1. Grain Boundary Engineering and its Applications
Convener/Guide :
Dr. Aniruddha Moitra (SO/G)
MMS/MTD/MMG
Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu , India
By:
Aditya Narayan Singh
JRF: Metallurgy (HBNI)
MS: Nanoscience Nanotechnology
B.Tech: Electrical & Electronics

2. Outlook Introduction Grain Boundary Engineering CSL boundary
Grain Boundary (GB) & its Postulates
Grain Boundary Engineering (GBE)
Introduction
Grain Boundary Engineering
CSL boundary
Processing Methods for GBE
Case Studies Using Different GBE Methods
Applications of GBE in Ni Based Alloys
A note on GB Characterization Technique
Construction
Representation
TMT
Magnetic Annealing
Electrical Field
Unidirectional Solidification
Representation
Summary

3. Introduction to Grain Boundaries
Grain: The arrangement of crystals in a particular direction .
Grain Boundary: Planar 2D defect that separates regions of different crystalline orientation (i.e. grains) within a polycrystalline solid.
Postulates
Dislocation Model (Burgers and Braggs)
The Ke’s Model or disordered atom group model
Amorphous Cement Theory ( W. Rosenhain-Ewen)
Transition-Lattice Theory ( Hargreaves-Hill)
→ There are voids between the two crystals
→ There is a zone in which some atoms are held in both crystal lattices
→ There exist a zone of disorganized/ amorphous metal
Stress distributions around disordered group are different from other places.
Disordered group
In the absence of an applied stress, the displacement produced in a disordered group is at random directions.
Bending of the crystal
More the dislocation, larger is the bending proportion due to increase in dislocation density

4. Introduction to Grain Boundary Engineering
Deliberate manipulation of GB structure in order to improve material properties
Mechanism of GBE relies on prolific increasing the overall proportion of special
boundaries in a material
Which boundary produce special properties?
Grain Boundary
Low-energy type
High-energy type
Low angle boundaries and CSL
General random Boundaries (R)
Physical Phenomenon
Segregation of impurities
Aging induced precipitation
Mechanical Properties
Ductility
Creep damage
Shift in DBTT
Fracture resistance
Corrosion Properties
SCC, sensitization
Intergranullar corrosion
Σ3  60°<111>
Σ11  50°<110>
The term special boundary is usually used in connection with GBE. A special boundary means one that has better properties than an average boundary, and often CSLs are referred to as special boundaries.
Examples:

5. Grain Boundary Engineering
Grain boundary engineering (GBE) is the methodology by which the local grain boundary structure is characterized and material processing variables adjusted to create an optimized grain boundary microstructure for improved material performance.
Introducing grain boundary planes:
Low-index planes are always related to Σ (Low energy)
GBE include:
Increasing the Interface connectivity:
Propagation of intergranular degradation ceases.
Enhancing the fraction of CSL boundaries
Restrict the intergranular failure

6. Coincidence Site Lattice (CSL)
→ There are some boundaries that have special properties, e.g. low energy.
→ In most known cases (but not all!), these boundaries are also special with respect to their crystallography.
→ When a finite fraction of lattice sites coincide between the two lattices, then one can define a coincident site lattice (CSL).
→ A boundary that contains a high density of lattice points in a CSL is expected to have low energy because of good atomic fit.

7. Rotation to Coincidence: 1/2tanθ = (y/x)
Construction of CSL Boundaries
CSL values in axis/angle
Σ = n(h2+k2+l2) S
θ(°)
uvw 3 60
111 5
36.86
100 7
38.21 9
38.94
110 11
50.47
13a
22.62
13b
27.79 15
48.19
210
17a
28.07
17b
61.9
221
19a
26.53
19b
46.8
21a
21.78
21b
44.41
211 23
40.45
311
25a
16.26
25b
51.68
331
27a
31.59
27b
35.43
29a
43.6
If odd then n=1
(h2+k2+l2)
If even then n=0.5
Example: Possible planes for Σ3 – (111) , (112)
Σ5 – (210), (310)
David Brandon [ 1966: "The structure of high-angle grain boundaries”, Acta metallurgica 14: ] originated a criterion for proximity to a CSL structure. vm = v0S-1/2 where the proportionality constant, v0, is generally taken to be 15°, based on the low-to-high angle transition.
Rotation Angle ?
Rotation to Coincidence: 1/2tanθ = (y/x)

