Material science Dr. Mohammad Sobuh. Material science involves investigating the relationships that...
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Material science Dr. Mohammad Sobuh. Material science involves investigating the relationships that exist between the structures and properties of materials
Material science involves investigating the relationships that
exist between the structures and properties of materials. Material
engineering is to create new products or systems using existing
materials, and/or to develop techniques for processing
materials.
Slide 4
The structure of a material is the arrangement of internal
components of the material. Structure levels: Subatomic structure
(electrons and nucleus interactions). Atomic structure
(organisation of atoms/ molecules relative to one another).
Microscopic structure (atoms agglomerated together and can be
observed by a microscope). Macroscopic structure (structure viewed
with the naked eye). Material property is a material trait in terms
of the kind and magnitude of response to a specific imposed
stimulus ( ex. forging metal/glass)
Slide 5
A single crystal Aluminium oxide may be transparent,
translucent, or opaque depending on the martial structure
Polycrystal: low porosity Polycrystal: high porosity
Slide 6
Properties of solid materials: Mechanical Electrical Thermal
Magnetic Optical Deteriorative
Slide 7
Classification of materials Based on chemical makeup and atomic
structure materials can be subdivided into: Metals. Ceramics.
Polymers. Composite. Advanced materials
Slide 8
Metals Contain one or more metallic elements and often also
non-metallic elements in a small amount (alloys). Atoms are
arranged in orderly manner, thus metals are usually dense. Are
relatively stiff and strong yet are ductile and resistant to
fracture. Metallic materials have large numbers of non- localized
electrons. Are extremely good conductors of electricity and heat
and are not transparent to visible light.
Slide 9
Slide 10
Of the 110 elements, 89 are classified as metals. Nonmetallic
elements: Oxygen, Chlorine, Iodine, Bromine, Nitrogen, Fluorine
Hydrogen, Phosphorus, Astatine. The inert gases: helium, neon,
argon, krypton, xenon, and radon. Intermediate elements
(metalloids): Boron, Carbon, Sulphur, Silicon, Arsenic, Germanium,
Tellurium
Slide 11
The most widely used metallic elements: Iron, copper, lead,
zinc, aluminium, tin, nickel, and magnesium. Metals can be used in
the pure state, but by far the largest amount is used in the form
of alloys. An alloy is a combination of elements that exhibits the
properties of a metal. The properties of alloys differ appreciably
from those of the underlying elements: Improvement of strength,
ductility, hardness, wear resistance, and corrosion resistance
Slide 12
Ceramics Compounds between metallic and non-metallic elements;
most frequently oxides, nitrides, and carbides (aluminium oxide (or
alumina)). Relatively stiff, strong (like metals), and very hard.
However, they are extremely brittle (lack ductility) and highly
subjected to fracture. Typically, insulative to the passage of heat
and electricity, and are more resistant to high temperatures and
harsh environments than metals and polymers. The traditional
ceramics: Clay minerals (i.e., porcelain), cement, and glass.
Slide 13
Polymers Include the familiar plastic and rubber materials.
Chain-like very large molecular structures, with carbon atoms as
backbone. Consist of organic compounds (i.e. carbon, hydrogen), and
other non-metallic elements (O,N, and Si). Low densities, and are
not as stiff nor as strong as metals and ceramics but they are
extremely ductile and pliable. They are relatively inert chemically
and un-reactive, however they have tendency to soften and/or
decompose at modest temperatures. Familiar polymers: Polyethylene
(PE), nylon, poly(vinyl chloride) (PVC), polycarbonate (PC),
polystyrene (PS), and silicone rubber. Polyethylene
Slide 14
Composites Composed of two (or more) individual materials
(metals, ceramics, and polymers). Used to: Achieve a combination of
properties that is not displayed by any single material.
Incorporate the best characteristics of each of the component
materials. Examples: Bone, wood and fibreglass, carbon fibres.
Slide 15
Fibreglass Small glass fibres are embedded within a polymeric
material (normally an epoxy or polyester). The glass fibres are
relatively strong and stiff, but also brittle. The polymer is
ductile, but also weak and flexible and has low density. The
resulting fibreglass is relatively stiff, strong, flexible, ductile
and it has a low density.
