High Entropy Alloys (HEAs) Sample Production

Charachterization Techniques

Properties and Future Applications Mechanical Properties

Arc Melting Alloy is produced from highly pure elements (Fe, Cu, Al, Ni, Ti) with Ti gathering in Ar atmosphere under vacuuum and melted 3 times to ensure homogenization. All of the components are 20% atomic percent to maximize mixing entropy.

Suction casting set-up

Melted alloys are casted into • 2mm • 3mm • 4mm Diameter cylindrical dies to see the effect of cooling rate on microstructural effects.

Common alloy systems are typically based on one or two elements mostly. However, a new

approach for alloy design , “High Entropy Alloys” was started in Tsing Hua University of Taiwan

since1995 by Yeh et al. HEA could be defined as having approximately equal concentrations

made up of a group of 5 to 11 major elements in the composition, mole fraction of each

major metallic element in the alloy is between 5% and 30%.

THERMODYNAMICS OF HIGH ENTROPY ALLOYS

Thermodynamics dictates that a system under constant temperature and pressure to

minimize its Gibbs free energy (G) and reach a stable state. For Gibbs free energy following

equilibrium exists: G = H – TS

On the other hand it is very hard to calculate Gibbs free energy in multicomponent systems at

a certain composition and temperature. In the pursuit of overcoming this problem Inoue and

Takeuchi proposed that for metallic glasses ΔG and ΔGmix are proportional. This assumption

is also valid for HEA alloys modifying the above relation to:

△Gmix = △Hmix - T△Smix

In which T is the absolute temperature, ΔHmix is the enthalpy of mixing and ΔSmix is the

entropy of mixing. [1]

Scanning Electron and Optical Microscopy

Energy Dispersive Spectroscopy

X-ray Diffraction DSC and DTA

References

SEM and 3D microscopy results have shown that dendtritic structure is present throughtout the material with a secondphase surrounding the dendrites. Alloys seemed to have a relatively low grain size under these conditions.

A-1 A-2

B-1 B-2

C-5 C-6

Fig. 1. A and B are (2mm, 4mm diameter respectively) SEM

micrographs of as cast (Fe20 Cu20 Al20 Ni20 Ti20), C are the 3D

micrographs.

EDS and mapping results indicate that the components Fe, Al, Ni, Ti are mostly evenly distributed along the grains with an obvious Cu shortage. Another EDS is taken from grain boundaries and it has been seen that Cu rejected from the grains piled up and created a secondary phase.

Fig. 2. D1, D2, D3, D4, D5 are the Al, Ti, Cu, Fe, Ni element maps respectively and D6 is the

control micrograph. .

D-1 D-2 D-3

D-4 D-5 D-6

As it can be seen from the x-ray diffraction patterns there are broad and intense BCC peaks joined by small FCC peaks. BCC peaks are formed by moslty Al, Fe, Ni, Tİ elements inside the grains that is the reason of high intensity. While small peaks are formed by FCC crystal structure of copper with smaller additions of other components in the grain boundaries. Another point of interest is that there are no peaks showing signs of intermetallic phases which provides brittle structures which weaken the alloys.

Fig. 3. XRD patterns of as cast (Fe20 Cu20 Al20 Ni20 Ti20) alloy

.

DTA and DSC have shown that alloy is stable up until 1100°C and also does not melt up to 1400°C. Making the alloy a strong candidate for high temperature applications.

Fig. 5. DSC curve of(Fe20 Cu20 Al20 Ni20 Ti20) alloy up to 500°C .

Fig. 4. DTA curve of (Fe20 Cu20 Al20 Ni20 Ti20) alloy up to 1400°C .

Compression Test

Fig. 6. Compressive stress strain curve of (Fe20 Cu20 Al20 Ni20 Ti20) alloy.

100 150 200 250 300 350 400 450

He

at F

low

(m

W)

Temperature(Co)

Exothermic

Heating

Cooling

400 600 800 1000 1200

Cooling

He

at F

low

(mW

)

Temparature(Co)

Heating

Exothermic

Engine materials --- better elevated-temperature strength, oxidation resistance, and hot corrosion (sulfidation) resistance.

Light transportation materials --- better strength, toughness, creep resistance, and workability

Functional coatings --- better wear resistance, anti-sticky, anti-finger print, anti-bacterial, and aesthetics

Superconductor --- higher critical temperature and critical current

Golf club head --- higher strength and resilience [2] 0 2 4 6 8 10 12

0

200

400

600

800

1.000

1.200

1.400

1.600

Tru

e S

tre

ss

True Strain

20 40 60 80 100 120

Inte

nsity (

a.u

.)

2

Compression data has revealed that the alloy doesnt yield up to 1100 Mpa and has ultimate tensile strength of 1400MPa.

[1] Inoue, A., & Takeuchi, A. (2002). Bulk Amorphous, NanoCrystalline and Nano-Quasicrystalline Alloys IV. Recent Progress in Bulk Glassy Alloys. Materials Transactions. [2] Yeh, J. (2006). Recent progress in high-entropy alloys. European Journal of Control.

Hardness of the alloy is found as 477 HV.

High Entropy Alloys (HEAs) Tolgahan Ulucan, Şeyma Koç, Fatih Sıkan

Academic Advisor: Assoc. Prof. Y. Eren Kalay

Metallurgical and Materials Engineering Department METU Ankara 06800 Turkey

Mechanical tests were applied to determine Yield strength Hardness Fracture behavior ductility of the produced alloys.

Tensile and hardness measurement set-ups.