A review of Strengthening and Deformation mechanism in High Entropy Alloy

1. A review on Strengthening and
Deformation mechanism in High Entropy
Alloy
School of Minerals, Metallurgical and Materials Engineering
Indian Institute ofTechnology Bhubaneswar
1
Gurudev Singh

2. Contents
• Introduction
• Previous Studies
• Deformation Mechanism
1.Dislocation slip mediated
2.MechanicalTwin mediated
3.Phase transformation mediated
4.Stacking Fault Mediated
• References

3. Introduction
• 5-13 Principle elements
• Composition of elements between
5% and 35%
• ∆H mix between -10kj to 5kj
• ∆S config > 1.5R
• Difference in atomic radii <6.6%
• Rely on maximization of
Configuration Entropy.
Previous Studies

4. Core
-
Effects
High Entropy Effect-
1) G=H-TS
2) Phase with high entropy is stable at
highT
3) Conventionally S ss > S IM
Sluggish Diffusion-
1) Difference in atomic configuration
lead to difference in local energy.
2)Trap in low energy region
3) Slowest element will be rate
determining.
Cocktail Effect
Properties are not just averaged
but also dependent on the inter
elemental reactions.
Lattice Distortion
1) Size difference leads to lattice
distortion.
2) Impedes dislocation and lead to solid
solution strengthening.
High Entropy
Alloys
Previous Studies

5. Strengthening mechanism
Strengthening Mechanism
Lattice
Resistance
Solid Solution
Strengthening
Dislocation
Strengthening
Grain
Boundary
Strengthening
Precipitation
Strengthening
CSRO
Discussed in Presentation -2

6. Lattice Resistance
Conventional Alloys-Only Solvent lattice produces lattice resistance, and solute atom contribute to SSS
In HEA- No as such concept of solute solvent, due to elemental size mismatch Lattice distortion is a core
effect
Thus, the lattice resistance in concentrated HEAs originates from the distorted lattice

7. Solid Solution Strengthening
• Interstitial solid solutions form by squeezing small
solute atoms into interstitial sites between the
solvent atoms.
• The invasion of the interstitial atoms deforms the
alloy’s lattice, leading to a local stress field that
impedes dislocation motion to strengthen the
alloy
Substitutional SSS Interstitial SSS
Where M = 3.06 is theTaylor factor, G is the shear modulus, c is the concentration of the interstitial solute, and Δε is the
difference in strains

8. • The mutual hindrance of dislocations to their motion
essentially requires higher applied stress to keep the plastic
flow continue compared to an ideal.
DISLOCATION STRENGTHENING GRAIN BOUNDARY STRENGTHENING
• The discontinuity of the slip plane from one grain to
another impediment of grain boundaries to dislocation
motion.

9. • Precipitation strengthening makes use of the declining second-phase solubility in the matrix
alloy with decreasing temperature to strength an alloy.
• Two-step heat treatment process i.e. firstly Solution-treated at an elevated temperature,
followed by aging at a lower temperature to precipitate out second-phase particles.
• The strengthening effect of precipitates essentially stems from their blockages to dislocation
motion.
Precipitation Strengthening

10. Enhancing Strength via SHORT RANGE ORDERING
HEA is energetically favorable to undergo short-range ordering (SRO), and the SRO leads to a pseudo-composite
microstructure, which surprisingly enhances both the ultimate strength and ductility.
For example, in bccTiZrHfNb HEA doped with 2 at.% oxygen.
• (Zr,Ti)-rich oxygen complexes.
• Changes the dislocation glide mode from planar slip to double cross slip leads to a simultaneous increase in
the tensile strength (by 48.5%) and ductility (by 95.2%)
• CSRO is defined by α, where
αij = 1 − Nij /NXj
• Where Ni j is the number of j-type atoms in the first nearest
neighboring around an i-type atom,
• N is the total number of atoms
• Xj is the atomic fraction

11. The CSRO regions are observed only in the fcc phase of
the present DP HEA
CSRO IN Fe50Mn30Cr10Co10
• Inverse FFT (IFFT) images of the FCC lattice
and CSRO regions.
• Evidence of CSRO regions in the FCC phase of
the sample annealed at 760 °C
• Close-up maps of Fe and corresponding binary map,
• Fe enrichment on alternating atomic planes.
• Solid white line: Fe-enriched; dashed white line: Mn-
/Co-/Cr-enriched.
Atomic-scale EDS composition map.

