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Eng 24 > Materials yapa yapa > Flashcards

Materials yapa yapa Flashcards

1
Q

3 points about metals

A

(metallic bonding)
– Strong, high modulus, ductile
– High thermal and electrical conductivity
– Crystalline, opaque, reflective

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2
Q

3 points on polymers (plastics)

A

(covalent and van der Waals bonding)
– Soft, ductile, low strength, low modulus, low density
– Thermal and electrical insulators
– Optically translucent or transparent

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3
Q

Ceramics

A

(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

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4
Q

elastic deformation

A

returns to original shape

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5
Q

plastic deformation

A

– structure retains some
permanent deformation
– many structures involve both
elastic and plastic responses

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6
Q

Tensile test

A

how does it work

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7
Q

Draw a stress strain curve for ceramics, metals and polymers

A
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8
Q

draw a tensile test diagram, engineering stress vs engineering strain

A
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9
Q

Name all stages of tensile test

A
  • initial elastic region
  • non linear elastic region
  • yield stress
  • plastic deformation
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10
Q

Describe Initial Elastic region

A

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

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11
Q

Where does yield stress occur

A

Material Yields at B
– Yield stress σy (yield strength) - The stress at which the onset of
plastic deformation occurs

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12
Q

Where does plastic deformation occur (and describe it etc)

A

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

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13
Q

Define toughness

A

TOUGHNESS – the amount of
energy a material can absorb
before it undergoes fracture

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14
Q

Poisson’s Ratio

A

as ratio of lateral (contraction) strain to
axial (extension) strain
𝑃𝑜𝑖𝑠𝑠𝑜𝑛 𝑅𝑎𝑡𝑖𝑜 = 𝜈 = − 𝑙𝑎𝑡𝑒𝑟𝑎𝑙 𝑠𝑡𝑟𝑎𝑖𝑛/𝑎𝑥𝑖𝑎𝑙 𝑠𝑡𝑟𝑎𝑖𝑛 = − ∆𝑑/𝑑 // ∆𝐿/𝐿

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15
Q

Shear Stress

A

shear stress (τ) acts tangential to the surface of a material element

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16
Q

Shear Modulus

A

shear modulus (G) is described in terms of ratio of shear stress
(τ) to the corresponding shear strain (γ)

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17
Q

Elastic isotropic material properties E, υ and G are not
independent and may be related to one another by,

A

𝐸 = 2(1 + 𝜐) 𝐺

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18
Q

Torsion

A

Angle of twist

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19
Q

Stiffness

A

resistance to elastic deformation

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20
Q

Yield strength

A

ability to resist plastic deformation

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21
Q

ductile

A

large plastic strain before fracture

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22
Q

brittle

A

low plastic strain before fracture

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23
Q

hardness

A

ability to resist localized plastic deformation

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24
Q

Formula for true stress related to engineering stress

A

σ true
= σ eng (1 + ε eng)

ε true
= ln (1 + ε eng)

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25
Q

Atomics bonding- what happens to atoms during elastic deformation?

A

– Atoms are stretched/separated but bonds are not broken
– Atoms return to original position on removal of load

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26
Q

Name the primary vs secondary bonding types

A

Primary (Chemical): Strong
– Ionic (Ceramics)
– Covalent (Polymers)
– Metallic

  • Secondary (Physical): Weak
    – van der Waals
    – Hydrogen
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27
Q

Ionic Bonding

A

Transfer of electron(s)
takes place between atoms
2Na + Cl2 → 2NaCl

– High melting and boiling points

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28
Q

Covalent Bonding

A

Covalent bonding - Outer electrons
(valence) are shared between atoms
weaker than Ionic Bond
– Polymers are primarily bonded
together through covalent bonds

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29
Q

Metallic Bonding

A

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

30
Q

Crystalline Materials

A

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

31
Q

Unit cell

A

Smallest repeat structure is called the

32
Q

Three types of metallic bonding for crystal structures

A

Three common types for metal
– Body Centred Cubic
– Face Centred Cubic
– Hexagonal Close Packed

33
Q

Is lattice structure seen in metals?

A

Lattice structure typically not seen in metallic materials
due to inefficient packing sequence… Polonium is only example

34
Q

Face Centered Cubic (FCC)

A

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

35
Q

Body Centered Cubic (BCC)

A

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

36
Q

Face centered Cubic - atoms and coordination number

A

4 atoms
12 coordination number

37
Q

Body centered Cubic - atoms and coordination number

A

2 atoms
8 coordination number

38
Q

Simple Cubic - atoms and coordination number

A

atoms 1
coordination 6

39
Q

Hexagonal Close Packed

A

Metals include Ti, Mg, Zn
* Closed Packed but with much fewer slip planes
* Normally brittle / low ductility

40
Q

Anisotropy

A

Anisotropy — Property value depends on
crystallographic direction of measurement
If grains textured (e.g.,
deformed grains have
preferential crystallographic
orientation):
properties anisotropic.

41
Q

Isotropy

A

properties homogenous
if grains randomly oriented:
properties isotropic

42
Q

Lattice Parameters

A

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

43
Q

Three point indices , lattice coordinate position

A

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

44
Q

Crystallographic Directions

A

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]
45
Q

Slip

A

sliding displacement along a plane

46
Q

Slip System

A

= 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

47
Q

Which is more packed, FCC or BCC?

A

No truly close packed planes like FCC so BCC less packed
BCC ductile, but typically less so than FCC

48
Q

HCP are the planes close packed and what are the slip systems like?

A

Closed-packed but with much fewer slip systems (typically
3 or 6)
– HCP metals tend to be quite brittle, with low ductility

49
Q

Schmid’s Law

A

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)

50
Q

Critical RSS

A

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

51
Q

Slip Stress for Polycrystalline Materials

A

Each crystal oriented differently to nominal loading
– Slip directions/planes are therefore randomly
oriented

52
Q

Crystal Lattice

A

Periodic arrangement of atoms

53
Q

defect

A

disrupts the order of the lattice

54
Q

Name the 3 types of lattice imperfections

A

Point
Line
planar

55
Q

Name the 3 types of Point defects

A

vacancy
self interstitial
substitutional

56
Q

Point defect

A
  • Point Defects – any defect that affects a few
    neighbouring atoms or lattice points
57
Q

Interstitial atom (Point Defect)

A

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

58
Q

Point Defect - Substitutional

A

Substitutional atom
– A point defect where an impurity atom
substitutes for a host atom
– Induce stress-field on the lattice

59
Q

Alloys

A

Solid solutions where one metal type is
dissolved within another to enhance
mechanical properties

60
Q

Point defect- vacancy

A

A location in a crystal lattice where an
atom is missing
Induce stress-field on the lattice

61
Q

Line Defects- name the three types of dislocation

A

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

62
Q

Edge Dislocation

A

Edge dislocation – Line defect where
an extra half-plane of atoms exists in
crystal lattice

63
Q

Screw dislocation

A

– 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

64
Q

Mixed dislocation

A

– Mixed Dislocations – Edge + Screw defects

65
Q

Dislocation - line defects

A

linear defect around which some of the atoms are misaligned
dislocation density influences strength and increases with applied stress

66
Q

Line defects - Burgers vector

A

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

67
Q

Dislocation Density

A

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

68
Q

Why are dislocations highest in metals??

A

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

69
Q

Grain Boundaries

A

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

70
Q
A

Eng 24 (7 decks)

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