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All Materials > Metals and Alloys > Flashcards

Metals and Alloys Flashcards

1
Q

What must be considered when selecting materials for design?

A

-Mechanical properties
-Raw material cost
-Transportation costs
-Maintenance costs
Aesthetic factors

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2
Q

What mechanical properties must be considered when selecting materials for design?

A

-Material stiffness (Young’s modulus)
-Resistance to plastic deformation (yield strength)
-Ductility

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3
Q

Name some physical properties of materials.

A

-Density
-Mass
-Melting and boiling points
-Electrical and thermal conductivity

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4
Q

Name two examples of mechanical property.

A

Elastic response (stiffness) and plastic flow (yield stress)

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5
Q

What determines the ability for a material to resist deformation?

A

Internal structure

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6
Q

For metals and alloys, describe the constitutive relation.

A

Stress will be a function of strain and material microstructure

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7
Q

What experimental methods can be used to determine microstructure characterisation?

A

-Optical microscopy
-Scanning electron microscopy
-Transmission electron microscopy

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8
Q

What experimental methods can be used to determine mechanical characterisation?

A

-Tensile/compressive test (Uniaxial loading)
-Fatigue test (Cyclic loading)
Creep test (time dependent behaviour)

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9
Q

What computational material modelling methods can be used to derive structure-property relations?

A

-Physics-bases, phenomenological or empirical methods
-Simulate process induced microstructures
-Simulate mechanical properties

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10
Q

What does a tensile test provide?

A

An experiment approach to studying basic material behaviour

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11
Q

What assumptions are made during a tensile test?

A

Loading is pure axial and the deformation takes place uniformly, both along the length and cross section of the specimen

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12
Q

How is a tensile test carried out?

A

By moving one end of the specimen at constant speed, while holding the other end fixed

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13
Q

What are the primary variables recorded during a tensile test?

A

Load (F) and extension (L)

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14
Q

What is required to determine true stress?

A

Knowledge of the instantaneous area at all times during deformation

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15
Q

What determines plastic deformation?

A

When stresses exceed the yield stress

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16
Q

What determines elastic behaviour?

A

Total strain is entirely recoverable - total strain is equal to the elastic strain

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17
Q

What is plastic strain?

A

A function of the stress on unloading

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18
Q

In the stress tensor matrix, which components are considered to represent normal stress?

A

Sigma_11, Sigma_22, Sigma_33

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19
Q

What components are present in the hydrostatic stress tensor?

A

Pure tensile or compressive stress

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20
Q

What components are present in the deviatoric stress tensor?

A

Shear stresses of the total stress state

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21
Q

What physical change is caused by hydrostatic strain?

A

Volume change

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22
Q

What physical change is caused by deviatoric strain?

A

Change in shape

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23
Q

What are the principle stresses?

A

Where the stress tensor has zero shear components and non-zero normal stresses

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24
Q

How do you define the stress state?

A

By using all components of the stress tensor

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25
Q

Name three materials with polycrystalline structures.

A

Steels, aluminium, titanium alloys

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26
Q

What is a polycrystalline structure?

A

Collection of crystals called grains that are joined together, in each grain atoms are arranged into long-range patterns of atomic order with a repetitive 3D pattern

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27
Q

Name three materials with hexagonal close packed (HCP) structure?

A

Alpha-titanium, zinc, zirconium

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28
Q

Name three materials with face-centred cubic (FCC) structure?

A

Nickel, aluminium, gamma-iron

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29
Q

Name four materials with body-centred cubic (BCC) structure?

A

Chromium, beta-titanium, alpha-iron, beta-iron

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30
Q

What causes lattice defects?

A

-Crystals are not perfect
-Disruptions in their lattice arrangements
-Atoms may be positioned in a non-lattice site, or an atom may be missing at a given lattice size

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31
Q

What are the different types of crystal defect?

A

-Point
-Line
-Surface
-Volume

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32
Q

What are some examples of point defect?

A

Vacancies, interstitial and substitutional atoms

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33
Q

How are vacancy point defects introduced?

