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Materials > Metals > Flashcards

Metals Flashcards

1
Q

Why do we use the yield strength with 0.2% proof stress (on a graph)?

A

Because, for metals and alloys the yield point is not always distinct

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2
Q

What accommodates plastic deformation?

A

Dislocation glide along crystallographic planes (slip planes)

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3
Q

What is a dislocation?

A

An extra half plane of atoms

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4
Q

What is a way of thinking about how dislocations move?

A

The movement of a caterpillar or moving a rug

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5
Q

What effect does restricting dislocation motion have on the material?

A

It make sit harder and stronger

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6
Q

What effect does restricting dislocation motion have on the material?

A

It make sit harder and stronger

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7
Q

What four main strategies can be used to increase the strength of metals and alloys?
(involving the micro structure)

A
  1. Strengthening through grain and phase boundries
  2. Work hardening
  3. Solid-solition strengthening
  4. Precipitation hardening
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8
Q

How do grain boundries affects dislocation glide?

A

They act as barriers

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9
Q

How does grain size affect dislocation glide?

A

Smaller grain = more grain boundries = more barrier to dislocation glide = increase in strength

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10
Q

What is the Hall-Petch equation?

A

Yield strength = yield0 +K*d^-1/2
Yield strength is the 0.2% proof strength
d is the average grain diameter
K gradient off d^-1/2 to yield strength graph

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11
Q

What’s is a Hall-Petch plot?

A 
Graph of d^-1/2 against yield strength
K = gradient
Straight line (not linear tho a sits not d)
When d^-1/2 = 0 there are no grain boundries
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12
Q

What happens to a ductile metals properties as it it plastic ally deformed?

A

Harder and stronger

Ductility is reduced

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13
Q

What is work hardening?

A

Work hardening is caused by the accumulation of dislocations generated by plastic deformation

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14
Q

Definition of dislocation density

A

The length of dislocation line per unit volume

m/m^3

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15
Q

What is the dislocation density in an annealed (not work hardened) soft metal

A

10^10 m/m^3

1cm cubed contains 10km of dislocation line!

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16
Q

What happens to dislocations when metals are deformed?

A

Dislocations multiply and the density grows
They get entangled and a forest of intersecting dislocation forms
This make sit harder for existing dislocations to move

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17
Q

What is solid solution strengthening

A

A technique used to strengthen and harden metals
by alloying a metal with impurity atoms
that are either substitutional or ingerstitial
E. G addition of zinc to copper to make brass

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18
Q

What happens to the yield strength and ductility when solid solution strengthening is used

A

Yield strength increases

Ductility decreases

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19
Q

How does solid-solution strengthening work?

A

If the impurity atoms are a different size to the main metal, they will locally distort the crystal lattice.
This results in crystal lattice strain field interactions between dislocations and the impurity atoms.
Therefor, dislocation movement is restricted.
The greater the difference in size and amount of alloying elements increases the strength of metal alloys.

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20
Q

What is precipitation strengthening?

A

When small, strong particles, of a second phase, form and disperse in the original phase matrix, in the path of dislocations.
Increases strength
but reduces ductility

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21
Q

How is precipitation strengthening achieved?

A

By subjecting an alloy, of appropriate chemical composition, to a series of heat treatments

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22
Q

Draw a precipitation strengthing, temperature against time graph

A 
Solution heat treatment
          \_\_\_\_
        /        | quench
      /          |             precipitation heat treatment
    /            |    \_\_\_\_\_
  /              |  /          \
/                |/              \
Time
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23
Q

What are the six families that we can classify engineering materials into?

A 
Metals
Polymers 
Elastomets
Glasses
Ceramics
Hybrids
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24
Q

What is the order used to categorise materials?

A 
Kingdom
Family
Class
Sub-class
Member
Attributes
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25
Q

What is the process a material must go through to reach the finished product?

A 
Raw material
Primary shaping
Secondary processes
Joining/ surface treatment
Finished product
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26
Q

Different types of shaping

A 
Casting
Molding
Deformation
Powder
Composite
Special
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27
Q

Types of secondary processes

A

Machining

Heat treat

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28
Q

Different types of joining

A

Fastening, riveting
Welding, heat bonding
Snap fits, friction bond
Adhesives, cements

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29
Q

Types of surface treatment

A

Polishing, texturing
Plating, metallizing
Anodize, chromizing
Painting, printing

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30
Q

What has a direct effect on the mechanical properties, and so the end application?

