High-Entropy AlloysBreakthrough Materials for Aero Engine Applications?

By Daniel Svensson, Gothenburg, 13/2 2015

Presentation Outline1. Introduction2. High-Entropy Alloys3. Aero Engine Materials4. Bridging the Gap5. Suggested Systems6. Summary

2

Introduction

● GKN Aerospace Engine Systems in Trollhättan manufactures engine parts

● Current superalloys (ρ> 8 g/cm3)● High-entropy alloys are potential

candidate materials3

Introduction

● High-entropy alloys is a new type of metallic materials

● Exciting properties○ Good strength○ Retain strength at elevated temperatures

4

Introduction

● Review high-entropy alloys● Review state-of-the-art aero

engine materials● Identify problems left to solve● Suggest potential high-

entropy alloy systems

5

Presentation Outline1. Introduction2. High-Entropy Alloys3. Aero Engine Materials4. Bridging the Gap5. Suggested Systems6. Summary

6

High-Entropy Alloys

1. Definition2. Four core effects3. Typical properties4. Processing routes

7

Definition● Conventional (low- and medium-entropy) alloys

○ 1-3 principal components with 1 or more minor componentsSteels, aluminium alloys...

● High-entropy alloys○ 5-13 principal components

○ (Not the only definition, they can also be defined according to their configurational entropy)AlCoCrFeNi, AlMo0.5NbTa0.5TiZr...

8

Four Core Effects

1. High mixing entropy effect○ Gibbs free energy ○ High configurational entropy can suppress ordered phases○ Especially at higher temperatures

9[High-Entropy Alloys - Murty B.S., Yeh J.W., Ranganathan S.]

Four Core Effects

2. Sluggish diffusion effect○ Fluctuating potential energy

due to many different elements

○ Much coordination of elements needed

Good elevated temperature properties10

[Sluggish diffusion in Co–Cr–Fe–Mn–Ni high-entropy alloys]

Four Core Effects

3. Lattice distortion effect○ Hinder dislocation movement

solid solution strengthening

○ Scatter propagating electrons and

phonons lowered electric and thermal conductivity

[Solid-Solution Phase Formation Rules for Multi-component Alloys]

Four Core Effects

4. Cocktail effect○ Properties of HEAs not

average of those of constituent elements

○ Interaction between

constituing elements

and lattice distortion will affect properties

12AlxCoCrCuFeNi

[High-Entropy Alloys - Murty B.S., Yeh J.W., Ranganathan S.]

Interesting Properties

● Most properties researched has been for some derivations of the Al-Co-Cr-Cu-Fe-Ni system

● Some research on refractory systems● (Often melted and cast)

13

Strength● The Al-Co-Cr-Cu-Fe-Ni system

○ Phase constitution varies with Al content

○ Strength dependent on the structure○ Retain strength at elevated

temperature, especially fcc type alloys○ Additional alloying elements (Ti,Mo,Mn,

Nb,Si…) also affect the phase composition

FCC

FCC + BCC

BCC

14

[Nanostructured High-Entropy Alloys with Multiple Principal Elements: Novel Alloy Design Concepts and Outcomes]

Strength● Refractory alloys

○ Mostly BCC type, some with ordered phases

○ Good elevated temperature strength

○ Mostly brittle, though some systems exhibit good compressive ductility

○ Also some Al containing systems with relativley low densities

15

[Mechanical properties of NbMoTaW and VNbMoTaW refractory high entropy alloys]

Fatigue● Limited research● One FCC type system

Al0.5CoCrCuFeNi

● One BCC type systemAl7.5Cr22.5Fe35Mn20Ni15

● Promising results, FCC type slightly better than BCC

● Scattered results, attributed to microstructural defects

16

[Fatigue behavior of Al0.5CoCrCuFeNi high entropy alloys]

Wear● Mainly Al-Co-Cr-Cu-Fe-Ni

system● Not linear with hardness

as opposed to for ferrous alloys

● Type of wear dependent on constituents (and crystal structure)