8. S5 relationship Red and Green lattices coincide
Points to be brought into coincidence

9. Rotating to the S5 relationship

10. Rotating to the S5 relationship

11. Rotating to the S5 relationship

12. Rotating to the S5 relationship

13. Rotating to the S5 relationship

14. Rotating to the S5 relationship

15. Rotating to the S5 relationship

16. Rotating to the S5 relationship

17. Rotating to the S5 relationship

18. Red and Green lattices coincide after rotation of
S5 relationship
Red and Green lattices coincide after rotation of
2 tan-1 (1/3) = 36.9°

19. Processing Methods for GBE Additional Processing Methods
The processing of GBE generally consists of repeated cycles of deformation and annealing, chosen so as to generate large fractions of “special boundaries”.
Proportion of special grain boundaries to 60%.
Thermal/Thermo-mechanical Treatment (TMT)
Applied during component forming/fabrication processes.
Increases Population of ‘Special’ Grain Boundaries
Reduces Average Grain Size
Enhances Microstructural Uniformity
Fully Randomizes Crystallographic Texture
Additional Processing Methods
Unidirectional & rotational solidification
Magnetic field application
Electric field application

20. Case Study: GBE Using TMT
Thermo-mechanical treatments to enhance S3
Iterative thermo-mechanical processing:
G 10% cold rolled 1,273 K for 0.5 h WQ 1
GG 10% cold rolled 1,273 K for 0.5 h WQ 2
GGG 10% cold rolled 1,273 K for 0.5 h WQ 3
GGGG 10% cold rolled 1,273 K for 0.5 h WQ 4
1st and 2nd iteration:
Strain accumulated at twin boundaries due to dislocation pile up.
Leads to increase internal stress
Also increase the driving force for GB migration
3rd iteration:
Subsequent strain increments leads to further increased driving force for GB migration.
As a result, boundaries move very rapidly through the microstructure
Annihilate few twins
4th iteration:
Less stored strain in lattice
Less driving force for grain boundary migration.
Boundaries move more slowly, at an intermediate velocity which is optimal for the nucleation and generation of annealing twins. M. Sumantra et al. J Mater Sci (2011) 46:275–284

21. Case Study: GBE Using Magnetic Annealing
Magnetic Annealing to Control Segregation induced Brittleness
Principle: When the difference in the magnetic susceptibility of one phase is > others, the driving force originating from the difference of the magnetization energy causes the migration of the interphase boundary.
Annealed without a magnetic field, segregation of Sn was almost 1.5 times higher than the concentration of Sn in the grain interior).
Magnetically annealed specimen, the segregation of Sn did not occur even at random boundary.

22. Case Studies: GBE Using Variation in Casting Process and
Electric Field Treatment
Unidirectional and Rotational Solidification
→ % pure silicon raw material was melted at 1773K in vacuum of 3.0x10-3 Pa.
→ After complete melting a crucible was moved down at a constant speed of 60mm/h.
→ During unidirectional solidification, the rotation of a crucible was applied with different rotational speed from non-rotation 0, to 90, 900 rotations per hour (rph).
Electric Field Treatment on Corrosion Behavior of a Ni-Cr-W-Mo Superalloy
→ With the rapid development of the aerospace engineering, the % of alloying elements is surged to meet demands & long-term service at elevated temperatures.
→ These alloying addition often brings complex microstructures , intermetallic and carbides at grain boundaries.
→ Carbides weaken the GB: sites now acts to promote crack initiation
→ Electric field treatment :coherent twin boundary :low energy

23. Case Studies: GBE Using Variation in Casting Process and
Electric Field Treatment
Anode: Nickel-based Ni-Cr-W-Mo/ Al2o3/ Cathode: Stainless steel
Set up was kept in a quartz tube+ heat source +N2 atmosphere
Electric field intensity was 1093 K for 0,120,300,600 min
→Formation of annealing twins (in Fig. b), is > normal ageing (in Fig.a)
→ GB energy decreased and the fraction of coherent boundaries increased.
→ The corrosion resistance of the alloy can be improved
Expln: The defects of vacancies and dislocation try to move towards grain boundary & forms a stack fault.
Stack fault exists stably to become the crystal nuclei of twins, because interfacial energy of coherent GB of twins is less than that of high angle GB.

