Materials yapa yapa Flashcards
3 points about metals
(metallic bonding)
– Strong, high modulus, ductile
– High thermal and electrical conductivity
– Crystalline, opaque, reflective
3 points on polymers (plastics)
(covalent and van der Waals bonding)
– Soft, ductile, low strength, low modulus, low density
– Thermal and electrical insulators
– Optically translucent or transparent
Ceramics
(ionic and covalent bonding)
– Metallic/non-metallic element compounds (oxides, carbides, etc.)
– Brittle, crystalline or amorphous, high Temp
– Strong, high modulus
– Electrically and thermally insulating
elastic deformation
returns to original shape
plastic deformation
– structure retains some
permanent deformation
– many structures involve both
elastic and plastic responses
Tensile test
how does it work
Draw a stress strain curve for ceramics, metals and polymers
draw a tensile test diagram, engineering stress vs engineering strain
Name all stages of tensile test
- initial elastic region
- non linear elastic region
- yield stress
- plastic deformation
Describe Initial Elastic region
recoverable (O - A)
– Linear Elastic
* Stress is directly proportional to strain
* When the force is removed the specimen returns to it’s original undeformed
shape
– Hooke’s Law
Where does yield stress occur
Material Yields at B
– Yield stress σy (yield strength) - The stress at which the onset of
plastic deformation occurs
Where does plastic deformation occur (and describe it etc)
Plastic Deformation after B
– Plastic deformation is not recoverable and it is permanent
– When the load is removed the specimen does not return to its
original undeformed shape. The material becomes damaged
– Work hardening – occurs during plastic deformation
Define toughness
TOUGHNESS – the amount of
energy a material can absorb
before it undergoes fracture
Poisson’s Ratio
as ratio of lateral (contraction) strain to
axial (extension) strain
𝑃𝑜𝑖𝑠𝑠𝑜𝑛 𝑅𝑎𝑡𝑖𝑜 = 𝜈 = − 𝑙𝑎𝑡𝑒𝑟𝑎𝑙 𝑠𝑡𝑟𝑎𝑖𝑛/𝑎𝑥𝑖𝑎𝑙 𝑠𝑡𝑟𝑎𝑖𝑛 = − ∆𝑑/𝑑 // ∆𝐿/𝐿
Shear Stress
shear stress (τ) acts tangential to the surface of a material element
Shear Modulus
shear modulus (G) is described in terms of ratio of shear stress
(τ) to the corresponding shear strain (γ)
Elastic isotropic material properties E, υ and G are not
independent and may be related to one another by,
𝐸 = 2(1 + 𝜐) 𝐺
Torsion
Angle of twist
Stiffness
resistance to elastic deformation
Yield strength
ability to resist plastic deformation
ductile
large plastic strain before fracture
brittle
low plastic strain before fracture
hardness
ability to resist localized plastic deformation
Formula for true stress related to engineering stress
σ true
= σ eng (1 + ε eng)
ε true
= ln (1 + ε eng)
Atomics bonding- what happens to atoms during elastic deformation?
– Atoms are stretched/separated but bonds are not broken
– Atoms return to original position on removal of load
Name the primary vs secondary bonding types
Primary (Chemical): Strong
– Ionic (Ceramics)
– Covalent (Polymers)
– Metallic
- Secondary (Physical): Weak
– van der Waals
– Hydrogen
Ionic Bonding
Transfer of electron(s)
takes place between atoms
2Na + Cl2 → 2NaCl
– High melting and boiling points
Covalent Bonding
Covalent bonding - Outer electrons
(valence) are shared between atoms
weaker than Ionic Bond
– Polymers are primarily bonded
together through covalent bonds
Metallic Bonding
Metallic bonding – Outer (valence)
electron not bound to any particular
atom
* Drift throughout entire structure
– Negative electron cloud
– Positive ion cores
* Typically high bond strength
– Plastic deformation ability (unlike brittle
ionic bonds)
* Free electrons
– High thermal and electrical conductivity
Crystalline Materials
A material in which the atoms are situated in a repeating or
periodic array over large atomic distances
* Upon solidification, the atoms will position themselves in a
repetitive three-dimensional pattern
* Each atom is bonded to its nearest neighbour atoms
* All metals, many ceramics and certain polymers form crystalline
structures
* Many material properties depend on crystal structure
* Structure of crystal minimises energy
Unit cell
Smallest repeat structure is called the
Three types of metallic bonding for crystal structures
Three common types for metal
– Body Centred Cubic
– Face Centred Cubic
– Hexagonal Close Packed
Is lattice structure seen in metals?
Lattice structure typically not seen in metallic materials
due to inefficient packing sequence… Polonium is only example
Face Centered Cubic (FCC)
atom at each corner + single atom in each cube face
Metals include aluminium, copper, gold, lead, nickel and silver
* Readily undergo plastic deformation
* Configuration allows atoms to slip past each other easily
Body Centered Cubic (BCC)
atom at each corner + single atom in each cube
Materials include lithium, alpha-iron and tungsten
* Tend to be harder and less malleable
Atomic Packing Factor = 𝑽 𝒂𝒕𝒐𝒎𝒔 // 𝑽 𝒖𝒏𝒊𝒕 𝒄𝒆𝒍𝒍
Face centered Cubic - atoms and coordination number
4 atoms
12 coordination number
Body centered Cubic - atoms and coordination number
2 atoms
8 coordination number
Simple Cubic - atoms and coordination number
atoms 1
coordination 6
Hexagonal Close Packed
Metals include Ti, Mg, Zn
* Closed Packed but with much fewer slip planes
* Normally brittle / low ductility
Anisotropy
Anisotropy — Property value depends on
crystallographic direction of measurement
If grains textured (e.g.,
deformed grains have
preferential crystallographic
orientation):
properties anisotropic.