17

[Mechanical performance of the AlxCoCrCuFeNi high-entropy alloy systemwith multiprincipal elements]

Oxidation

● Not much researchAl +(Cr +)Fe -

18

AlxCo1.5CrFeNi1.5Tiy

[Microstructure and wear behavior of AlxCo1.5CrFeNi1.5Tiy high-entropy alloys]

CorrosionVarying corrosion properties, in both H2SO4 and NaCl

19[Alloying and Processing Effects on the Aqueous Corrosion Behavior of High-Entropy Alloys]

Thermal Properties● AlxCoCrFeNi

○ Thermal conductivity lower than in pure metals■ Lattice distortion effect■ Precipitates■ Nanograins

○ Thermal conductivity increase with temperature■ Lattice distortion■ Increase in lattice size

20

[Microstructure, thermophysical and electrical properties in AlxCoCrFeNi (0≤x≤2) high-entropy alloys]

Processing

● Casting○ Most common processing

route

○ Vacuum arc melting or vacuum induction melting

○ Copper mold casting

○ Microstructure depends on

cooling-rate, heat-treatments, forging 21

[High-Entropy Alloys - Murty B.S., Yeh J.W., Ranganathan S.]

High-Entropy Alloys

● Powder metallurgy○ More homogeneous

○ Good when having a wide

range of evaporation temperatures

22http://what-when-how.com/materialsparts-and-finishes/mechanical-alloying/

Processing● Thin films/coatings

○ From vapor state: magnetron sputtering or plasma nitriding

○ From liquid state: tungsten inert gas/gas tungsten arc welding or laser cladding

● Additive manufacturing○ FeCoCrNi from selective laser

melting○ Better tensile properties than as-

cast alloys, attributed to the fine microstructure

23

http://www.laserstoday.com/2011/05/high-performance-laser-cladding/#more-1035

Presentation Outline1. Introduction2. High-Entropy Alloys3. Aero Engine Materials4. Bridging the Gap5. Suggested Systems6. Summary

24

The Aero Engine

1. Suck2. Squeeze3. Bang4. Blow

http://en.wikipedia.org/wiki/File:Jet_engine.svg

25

Aero Engine Materials● Today’s aero engines made

mostly out of four types of alloys○ Aluminium alloys○ Steels○ Titanium alloys○ Nickel alloys (superalloys)

● Other exciting new materials○ Ceramics○ Composites○ Intermetallics

26[Manufacturing Technology for Aerospace Structural Materials]

Aluminium Alloys and Steels

● Aluminium alloys+ Light-weight (Al density 2.7 g/cm3)− Low temperatures− Low stiffness

● Steels+ Cheap+ Higher stiffness– Not to high temperatures

27

Titanium Alloys

+ High strength to weight ratio+ Good fatigue strength+ Good corrosion resistance− Not higher temperatures than ~550o C

28

Nickel Alloys (Superalloys)

+ Able to withstand higher temperatures than Ti alloys

+ High strength+ Good fatigue and creep

resistance+ Good corrosion and

oxidation resistance− High density

29

[Application of alloy 718 in GE aircraft engines: past, present and next ve years, Superalloys 718, 625, 706 and various derivatives]

Coatings

● Diffusion coatings (CoAl, NiAl...)● Overlay coatings (MCrAlY, WC-

Co...)● Thermal barrier coatings (Y2O3-

stabilized ZrO2…)

30

[Tbc experience on ge aircraft engines]

Other Exciting New Materials

● Ceramics (SiC,Al2O3...)● Composites (CMC,MMC…)● Intermetallics (NiAl,TiAl...)