Slide 16
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Slide 21
ADVANCED MATERIALS Materials that are utilized in
high-technology applications. Advanced materials are typically:
Traditional materials whose properties have been enhanced. Newly
developed high-performance materials. They may be of all material
types (e.g., metals, ceramics, polymers), and are normally
expensive. Examples: liquid crystal displays (LCDs), and fibre
optics Categories: Semiconductors: have electrical properties that
are intermediate between the electrical conductors (viz. metals and
metal alloys) and insulators (viz. ceramics and polymers).
Biomaterials: materials employed in components implanted into the
human body for replacement of diseased or damaged body parts. Smart
Materials: materials are able to sense changes in their
environments and then respond to these changes in predetermined
manners. Example: Memory alloys
Slide 22
Properties of solid materials Mechanical, electrical, thermal,
magnetic, optical, deteriorative. The mechanical behavior of a
material reflects the relationship between its response or
deformation to an applied load or force. Deformation: Shaping
materials by applying forces by various means. The term plastic
relates to permanent changes, whereas elastic relates to reversible
changes.
Slide 23
Strain: The degree of deformation to which the material is
subjected. Types of strain: tensile, compressive, shear.
Stress
Slide 24
Important mechanical properties: Strength, ductility,
stiffness. Proportional (linear) change Yield point (Hookes law)
Modulus of resilience
Slide 25
Engineering stress ()= Applied load (P)/ original
cross-sectional area (A 0 ) Modulus of elasticity or Youngs modulus
(E)= /e. and E have a unit of pressure; Pascal (Pa or N/m or m 1
kgs 2 ) E is a measure of the stiffness of an elastic material.
Stiffness is the rigidity of an object - the extent to which it
resists deformation in response to an applied force.
Slide 26
Poissons ratio (v): The absolute value of the ratio of the
lateral strain to longitudinal strain.
Slide 27
Modulus of resilience: The area under stress- strain curve up
to the yield point Y. Resilience is the ability of a material to
absorb energy when it is deformed elastically, and release that
energy upon unloading. Modulus of resilience =Y 2 /2E. Modulus of
resilience has the units of energy per unit volume (N-m/m 3 ). In
words Modulus of resilience is the specific energy that the
material can store.
Slide 28
With increasing load beyond Y: Plastic deformation begins to
take place. the relationship between stress and strain is no longer
linear. Yield strength: is the stress at Y point. Yielding does not
necessarily mean failure; yielding is necessary in metalwork
(forging and bending).
Slide 29
Yield strength is a measure of its resistance to plastic
deformation. Increasing load beyond Y results first in a continuous
elongation in which the A 0 decreases permanently and uniformly
through the whole length.
Slide 30
Slide 31
Further elongation requires increased load (thus stress), up to
a maximum stress point after which less stress is required for
further deformation. The maximum stress is known as ultimate
tensile strength (UTS). UTS is a practical measure of the overall
strength of a material. Elongation before reaching UTS is
uniformed. Strain up to the UTS is called uniform strain. Increased
load beyond the UTS results in a change in the A 0 that is no
longer uniformed but is concentrated locally in a neck.
Slide 32
Following necking, the stress required for elongation deceases
further. Finally, fracture happens within the necked region.
Breaking or fracture stress is the final stress level in the
stress/strain curve at which fracture takes place. Ductility is how
large a strain the material withstands before fracture. A material
that experiences A material that experiences very little or no
plastic deformation upon fracture is termed brittle. Strain at
fracture is a measure of ductility. The elongation at fracture is
the total elongation.
Slide 33
Two quantities are used to defined ductility: Percent
Elongation. Percent Reduction of area. Elongation = (l f -l 0 )/l 0
x 100 Reduction of area = (A 0 -A f )/A 0 x100 Elongation
percentage has a inversed relationship with the original length.
Toughness is a mechanical term that is a measure of the ability of
a material to absorb energy up to fracture. It is the area under
the stressstrain curve up to the point of fracture. Hardness is a
measure of a materials resistance to localized plastic deformation
(e.g., a small dent or a scratch)
Slide 34
Engineering stressstrain behavior for iron at three
temperatures.
Slide 35
Crystal Structures Solid materials may be classified according
to the regularity with which atoms or ions are arranged with
respect to one another into: Crystaline materials Non-Crystaline
materials A crystalline material is one in which the atoms are
positioned in a repetitive 3D pattern, in which each atom is bonded
to its nearest-neighbour atoms. Crystalline structure: the manner
in which atoms, ions, or molecules are spatially arranged. All
metals form crystalline structures under normal solidification
conditions. Some of the properties of crystalline solids depend on
the crystal structure of the material.