12. Strength and
Ductility
Composition
Effect
Effect of Mn Effect of Co Effect of Cr Effect of Al
Processing
Effect
Microstructural
Effect
Temperature
Strength
ductility
tradeoff
Discussed in Presentation -3
Strength and Ductility

13. Impact of the supplementary elements
• HighVEC lead to greater inter atomic forces
→ atoms arrange themselves into a more
closed packed structure i.e. FCC
• LowVEC lead to smaller inter atomic forces
→ atoms arrange themselves into a more
open structure i.e. BCC

14. 14
Effect of Fe (Substitution ofCo)
• Alloy= Fe60-xMn30Cr10Cox
• The substitution of Iron by Cobalt atoms leads to an increase of the
martensitic transformation temperature as Co content increases.
• Co contents smaller than approximately 3 at % the FCC-HCP
transition is completely inhibited.

15. 15
• CoCrFeNi alloy demonstrates the presence of a single
phase with the FCC lattice (a = 3.577 Å). Only one FCC
phase, with a slightly higher lattice parameter (a = 3.602
Å) is also found in the CoCrFeNiMn alloy.
• Dendritic Structure are formed.
• The yield strength of the CoCrFeNiMn alloy is 215 MPa in as-
solidified state and 162 MPa after annealing is slightly higher
than that of the CoCrFeNi alloy which is 140 MPa.
• Structure is FCC and strength is almost same
https://www.researchgate.net/publication/260014088_Effect_of_Mn_and_V_on_structure_and_mechanical_properties_of_high-entropy_alloys_based_on_CoCrFeNi_system
Effect of Mn andV
Note- Addition of V givesTetragonal structure and increases hardness significantly

16. • (FeNiCrMn)(100x)Cox (x ¼ 5, 10 and 20) alloys annealed at 850 C.
• All the investigated HEAs exhibit a single FCC phase in the as-
cast state.
• A Cr-rich sigma phase precipitates after annealing for the Co5
and Co10 alloys.The Co20 alloy remains a single FCC phase in
the as-annealed state.
• The hardness and yield strength increase while tensile ductility
decreases for the Co5 and Co10 alloys due to the precipitation
of hard yet brittle Cr-rich phase after annealing.
• For the Co20 alloy, the hardness and yield strength decreased
due to a more homogeneous distribution of constituting
elements and grain growth while the tensile elongation keeps
almost unchanged due to high phase stability.
Compositional Effect
Effect ofCo

17. (FeCoNiCrMn)100xAlx (x = 0–20 at.%)
• Al < 8%- solid solution alloy, single fcc region
• 8% < Al < 16% bcc phases begin to appear and both the
fracture and yield strength are drastically increased
• (Al > 16%), alloys consist of (disordered bcc)A2
precipitates embedded in an ordered B2 (ordered BCC)
matrix
Compositional Effect
Effect ofAlSubstitution

18. 18
Effect ofCo andCr (Substitution of Ni)
Increasing Co concentration increases
Strength
Increasing Cr increases strength at first
then decreases (Trend for E and G)
https://www.jmst.org/article/2020/1005-0302/1005-0302-48-0-146.shtml

19. 19
Effect of Cr
substitution
Effect of Co
substitution

20. Deformation Mechanisms
Dislocation
Mediated
Twinning
Mediated
Phase
Transformation
Stacking Fault
Mediated
Combination of
these
Deformation Mechanisms

