based, aluminum-based, magnesium-based, and titanium-based alloys [1, 2]. High-entropy alloys (HE alloys) are a novel alloy concept beyond the realm of traditional alloys that consist of at least five major metallic elements and therefore have a huge number of possible compositions [3-11]. After a decades research, HE alloys have been developed from various alloy compositions to give them combinations of functional properties such as high hardness [7], work hardening capacity, wear resistance [8], high-temperature softening resistance, anti-oxidation [9], anti-corrosion [10, 11], and electrical resistance. These merits are found to originate from the tendency of HE alloys to form simple nanostructured solid-solutions and the cocktail effect of these multi-element mixtures [3, 12]. The special features of HE alloys could be listed as follows:

1. The tendency to form simple FCC and BCC solid-solution phases with nanoscale or

even amorphous structures. 2. Hardnesses that range from 100 to 1100 Hv. 3. Deformation in FCC phases by a nano-twining mechanism. 4. Thermally stable microstructures. 5. Excellent resistance to temper softening. 6. High-temperature precipitation hardening between 600 and 1000 oC. 7. A positive temperature coefficient of strength, and hence maintain a high strength

level at elevated temperatures. 8. Can possess excellent corrosion resistance, wear resistance and oxidation resistance. 9. Can have a high electrical resistivity with a low or negative temperature coefficient. 10. Can have good high-frequency soft magnetic properties. 11. Potential of good thermoelectric properties.

By exploiting the special features listed above, HE alloys have been developed for numerous industrial applications, which will now be reviewed. 2. THERMAL BARRIER COATINGS OF HIGH-ENTROPY ALLOYS

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Nano-precipitated structures are rarely observed in conventional castings of traditional alloys. A rapid solidification process or precipitation treatment is required to form nanostructures in specific traditional alloys. On the contrary, HE alloys readily form nano-precipitated structures, even in the as-cast state, due to the sluggish elemental diffusion [2, 5, 7]. During thermal spray deposition, the higher cooling rate allows HE alloys to easily form supersaturated states [9]. For some thermally sprayed HE alloy coatings (table I), 5-20 nm precipitates and structures with 50-100 nm grains were produced. It is noted from table I that the thermal conductivities (K) of the HE alloy coatings, measured by the hot disk method, are 3.24 W/mK and 3.14W/mK, which are much lower than those of their as-cast bulk states. Since phonons and electrons are the carriers in solid-state thermal conduction, their scattering by point defects and boundaries are the main causes for a reduction in thermal conductivity. For HE alloys, large mass and size differences are easy to obtain among the different atoms, and these induce large phonon- and electron-point defect scattering. This results in HE alloys with low thermal conductivities. Furthermore, the nano-sized grains and precipitates inside HE alloy coatings provide a large amount of boundaries and interfaces that further retard the movement of the heat-transferring phonons and electrons. Scattering by these different interactions is shown schematically in figure 1. From table I, it can also be seen that the measured thermal conductivity of commercial hot-mold steel SKD61, in its wrought form, is far higher than that of the HE alloys. This suggests that HE alloy coatings are good candidates for the thermal management of casting molds.

Table I. Thermal conductivities of HE alloys and SKD61 steel measured by the hot disk method.

The casting of thin-gage and large-area castings requires molds of low thermal conduction to

keep the liquid hot and slow down solidification. Steel molds are thus limited in producing very thin-gage and large-area castings, such as notebook and mobile-phone cases made of Al or Mg alloys. Although this might be solved by high-speed injection, additional drawbacks arise, such as more turbulence, more pores, and heavier erosion of the gating system. These defects might result in a rejection rate of 50%. Thus, low-K HE alloy coatings present a potential solution to this problem by reducing the overall heat transfer rate at the cavity surfaces of the molds.

Material HE alloy A HE alloy B SKD61

Thermal conductivity (W/mK) Bulk 7.94 6.69

28 Coating 3.24 3.14

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.

Figure 1. Schematic diagram depicting the mechanisms that reduce thermal conductivity in HE alloy coatings.