Slide 36
When describing crystalline structures: Atoms (or ions) are
thought of as being solid spheres having well-defined diameters.
Spheres touch one another. Lattice means a three-dimensional array
of points coinciding with atom positions (or sphere centres). Unit
cells: Small groups of atoms form a repetitive pattern within the
crystal. All the atom positions in the crystal may be generated by
translations of the unit cell integral distances along each of its
edges. The unit cell is the basic structural unit or building block
of the crystal structure. Atomic hard sphere model Crystal Unit
cell
Slide 37
METALLIC CRYSTAL STRUCTURES Three relatively simple crystal
structures are found for most of the common metals: Face-centred
cubic, body-centred cubic, and hexagonal close-packed. The
Face-Centred Cubic Crystal Structure (FCC): A unit cell of cubic
geometry, with atoms located at each of the corners and the centres
of all the cube faces.
Slide 38
In the FCC, spheres or ion cores touch one another across a
face diagonal. The cube edge length (unit cell length) a and the
atomic radius R are related through: For the FCC crystal structure:
Each corner atom is shared among eight unit cells. A face-centred
atom belongs to only two. One-eighth of each of the eight corner
atoms and one-half of each of the six face atoms, or a total of
four whole atoms, may be assigned to a given unit cell. Some of the
familiar metals with FCC crystal structure: Copper, aluminium,
silver, and gold.
Slide 39
Coordination number: The number of nearest- neighbour or
touching atoms. For metals, each atom has the same coordination
number. In FCC structure, the coordination number is 12. Atomic
packing factor (APF): is the fraction of volume in a crystal
structure that is occupied by atoms. APF = the sum of the sphere
volumes of all atoms within a unit cell/ the unit cell volume. For
the FCC structure, APF is 0.74, which is the maximum packing
possible for spheres all having the same diameter. Metals typically
have relatively large atomic packing factors
Slide 40
The Body-Centred Cubic Crystal Structure (BCC): Has a cubic
unit cell with atoms located at all eight corners and a single atom
at the cube centre. Centre and corner atoms touch one another along
cube diagonals. Unit cell length a and atomic radius R are related
through:
Slide 41
Examples of metals with BCC structure: Chromium, iron (),
tungsten, and tantalum. Two atoms are associated with each BCC unit
cell. The coordination number for the BCC crystal structure is 8.
The atomic packing factor for BCC is 0.68.
Slide 42
The Hexagonal Close-Packed Crystal Structure (HCP): The top and
bottom faces of the unit cell consist of six atoms that form
regular hexagons and surround a single atom in the centre. Another
plane that provides three additional atoms to the unit cell is
situated between the top and bottom planes.
Slide 43
The equivalent of six atoms is contained in each unit cell:
One-sixth of each of the 12 top and bottom face corner atoms.
One-half of each of the 2 centre face atoms All 3 midplane interior
atoms. If a and c represent, respectively, the short and long unit
cell dimensions, the c/a ratio should be 1.633. However, for some
HCP metals this ratio deviates from the ideal value. Coordination
number and the atomic packing factor for the HCP crystal structure
are the same as for FCC. The HCP metals include: Cadmium,
magnesium, titanium (), zinc and cobalt.
Slide 44
DENSITY COMPUTATIONS A knowledge of the crystal structure of a
metallic solid permits computation of its theoretical density ()
through: Where: n = number of atoms associated with each unit cell.
A = atomic weight V C = volume of the unit cell N A = Avogadros
number (6.023 10 23 atoms/mol)
Slide 45
Polymorphisms Some metals may have more than one crystal
structure, a phenomenon known as polymorphism. In elemental solids,
the condition is often termed allotropy. Crystal structure depends
on both the temperature and the external pressure Pure iron has a
BCC crystal structure at room temperature, which changes to FCC
iron at 912C. A modification of the density and other physical
properties accompanies a polymorphic transformation.
Slide 46
CRYSTAL SYSTEMS Crystal can be divided into groups according to
unit cell configurations (unit cell geometry) and/or atomic
arrangements. In the unit cell geometry based system, the shape of
the appropriate unit cell parallelepiped without regard to the
atomic positions in the cell. The unit cell geometry is completely
defined in terms of six parameters (sometimes termed the lattice
parameters): The three edge lengths a, b, and c. The three
interaxial angles ,,and .
