UNIT 1 CRYSTALLOGRAPHY
Introduction to Crystallography
Amorphous solids are homogeneous and isotropic because there is
no long range order or periodicity in their internal atomic arrangement.
By contrast, the crystalline state is characterized by a regular arrangement
of atoms over large distances. Crystals are therefore an isotropic
– their properties vary with direction.
For example, the inter atomic spacing varies with orientation within the crystal, as does the elastic
response to an applied stress.
Engineering materials are usually aggregates of many crystals of
varying sizes and shapes; these poly crystalline materials have properties
which depend on the nature of the individual crystals, but also
on aggregate properties such as the size and shape distributions of the
crystals, and the orientation relationships between the individual crystals.
The randomness in the orientation of the crystals is a measure of
texture, which has to be controlled in the manufacture of transformer
steels, uranium fuel rods and beverage cans.
The crystallography of interfaces connecting adjacent crystals can
determine the deformation behavior of the poly crystalline aggregate;
it can also influence the toughness through its effect on the degree of
segregation of impurities to such interfaces.
Crystallographic methods now depend on analysis of the diffraction patterns of a sample targeted by a beam of some type. X-rays are most commonly used; other beams used include electrons or neutrons. This is facilitated by the wave properties of the particles.
Crystallography often explicitly state the type of beam used, as in the terms X-ray crystallography, neutron diffraction and electron diffraction. These three types of radiation interact with the specimen in different ways.
- X-rays interact with the spatial distribution of electrons in the sample.
- Electrons are charged particles and therefore interact with the total charge distribution of both the atomic nuclei and the electrons of the sample.
- Neutrons are scattered by the atomic nuclei through the strong nuclear forces, but in addition, the magnetic moment of neutrons is non-zero. They are therefore also scattered bymagnetic fields. When neutrons are scattered from hydrogen-containing materials, they produce diffraction patterns with high noise levels. However, the material can sometimes be treated to substitute deuterium for hydrogen.
Lattice crystal :
The Lattice
Crystals have translational symmetry: it is possible to identify a
regular set of points, known as the lattice points, each of which has an
identical environment.
The set of these lattice points constitutes a three dimensional lattice.
A unit cell may be defined within this lattice as a space–filling
parallelepiped with origin at a lattice point, and with its edges defined
by three non-coplanar basis vectors a1
, a2
and a3
, each of which represents
translations between two lattice points.
The entire lattice can
then be generated by stacking unit cells in three dimensions. Any vector
representing a translation between lattice points is called a lattice
vector.
The unit cell defined above has lattice points located at its corners.
Since these are shared with seven other unit cells, and since each cell
has eight corners, there is only one lattice point per unit cell. Such a
unit cell is primitive and has the lattice symbol P.
Non–primitive unit cells can have two or more lattice points, in
which case, the additional lattice points will be located at positions
other than the corners of the cell. A cell with lattice points located at
the centres of all its faces has the lattice symbol F; such a cell would
contain four lattice points. Not all the faces of the cell need to have
face–centering lattice points; when a cell containing two lattice points
has the additional lattice point located at the centre of the face defined
by a2
and a3
, the lattice symbol is A and the cell is said to be A-centred.
B-centred and C-centred cells have the additional lattice point located
on the face defined by a3 & a1
or a1 & a2
respectively.
POINT DEFECT
Imperfections
in Solids
Introduction
Materials are often
stronger when they have defects. The study of defects is divided
according to their dimension:
0D (zero dimension) –
point defects: vacancies and interstitials. Impurities.
1D – linear defects:
dislocations (edge, screw, mixed)
2D – grain boundaries,
surfaces.
3D – extended defects:
pores, cracks.
Point Defects
Vacancies and Self-Interstitials
A vacancy is a lattice position that is vacant
because the atom is missing. It is created when the solid is formed. There are
other ways of making a vacancy, but they also occur naturally as a result of
thermal vibrations.
An interstitial is an atom that occupies a place
outside the normal lattice position. It may be the same type of atom as the
others (self interstitial) or an impurity atom.
In the case of vacancies and interstitials,
there is a change in the coordination of atoms around the defect. This means
that the forces are not balanced in the same way as for other atoms in the
solid, which results in lattice distortion around the defect.
The number of vacancies formed by thermal
agitation follows the law:
NV = NA × exp (-QV/kT)
where NA is the total number of atoms in the solid, QV is the energy required to form a vacancy, k is
Boltzmann constant, and T the temperature in Kelvin (note, not in oC or oF).