A

Thermal vibrations of atoms or from imperfect packing during crystallisation

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34
Q

How can a solute atom be incorporated into a host lattice?

A

Substitutionally, Interstitially

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35
Q

What happens when an interstitial atom is inserted into a crystal lattice?

A

Neighbouring atoms become displaced radially away from the interstitial solute atom

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36
Q
A
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37
Q

What are line defects?

A

Dislocations that are characterised by a chain of atoms that are incorrectly places in the lattice

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38
Q

Name three types of line defect/dislocation.

A

-Edge
-Screw
-Mixed

39
Q

How do dislocations interact with each other?

A

Through their elastic fields, this self induces stress and strains within the crystal

40
Q

What is glide (in terms of dislocations)?

A

Applied shear stresses induce a force on the dislocations, causing them to move on close-packed planes

41
Q

How are plastic deformations induced by dislocations?

A

Shear stresses lead to a displacement of the dislocation through repeated breaking an formation of bonds

42
Q

How are slip bands measured experimentally?

A

Using electron-back-scatter-diffraction (EBSD) and digital image correlation (DIC), where the slip bands are revealed as ‘lines’ of high strain

43
Q

Name three types of surface defect.

A

-Twins
-Staking faults
-Grain boundaries

44
Q

What physical change can alter grain boundaries?

A

Temperature

45
Q

Name three types of volume defect.

A

-Voids
-Cracks
-Foreign inclusions

46
Q

How are volume defects introduced into a crystal lattice?

A

Unintentionally during processing or fabrication

47
Q

How is the orientation of a dislocation specified?

A

By the direction of the tangent vector

48
Q

What is a Burgers vector?

A

The vector needed to close a circuit (around a dislocation), from its final point to the starting point

49
Q

How does a Burger vector change for two dislocations?

A

The circuit measure the total Burgers vector inside the contour

50
Q
A
51
Q

Describe screw dislocations.

A

A ‘cutting-displace-gluing’ mechanism, where the displacement faces are parallel to the edge of the cut

52
Q

Describe a mixed dislocation.

A

Tangent vector varies along the dislocation line, the Burgers vector will be expressed in terms of the perpendicular and parallel components

53
Q

Describe how we know that the Burgers vector of dislocations is conserved.

A

-Consider a dislocation segment with Burgers vector b1
-If the dislocation splits into two other segments with Burgers vectors b2 and b3
-Since the Burgers circuit measure the Burgers vector of dislocations contained within it, b1 = b2 + b3

54
Q

What parameters determine the energy of a dislocation?

A

Shear modulus and magnitude of the Burgers vector

55
Q

What are partial dislocations?

A

Dislocation which has Burgers vector whose magnitude is a fraction of the lattice spacing, the atoms between the partials form a stacking fault

56
Q

Why do partial dislocations form?

A

When there is a reduction in enegry

57
Q

How do we determine if a dissociation will occur?

A

If the energy change associated with the reaction at the first dislocation is greater than that associated with the split dislocations

58
Q

How do we determine if two dislocations will combine to form a new segment?

A

If the energy associated with with the reaction at the first dislocation is lower than that associated with the split dislocations

59
Q

What does the Orowan-Bailey relation describe?

A

Shear strain increment

60
Q

What are isomorphous alloys?

A

Characterised by complete solubility of the two components

61
Q

What can be determined from a phase diagram?

A

The fraction of phase present at equilibrium

62
Q

What is the composition and phase fraction of a single phase?

A

Composition is C_0 and the phase fraction is 1

63
Q

How is the phase fraction for a 2-phase region determined?

A

Using the tie-line and Lever rule

64
Q

What parameter is determined by the lever rule?

A

Mass fraction

65
Q

What is a eutectic reaction?

A

A reaction in which a single liquid changes phase into a 2-phase solid

66
Q

Describe ferrite.

A

-At room temperature the structure is BCC
-Low solubility of C atoms in interstitial sites
-Can dissolve Cr, Si, W and Mo by substitution

67
Q

Describe Austenite.