Slide 47
There are seven different possible combinations of a, b, and c,
and ,,and ; each of which represents a distinct crystal system. The
cubic system, for which and has the greatest degree of symmetry.
Least symmetry is displayed by the triclinic system, since a b c
and .
Slide 48
Slide 49
Slide 50
Crystallographic Points, Directions, and Planes The basis for
determining index values is the unit cell, with a right-handed
coordinate system consisting of three (x, y, and z) axes situated
at one of the corners and coinciding with the unit cell edges.
Slide 51
POINT COORDINATES: The position of any point located within a
unit cell may be specified in terms of its coordinates as
fractional multiples of the unit cell edge lengths The position of
P is designated using coordinates q r s with values that are less
than or equal to unity. Coordinates are not separated by commas or
any other punctuation marks
Slide 52
Slide 53
CRYSTALLOGRAPHIC DIRECTIONS Crystallographic direction is
defined as a line between two points, or a vector. The following
steps are utilized in the determination of the three directional
indices: 1.A vector of convenient length is positioned such that it
passes through the origin of the coordinate system. Any vector may
be translated throughout the crystal lattice without alteration, if
parallelism is maintained. 2.The length of the vector projection on
each of the three axes is determined; these are measured in terms
of the unit cell dimensions a, b, and c. 3.These three numbers are
multiplied or divided by a common factor to reduce them to the
smallest integer values. 4.The three indices, not separated by
commas, are enclosed in square brackets, thus: [uvw]. The u, v, and
w integers correspond to the reduced projections along the x, y,
and z axes, respectively. 5.For each of the three axes, there will
exist both positive and negative coordinates. Negative indices are
represented by a bar over the appropriate index ex.
Slide 54
Lets derive the following directions: [010], [011], [012],
[10], [212], [021]
Slide 55
To draw the direction vector 1.Draw x, y and z axes 2.Determine
the origin as the tail of your direction vector (if there is a
negative number in the direction vector move the origin in the
positive direction of this axis). 3.Determine the endpoint of the
vector (remove the a common factor if any of the indices are larger
than 1)
Slide 56
Slide 57
For some crystal structures, several nonparallel directions
with different indices are actually equivalent. For example, in
cubic crystals, all the directions represented by the following
indices are equivalent: [100], [010], [00], [001], [00]. The
indices are equivalent because the spacing of atoms along each
direction is the same. Equivalent directions are grouped together
into a family, which are enclosed in angle brackets 100. directions
in cubic crystals having the same indices without regard to order
or sign, for example, [123] and [23 ], are equivalent.
Slide 58
CRYSTALLOGRAPHIC PLANES In all but the hexagonal crystal
system, crystallographic planes are specified by three Miller
indices as (hkl). Each index denotes a plane orthogonal to a
direction [hkl]. Any two planes parallel to each other are
equivalent and have identical indices.
Slide 59
Determining indices from a given plane 1.Choose origin (O) in
unit cell OR a new O with axes x, y, z. 2.Find intercepts from O to
(a) intercept; from O to (b) int.; from O to (c) int. 3.Take
reciprocals of intercept position 4.Clean up: a.Reduce multiples;
b.Eliminate fractions; c.Put above negative integer in indices;
d.Place round brackets around integers.
Slide 60
1. Choose Origin Unit Cellabc 2. Intercept 3. Reciprocal 4.
Reciprocal Value 5. Miler indices (001) (110) (111) (???) 1.Plane
cannot touch the origin. 2.Choose the closest corner. (121)
Slide 61
Slide 62
Determining plane from a given indices 1.Select origin O at 000
Or if there is nigatives at O xyz. 2.Take the reciprocal of the
indices which will be the intercepts. 3.Mark intercepts along x,y,z
axes
Slide 63
Plot the plane of the given indices: (011) (100) 1. Choose
Origin 2. Reciprocal 3. Reciprocal Value Unit Cellabc 4. Mark
intercepts 5. Draw
Slide 64
Atomic Arrangements The atomic arrangement for a
crystallographic plane depends on the crystal structure. A family
of planes contains all those planes that are crystallographically
equivalent; having the same atomic packing. A family is designated
by indices that are enclosed in braces {}. (110) Reduced-sphere
unit cells
Slide 65
For example, in cubic crystals (111), (111), (111), (111),
(111), (111), (111), (111) planes all belong to the {111} family.
In the cubic system only, planes having the same indices,
irrespective of order and sign, are equivalent. For example, both
(123) and (312 ) belong to the {123} family.