When QV is given in joules, k = 1.38 × 10-23 J/atom-K. When using eV as the unit of energy, k
= 8.62 × 10-5 eV/atom-K.
Note that kT(300 K) = 0.025 eV (room
temperature) is much smaller than typical vacancy formation energies. For
instance, QV(Cu) = 0.9 eV/atom. This means that NV/NA at room temperature is exp(-36) = 2.3 × 10-16,
an insignificant number. Thus, a high temperature is needed to have a high thermal concentration of vacancies. Even so, NV/NA is typically only about 0.0001 at the melting
point.
Impurities in Solids
All real solids are impure. A very high purity
material, say 99.9999% pure (called 6N – six nines) contains ~ 6 × 1016 impurities per cm3.
Impurities are often added to materials to
improve the properties. For instance, carbon added in small amounts to iron
makes steel, which is stronger than iron. Boron impurities added to silicon
drastically change its electrical properties.
Solid solutions are made of a host, the solvent
or matrix) which dissolves the solute (minor component). The ability to
dissolve is called solubility. Solid solutions are:
- homogeneous
- maintain
crystal structure
- contain
randomly dispersed impurities (substitutional or interstitial)
Factors for high solubility
- Similar
atomic size (to within 15%)
- Similar
crystal structure
- Similar
electro negativity (otherwise a compound is formed)
- Similar
valence
Composition can be expressed in weight percent,
useful when making the solution, and in atomic percent, useful when trying to
understand the material at the atomic level.
Miscellaneous
Imperfections
Dislocations—Linear Defects
Dislocations—Linear Defects
Dislocations are abrupt changes in the regular
ordering of atoms, along a line (dislocation line) in the solid. They occur in
high density and are very important in mechanical properties of material. They
are characterized by the Burgers vector, found by doing a loop around the
dislocation line and noticing the extra interatomic spacing needed to close the
loop. The Burgers vector in metals points in a close packed direction.
Edge dislocations occur when an extra plane is
inserted. The dislocation line is at the end of the plane. In an edge
dislocation, the Burgers vector is perpendicular to the dislocation line.
Screw dislocations result when displacing planes
relative to each other through shear. In this case, the Burgers vector is
parallel to the dislocation line.
Interfacial Defects
The environment of an atom at a surface differs
from that of an atom in the bulk, in that the number of neighbors
(coordination) decreases. This introduces unbalanced forces which result in relaxation (the lattice spacing is decreased) or reconstruction (the crystal structure changes).
The density of atoms in the region including the
grain boundary is smaller than the bulk value, since void space occurs in the
interface.
Surfaces and interfaces are very reactive and it
is usual that impurities segregate there. Since energy is required to form a
surface, grains tend to grow in size at the expense of smaller grains to
minimize energy. This occurs by diffusion, which is accelerated at high
temperatures.
Twin boundaries: not covered
Bulk or Volume
Defects
A typical volume defect is porosity, often
introduced in the solid during processing. A common example is snow, which is
highly porous ice.
Atomic Vibrations
Atomic vibrations occur, even at zero
temperature (a quantum mechanical effect) and increase in amplitude with
temperature.
POINT DEFECT
Point defects are
defects that occur only at or around a single lattice point. They are not
extended in space in any dimension. Strict limits for how small a point defect
is are generally not defined explicitly, typically; however, these defects
involve at most a few extra or missing atoms. Larger defects in an ordered
structure are usually considered dislocation loops. For historical reasons, many point defects,
especially in ionic crystals, are called centers: for example a
vacancy in many ionic solids is called a luminescence center, a color center,
or F-center. These dislocations permit ionic transport through crystals
leading to electrochemical reactions. These are frequently specified using Kröger–Vink Notation.
·
Vacancy defects are lattice sites which would be occupied in a perfect
crystal, but are vacant. If a neighboring atom moves to occupy the vacant site,
the vacancy moves in the opposite direction to the site which used to be
occupied by the moving atom. The stability of the surrounding crystal structure
guarantees that the neighboring atoms will not simply collapse around the
vacancy. In some materials, neighboring atoms actually move away from a
vacancy, because they experience attraction from atoms in the surroundings. A
vacancy (or pair of vacancies in an ionic solid) is sometimes called a Schottky defect.
·
Interstitial defects are atoms that occupy a site in the
crystal structure at which there is usually not an atom. They are generally
high energy configurations. Small atoms in some crystals can occupy interstices
without high energy, such as hydrogen in palladium.
