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8/13/2019 Solidification Heat Treatment
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Sp’ 05 W. Li
Solidification and HeatTreatment
Solidification
Crystal structures
Structure-property relationships
Heat treatment
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Pure Metal Solidification
• Temperature remains
constant while grains
grow.
• Some metals undergo
allotropic transformation
in solid state.
• For example on cooling
bcc -iron changes tofcc -iron at 1400 C,
which again to bcc -
iron at 906 C.
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Nucleation and Grain Growth
Nucleation;
Homogeneous nucleation: very pure metal, substantialundercooling (0.2Tm)
Heterogeneous nucleation: nucleation agents (5ºC
undercooling) Grain growth
Planar: pure metal
Dendritic: solid solution
Grain size
depends on number of nuclei and cooling rate.
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Crystal Nucleation and Growth
“ Manufacturing Processes for Engineering Materials,” by Serope Kalpakjian
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Crystal Structure of Metals
Atoms arrange themselves into various orderlyconfiguration, called crystals.
The arrangement of the atoms in the crystal is
called crystalline structure.
The smallest group of atoms showing the
characteristic lattice structure of a particular
metal is known as a unit cell.
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Crystal Structure of Metals
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Slip Systems
Deformation (dislocation) occurs on preferentialcrystallographic planes and directions, called slip
systems.
The slip plane/direction is the plane/direction with the
most closely packed atoms.
6x2=12 4x3=12 1x3=3
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Slip Systems
BCC has 6 slip planes and 2 slip directions per plane (12slip systems), but distance between slip planes is small,
therefore the required stress is high. Good Strength and
moderate ductility, e.g. Steel, Titanium, Molybdenum,
Tungsten. FCC has 4 slip planes and 3 slip directions per plane (12
Slip Systems), but distance between slip planes is larger
than BCC. Therefore, probability of slip is moderate,
shear stress to cause slip is low. Moderate Strength andGood Ductility, e.g., Aluminum, Copper, Gold, Silver
HCP has 1 slip plane and 3 slip directions on that plane
(3 systems). Low probability of slip. Generally brittle
materials, e.g., Beryllium, Magnesium, and Zinc
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Plastic Deformation of Single
Crystals
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Theoretical Shear Strength and
Tensile Strength
Theoretical shear stress is the shear stress tocause permanent deformation in a perfectcrystal.
Theoretical or ideal tensile strength of material is
the tensile stress required to break the atomicbonds between two neighboring atomic planes.
30/~10/2
max GGbetween
a
bG
10/max
E
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Solid Solutions
Most metals are not pure but contain a number of
other metallic or non-metallic elements, eitheralloying elements or contaminants. Alloying elements
are uniformly distributed in the base metal, forming a
solid solution.
Substitutional solid solution
Interstitial solid solution
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Effect of Imperfections
Pure metal: dislocation
Solid solutions
Solute atoms of slightly different size distort the lattice and makesdislocation propagation more difficult, thus strength increaseswithout necessarily reducing ductility.
Interstitial elements play a similar role in impeding dislocationmobility although they can have an embrittling effect.
Interfaces, inclusions, gases
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Grain Size Effect
Grain boundaries presentobstacles to dislocation
propagation. Therefore, it is
generally found that the yield
strength of a material
increase with decreasinggrain size according to the
Hall-Petch equation.
However at low strain rate
and close to Tm, dislocation
is resolved by diffusion.Material deforms by sliding
of grains or reshaping of
grains. Both processes are
easier if grain size is small.
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Phase Diagrams
• A phase diagram, also called equilibrium diagram or a constitutionaldiagram, graphically illustrates the relationships among temperature,
composition, and the phases present in a particular alloy system.
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Lever Rule
The composition of various phases in a phase
diagram can be determined by a procedure
called the lever rule.
Example: Calculate the relative proportions of
the phases in a Cu-Ag alloy of eutectic
composition just below the eutectic temperature.
L s
s
L s
L
C C
C C
LS
Lor C C
C C
LS
S
00
%2.232.919.7
2.919.71
C C
C C E
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The Structure of a Cu-Ag Solid
Solution with 20% Ag
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Iron/Iron Carbide Phase Diagram
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Nonequilibrium Solidification
Microsegregation or coring
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Heat Treatment
Most parts will require heat treatment either after orduring the processing for proper in-service properties
Annealing
Heat to elevated temp, hold, cool
Softens the material and removes stress
Precipitation Hardening
Diffusion of alloys to produce two phase structure that
promote good strength and ductility
(Aging – Aluminum for example)
Heat Treatment of Steel
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Heat Treatment of Steel (TTT
Diagram)
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Summary
Solidification process affects crystal structures
which in turn affect material properties.
Single crystal materials behave very differently
than metal alloys.
The effect of imperfections and grain size in
solid solutions.
Heat treatment can modify material properties by
changing the crystal structure.