31

Density Comparison

32

AlCoCrCuFeNi 7.1* g/cm3 Ti-6Al-4V 4.43 g/cm3

AlCoCrFeNi 6.7* g/cm3 Inconel 718 8.19 g/cm3

AlMo0.5NbTa0.5TiZr 7.4 g/cm3 Haynes 230 8.97 g/cm3

VNbMoTaW 12.36 g/cm3 Waspaloy 8.20 g/cm3

High-entropy alloys Conventional alloys

* Calculated using rule-of-mixtures with room temperature data

Specific Parts

● Lower densities than superalloys● Elevated temperature strength

● Hot structural components○ Turbine Exhaust Case, Mid Turbine Frame, Exhaust

Nozzle and Cone

33

Turbine Exhaust Case● Situated downstream of the

final turbine● Support the low pressure rotor● Mount engine to aircraft body● Remove angular component of

outgoing flow● Exposed to high temperatures● Inconel 718

34[Weld sequence optimization:The use of surrogate models for solving sequential combinatorial problems]

Turbine Exhaust Case

● Separation of functionalities○ Load Carrying Structure

■ Limited by LCF, strength, stiffness, creep/thermo mechanical fatigue and oxidation

■ Today Inconel 718○ Heat Shielding Fairing

■ Limited by temperature capability, formability and oxidation

■ Working temperature 670oC, peak temperatures of 760oC

■ Solution hardened alloy 35

Mid Turbine Frame● Situated in between high pressure

and low pressure turbines● Houses the mid turbine bearing,

supporting low and high pressure rotors

● Similar demands as on the TEC, with a similar separation of functionalities○ Load carrying structure in Inconel

718○ Heat shielding fairings in Mar-M-

247 or Mar-M-509

36

Exhaust Nozzle and Cone● Integrate with TEC to

avoid interfaces● Limited by creep,

temperature capability, surface stability and weight

● Today often titanium alloys

● Research into CMCs© Boeing

37

Presentation Outline1. Introduction2. High-Entropy Alloys3. Aero Engine Materials4. Bridging the Gap5. Suggested Systems6. Summary

38

Bridging the Gap

● Fatigue and creep○ Little fatigue research, only two systems○ Only HCF, not LCF○ No creep research

○ Good creep resistance can be expected from the sluggish diffusion and lattice distortion core effects

○ Conventionally creep resistance is increased by coarsening the grains

39

Bridging the Gap

● Oxidation○ Little research

○ Al and Cr conventionally gives good resistance by forming protective layers

● Property and alloy optimization○ Balancing properties against each other (e.g. strength

and ductility)

○ Be aware of eventual problems with the used elements (expensive, rare, hazardous etc.) 40

Bridging the Gap

● Thermal stability○ Not well researched○ Many alloys have been in a metastable

state○ Alloys will be exposed to high

temperatures for extended periods of time

● Manufacturability○ Materials must be formable and

possible to join with other materials○ Property scattering from defects needs

to be removed/minimized 41

http://forthillhs.com/Students/Student%20subject%20wepages/Welding/index.html

Presentation Outline1. Introduction2. High-Entropy Alloys3. Aero Engine Materials4. Bridging the Gap5. Suggested Systems6. Summary

42

Suggested Systems

Load Carrying StructureAl-Co-Cr-Fe-Ni-Mo

Heat Shielding FairingAl-Co-Cr-Fe-Ni

Exhaust Nozzle and ConeAlNbTiV

43

Presentation Outline1. Introduction2. High-Entropy Alloys3. Aero Engine Materials4. Bridging the Gap5. Suggested Systems6. Summary

44

Summary

● High-entropy alloys: new exciting material● Four core effects of high-entropy alloys

○ High mixing entropy○ Sluggish diffusion○ Lattice distortion○ Cocktail effect

45

Summary

● Potential for low density metallic alloys with good elevated temperature properties

● Candidates for structural components in the hotter parts of aero engines

● Many problems left to solve●

46

Acknowledgements

● Chalmers○ Sheng Guo

● GKN○ Magnus Hörnqvist○ Bengt Pettersson○ Anders Hellgren

47