Schematic illustration of some simple point defect types in a
monatomic solid
·
A nearby pair of a
vacancy and an interstitial is often called a Frenkel defect or Frenkel pair. This is caused when an ion moves into an
interstitial site and creates a vacancy.
·
Due to fundamental
limitations of material purification methods, materials are never 100% pure,
which by definition induces defects in crystal structure. In the case of an
impurity, the atom is often incorporated at a regular atomic site in the
crystal structure. This is neither a vacant site nor is the atom on an
interstitial site and it is called a substitutional defect.
The atom is not supposed to be anywhere in the crystal, and is thus an
impurity. In some cases where the radius of the substitutional atom (ion) is
substantially smaller than that of the atom (ion) it is replacing, its
equilibrium position can be shifted away from the lattice site. These types of
substitutional defects are often referred to as off-center ions. There are two different types of substitutional defects:
Isovalent substitution and aliovalent substitution. Isovalent substitution is
where the ion that is substituting the original ion is of the same oxidation
state as the ion it is replacing. Aliovalent substitution is where the ion that
is substituting the original ion is of a different oxidation state than the ion
it is replacing. Aliovalent substitutions change the overall charge within the
ionic compound, but the ionic compound must be neutral. Therefore, a charge
compensation mechanism is required. Hence either one of the metals is partially
or fully oxidised or reduced, or ion vacancies are created.
·
Antisite
defects occur in an ordered alloy or compound when
atoms of different type exchange positions. For example, some alloys have a
regular structure in which every other atom is a different species; for
illustration assume that type A atoms sit on the corners of a cubic lattice,
and type B atoms sit in the center of the cubes. If one cube has an A atom at
its center, the atom is on a site usually occupied by a B atom, and is thus an
antisite defect. This is neither a vacancy nor an interstitial, nor an
impurity.
·
Topological defects
are regions in a crystal where the normal chemical bonding environment is
topologically different from the surroundings. For instance, in a perfect sheet
of graphite (graphene) all atoms are in rings containing six atoms.
If the sheet contains regions where the number of atoms in a ring is different
from six, while the total number of atoms remains the same, a topological
defect has formed. An example is the Stone Wales defect in nanotubes, which consists of two
adjacent 5-membered and two 7-membered atom rings.
Schematic illustration of defects in a compound solid, using Ga As
as an example.
·
Also amorphous solids may contain defects. These are naturally somewhat
hard to define, but sometimes their nature can be quite easily understood. For
instance, in ideally bonded amorphous silica all
Si atoms have 4 bonds to O atoms and all O atoms have 2 bonds to Si atom. Thus
e.g. an O atom with only one Si bond (a dangling bond) can be considered a defect in silica. Moreover, defects can
also be defined in amorphous solids based on empty or densely packed local
atomic neighbourhoods, and the properties of such 'defects' can be shown to be
similar to normal vacancies and interstitials in crystals,.
·
Complexes can form
between different kinds of point defects. For example, if a vacancy encounters
an impurity, the two may bind together if the impurity is too large for the
lattice. Interstitials can form 'split interstitial' or 'dumbbell' structures
where two atoms effectively share an atomic site, resulting in neither atom
actually occupying the site.
Line defects
Line defects can be
described by gauge theories.
Dislocations are linear defects around which some of the
atoms of the crystal lattice are misaligned There are two basic types of
dislocations, the edge dislocation and the screwdislocation.
"Mixed" dislocations, combining aspects of both types, are also
common.
An edge dislocation is shown. The dislocation
line is presented in blue, the Burgers vector b in black.
Edge dislocations are
caused by the termination of a plane of atoms in the middle of a crystal. In
such a case, the adjacent planes are not straight, but instead bend around the
edge of the terminating plane so that the crystal structure is perfectly
ordered on either side. The analogy with a stack of paper is apt: if a half a
piece of paper is inserted in a stack of paper, the defect in the stack is only
noticeable at the edge of the half sheet.
The screw dislocation
is more difficult to visualise, but basically comprises a structure in which a
helical path is traced around the linear defect (dislocation line) by the
atomic planes of atoms in the crystal lattice.
The presence of
dislocation results in lattice strain (distortion). The direction and magnitude
of such distortion is expressed in terms of a Burgers vector (b). For an edge type, b is perpendicular to the
dislocation line, whereas in the cases of the screw type it is parallel. In
metallic materials, b is aligned with close-packed crystallographic directions
and its magnitude is equivalent to one interatomic spacing.