Table II reveals the results of die-casting tests under various press rates. It can be seen that low-K HE alloy coatings significantly improve the filling ratio of magnesium alloy plates and can improve the yield. By combining the low-K, good high-temperature hardness, wear resistance and oxidation resistance of HE alloys, they appear to have potential use for other high-temperature applications, such as turbines, burners, and reactors. Table II. Filling tests of molds with and without HE alloy coatings

Mold condition Pressing rate 0.1 m/s 0.2 m/s 0.3 m/s

Without HE alloy coating Plates (kg) 0.185 0.207 0.26 Filling ratio 70% 76% 95%

With HE alloys coating Plates (kg) 0.265 0.27 0.275 Filling ratio 96% 98% 100%

3. HARDFACING APPLICATIONS OF HIGH-ENTROPY ALLOYS In order to protect the surface of machine components and tools, hardfacing technology can be employed. In hardfacing a thick layer of wear and/or corrosion resistant material is welded, thermal spray welded, or plasma arc welded to the surface. HE alloys are very suitable candidates for hardfacing because of their high hardness, wear resistance, high-temperature softening resistance, anti-corrosion, and combinations of the aforementioned properties. Some HE alloys are now fabricated into rods and powders and then welded or thermally-sprayed onto the surface of tools and other components. Additional high-temperature aging can be used to

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200 300 400 500 600 700 800 900 1000

1E-7

1E-6

1E-5

1E-4

Oxidation at high temperature

AlCrX HE alloys

Traditional alloys

Wea

r Rat

e (m

m3 /N

.m)

Hardness (HV)

Al-0.3 Casting, Homogenization, Quenching Al-0.3 Casting, Homogenization, Air-cooling Al-0.3 Casting, Homogenization, Furnace-cooling Al-0.3 Forging, Homogenization, Air-cooling Al-0.3 Forging, Homogenization, Furnace-cooling Al-0.5 Casting Al-0.3 600oC 50h Aging Al-0.3 1100oC 2h Furnace-cooling, Oxidation 316 Stainless Steel SKD61 SUJ2 SKH51

enhance the hardness and wear resistance of the layers. As well as the above, high-temperature age-hardening of AlCrX HE alloys can be exploited

to produce graded materials for surface protection, as illustrated in figure 2. This is accomplished by heating the required area to a suitable temperature for an appropriate time. The surface hardness (from Hv350 to Hv850) and the thickness can be controlled by the temperature gradient and time period. As shown in figure 3, the wear resistances of two AlCrX HE alloys that have undergone various treatments are better than the base line connected by several conventional alloys. Oxidizing and nitriding treatments can further increase the wear resistance by more than 100 times. Moreover, these alloys have better hot hardnesses than those of

Base: Hv300~400

Hardfacing layer after aging hardening, nitridizing, or oxidizing treatment: >Hv800

Figure 2. Schematic diagram showing a HE alloy graded mold with a hardfaced layer on the mold-cavity surface.

Figure 3. The comparison of wear-resistance and hardness of several HE alloys and traditional alloys.

Hardfaced layer after age hardening, nitriding or oxidizing

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high-carbon steels and high speed steels. Through different combinations of process and surface treatments, AlCrX alloys could be made more versatile in tool and mold applications.

Figure 4 is an example showing a cast-mold of AlCrX alloy that had been treated by age hardening and oxidizing to give it a graded hardness. In a real forging test, it was found that the mold showed negligible wear after forging 300 pieces of low-carbon steel rods at 1000oC. These different hardfacing techniques using HE alloy may be applied to a wide range of components, including golf club heads, walls of steel tubes, roller surfaces, knives, shafts, molds.

