The document reviews high entropy alloys (HEAs), emphasizing their unique properties and applications, including findings from literature on phase formation rules and mechanical properties. It discusses the 'four core effects' that contribute to HEAs' strength and ductility, alongside various processing routes and examples of specific alloys. Finally, it highlights the applications of HEAs, such as protective coatings and bulk metallic glass, and lists relevant references.

1. HIGH ENTROPY ALLOYS
DEVANG GANDHI
M.TECH 1ST YR
IIT ROORKEE
16544016

2. INDEX
 L i te ra t u re Re v i e w
 I n t ro d u c t i o n
 P h a s e Fo r m a t i o n R u l e s
 Fo u r C o re Effe c t
 P ro c e s s i n g Ro u te
 M e c h a n i c a l P ro p e r t i e s
 E xa m p l e s
 A p p l i c a t i o n s
 Re fe re n c e s

3. 2004
2006
2008
2014
2016
LITERATURE REVIEW
Applications of CoCrFeMnNi in Sub-Zero Region.
Four Core Effects were Observed in High Entropy Alloys
Phase Formation Rules were framed by Zhang et al.
High Entropy Alloys were discovered by Yeh and Cantor.
Future Prospects of HEA being considered.

4. INTRODUCTION
 5-13 principal elements
 % of Elements between 5% and 35%
 Density – 6.7-7.3 gm/cm3
 ∆Hmix between -10kJ/mol and 5kJ/mol
 ∆Sconfig >1.5R
 Difference in Atomic Radii ∂ < 6.6%
Fig 1. CoCrFeMnNi FCC High Entropy Alloy
N 1 2 3 4 5 6 7 8 9 10
∆Sconfig
(-R 𝒊=𝟏
𝒏
𝑿𝒊𝒍𝒏𝑿𝒊)
0 0.69 R 1.1 R 1.39 R 1.61 R 1.79 R 1.95 R 2.08 R 2.2 R 2.3 R
Fig 1 Ref : Wang S. (2012) “Atomic Structure Modeling of Multi-Principal Element Alloys by the Principal of Maximum Entropy”, Entropy 15, 5536-5548
Table 1 : ∆Sconfig vs N

5. PHASE FORMATION RULES
Fig 2. Ref : Tsai MH, Yeh J (2014) “High Entropy Alloys: A Critical Review” Mater. Res. Lett. 2, 107-123
∆Hmix = 𝒊=𝟏,𝒊≠𝒋
𝒏
𝟒∆𝑯𝒎𝒊𝒙𝒄𝒊𝒄𝒋
∆Sconfig = -R 𝒊=𝟏
𝒏
𝑿𝒊 𝒍𝒏𝑿𝒊
∆Gmix = ∆Hmix - T∆Smix
∂ = 𝟏𝟎𝟎 𝒊=𝟏
𝒏
𝒄𝒊(𝟏 −
𝒓𝒊
𝒓
) 𝟐
Ω =
𝑻∆𝑺𝒄𝒐𝒏𝒇𝒊𝒈
∆𝑯
(where i represents ith element and ci represents the atomic
percentages and ri denotes the atomic radius)
Fig 2. Variation of ∆Hmix with ∂ for a number of alloys

6. PHASE FORMATION RULES
Solution Parameter Crystal Structure
Single Solid
Solution
Ω > 1.1 BCC
∂ < 6.6 VEC < 6.87
FCC
-15 kJ/mol < ∆Hmix < 5 kJ/mol VEC > 8
Ordered and
Disordered Solid
Solution
∂ < 6.6 BCC+FCC
-22 kJ/mol < ∆Hmix < 7 kJ/mol 6.87 < VEC < 8
Possible States Elemental
Phase
Compounds Random Solid
Solutions
Partially
Ordered Solid
Solutions
∆Hmix ~0 Large Negative Medium
Negative
Medium
Negative
-T∆Smix ~0 ~0 -RTln(n) <-RTln(n)
∆Gmix ~0 Large Negative Large Negative Large Negative
∆Hmix = 𝒊=𝟏,𝒊≠𝒋
𝒏
𝟒∆𝑯𝒎𝒊𝒙𝒄𝒊𝒄𝒋
∆Sconfig = -R 𝒊=𝟏
𝒏
𝑿𝒊 𝒍𝒏𝑿𝒊
∆Gmix = ∆Hmix - T∆Smix
∂ = 𝟏𝟎𝟎 𝒊=𝟏
𝒏
𝒄𝒊(𝟏 −
𝒓𝒊
𝒓
) 𝟐
Ω =
𝑻∆𝑺𝒄𝒐𝒏𝒇𝒊𝒈
∆𝑯
(where i represents ith element and ci represents the atomic
percentages and ri denotes the atomic radius)Table 2 : Determination of Crystal Structure

7. FOUR CORE EFFECT

8. FOUR CORE EFFECT HIGH ENTROPY EFFECT
Fig 3. Ref : Yeh J. (2015) "Physical metallurgy of high-entropy alloys" JOM, 67, 2254-2261.
Fig 3. Variation of Potential Energy with distance between Lattice Sites
 Formation of Solution Phase to render Simple
Microstructure.
 High Configuration Entropy suppresses
Ordered phase formation, especially at Higher
Temperatures.
 Enhances formation of solution phase,
increase strength and ductility of solution
phase due to solution hardening.
 Stronger Bond Energies – Enhance Solid
Solution & Inhibits formation of Intermetallics.

