High entropy alloys

HIGH ENTROPY ALLOYS

SOUNAK GUHAM-TECH- 1st Year

ROLL- 16MT61R38METALLURGICAL & MATERIALS

ENGINEERING DEPARTMENTIIT KHARAGPUR

ContentsIntroductionCriterion of forming HEAPhase stabilityCore effectsDifferent propertiesSynthesis of HEAApplicationsConclusionReference

Introduction

HEAs are defined as alloys with five or more principal elements having equal atom

percentage. Each principal element should have a concentration between 5 and 35 atom%. Existence of HEAs were firstly reported by Scientists Yeh J.W & Huang K.H in 1996.

Criterion of forming HEAs

According to Zhang & Suang there is another parameter (Ω ≥ 1.1) This parameter comes from

competing action between entropy & enthalpy of mixing.According to Guo & C.T Liu there are 2 more parameters.

(3) Electronegitivity difference(Δχ,)= ………………………… g

where χi = Pauling’s electronegativity for ith component

(4) Valence electron concentration (VEC): It control the crystal structures of HEAs. It is denoted as

ΔHmix, δ, Δχ, ΔSmix and VEC for a series ofequiatomic or nearly equiatomic alloys were calculated

and are listed in Table 1. By statistically analyzing the results Scientist Zhang formulated the criterion of

forming solid solution of HEAs.

Results

Criterion as per results obtained

0≤δ≤8.5 −22≤∆Hmix≤7 kJ/mol

11≤∆Smix≤19.5 J/(K·mol)

Phase stability

Conditions of forming Fcc or Bcc structure in HEAs are governed by Valence Electron Concentration.

VEC (<6.87)- Bcc structureVEC ≥ (8.0)- Fcc structure

Sometimes TCP form in HEAs. Calculation of ‘’d’’ orbital energy is the most accurate method of predicting TCP.

Examples of TCPs are laves phase, σ phase etc. Formation of TCP is important in every HEAs ( for ex-

AlCoCrFeNiNbx, CoCrFeNiMox)

There are 2 electron concentration theories to represent phase stability in HEAs.

(1) Average no of itinerant electrons/atom – e/a theory(2) Total no of electrons (VEC) including d orbital .

e/a theory(Hume Rothery)

VEC theory

Core effects

(1) High entropy of mixing

△Gmix = H△ mix -T S△ mix

If △Smix

△Gmix

If Smix> 1.61R, solid solution occurs.△This factor stabilize one single phase rather than multiple

phases predicted by Gibbs phase rule.

(2) Lattice distortion effect

Volume strain energy is the outcome of lattice distortion which occurs due to mismatch between atomic size &

shear modulus of multicomponents constituting HEAs. Atomic size difference is the key parameter δ.

Figure 5. Examples of (a) BCC and (b) FCC crystal structures with five principal elements.

(3) Sluggish diffusion2 factors are responsible for this. (1) Tendency to

form substitutional solid solution (2) Fluctuation of lattice potential energy (3) sharp local stress gradient.

Cocktail effect

This term was first coined by Indian professor S.Ranganathan. It means a synergistic mixture leading

to an unpredictable result which is greater than the sum of the effect of constituting elements. These

properties depend on material composition, electronic structure, microstructure etc.

Typical properties

HEAs show outstanding mechanical properties compared to other conventional alloys. Ultra high strength with excellent fracture toughness, high

thermal stability, high corrosion resistance, high fatigue resistance etc are some of the properties.

Strength

Wear resistance

The type of wear has been shown to be dependent on aluminium concentration

and titanium concentration where for low Al and Ti concentrations it is delamination

wear and for higher concentrations it is oxidation wear.

Ex- Al0.2Co1.5CrFeNi1.5Ti1.0

DBTT

Corrosion resistance

Synthesis of HEA

CoCrFeCuNi and CoCrFeMnNi were prepared by MA & HPS.

(1) Powder particle (<45µm)(2) 99.7 % purity

(3) Tungsten carbide balls, toluene ( media)(4) Planetary ball mill

(5) Ball to powder weight is 10:1 (6) Milling speed - 450 r.p.m

(7) Milling time 30 hours (8) P.C.A used after 25 hours

(9) Sintering at 1273k & 5 Gpa

Applications

Due to their superior properties HEAs are used in Aerospace industry (Turbine blades),high temperature refractory

applications, cryogenics , Hard coating industries. Even A low-density (2.67 g cm−3) high-entropy alloy

with composition Al20Li20Mg10Sc20Ti30 has been preparedby mechanical alloying of elemental powders.

Conclusion

There has been a lot of misconceptions about HEAs. If we can simulate their properties

through different modelling processes, it will be an indispensable part in Materials Engineering.

Thank you