A

-At elevated temperature the structure is FCC
-Can dissolve more carbon interstitially than Ferrite (Alpha-iron)
-Can dissolve by substitution large amounts of Ni and Mn

68
Q

Describe Carbide.

A

-Iron and carbon form the compound cementite
-Hard and brittle phase
-There can be other alloying elements in the cementite such as Cr, Mn and Mo (carbides)

69
Q

What is a eutectoid reaction?

A

Involves the transformation of solid solution to two phases

70
Q

Describe hypoeutectoid steels.

A

-At 1000 degrees there is a single austenite phase
-At 800 degrees there are two phases, austenite and proeutectoid ferrite
-At 730 degrees the grain boundaires are plated by proeutectoid ferrite
-Cooling below 730 degrees, forms pearlite (a mixture of ferrite and carbide)

71
Q

What does the mean melting temperature tell us about Gibbs free energy?

A

Above the mean melting temperature, the liquid has a lower free energy.
For temperatures below the mean melting temperature, the Gibbs free energy of a solid is higher than the Gibbs free energy for a liquid.

72
Q

What is the process for solidification in real alloys?

A

Nucleation and growth of solid particles

73
Q

What two contributions are associated with the change in free enery?

A

-Formation of a volume of solid
-Formation of interface separating the solid and liquid phase

74
Q

How does a large particle radius affect the nucleation energy?

A

Decreases it

75
Q

What needs to be done by a system with a lower radius compared to critical radius?

A

Lower its energy by dissolution of the particle

76
Q

What happens to a system with a radius is higher than the critical radius?

A

Free energy decreases by growth of the particle

77
Q

How can the critical radius and free energy in terms of undercooling an latent heat of fusion be decreased?

A

Increasing the undercooling

78
Q

How does particle growth of a nucleated phase occur?

A

Via diffusion of atoms

79
Q

What parameter determined the growth rate of particles?

A

Diffusion coefficient of atomic species making up the phase of the nuclei

80
Q

What equation (name) can be used to describe the growth rate of particles?

A

Arrhenius equation

81
Q

Describe the system of particles at a temperature close to the melting temperature.

A

Nucleation rate is slow with high growth rates, the resulting microstructure will contain relatively few but coarse particles

82
Q

Describe a system of transforming particles at lower temperatures (below melting).

A

Nucleation rate is high and growth rates are slow, resulting in a fine distribution of particles

83
Q

What three factors induce phase transitions?

A

-Change in temperature
-Change in composition
-Change in applied pressure

84
Q

How is Martensite formed?

A

Steels are rapidly cooled from the austenite phase field to low temperatures

85
Q

What is the change in crystal structure that takes place when Martensite is formed?

A

FCC-BCT (Body centered tetragonal)

86
Q

Describe what the yield stress associated with the obstacles in a dislocation line shows.

A

Stress needed to be applied in order for the dislocation to by-pass the obstacles

87
Q

Describe the Frank-Read mechanism.

A

A dislocation segment is pinned at its ends will blow out under the action of an applied stress (this is an example of a dislocation source)

88
Q

Describe solid solution strengthening.

A

-Solute atoms induce distortions, which inside a stress state
-The distortions can result in tensile or compressive stresses
-Interstitial atoms ‘pull’ neighbouring host atoms towards them, this can either induce a tensile stretch or a compressive stress
-The elastic field associated with solute atoms can interact with the dislocations, these can pin the dislocation line and restrict motion

89
Q

How does yield stress change with the concentration of solute atoms during solid solution strengthening?

A

As the yield stress of alloys increases, the concentration of solute atoms also increases

90
Q

What influence do elastic interactions have on dislocations?

A

They can pin them

91
Q

What effect does plastic deformation have on the number of dislocations?

A

Number of dislocations increase, as shown by the Orowan-Bailey relation

92
Q

How are yield stress and dislocation density related?

A

Yield stress increases parabolically with the dislocation density

93
Q

What is told about the yield stress of a material if is is said to work harden?

A

Yield stress increases with a decreasing positive gradient

94
Q

Describe the Hall-Petch effect.

A

Smaller grains have higher yield stress compared to larger grains

All Materials (2 decks)

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