Dislocations can move
if the atoms from one of the surrounding planes break their bonds and rebond
with the atoms at the terminating edge.
It is the presence of
dislocations and their ability to readily move (and interact) under the
influence of stresses induced by external loads that leads to the
characteristic malleability of metallic materials.
Dislocations
can be observed using transmission
electron microscopy, field ion microscopy and atom probe techniques. Deep
level transient spectroscopy has been used for studying the
electrical activity of dislocations in semiconductors, mainly silicon.
Disclinations are line defects
corresponding to "adding" or "subtracting" an angle around
a line. Basically, this means that if you track the crystal orientation around the line defect, you
get a rotation. Usually, they were thought to play a role only in liquid
crystals, but recent developments suggest that they might have a role also in
solid materials, e.g. leading to the self-healing of crack.
Planar defects
Origin of stacking faults: Different stacking sequences of
close-packed crystals
·
Grain boundaries occur where the crystallographic
direction of the lattice abruptly changes. This usually occurs when two
crystals begin growing separately and then meet.
·
Antiphase
boundaries occur in ordered alloys: in this case, the crystallographic
direction remains the same, but each side of the boundary has an opposite
phase: For example, if the ordering is usually ABABABAB (hexagonalclose-packed crystal), an
antiphase boundary takes the form of ABABBABA.
·
Stacking faults occur in a number of crystal
structures, but the common example is in close-packed structures. They are formed by a local
deviation of the stacking sequence of layers in a crystal. An example would be
the ABABCABAB stacking sequence.
·
A twin boundary is a defect that introduces a plane of
mirror symmetry in the ordering of a crystal. For example, in cubic close-packed crystals, the
stacking sequence of a twin boundary would be ABCABCBACBA.
·
On surfaces
of single crystals, steps between atomically
flat terraces can also be regarded as planar defects. It has been shown that
such defects and their geometry have significant influence on the adsorption of
organic molecules[14]
·
three-dimensional
macroscopic or bulk defects, such as pores, cracks, or inclusions
·
Voids
— small regions where there are no atoms, and which can be thought of as
clusters of vacancies
·
Impurities
can cluster together to form small regions of a different phase. These are
often called
DEFORMATION
OF METAL
A temporary shape change that is self-reversing
after the force is removed, so that the object returns to its original shape,
is called elastic deformation.
In other words, elastic
deformation is a change in
shape of a material at low stress that is recoverable after the stress is
removed.
Elastic/Plastic Deformation
When a sufficient load is applied to a
metal or other structural material, it will cause the material to change shape.
This change in shape is called deformation. A temporary shape change that is
self-reversing after the force is removed, so that the object returns to its
original shape, is called elastic deformation. In other words, elastic
deformation is a change in shape of a material at low stress that is
recoverable after the stress is removed. This type of deformation involves
stretching of the bonds, but the atoms do not slip past each other.
When the stress is sufficient to
permanently deform the metal, it is called plastic deformation. As discussed in
the section on crystal defects, plastic deformation involves the breaking of a
limited number of atomic bonds by the movement of dislocations. Recall that the
force needed to break the bonds of all the atoms in a crystal plane all at once
is very great. However, the movement of dislocations allows atoms in crystal
planes to slip past one another at a much lower stress levels. Since the energy
required to move is lowest along the densest planes of atoms, dislocations have
a preferred direction of travel within a grain of the material. This results in
slip that occurs along parallel planes within the grain. These parallel slip
planes group together to form slip bands, which can be seen with an optical microscope.
A slip band appears as a single line under the microscope, but it is in fact
made up of closely spaced parallel slip planes as shown in the image.
Metal forming is the backbone of modern manufacturing
industry besides being a major industry in itself. Throughout the world
hundreds of million tons of metals go through metal forming processes every
year. As much as 15–20% of GDP of industrialized nations comes from metal
forming industry. Besides, it fulfils a social cause by providing job opportunities
to millions of workers. Metal forming industry, in general, is a bulk producer
of semi-finished and finished goods and this is one reason that it is viable to
undertake large scale research and development projects because even a small
saving per ton adds up to huge sums. In metal forming processes, the product
shapes are produced by plastic deformation. Hence it is important to know the
plastic flow properties of metals and alloys for optimizing the processes. Also
the resulting component properties depend upon the intensity and the conditions
of plastic deformation during forming. Many forming processes produce raw
materials for other processes which in turn produce finished or semi-finished
products.
