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Sub-topics
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Processing for properties
Microstructure
Microstructure evolution during processing
Non-metals processing
HOW MANY OPTIONS?
Often a materials problem is really one of selecting
the material that has the right combination of
characteristics for a specific application.
Intrinsic properties
depend on
microstructure
and
microstructure
depends on processing
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HISTORICAL REVIEW OF
AN ALUMINUM BIKE
FRAME
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Materials processing
involves more than
one step
Each process step has
a characteristic
history
Designers should
watch out for
unintended side-effects
in the joining stage
Design focuses on the
properties of finished
products, but some of
these properties are
also critical during
In choosing materials for a component
processing
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it is important to examine their suitability for processing as
well as for performance in service.
METALS
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CERAMICS
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POLYMERS
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PHASES
Phase: region of a material with a specified
atomic arrangement
Phase diagram: maps showing the phases
expected as a function of composition
and temperature, if the material is in its
lowest free energy state
Phase transformation: occur when the phases
present change – requires a driving force
and a mechanism
Phase diagrams
show us when changes are possible.
Phase transformations
are central to microstructural control.
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THERMODYNAMICS OF PHASES
Microstructures can only evolve from one state
to another if it is energetically favorable to do so
The Gibbs free energy G helps to determine what
phase an alloy will be in at a given composition
held at a fixed temperature
U – intrinsic energy of the material
p – pressure
V – volume
U + pV – enthalpy
T – temperature
S – entropy
The state of lowest free energy
is the state of thermodynamic
equilibrium – at equilibrium no
phase change is physically possible
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PHASE DIAGRAM: LEAD-TIN SYSTEM
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The diagram divides up into single- and
two-phase regions, separated by grain
boundaries
•
At any point in the two-phase region, the
present phases are those found at the
phase boundaries at either end of the of a
horizontal tie-line through the point
defining the composition and temperature
concerned
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Both Pb and Sn will dissolve in one
another to some extent, with the maximum
solubility in both cases being at the same
temperature
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The pure elements have a unique melting
temperature
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Alloys show a freezing range between the
boundaries known as the liquidus and
solidus, so there is no longer a single
melting point
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At the eutectic point, the alloy can change
from 100% liquid to 100% two-phase solid
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COMMERCIAL CAST IRON
Pearlite - the lamellar structure in
carbon steels and cast irons that
consists of plates of pure iron and
iron carbide
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OPTICAL PHOTOMICROGRAPHS OF VARIOUS
CAST IRONS
Gray iron: the dark
graphite flakes are
embedded in an ferrite
matrix
Nodular (ductile) iron: the
dark graphite nodules are
surrounded by an ferrite
matrix.
Malleable iron: dark graphite rosettes
(temper carbon) in an ferrite matrix
White iron: the
light cementite regions
are surrounded by
pearlite, which has the
ferrite–cementite layered
structure
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KINETICS OF PHASE TRANSFORMATIONS
The boundaries on phase diagrams show where two
different states can both be in equilibrium.
At the melting point, both
phases have the same free
energy and can co-exist
Above the melting point,
liquid is the state of lower
free energy – if a liquid is
cooled beyond its melting
point, its free energy as a
liquid is greater than that of
a solid – the system can
release energy if it solidifies
– this is the driving force
for phase transformation
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RATE OF PHASE TRANSFORMATION
The rate of phase transformation depends on both
the driving force and the diffusion rate – both are
dependant on but have opposite reaction to temperature
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NUCLEATION AND GROWTH
When a liquid solidifies, solid first has to appear from
somewhere, after which the interface between solid
and liquid can migrate to enable atoms to switch from
one phase to the other at the boundary –
the two stages are nucleation and growth
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TIME-TEMPERATURE-TRANSFORMATION
(TTT) DIAGRAMS
TTT diagrams are used to study diffusion-controlled
transformation:
C-curves reflect the transformationrate as a function of
temperature;
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TTT curves assume rapid cooling from above the
transformation temperature
METALS PROCESSING
Most alloys offer both cast and wrought variants
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Metals with an HCP structure are primarily cast at
high temperatures because of their inherent lack of
ductility
Casting and wrought alloys in a given system tend to
have different compositions
Casting leads to coarser microstructures and poorer
strength and toughness than a wrought alloy
Good castability requires higher levels of alloying
additions than wrought alloys to lower the melting
temperature
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SOLIDIFICATION: METAL CASTING
In casting, a liquid above its melting point is poured into
a mold where it cools by thermal conduction – it is
relatively cheap and well suited for complex 3-d shapes
New solid forms by nucleation –
new crystals form in the melt, on
the walls of the mold, or on
foreign particles
Crystals grow in opposing
directions and impinge on
one another to form grain
boundaries
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DEFORMATION PROCESSING OF METALS
Deformation processes exploit the plastic response of
metals, that is, their ability to remain intact without damage
when subjected to large strains and shape changes
Deformation processes influence
Microstructure by various means:
• Temperature – determines the phases present and is
relevant during forming and during cool-down
• Grain size – forming process can strengthen the metal
by changing both the size and the shape of the grains
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FORMING PROCESSES
increasing the strength of metals often reduce their ductility
–
it is necessary to follow forming with an annealing heat
treatment to restore some of the materials ductility
Grain structure evolution through deformation and annealing
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MATERIAL ALLOYS USED FOR EURO
COINS
On January 1st, 2002 the euro became the single legal
currency in some European countries.
Euro coins are minted in
eight different
denominations:
2 and 1 euros, as well as
50, 20, 10, 5, 2, and 1 cent
euros.
Outer part: copper-nickel
Inner part: Layers of nickel
brass, nickel, nickel brass
Nordic gold - 89% copper, 5%
aluminium, 5% zinc, and 1% tin
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HEAT TREATMENT OF METALS
In a typical heat treatment, a component is
heated to high temperature,
cooled at a controlled rate, and
usually reheated to an intermediate temperature
The thermal profile and
resulting microstructure for a
heat treatment process
A solid solution is
formed at
high temperature
followed
by precipitation
hardening
at an intermediate
temperature
after being cooled
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EFFECTS OF HEAT TREATMENT ON
MECHANICAL PROPERTIES
Normalizing – slow cooling
from high temperature
producing a microstructure of
lower strength but high
toughness – no follow-up heat
treatment
Quench and temper –
cooled faster than the critical
cooling rate producing a
martensitic microstructure
with high strength and low
toughness – then
reheated at an intermediate
temperature to restore
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toughness
JOINING PROCESSES
Thermal welding of metals involves heating and cooling
which may cause phase transformations in the weld
metal and in the heated regions of the weld
A weld cross-section with
corresponding
thermal histories in the weld
metal and heat-affected zone.
Typical hardness profiles
induced across welds in
aluminum alloys, low carbon
steel, and low alloy steel
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SURFACE ENGINEERING
Surface treatments exploit many
different mechanisms and
processes to change the surface
microstructure and properties
Laser hardening is a surface treatment process
that modifies microstructure –
the traversing laser beam induces a rapid thermal cycle,
causing phase changes on both heating and cooling –
the track below the path of the laser has a
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different microstructure of high hardness
FERROUS METALS
Strength and toughness
tend to dominate the
property profile
desired for applications that
use ferrous metals
The range of mechanical
properties able to be
achieved through
alloying and processing
makes ferrous metals the
most versatile group of
engineering material
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