A

The chemical composition ie raw material
It’s manufacturing and processing
The microstructure

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31
Q

What is the crystal structure?

A

The way atoms are arranged in a particular way

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32
Q

3 types of crystal structures

A

Hcp, hexagonal close packed
Fcc, face centred cubic
Bcc, body centred cubic

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33
Q

4 types of crystal defects

A 
Point defects
Linear defects
Interfacial
Defects
Bulk or volume defects
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34
Q

Point defects

A 
Vacancies
Self-interstitials
Interstitial impurity atoms
Substitutional impurity atoms
ie impurities

Atomic level/nanometres

35
Q

Linear defects

A

Dislocations

  1. edge type
  2. screw type
  3. mixed type
36
Q

Interfacial defects

A

Grain boundries
Twin boundries
Stacking faults
Phase boundries

37
Q

Bulk or volume defects

A

Voids/Porosity
Cracks
Foreign bodies

Largest

37
Q

Bulk or volume defects

A

Porosity
Cracks
Foreign bodies

Largest

38
Q

Bulk or volume defects

A

Porosity
Cracks
Foreign bodies

Largest- mm or more

39
Q

What are vacancies

A

Voids or a missing atom in the pattern of atoms

40
Q

Self interstitials

A

atoms, of the same type, that can fit in between the regular pattern of atoms

41
Q

Interstitial impurity atoms

A

Smaller atoms of a different type fit in between other atoms

42
Q

Substitutional impurity atoms

A

When the impurity atom is a similar size it is a substitute

43
Q

Dislocation

A

A linear defect around which the atoms are misaligned.

Leads to localised lattice distortion.

44
Q

Grain boundary

A

The boundary separating two small crystals that have different crystallographic orientations

45
Q

Two types of grain boundarys

A

Small angle

High angle

46
Q

Twin boundary

A

A special type of grain boundary where there is a specific crystal lattice mirror symmetry

47
Q

Stacking faults

A

Found in Fcc metals when there is an interruption in the abcabc stacking sequence of the close packed crystal planes

48
Q

Phase boundaries

A

Exist in multi-phase materials across which there is a sudden change in physical and/or chemical characteristics

49
Q

When are volume defects introduced?

A

Unintentionally during processing and fabrication

50
Q

5 ways steels can be classified

A

The chemical composition (carbon, low alloy, stainless steels)

The product form (bar, plate, sheet)

The microstructure (ferrite, austenite martensite)

The required strength (yeild or tensile)

How it is heat treated (annealed, quenched and tempered)

51
Q

Hwta elements does/can steel contain

A

Iron(Fe)
Carbon(C)

Nickel
Molybdenum

52
Q

Hwo doe stye AISI/SAE Steel numbering system work?

XXXX(X)

A

First two digits = major allow additions made to the iron
Last two/three = ÷100 is the carbon concentration in wt%

XX20 = 0.2wt% carbon

53
Q

How does the carbon content of steel influence the yield, tensile strength and ductility ?

A

As carbon content increases
Yield and tensile strength increases
Ductility reduces

54
Q

At what two temperatures does pure iron under go a solid state transformation?
What temperature does it melt?

A

912°C (BBC ferrit to Fcc austenite)
1394°C (Fcc austenite to BBC delta ferrite)
1538°C - - > liquid

55
Q

What effect does carbon have on phase transformations in iron?

A

Carbon is soluble as an interstitial impurity in all three phases of iron
(less so in alpha and delta)
The addition of carbon changes the temperature at which the phase tranformations take place

56
Q

What three phases does pure iron go through its two solid state phase transformations?

A 
Low temp.--- BCC ferrite/alpha iron
912°C
High temp.--- Fcc austenite/gamma iron
1394°C
Higher temp.--- BCC delta ferrite
1539°C
Liquid
57
Q

What is a Eutectoid point?

A

The temperature and wt%C at which a Eutectiod reaction occurs
The lowest temp. at which a solid becomes another solid
The point at which 3 solid phases exist

58
Q

What is a Eutectic point?

A

It is the lowest temp. at which a liquid can exist in this system / solidification occurs
Two liquid curves meet
A liquid becomes a solid

59
Q

What is pearlite ?