9. FOUR CORE EFFECT LATTICE DISTORTION EFFECT
Fig 4. Distorted or Strained Lattice of High Entropy Alloy
 Every atom in multi-principal element matrix
is surrounded by different kinds of atom and
suffer lattice strain.
 Severe Lattice Distortion can increase
Hardness and Strength by Solid Solution
Hardening.
 Factors contributing to the Lattice Strain
include :
 Atomic Size Difference
 Difference in Bonding Energies
 Crystal Structure
Fig 4. Ref : Yeh J. (2015) "Physical metallurgy of high-entropy alloys" JOM, 67, 2254-2261.

10. FOUR CORE EFFECT SLUGGISH DIFFUSION EFFECT
Fig 5 Ref : Tsai KY, Tsai MH, Yeh JW (2013) “Sluggish diffusion in Co-Cr-Fe-Mn-Ni high-entropy alloys.” Acta Mater 61:4887–4898
Fig 5. Normalized Activation Energy of Diffusion for Cr, Mn, Fe,
Co and Ni in different matrix
 Vacancy Concentration for Substitutional
Diffusion is limited in HEA.
 Positive Enthalpy of Formation and Excess of
Mixing Entropy needed.
 Higher Activation Energy needed, Slower
Diffusion. Only one HEA tested till date,
CoCrFeMnNi.
 Slower Diffusion indicates Slower Kinetics or
Slower Phase Transformation. Hence, better
Microstructure and Property control.

11. FOUR CORE EFFECT COCKTAIL EFFECT
‘Alloyed Pleasures : Multi-Metallic Cocktails’
S. Ranganathan: ‘Alloyed pleasures: multimetallic cocktails’, Curr. Sci., 2003, 85, 1404–1406.
‘overall effect resulted from mutual interactions among composing elements, which would bring
excess quantities to the average values simply predicted by the mixture rule’
E. J. Pickering, N. G. Jones: ‘High-entropy alloys: a critical assessment of their founding principles and future prospects’, Int. Mat. Reviews, 2016, 61, 183-202
‘the overall effect from composition, structure, and microstructure’
J.W. Yeh: ‘Physical metallurgy of high-entropy alloys’, JOM, 2015, 67, 2254–2261.

12. PROCESSING ROUTES
Schematic of Fabrication Process of High Entropy Alloys

13. PROCESSING ROUTES
Fig 6. Ref : Ioannis S.Aristeidakis Maria-Ioanna T.Tzini (2016) “High Entropy Alloys”, Univ. of Thessaly, Jan. 2016
(a) Arc Melting (b) Bridgman Solidification
Fig 6. Processing from the Liquid State

14. PROCESSING ROUTES
Mechanical Alloying Sputter Deposition Process
Fig 7. Processing from the Solid State Fig 8. Processing from Gaseous State
Fig 7. & Fig 8. Ref : Ioannis S.Aristeidakis Maria-Ioanna T.Tzini (2016) “High Entropy Alloys”, Univ. of Thessaly, Jan. 2016

15. MECHANICAL PROPERTIES
Fig 9. Ref : Gludovatz B. et al. (2016) “Exceptional damage-tolerance of a medium entropy alloy CrCoNi at cryogenic temperatures”, Nature Comm, 7, 10602
 High Strength
 High Hardness
 Good Fracture Toughness
 Good Creep Strength
 Excellent Wear Resistance
 Excellent Corrosion Resistance
 High Thermal Stability
Fig 9. Fracture Toughness and Yield Strength Comparison in Materials

16. CoCrFeMnNi
Fig 10. & Fig 11. Ref : Yeh J. (2015) "Physical metallurgy of high-entropy alloys" JOM, 67, 2254-2261.
As the Number of Elements Increase
Peak Intensity Decreases & Number of Peaks Increase
As the Number of Elements Increase
Stacking Fault Energy decreases continuously
Fig 10. XRD Pattern of CoCrFeMnNi Fig 11. SFE vs Number of Elements in CoCrFeMnNi