For example, steel plants produce sheet metal which is used by automobile industry to manufacture components of automobiles and their bodies. In fact sheet metal is used by a number of manufacturers for producing a large variety of household and industrial products. Similarly billets produced by steel plants are used by re-rolling mills for rolling into products like angles, channels, bars etc. Bars may be further used for manufacturing forgings, wires, bright bars and machined products. Similarly the manufacturers of rivets, screws, bolts and nuts buy wire from wire manufacturers and process them further. Therefore, the producers of semi-finished materials such as sheet metal, bar stock and wires, etc. have to consider that they produce such properties in their products which are required by down stream industry engaged in further processing of these products.
For example, deep drawability of sheet metal increases with increase in anisotropy ratio (see Section 1.9), therefore, rolling parameters such as finishing temperature, cold reduction etc, are adjusted to produce higher anisotropy ratio in the sheet metal which is to be used for deep drawing. The properties of metals and alloys are highly influenced by their microstructure which may be modified or altered by alloying elements, by heating or heat treatment or by plastic deformation. For example, metals and alloys may be hardened by plastic deformation. It would, therefore, be helpful if we look at metals at the micro level.
UNIT 3 HEAT TREATMENT PROCESS
Heat
Treatment Processes
For example, steel plants produce sheet metal which is used by automobile industry to manufacture components of automobiles and their bodies. In fact sheet metal is used by a number of manufacturers for producing a large variety of household and industrial products. Similarly billets produced by steel plants are used by re-rolling mills for rolling into products like angles, channels, bars etc. Bars may be further used for manufacturing forgings, wires, bright bars and machined products. Similarly the manufacturers of rivets, screws, bolts and nuts buy wire from wire manufacturers and process them further. Therefore, the producers of semi-finished materials such as sheet metal, bar stock and wires, etc. have to consider that they produce such properties in their products which are required by down stream industry engaged in further processing of these products.
For example, deep drawability of sheet metal increases with increase in anisotropy ratio (see Section 1.9), therefore, rolling parameters such as finishing temperature, cold reduction etc, are adjusted to produce higher anisotropy ratio in the sheet metal which is to be used for deep drawing. The properties of metals and alloys are highly influenced by their microstructure which may be modified or altered by alloying elements, by heating or heat treatment or by plastic deformation. For example, metals and alloys may be hardened by plastic deformation. It would, therefore, be helpful if we look at metals at the micro level.
UNIT 3 HEAT TREATMENT PROCESS
Engineering
properties are modified by heat treatment processes so that structural
components are able withstand specified operating conditions and have desired
useful life.
· Heat treatment is
the heating and cooling of metals to change their physical and mechanical
properties, without letting it change its Heat
treatment could be said to be a method for strengthening materials but could
also be used to alter some mechanical properties such as improving formability,
machining, etc. The most common application is metallurgical but heat treatment
can also be used in manufacture of glass, aluminum, steel and many more
materials.
The process of heat treatment involves the use of heating or
cooling, usually to extreme temperatures to achieve the wanted result. It is
very important manufacturing processes that can not only help manufacturing
process but can also improve product, its performance, and its characteristics
in many ways.
Heat
Treatment Processes
Hardening
Hardening involves heating of steel, keeping it at an appropriate
temperature until all pearlite is transformed into austenite, and then
quenching it rapidly in water or oil. The temperature at which austentizing
rapidly takes place depends upon the carbon content in the steel used. The
heating time should be increased ensuring that the core will also be fully
transformed into austenite. The microstructure of a hardened steel part is
ferrite, martensite, or cementite.
Tempering
Tempering involves heating steel that has been quenched and hardened
for an adequate period of time so that the metal can be equilibrated. The
hardness and strength obtained depend upon the temperature at which tempering
is carried out. Higher temperatures will result into high ductility, but low
strength and hardness. Low tempering temperatures will produce low ductility,
but high strength and hardness. In practice, appropriate tempering temperatures
are selected that will produce the desired level of hardness and strength. This
operation is performed on all carbon steels that have been hardened, in order
to reduce their brittleness, so that they can be used effectively in desired
applications.
Annealing
Annealing involves
treating steel up to a high temperature, and then cooling it very slowly to
room temperature, so that the resulting microstructure will possess high
ductility and toughness, but low hardness. Annealing is performed by heating a
component to the appropriate temperature, soaking it at that temperature, and
then shutting off the furnace while the piece is in it. Steel is annealed
before being processed by cold forming, to reduce the requirements of load and
energy, and to enable the metal to undergo large strains without failure.