21. Dislocation Mediated
TEM image of the deformed (CoCrNi)94Al3Ti3 alloy,
indicating the planar slip of dislocations along with
intersecting slip lines in two different {111} planes.
• In FCC structured HEAs deformation is highly
planar involving slip of ½〈110〉dislocations on
the {111} slip planes.
• In BCC structured HEAs motion of screw
dislocation with b = a/2 〈111〉.
• Cross-slip of screw dislocations is observed in
BCC HEAs
• In the low stacking fault energy (SFE) in these
FCC-structured HEAs often results in the
dissociation of perfect dislocations into Shockley
Partials.
• Lower SFE materials display wider stacking faults
and have more difficulties for cross-slip

22. Dislocation Movement in HEAs
∆𝐸𝐷𝐴= Local potential energy for a dislocation segment in DiluteAlloy,
∆𝐸𝐻𝐴= Local potential energy for a dislocation segment in HEAs.
∆𝐸𝑃𝑀= Local Peierls potential energy
∆𝐸𝐿𝐷= Energy contribution of lattice distortion
∆𝐸𝑒𝑒= Extra Activation Energy due to local arrangement of different lead to change in bonding energy, and lattice strain
energy.
Conclusion- Additional strengthening effect
in high-entropy.

23. Twinning Mediated
TEM micrographs showing the evolution of twins
with the true tensile strain in the CoCrFeMnNi
alloy
• Another most common way of deformation in the
metals and alloys aside from slipping is the
mechanical twinning
• A clear transition is observed in the materials from
dislocation slip to mechanical twins when material is
deformed at higher a strain values
• The material shows these mechanical twins
formation only at critical strain level
𝜏𝑡𝑤𝑖𝑛 =
𝛾
𝐹𝑏
+
𝑘𝑇
𝑑
𝜏𝑡𝑤𝑖𝑛 = CRSS, γ = SFE, F = fitting parameter,
b =Burgers vector, 𝑘𝑇 =Hall-Patch constant, d = grain size

24. PhaseTransformation Mediating Deformation
TRIP behavior is shown with FCC to HCP transformation in
FeMnCrCo alloy
• Transformation-induced plasticity (TRIP) → extends the uniform plastic ductility by delaying the onset of necking, →
increasing overall toughness of the material.
• In the case of FeMnCrCo alloy,TRIP behavior results in the FCC to HCP transformation thus increasing the overall
toughness
Alloy Transformation Behavior
Fe20Co30Ni10Cr20Mn20 Cold-rolled, annealed transforms
from FCC→HCP
Fe60Co15Ni15Cr10 Cold-rolled, annealed, and
water-quenched transforms from
FCC→ BCC
Co20Cr20Fe34Mn20Ni6 Cold-rolled, and tempered, plus
water-quenched transforms from
FCC→HCP

25. Stacking faults Mediated
Bright-field TEM images of dislocation structures and
stacking faults (SFs) in the precipitation-strengthened
FeCoNiCrTi0.2 alloy after deformed to a true strain of (a)
~2.5%, (b) ~10%
• In low SFE materials, Stacking fault mediated
transformation is observed.
• Stacking faults (SFs) are important two dimensional crystal
defect impacting the mechanical and deformational
behavior of materials
• Distance between two SFs can be considered as the mean
free path for dislocation movement
• SFs help the strength of alloys due to the development of
intersecting stacking-fault bands
• SFs can prevent the planar dislocation glide, resulting in the
cross slip of dislocations and high work hardening

26. Conclusion
• In FCC HEAs slip of ½〈110〉dislocations and in BCC HEAs motion of screw dislocation
with b = a/2 〈111〉describes the deformation mechanism.
• In the low stacking fault energy (SFE) in these FCC-structured HEAs often results in the
dissociation of perfect dislocations into Shockley Partials.
• The material shows these mechanical twins formation only at critical strain level. From
slip at low strain rates to twin at high strain rates.
• Phase transformation mediated transformation extends the uniform plastic ductility by
delaying the onset of necking
• In low SFE stacking fault mediated transformation is observed and mechanical
properties are determined by these stacking faults.

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31. THANKYOU