Slide 66
How the crystal structures arrive (Stacking sequence) (52%)
(68%) = = (74%) A A A A A B A A CB A C B
Slide 67
FCC HCP
Slide 68
LINEAR AND PLANAR DENSITIES Linear and planar densities are
important considerations relative to the process of slip. Slip is
the mechanism by which metals plastically deform. Slip occurs on
the most densely packed crystallographic planes and, in those
planes, along directions having the greatest atomic packing.
Slide 69
Linear density (LD) is the number of atoms per unit length
whose centres lie on the direction vector for a specific
crystallographic direction; that is The units of linear density are
reciprocal length i.e. nm -1
Slide 70
Example Determine the linear density of the [110] direction for
the FCC crystal structure. There is an equivalence of two atoms
along the [110] direction vector in the unit cell. The direction
vector length is equal to 4R LD 110 = 2/4R = 1/2R atom 1 atom
Slide 71
Planar density (PD) is taken as the number of atoms per unit
area that are centred on a particular crystallographic plane, or
The units for planar density are reciprocal area i.e. nm -2
Slide 72
Example Determine the planner density of the direction shown in
the following figure for the FCC crystal structure. 2 atoms in the
plane Length = 4R Width= (110)
Slide 73
Crystallisation Single crystal: All unit cells interlock in the
same way and have the same orientation. Single crystal has regular
geometric shape having flat faces.
Slide 74
Polycrystalline: A collection of many small crystals (grains).
Grain boundary: The region where two crystallographically
mismatched grains meet (see (3) in the following slide). Sometimes
the grains in polycrystalline materials have a preferential
crystallographic orientation (not randomly oriented); this material
is said to have a texture.
Slide 75
Stages of solidification Small crystals or nuclei form at
various positions in random crystallographic orientations. The
small grains grow by the successive addition from the surrounding
liquid of atoms to the structure of each grain. The extremities of
adjacent grains impinge on one another as the solidification
process approaches completion. 1 2 3
Slide 76
ANISOTROPY The physical properties of single crystals of some
substances depend on the crystallographic direction in which
measurements are taken. This is due to the variance of atomic or
ionic spacing with crystallographic direction. For example, the
elastic modulus and the electrical conductivity may have different
values in the [100] and [111] directions. This directionality of
properties is termed anisotropy
Slide 77
Isotropic substances: are these in which measured properties
are independent of the direction of measurement. The degree of
anisotropy increases with decreasing structural symmetry. Triclinic
vs. cubic structure.
Slide 78
Imperfections in Solids Perfect order throughout crystalline
materials on an atomic scale does not exist. All contain large
numbers of various defects or imperfections. Crystalline defect is
referred to the lattice irregularity at one or more of its
dimensions. Classification of crystalline imperfections (based on
geometry or dimensionality of the defect): Point defects: those
associated with one or two atomic positions). Linear defects:
one-dimensional defects interfacial defects: two-dimensional
defects
Slide 79
Point Defects The simplest of the point defects is a vacancy
(vacant lattice site). All crystalline solids contain vacancies.
The equilibrium number of vacancies (N v ) for a given quantity of
material depends on and increases with temperature (exponential).
Self-interstitial: the presence of an extra atom in an otherwise
complete lattice.
Slide 80
In metals, a self-interstitial introduces relatively large
distortions in the surrounding lattice.
Slide 81
IMPURITIES IN SOLIDS A pure metal consisting of only one type
of atom just is not possible! Impurity (existence of foreign atoms)
will always be present. Most familiar metals are not highly pure;
rather, they are alloys. In alloys impurity atoms have been added
intentionally to impart specific characteristics to the material.
The addition of impurity atoms to a metal will result in the
formation of a solid solution.
Slide 82
Solvent represents the element or compound that is present in
the greatest amount. Solvent atoms are also called host atoms.
Solute is used to denote an element or compound present in a minor
concentration. A solid solution forms when the solute atoms are
added to the host material. In solid solution, the crystal
structure is maintained, and no new structures are formed. Solid
solution is homogeneous (the impurity atoms are randomly and
uniformly dispersed within the solid).
Slide 83
Impurity point defects: Substitutional solid solutions,: Solute
or impurity atoms replace or substitute for the host atoms.
Interstitial solid solutions: impurity atoms fill the voids or
interstices among the host atoms.