A

A mixture of two phases
Cementite Fe3C
Ferrite

NOT a phase

60
Q

Mechanical properties of pearlite

A

Intermediate between
Soft, ductile ferrite
Hard, brittle cementite

61
Q

What is a hypoeutectoid steel?

A

The carbon content is less than the Eutectoid composition

62
Q

What is the lever rule for:
Proeutectoid ferrite/alpha (Walpha)
Pearlite (Wp)

A 
Walpha= U/(T+U)
Wp= T/(T+U)
63
Q

What is hypereutectoid steel?

A

The carbon concentration is greater than the Eutectoid composition

64
Q

What influences the strength of an engineering alloy with a Eutectic microstructure?

A

The amount and length-scale of the Eutectic structure

Max strength is when the microstructure= 100% Eutectic

65
Q

What does a Eutectic phase diagram show?

A

The development of microstructure in the alloy during slow cooling
The phase transformations under equilibrium conditions

66
Q

What are the two types of phase transformation diagrams?

A
  1. Under equilibrium conditions
    Transformations require atomic diffusion to take place and are called nucleation and growth processes

2.non-equilibrium conditions
Transformations occur at fast cooling rates
Equilibrium phase diagrams can not predicts these
You need to use Time-Temperate- Transformation Diagrams
(TTT) Diagrams

67
Q

What does a TTT diagram show?

A

Time- Temperature–Transformation diagram

Graphically represent the nucleation of growth kinetics of transformation products e.g. Pearlite associated with the decomposition of Austenite.

It is produced from a percentage transformed versus time diagram for a range of temperatures.
These diagrams show the percentage transformed for a steel of Eutectoid composition that was rapidly cooled and held at that temperature.

68
Q

Does fine pearlite or coarse pearlite have greater tensile strength?

A

Fine pearlite as it has a higher carbon content

69
Q

Definition of componants

A

Components are pure metals and/or compounds of which an alloy is composed

70
Q

System

A

A system refers to a specific body of material under consideration
Or
A series of possible alloys consisting of the same componats (e.g. The iron carbon system)

71
Q

Phase

A

A phase may be defined as a homogeneous portion of the system that has uniform physical and chemical characteristics

72
Q

What cases do binary isomorphous phase diagrams apply for?

A

Cases of unlimited solid-solubility

Conditions for this is given by the Hume-Rothery rules

73
Q

What is the difference between the melting point of a pure metal and an alloy or mixture?

A

Pure metals have a very drfinate melting point

Alloys and mixture have a freezing range (a range of temperatures)

74
Q

What are the Hume-Rothery rules?

A

Must satisfy 1 or more

  1. Size factor: the atoms/ions should be of a similar size, with less than 15% difference in radii
  2. Crystal structure: the material must have the same crystal structure
  3. Valence: the ions should have the same valence otherwise they prefer to form conoounds
  4. Electronegativity: the ions/atoms should have similar electronegavity, otherwise compound formation is preferred

These are necessary but not always sufficient to guarantee solid solution formation

75
Q

How are phases distinguished?

A

By their
State of aggregation
Crystal structure
Chemical composition

76
Q

Phase transformation

A

Any rearrangement within the assembly of atoms or molecules, which carries the system from one configuration to another.

The configuration is defined my its state of aggregation, crystal structure or composition to another

77
Q

What is at% and wt%

A

at% = atomic fraction/percentage

wt% weight fraction/percentage

78
Q

Conversion from at% to wt%

A

wt%A=

(at%A x MA) /(at%A x MA + at%B x MB) x 100

79
Q

Conversion from wt% to at%

A

at%A =

((wt%A/MA) /MA) /((wt%A/MA) + (wt%B/MB)) * 100

80
Q

What are isomorphous phase diagrams?

A

The simplest type of binary phase equilibrium diagrams

They show the phases and the compositions at anu combination of temperature and composition

81
Q

How many solid phases are present in an isomorphous system?

Why?

A

One solid phase

Because the alloying elements have unlimited solubility in each other

82
Q

Liquidus

A

Defines the temperature above which 100% liquid would be present for any given alloy composition
Apears as I line on a phase diagram

83
Q

Solidus

A

Defines the temperature below which 100% solid would be present for any given alloy composition

Materials (2 decks)

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