17. CoCrFeMnNi
Fig 12 & Fig 13. Ref : Gludovatz B. et al. (2015) "Processing, microstructure and mechanical properties of the CrMnFeCoNi high entropy alloy" JOM, 67, 2262-2270.
Presence of Twins in Microstructure
XRD Pattern shows FCC Crystal Structure
As the Temperature Decreases
Ductility and Strength Increases
Fig 12. Micrograph of CoCrFeMnNi Fig 13. Stress Strain Curve of CoCrFeMnNi

18. EXAMPLES
Alloy Structure Property Application
FeCoCrAlNi Single BCC Solid Solution Less Corrosive Enhance Corrosion and
Cavitation Erosion Resistance
Coating on 304 Stainless
Steel
TiNbTaZrMo Two BCC Solid Solution Considerable Strength with Superior
Biocompatibility
Metallic BioMaterial
AlxCoCrCuFeNi FCC+BCC Change of Crystal Structure,
High Strength to Weight Ratio
Binder in WC and
Inhibit WC Coarsening
MoNbTaVW BCC Retain Strength at Elevated Temperature Refractory HEA
HfNbTaTiZr BCC Excellent Compression Ductility Refractory HEA
Table 3 : Examples of High Entropy Alloys

19. APPLICATIONS
High Entropy
Alloy Coatings
High Entropy
Bulk Metallic Glass
Refractory
High Entropy Alloys
Carbides and Cermets
with HEA Binders
 HEA deposited on substrates for protection against wear, corrosion and heat.
 Thin Film Coating processes include Electroless Plating, Physical Vapor Deposition, Chemical Vapor Deposition.
 Example : AlCrSiTiV HEA Coating on Ti-6Al-4V substrate
Fig 14. Ref : Huang C et al. (2012) “Dry sliding wear behavior of laser clad TiVCrAlSi high entropy alloy coatings on Ti-6Al-4V substrate.” Mater Des 41:338–343
Fig 14. Surface Morphology and SEM Micrograph of
AlCrSiTiV HEA Coating on Ti-6Al-4V substratettim

20. APPLICATIONS
High Entropy
Alloy Coatings
High Entropy
Bulk Metallic Glass
Refractory
High Entropy Alloys
Carbides and Cermets
with HEA Binders
Fig 15. Ref : http://www.techbriefs.com/component/content/article/ntb/tech-briefs/materials/23866
 High Entropy feature along with Amorphous nature of Bulk Metallic Glass.
 Small ∆Hmix , Large ∂ (greater than 6.6%)
 Example : Ti40Zr20Cu5Al5Be30
Fig 15. High Entropy Bulk Metallic Glass Ti40Zr20Cu5Al5Be30

21. APPLICATIONS
High Entropy
Alloy Coatings
High Entropy
Bulk Metallic Glass
Refractory
High Entropy Alloys
Carbides and Cermets
with HEA Binders
 Improved Elevated Temperature Strength, Reduced Density and High Melting Point.
 Lower Yield Strength, High Ductility and High Strain Hardening.
 Example : MoNbTaW and MoNbTaVW
Fig 16. Ref : Zou Y. et al. (2015) “Ultrastrong ductile and stable high-entropy alloys at small scales.” Nature Comm 6, 8748
Fig 16. Refractory HEA MoNbTaW

22. APPLICATIONS
High Entropy
Alloy Coatings
High Entropy
Bulk Metallic Glass
Refractory
High Entropy Alloys
Carbides and Cermets
with HEA Binders
Fig 17. Ref : Chen CS et al. (2014) “Novel cermet material of WC/multielement alloy.” Int J Refract Hard Met 43:200–204
 High Hardness, Softening Resistance at High Temperature, Wear Resistance, Corrosion Resistance.
 Provides higher hot hardness due to finer WC grain size of WC/HEA carbide than WC/Co.
 Example : Al0.5CoCrCuFeNi HEA as Binder.
Fig 17. HEAAs Binder

23. REFERENCES
S. Ranganathan, "Alloyed pleasures: Multimetallic cocktails," Curr. Sci. , vol. 85, pp. 1404-1406, 2003.
J. Yeh, S. Chen, S. Lin, G. J.Y., T. Chin, T. Shun, T. C.H. and S. Chang, "Nanostructured high-entropy alloys with
multiple principal elements: Novel alloy design concepts and outcomes.," Adv. Eng. Mater, vol. 6, no. 5, p. 299–
303, 2004.
B. Cantor, I. T. H. Chang, P. Knight and A. J. B. Vincent, "Microstructural development in equiatomic
multicomponent alloys," Mater. Sci. Eng. A, pp. 375-377,213-218, 2004.
J. Yeh, "Physical metallurgy of high-entropy alloys," JOM, vol. 67, pp. 2254-2261, 2015.
B. Gludovatz, A. Hohenwarter, C. D., E. H. Chang and R. O. Ritchie, "A fracture-resistant high-entropy alloy for
cryogenic applications.," Science, vol. 345, pp. 1153-1158, 2014.

24. THANK YOU