Normalizing
Normalizing involves heating steel, and then keeping it at that
temperature for a period of time, and then cooling it in air. The resulting
microstructure is a mixture of ferrite and cementite which has a higher strength
and hardness, but lower ductility. Normalizing is performed on structures and
structural components that will be subjected to machining, because it improves
the machinability of carbon steels.
Carburization
It is a heat treatment process in which steel or iron is heated to a
temperature, below the melting point, in the presence of a liquid, solid, or
gaseous material which decomposes so as to release carbon when heated to the
temperature used. The outer case or surface will have higher carbon content
than the primary material. When the steel or iron is rapidly cooled by
quenching, the higher carbon content on the outer surface becomes hard, while
the core remains tough and soft.
Surface Hardening
In many engineering applications, it is necessary to have the
surface of the component hard enough to resist wear and erosion, while
maintaining ductility and toughness, to withstand impact and shock loading.
This can be achieved by local austentitizing and quenching, and diffusion of
hardening elements like carbon or nitrogen into the surface. Processes involved
for this purpose are known as flame hardening, induction hardening, nitriding
and carbonitriding
PHASE DIAGRAM
IRON CARBON DIAGRAM
Introduction
Cooling curve for pure iron
Definition of structures
Iron-Carbon equilibrium phase diagram – Sketch
The Iron-Iron Carbide Diagram -
Ø The Austenite to ferrite / cementite
transformation
Ø Nucleation & growth of pearlite
Ø Effect of C %age on the
microstructure of steel
Ø Various phases that appear on the Iron-Carbon
equilibrium phase diagram are as under:
Ø Austenite
Ø Ferrite
Ø Pearlite
Ø Cementite
Ø Martensite*
Ø Ledeburite
Ø Relationship b/w C %age &
mechanical properties of steel
Ferrite is known as α solid solution.
Ø It is an interstitial solid solution
of a small amount of carbon dissolved in α (BCC) iron.
Ø stable form of iron below 912 deg.C
Ø The maximum solubility is 0.025 % C
at 723°C and it dissolves only 0.008
% C at room temperature.
Ø It is the softest structure that
appears on the diagram.
Pearlite is the eutectoid mixture containing 0.80 % C and is formed at
723°C on very slow cooling.
Ø It is a very fine platelike or
lamellar mixture of ferrite and cementite.
Ø The white ferritic background or
matrix contains thin plates of cementite (dark).
Cementite or
iron carbide, is very hard, brittle intermetallic compound of iron &
carbon, as Fe3C, contains 6.67 % C.
It is the hardest structure that
appears on the diagram, exact melting point unknown.
Its crystal structure is
orthorhombic.
It is has
q low tensile strength (approx. 5,000
psi), but
q high compressive strength.
Peritectic, at 1490 deg.C, with low wt% C alloys (almost no
engineering importance).
Eutectic, at 1130 deg.C, with 4.3wt%
C, alloys called cast irons.
Eutectoid, at 723 deg.C with eutectoid composition of 0.8wt%
C, two-phase mixture (ferrite & cementite). They are steels.
In order to understand the
transformation processes, consider a steel of the eutectoid composition. 0.8%
carbon, being slow cooled along line x-x‘.
At the upper temperatures, only
austenite is present, with the 0.8% carbon being dissolved in solid solution within
the FCC. When the steel cools through 723°C, several changes occur
simultaneously.
The net reaction at the
eutectoid is the formation of pearlitic
structure.
Since the chemical
separation occurs entirely within crystalline solids, the resultant structure
is a fine mixture of ferrite and cementite.
As the carbon-rich phase nucleates
and grows, the remaining austenite decreases in carbon content, again reaching
the eutectoid composition at 723°C.
This austenite transforms to pearlite
upon slow cooling through the eutectoid temperature.
The resulting structure consists of primary
cementite and pearlite.
The continuous network of primary
cementite will cause the material to be extremely brittle.
-Iron-Carbon alloys of 2.11%C or more
are cast irons.
-Typical composition:
2.0-4.0%C,0.5-3.0% Si, less than 1.0% Mn and less than 0.2% S.
n
-Si-substitutes
partially for C and promotes formation of graphite as the carbon rich component
instead.
Material science book by Michael F. Ashby, David R H Jones
Material science book by Michael F. Ashby, David R H Jones