Isotropy
properties homogenous
if grains randomly oriented:
properties isotropic
Lattice Parameters
The unit cell geometry is completely defined in terms of six
parameters: the three edge lengths a, b, and c, and the three
interaxial angles α, β, and γ
Three point indices , lattice coordinate position
Coordinate specifications are possible using three point
indices: q, r, and s
* These indices are fractional multiples of a, b, and c unit cell lengths
– q is some fractional length of a along the x axis,
– r is some fractional length of b along the y axis,
– s is some fractional length of c along the z axis
example
Crystallographic Directions
A crystallographic direction is defined as a line directed between
two points, or a vector
– x, y, z coordinate system in unit cell corner
– determine coordinates of two points on the direction vector (xi , y i , zi )
– determine length by subtracting coordinates (head – tail)
– normalize by dividing by their respective lattice parameters
- Convert to smallest integer
- They are expressed in square brackets [u v w]
Slip
sliding displacement along a plane
Slip System
= slip plane and slip direction
Slip occurs on densely or close-packed planes, in close-packed directions
– Lower shear stress/energy is required for slip to occur in close-packed
planes and in close-packed directions
Which is more packed, FCC or BCC?
No truly close packed planes like FCC so BCC less packed
BCC ductile, but typically less so than FCC
HCP are the planes close packed and what are the slip systems like?
Closed-packed but with much fewer slip systems (typically
3 or 6)
– HCP metals tend to be quite brittle, with low ductility
Schmid’s Law
Show that the Resolved Shear Stress (RSS)
(t R) on a slip plane in the slip direction is
given by,
t R = σ cosλ cosf (Schmid’s Law)
Critical RSS
Slip will occur
The shear stress for initiating slip in different materials
– This is a material property
– Slip will happen on the plane with highest RSS (
tR )
* For a single crystal – the engineering stress (
σ) required to cause slip depends on angle of slip plane to loading
* Engineering yield stress may therefore vary for a single crystal depending on the loading direction
Slip Stress for Polycrystalline Materials
Each crystal oriented differently to nominal loading
– Slip directions/planes are therefore randomly
oriented
Crystal Lattice
Periodic arrangement of atoms
defect
disrupts the order of the lattice
Name the 3 types of lattice imperfections
Point
Line
planar
Name the 3 types of Point defects
vacancy
self interstitial
substitutional
Point defect
- Point Defects – any defect that affects a few
neighbouring atoms or lattice points
Interstitial atom (Point Defect)
Interstitial atom
– A point defect where a smaller atom fits in
between other atoms in the crystal lattice
– Can be an Impurity or intentional alloying
element
– Induce minor stress-field on the lattice
Point Defect - Substitutional
Substitutional atom
– A point defect where an impurity atom
substitutes for a host atom
– Induce stress-field on the lattice
Alloys
Solid solutions where one metal type is
dissolved within another to enhance
mechanical properties
Point defect- vacancy
A location in a crystal lattice where an
atom is missing
Induce stress-field on the lattice
Line Defects- name the three types of dislocation
Edge dislocation – Line defect where
an extra half-plane of atoms exists in
crystal lattice
– Screw Dislocation – Line defect, where
path spirals around a dislocation line
penetrating otherwise parallel planes
– Mixed Dislocations – Edge + Screw defects
Edge Dislocation
Edge dislocation – Line defect where
an extra half-plane of atoms exists in
crystal lattice
Screw dislocation
– Screw Dislocation – Line defect, where
path spirals around a dislocation line
penetrating otherwise parallel planes
Screw dislocations occur when the
lattice itself is sheared such that there is
a misalignment of the atoms
* Dislocation line is the edge, or where
this misalignment begins
Mixed dislocation
– Mixed Dislocations – Edge + Screw defects
Dislocation - line defects
linear defect around which some of the atoms are misaligned
dislocation density influences strength and increases with applied stress
Line defects - Burgers vector
Magnitude of dislocation given by
the Burgers Vector, b
– Draw loop around defect
– Repeat the loop steps in a perfect
crystal
– b is the vector required to close the
loop
* b is always perpendicular to
dislocation line for edge
dislocations
– Magnitude of lattice distortion
Dislocation Density
Dislocation Density = Total dislocation line length per unit volume
– Highly deformed metal – cold worked (rolling, drawing etc.):
– Means there has been significant amount of plastic deformation
Why are dislocations highest in metals??
The number of dislocations is highest
in metals
* Atoms easily move to another
position with dislocation movement
* Motion of dislocations is easiest
because metals have non-directional
bonding and close-packed directions
* Covalent – directional, angular
bonding – need to break bond
* Ionic – charged ions don’t want to
change position
Grain Boundaries
Grain Boundaries are planar defects a 2D
interface between adjacent grains (single
crystals) in a polycrystalline material
* Most common type of “planar” defect
* Characteristics can strengthen or weaken a
material
* Grains randomly oriented
* Each grain has it’s defined crystal structure, i.e.
FCC, etc