Figure 4. HE alloy graded mold (right side) and forgings (left side) of the forging test. No tear cracks or wear were observed after forging 300 low-carbon steel rods at 1000oC. 4. HARD METALS USING HIGH-ENTROPY-ALLOY BINDERS

Hard metals, because of their excellent combinations of high hardness, wear-resistance, toughness, and high-temperature stability [13], have been widely used in a lot of industrial products, such as molds, tools and nozzles. Hard metals are classed as composites comprising of binder metals, such as Fe, Co and Ni, and ceramic particles such as WC and TiC. For WC, Co is the most common binder metal, whereas for TiC, Ni and Ni13Mo7 are the common binders. Grain refiners such as VC, TaC are usually added to improve the strength and toughness of the sintered hard metal by preventing coarsening of the carbide grains. Numerous compositions of binder metals have been investigated and developed for grain refinement and to reduce costs. HE alloys, designated as AlCoX and CoCrX, have potential to replace the conventional binders in hard metals as they have low contents of the expensive cobalt and are composed of an FCC phase that exhibits a better high-temperature strength than Co, Ni, or Ni13Mo7. It has been found that the grain size of WC and TiC can be effectively reduced in the HE alloy binders after liquid-phase sintering, even without the presence of grain refiners. Furthermore, the effective grain refinement was found to lead to improvements in the hardness and wear resistance.

Figure 5 shows the comparison of the hot hardness between sintered WC-10%AlCoX, WC-25%AlCoX, WC(2.5 m)-20%Co and WC(0.5 m)-9.5%Co. The 25%-AlCoX, alloy is

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seen to have a higher hot hardness than that of 20%Co. This is attributed to the greater grain refinement in the HE alloy binder and its higher hot hardness compared to Co. The higher hardness at lower temperatures for WC(0.5 m)-9.5%Co alloy is through the use of smaller WC grains, however, the difference in its hot hardness with the WC-10%AlCoX alloy at higher temperatures is small, reflecting the higher hot hardness of the AlCoX HE alloy binder.

0 200 400 600 800 10000

200

400

600

800

1000

1200

1400

1600

1800

2000

Hard

ness

HV3

0

Temperature(OC)

WC-25AlCoX WC-10AlCoX WC(2.5mm)-20% Co WC(0.5mm)-9.5% Co

Figure 5. Hot hardness as a function of temperature for WC-AlCoX and WC-Co hard metals

Table III displays the combination of superior hardness and fracture toughness of WC-AlCoX compared to the other WC-Co hard metals which contain modifiers [14, 15]. Table III. Hardness and toughness of WC-AlCoX and WC-Co hard metals.

Composition Hardness (Hv) K1C (MNm-1.5) Total crack length (m)

WC+10%AlCoX 1446 11.89 230 WC+10 (TiC,TaC,NbC)+11.5Co 1380 10.9 261

WC+31(TiC,TaC,NbC)+9Co 1560 8.1 535

Cermets employing AlCoX and CoCrX binders have also been found to exhibit superior combinations of hardness and toughness to those of other cermets [16, 17], as shown in table IV. In figure 6, the high-temperature softening resistances of TiC-AlCoX and TiC-CoCrX cermets can also be seen to be better than the other TiC or WC hard metals [18, 19]. This is attributed to the fact that HE alloy binders significantly inhibit TiC grain growth and inherently provide

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better elevated-temperature strength. In summary, HE alloy binders have several advantages over conventional binders in terms of low Co contents, lower cost, higher hardness, increased toughness and improved high-temperature softening resistance. They therefore have potential to be used commercially in components such as cutting knives, tools, press dies etc. Table IV. Hardness and toughness of TiC-AlCoX, TiC-CoCrX and other non-HE alloy cermets.

0 200 400 600 800 1000200

400

600

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1000

1200

1400

1600

1800

2000

Har

dnes

s (H

V)

Temperature (oC)

TiC + 20% CoCrX TiC + 20% AlCoX TiC + 20% Ni TiC + 20% (Ni,Mo) WC + 9.5% Co WC + 9.5% (60Co/20Ni/20Fe) WC-based submicron hardmetal (0.5-0.83) mm WC-based fine grain hardmetal (0.8-1.3) mm WC + Co + 1.0% Ru

High-temperautre hardness of hardmetals

Figure 6. Hot hardness as a function of temperature for TiC-HE alloy, TiC non-HE alloy and WC non-HE alloy hard metals.