24. Applications of GBE Lead-Acid Battery Electrodes
→Lead-acid batteries life is limited by the resistance of the positive electrodes (grids)
→ Intergranular corrosion contribute to weight loss
→ Creep-cracking promotes grid growth
Comparison of conventional Pb-Ca-Sn battery (a) vs. GBE-processed grid (b)
→ GBE-processed grids containing special grain boundary fractions in excess of 63% remain fully intact, conventional grids undergo a complete loss in structural integrity due to extensive through wall creep cracking and grain dropping (via intergranular corrosion)
→ GBE processing also improves reduction of grid thicknesses by 50% leading to a better enhanced life and durability.
G Palumboet.al. JOM 50, (1998).

25. Applications of GBE in Ni Based Alloys
Crack Propagation
Austenitic stainless alloy, Ni: 72, Cr:14-17, Fe:6-10, Cu:0.50, C:0.15, Mg: 1, S: 0.015, Si: 0.50
Alloy 600 (Ni-base alloy)
Several 3 boundaries with larger deviations from ideal misorientation cracked.
Gertsman et al., Acta Mater., 49 (9): (2001).

26. Applications of GBE in Ni Based Alloys
Creep Resistance
Inconel 600: Ni-16Cr
Standard Material
Grain Boundary Engineered
Constant load creep curves show dramatic improvement in creep resistance in GBE samples with a high fraction of CSL boundaries compared to samples with normal boundaries.
Dislocations (extrinsic grain boundary dislocations) accumulate in CSL boundaries giving rise to back stresses that oppose creep.
Mechanism:
Lin et al., Acta Metall. Mater., 41 (2): (1993).

27. Applications of GBE in Ni Based Alloys
Fatigue 2 4 6 8 1 A l o y 5 / k s i 7 3 V R m T e p r a t u F g P f n c S d N B C v G E
Cycles to Failure M
Alexandreanu, B. et al. (2003) Acta materialia

28. Grain Boundary Cracking
Example: Application of GBE
Grain Boundary Cracking
Cracking at grain boundaries in corrosion testing post-creep shows strong sensitivity to boundary type: CSL boundaries are less prone to corrosion attack.
V. Thaveepringsriporn et al. (1997) Metall. Trans. 28A: 2101.

29. GB Characterization Technique
GBE resulted in optimise
grain boundary and triple junction character distributions.
EBSD Crystal Orientation Mapping/Orientation Imaging Microscopy
Grids of orientation measurements are acquired over a selected area of a sample,
It allows the investigation of large surface areas
analysis of the microstructure concurrently with microtexture, to create a spatial map of crystal orientation.
Correlation of neighbouring grid orientation used for investigation of grain boundary characteristics.
Preceding GB Characterization Technique
X-ray and Neutron Diffraction
Transmission Electron Microscopy
High Resolution Electron Microscopy

30. Summary
Grain Boundary Engineering (GBE) is the practice of obtaining microstructures with a high fraction of boundaries with desirable properties
In general, desirable properties are associated with boundaries that have simple, low energy structures. Such low energy structures are, in turn, associated with CSL boundaries.
GBE generally consists of repeated cycles of deformation and annealing, chosen so as to generate large fractions of “special boundaries”.
Grain Boundary Engineering relies upon CSL analysis. By incorporating the CSL boundaries through GBE can enhance the properties of the structural materials.
Several properties viz.: segregation of impurities, intergranular corrosion, aging induced precipitation etc can be improved by the application of grain boundary engineering.

31. Thank You