Slide 84
Features of the solute and solvent atoms that determine the
degree to which the solute dissolves: 1.Atomic size factor:
Appreciable quantities of a solute may be accommodated in the solid
solution only when the difference in atomic radii between the two
atom types is less than about 15%. 2.Crystal structure: For
appreciable solid solubility the crystal structures for metals of
both atom types must be the same. 3.Electronegativity: The more
electropositive one element and the more electronegative the other,
the greater is the likelihood that they will form an intermetallic
compound instead of a substitutional solid solution. 4.Valences: A
metal will have more of a tendency to dissolve another metal of
higher valency than one of a lower valency.
Slide 85
Interstitial solid solutions For metallic materials that have
relatively high atomic packing factors, these interstitial
positions are relatively small (thus smaller atomic diameter is
required). the maximum allowable concentration of interstitial
impurity atoms is low (less than 10%). Even very small impurity
atoms introduce some lattice strains on the adjacent host
atoms.
Slide 86
Miscellaneous Imperfections DISLOCATIONSLINEAR DEFECTS: A
dislocation is a linear or one-dimensional defect around which some
of the atoms are misaligned. Virtually all crystalline materials
contain some dislocations introduced: During solidification During
plastic deformation Consequence of thermal stresses that result
from rapid cooling.
Slide 87
Types of dislocation Edge dislocation: An extra portion of a
plane of atoms, or half-plane, the edge of which terminates within
the crystal. The defect centres around the line that is defined
along the end of the extra half-plane of atoms (dislocation line).
The atoms above the dislocation line are squeezed together, and
those below are pulled apart. The magnitude of this distortion
decreases with distance away from the dislocation line. The
magnitude and direction of the lattice distortion associated with a
dislocation is expressed in terms of a Burgers vector (b)
(perpendicular).
Slide 88
Slide 89
Screw dislocation: A dislocation formed by a shear stress in
which the upper front region of the crystal is shifted (ex. one
atomic distance to the right) relative to the bottom portion. The
screw dislocation is linear and along a dislocation line (line
AB).
Slide 90
Slide 91
Mixed dislocations: Most dislocations found in crystalline
materials exhibit components of both edge and screw dislocation.
Though a dislocation changes direction and nature within a crystal
(e.g. from edge to mixed to screw), the Burgers vector will be the
same at all points along its line. For metallic materials, the
Burgers vector for a dislocation will point in a close-packed
crystallographic direction and will be of magnitude equal to the
interatomic spacing.
Slide 92
Slide 93
The nature of a dislocation (i.e., edge, screw, or mixed) is
defined by the relative orientations of dislocation line and
Burgers vector: For an edge, they are perpendicular. For a screw,
they are parallel. They are neither perpendicular nor parallel for
a mixed dislocation, but lies somewhere in between.
Slide 94
High-magnification transmission electron micrograph
Dislocations can be observed in crystalline materials using
electron-microscopic techniques.
Slide 95
INTERFACIAL DEFECTS Are boundaries that have two dimensions and
normally separate regions of the materials that have different
crystal structures and/or crystallographic orientations.
Interfacial defects include: External surfaces. Grain boundaries.
Twin boundaries. Stacking faults. Phase boundaries
Slide 96
External Surfaces Surfaces along which the crystal structure
terminates. Surface atoms are not bonded to the maximum number of
nearest neighbours, therefore in a higher energy state than the
atoms at interior positions. To reduce this energy, materials tend
to minimize, if at all possible, the total surface area.
Slide 97
Grain Boundaries The boundary separating two small grains or
crystals having different crystallographic orientations in
polycrystalline materials. Various degrees of crystallographic
misalignment between adjacent grains are possible: Small- (or low-
) angle grain boundary: When this orientation mismatch is slight,
on the order of a few degrees. high-angle boundaries. Tilt
boundary: When several edge dislocations are aligned. Twist
boundary: An array of screw dislocations (The angle of
misorientation is parallel to the boundary).
Slide 98
Slide 99
Angle of misorientation Tilt boundary Twist boundary
Slide 100
The atoms are bonded less regularly along a grain boundary
(high grain boundary energy). The magnitude of this energy is a
function of the degree of misorientation. This results in: Grain
boundaries are more chemically reactive than the grains themselves.