composition Hardness (Hv) K1C (MNm-1.5) Total crack length (m) TiC+20%AlCoX 193719 8.80.1 562 TiC+20%CoCrX 1876 9.0 518

TiC+20%Ni 137231 11.80.3 222 TiC+20%Ni ~1300 ~11.1 237

TiC+20%Ni13Mo7 163918 8.50.2 510 TiC+20%Ni13Mo7 ~1430 ~11.8 231

Commercial TiC-based cermet 168511 9.10.1 458

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5. ANTIBACTERIAL AND ELECTROMAGNETIC INTERFERENCE SHIELDING COATINGS

Other functional characteristics of HE alloys that have been studied include their

electromagnetic and biochemistry properties. Table V illustrates the results of 4 different bacteria antimicrobial tests on the surfaces of HE alloy coatings. Colony-forming units on HE alloy coatings are seen to be considerably inhibited, and the antibacterial rates for all 4 different bacteria exceed 99.999%. This indicates that the HE alloy coatings have efficient antibacterial abilities. Thus, HE alloy coatings have potential to be applied to various daily appliances like tableware, kitchenware etc. Accompanied with high hardness, wear-resistance, anti-oxidation, and anti-corrosion, multifunctional HE alloy coatings will turn daily appliances into durable, stable and germless products. Table V. Antibacterial analyses of HE alloys.

Bacterium name

Colony-forming unit of

comparison group after 24h

Colony-forming unit of sample group after 24h

Antimicrobial activity

Antibacterial rate (%)

S. aureus 2.1 x 106 < 10 5.32 >99.999

Escherichia coli 1.1 x 106 < 10 5.04 >99.999

Klebsiella pneumoniae 1.6 x 10

6 < 10 5.20 >99.999

Pseudomonas aeruginosa 1.9 x 10

7 < 10 6.28 >99.9999

How to efficiently suppress electromagnetic interference (EMI) has became a very important

issue in the rapidly developing field of electronics. It has been found that HE alloy coatings can efficiently suppress electromagnetic interference (EMI), as shown in figure 7. The EMI shielding efficiency of HE alloy coatings increases with the frequency of the electromagnetic wave. Moreover, thicker HE alloy coatings have higher EMI shielding efficiency. At high frequencies, above 1300MHz, the EMI shielding efficiency of 1 m-thick HE alloy coatings meets the standard required for commercial applications. Therefore, through careful alloy design and selected coating processes, HE alloys used to coat electronic products may have multiple functions, such as, wear-resistance, anti-oxidation, anti-bacteria and EMI shielding.

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-200 0 200 400 600 800 1000 1200 1400 1600 1800 200010

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35 EMI Analyses of HEA thin films

Qualified

Sh

ield

ing

Effe

ctiv

enes

s (d

b)

Frequency (MHz)

~1 mm 100 nm

Figure 7. EMI shielding efficiency as a function of frequency for HE alloy coatings of different thicknesses. 6. CONCLUSIONS

By exploiting the special features of HE alloys, they can be designed and developed for numerous commercial applications, including low-K thermal barriers, hardfacings, graded materials, hard metals, antibacterial coatings, and EMI coatings. HE alloys exhibit a high potential for huge commercial and industrial usage. For certain through future research and development, HE alloys will find even more commercial applications. 7. REFERENCES [1] J. R. Davis (Ed.), Metals Handbook, 10th edn., Vol. 1, ASM International, Metals Park, OH,

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Abstract- High-entropy alloys (HE alloys) are composed of at least five major metal elements, as opposed to traditional alloy systems that are typically based on only one or two major elements. Dependent upon their composition and/or processing route,...1. INTRODUCTION2. THERMAL BARRIER COATINGS OF HIGH-ENTROPY ALLOYS3. HARDFACING APPLICATIONS OF HIGH-ENTROPY ALLOYS4. HARD METALS USING HIGH-ENTROPY-ALLOY BINDERS5. ANTIBACTERIAL AND ELECTROMAGNETIC INTERFERENCE SHIELDING COATINGS6. CONCLUSIONS7. REFERENCES