High Entropy Alloys

HIGH ENTROPY ALLOYS

DEVANG GANDHI

M.TECH 1ST YR

IIT ROORKEE

16544016

INDEX

L i t e r a t u r e R e v i e w

I n t r o d u c ti o n

P h a s e F o r m a ti o n R u l e s

F o u r C o r e E ff e c t

P r o c e s s i n g R o u t e

M e c h a n i c a l P r o p e r ti e s

E x a m p l e s

A p p l i c a ti o n s

R e f e r e n c e s

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.

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)

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

PHASE FORMATION RULES

Fig 2. Ref : Tsai MH, Yeh J (2014) “High Entropy Alloys: A Critical Review” Mater. Res. Lett. 2, 107-123

∆Hmix =

∆Sconfig = -R

∆Gmix = ∆Hmix - T∆Smix

∂ =

Ω =

(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

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 =

∆Sconfig = -R

∆Gmix = ∆Hmix - T∆Smix

∂ =

Ω =

(where i represents ith element and ci represents the atomic percentages and ri denotes the atomic radius)

Table 2 : Determination of Crystal Structure

FOUR CORE EFFECT

High Entro

py Alloy

s

Thermodynamics

High Entropy

Properties Cocktail

Effect

Kinetics Sluggish Diffusion

Structure Lattice Distortion

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.

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.

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.

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.

PROCESSING ROUTES

Schematic of Fabrication Process of High Entropy Alloys

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

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

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

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

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 MicrostructureXRD 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

EXAMPLES

Alloy Structure Property Application

FeCoCrAlNi Single BCC Solid Solution Less Corrosive Enhance Corrosion and

Cavitation Erosion ResistanceCoating 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

APPLICATIONS

High EntropyAlloy Coatings

High EntropyBulk 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

APPLICATIONS





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

APPLICATIONS





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

APPLICATIONS





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. HEA As Binder

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.

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High Entropy Alloys

Engineering

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