Impurity atoms often preferentially segregate along these
boundaries The total energy is lower in large or coarse- grained
materials than in fine-grained ones: Grains grow at elevated
temperatures to reduce the total boundary energy
Slide 101
Twin Boundaries A special type of grain boundary across which
there is a specific mirror lattice symmetry. The region of material
between these boundaries is termed a twin. Twins result from atomic
displacements that are produced from: Mechanical shear forces
(mechanical twins) During annealing heat treatments following
deformation (annealing twins) Twinning occurs on a definite
crystallographic plane and in a specific direction depending on the
crystal structure: Annealing twins are typically found in metals
that have the FCC crystal structure. Mechanical twins are observed
in BCC and HCP metals
Slide 102
Slide 103
ATOMIC VIBRATIONS Every atom in a solid material is vibrating
very rapidly about its lattice position within the crystal. At any
instant of time not all atoms vibrate at the same frequency and
amplitude, nor with the same energy. Therefore these atomic
vibrations may be thought of as imperfections or defects. At room
temperature, a typical vibrational frequency is on 10 13 Hz,
whereas the amplitude is a few thousandths of a nanometer.
Slide 104
BULK OR VOLUME DEFECTS These include pores, cracks, and foreign
inclusions. They are normally introduced during processing and
fabrication steps. Some of these defects and their effects on the
properties of materials
Slide 105
Dislocations mechanisms Plastic deformation corresponds to the
net movement of large numbers of atoms in response to an applied
stress. Thus plastic deformation corresponds to the motion of large
numbers of dislocations. During this process, interatomic bonds
must be ruptured and then reformed.
Slide 106
Slide 107
Shear stress
Slide 108
The process by which plastic deformation is produced by
dislocation motion is termed slip The crystallographic plane along
which the dislocation line traverses is the slip plane. An edge
dislocation: Moves in response to a shear stress applied in a
direction perpendicular to its line. The direction of movement is
parallel to the stress direction. A screw dislocation: Moves in
response to a shear stress applied in a direction parallel to its
line. The direction of movement is perpendicular to the stress
direction.
Slide 109
The number of dislocations, or dislocation density: The total
dislocation length per unit volume or, equivalently, the number of
dislocations that intersect a unit area of a random section. For
heavily deformed metals, the density may run as high as 10 9 to 10
10 mm -2. Dislocation densities as low as 10 3 mm -2 are typically
found in carefully solidified metal crystals. typical dislocation
density for ceramic materials is between 10 2 and also 10 4 mm
-2
Slide 110
CHARACTERISTICS OF DISLOCATIONS When metals are plastically
deformed, some fraction of the deformation energy (~ 5%) is
retained internally; the remainder is dissipated as heat. The major
portion of this stored energy is as strain energy associated with
dislocations. As a consequence of dislocation, there are regions in
which compressive, tensile, and shear lattice strains are imposed
on the neighbouring atoms. The strains extend into the surrounding
atoms, and their magnitude decreases with radial distance from the
dislocation.
Slide 111
Shear
Slide 112
The strain fields surrounding dislocations may interact Forces
are imposed on each dislocation by the combined interactions of all
its neighbouring dislocations. Same plane Same direction Same plane
Different direction
Slide 113
Dislocation interactions are possible between edge, screw,
and/or mixed dislocations, and for a variety of orientations.
During plastic deformation, the number of dislocations increases
dramatically due to: Existing dislocations, which multiply Grain
boundaries Internal defects and surface irregularities such as
scratches and nicks
Slide 114
SLIP SYSTEMS Dislocations do not move with the same degree of
ease on all crystallographic planes of atoms and in all
crystallographic directions. There is a preferred plane (slip
plane), and in that plane there are specific directions (slip
direction) along which dislocation motion occurs. This combination
of the slip plane and the slip direction is termed the slip
system.
Slide 115
The slip system depends on: The crystal structure of the metal
The atomic distortion that accompanies the motion of a dislocation
(minimal). For a particular crystal structure : The slip plane is
the plane that has the most dense atomic packing. The slip
direction corresponds to the direction, in this plane, that is most
closely packed with atoms. The number of independent slip systems
represents the different possible combinations of slip planes and
directions. Ex. there are 12 slip systems in FCC: four unique {111}
planes and, within each plane, three independent directions.
Slide 116
{111} 110
Slide 117
Metals with FCC or BCC crystal structures have a relatively
large number of slip systems (at least 12). These metals are quite
ductile because extensive plastic deformation is normally possible
along the various systems. Conversely, HCP metals, having few
active slip systems, are normally quite brittle.