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Crystal-Air surface PowerPoint PPT Presentation


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2D DEFECTS (Surface / Interface). Anti-phase Boundary. Crystal-Air surface. Interphase boundary. Crystal Boundary. Crystal-Crystal. Low angle. Grain boundary. High angle. Stacking Faults. Twin Boundary. 2D DEFECTS (Surface / Interface). Anti-phase Boundary. Alternate view.

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Crystal-Air surface

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2D DEFECTS(Surface / Interface)

Anti-phase Boundary

Crystal-Air surface

Interphaseboundary

Crystal Boundary

Crystal-Crystal

Low angle

Grainboundary

Highangle

Stacking Faults

Twin Boundary


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2D DEFECTS(Surface / Interface)

Anti-phase Boundary

Alternate view

Low angle

External

Grain boundary

Highangle

Rotation

Homophase

Twin Boundary

Mirror

Internal

Stacking Faults

Translation

Heteropase

(From lecture notes of Dr. Rajesh Prasad)


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Low angle

High angle

Based on angle of rotation

Twist

Homophase

Tilt

Based on axis

Mixed

Based on Lattice Models

Special

CSL

Random

Based on Geometryof the Boundary plane

Curved

Faceted

Mixed


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Coherent

Heteropase

Semi-coherent

Incoherent


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Surface Imperfections

  • 2D in a mathematical sense

  • The region of distortion is ~ few atomic diameters in thickness


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External surface of the crystal

  • External surfaces have energy related to the number of bonds brokenat the surface

No. of atoms/ unit area

Surface Energy/unit area (J/m2)

No. of bonds broken/ unit area

Bond energy / bond

As two surfaces are created / bond broken


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Interphaseboundary

Grainboundary


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Grain Boundary

  • The grain boundary region may be distorted with atoms belonging to neither crystal

  • The thickness may be of the order of few atomic diameters

  • The crystal orientation changes abruptly at the grain boundary

  • In an low angle boundary the orientation difference is < 10º

  • In the low angle boundary the distortion is not so drastic as the high-angle boundary → can be described as an array ofdislocations

  • Grain boundary energy is responsible for grain growth on heating~ (>0.5Tm)

  • Large grains grow at the expense of smaller ones

  • The average no. of nearest neighbours for an atom in the grain boundary of a close packed crystal is 11


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Grain boundaries in SrTiO3


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Low angle grain boundaries (misorientation < 10º)

Two extremes

TILT

An array of edge dislocations

TWIST

An array of screw dislocations

  • Rotation axis lies on the boundary plane

  • Rotation axis lies  to the boundary plane


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Low angle tilt grain boundary


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b

2h

Book


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No visible Grain Boundary

2.761 Å

Fourier filtered image

Dislocation structures at the Grain boundary

~8º TILT BOUNDARY IN SrTiO3 POLYCRYSTAL


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Twin Boundary

  • The atomic arrangement on one side of the twin boundary is related to the other side by a symmetry operation (usually a mirror)

  • Twin boundaries usually occur in pairs such that the orientation difference introduced by one is restored by the other

  • The region between the regions is called the twinned region

Annealing twins (formed during recrystallization)

Twin

Deformation twins (formed during plastic deformation)


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Twin boundary in Fe doped SrTiO3 bicrystals (artificially prepared)

High-resolution micrograph

Mirror related

variants

Twin plane

[1] S. Hutt, O. Kienzle, F. Ernst and M. Rühle, Z Metallkd, 92 (2001) 2


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Stacking Fault

  • Error in the sequence of stacking atomic planes → Stacking fault

  • Defined by a shift vector

…ABC ABC ABC ABC…

FCC stacking

FCC stacking with a stacking fault

…ABC AB AB ABC…

Thin region of HCP type of stacking

  • In above the number of nearest neighbours remains the samebut next-nearest neighbours are different than that in FCC

  • Stacking fault energy ~ 0.01 – 0.05 J/m2

  • Stacking fault in HCP can lead to thin region of FCC kind of stacking


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Comparison of Energy of Various 2D Defects


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Comparison of Interfacial Energies of Various 2D Defects


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