metallurgical structures
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METALLURGICAL STRUCTURES
Ilyas (15006)Mithun (15010)
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Grains
Crystal Structure
Basic Metallurgy
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X
Y
Z
Space Lattice: A collection of points that divided space into smaller sized segments.
Unit Cell: A subdivision of the lattice that still retains the overall characteristics of the entire lattice.
Crystal Structure(Atomic Arrangement)
Basic Metallurgy
Atom
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Formation of Polycrystalline Material
Liquid
a b
c d
Solid (Unit Cell)
Grain Boundaries
a) Small crystalline nuclei b) Growth of Crystalsc) Irregular grain shapes formed upon completion of solidification
d) Final grain structure
Grain Boundary: The zone of crystalline mismatch between adjacent grains. The lattice has different orientation on either side of the grain boundary
Basic Metallurgy
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Grain Boundary
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BCC - Delta Iron (d)
FCC - Gamma Iron (g)
BCC - Alpha Iron (a)
Tem
p er a
ture
1540 oC
1400 oC
910 oC
Atomic Packing in Iron (Allotropic)
Basic Metallurgy
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Body Centered Cubic (BCC)
Alpha & Delta Iron (a , d)Total 2 Atoms/Unit Cell
α Lattice Parameter (a) = 0.287 nmδ Lattice Parameter (a) = 0.293 nm
a
Squared Packed Layer
Basic Metallurgy
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Face Centered Cubic (FCC)
Gamma Iron (g)Total 4 Atoms/Unit Cell
g Lattice Parameter (a) = 0.359 nm
a
Close Packed Layer
Basic Metallurgy
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1) Point defects: a) vacancies, b) interstitial atoms, c) small substitional atoms, d) large substitional atoms, … etc.
2) Surface defects: Imperfections, such as grain boundaries, that form a two-dimensional plane within the crystal.
Microstructural DefectsBasic Metallurgy
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3) Line defects: dislocations (edge, screw, mixed)
Dislocation: A line imperfection in the lattice or crystalline material
Movement of dislocations helps to explain how materials deform. Interface with movement of dislocations helps explain how materials are strengthened.
They are typically introduced into the lattice during solidification of the material or when the material is deformed.
Microstructural DefectsBasic Metallurgy
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Motion of Dislocation
When a shear stress is applied to the dislocation in (a), the atoms displaced, causing the dislocation to move one step (Burger’s vector) in the slip (b). Continued movement of the dislocation eventually creates a step (deformation) direction (C)
Basic Metallurgy
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STEEL = IRON + Alloying Elements ( C + Mn, Si, Ni, …)
IRON + < 2 % Carbon = STEEL
IRON + > 2 % Carbon = CAST IRON
What is the difference between “STEEL” and “CAST IRON” ?
Basic Metallurgy
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Atomic Packing in Iron (Allotropic)
BCC - Delta Iron (d)
FCC - Gamma Iron (g)
BCC - Alpha Iron (a)
Basic Metallurgy
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Tranformations
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TTT Diagram
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Basic Metallurgy
Weight Percentage Carbon
Tem
pera
ture
(o
C)
( g + Fe3C )
( g + L )
Austenite (g )
Liquid (L)15401495
727 °C
910
Cementite (Fe3C)+ Pearlite
( a + g )
Ferrite + Pearlite
Ferrite (a )
1000 -
1200 -
1400 -
1600 -
1.0
800 -
2.0
400 -
600 -
200 -
0 -0.8%
Eutectoid
Delta Ferrite ( d )
( d + g )Peritectic
( d + L )
1150 °C
FerriteCementite
~0% C
0.2% C
0.35% C
0. 5% C
0. 7% C
0. 8% C
1.2% C
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Microstructures in Fe-C Diagram
MartensiteGray Cast Iron
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Questions..?
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Thank you..!