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MS3 > Microstructure And Procesing I > Flashcards

Microstructure And Procesing I Flashcards

1
Q

What is an alloy?

A

A metallic allows is a mixture of metals with other metals or non-metals. Ceramics can be mixed to form ceramic alloys.

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2
Q

What are the components of an alloy?

A

The chemical elements that make up the alloy.

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3
Q

What is a binary alloy?

A

An alloy that has two components.

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4
Q

What are the two measures for concentration of components in an alloy?

A

Weight percent and atomic percent.

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5
Q

In the solid state, for metals, what microstructures usually form?

A

A single solid solution.
Two separate solid solutions.
A chemical compound, with a separate solid solution.

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6
Q

What is a solid solution?

A

A solid in which one or more elements are ‘dissolved’ in another so they are homogeneously dispersed at an atomic scale.

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7
Q

What is a phase?

A

A region of a material which has a homogeneous structure at the atomic scale.

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8
Q

What is the constitution of an alloy?

A

The constitution of an alloy is described as:
The phases present.
The weight fraction of each phase.
The composition of each phase.

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9
Q

At thermodynamic equilibrium what can be said about the constitution?

A

The constitution is stable and there is no tendency for it to change.

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10
Q

What are the independent state variables that determine the constitution?

A

Temperature, pressure and composition.

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11
Q

How is the equilibrium constitution defined (conditions)?

A

At constant temperature and pressure for a given alloy composition.

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12
Q

Equation for free energy at fixed temperature and pressure:

A

G = U + pV - TS = H - TS

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13
Q

Each possible state of the system has an associated free energy, which are present at thermodynamic equilibrium?

A

The lowest free energy state.

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14
Q

What is the definition of equilibrium constitution?

A

The state of lowest free energy for a given composition, temperature and pressure.

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15
Q

If an alloy is stable it shows no tendency to change, why?

A

It is thermodynamically stable.

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16
Q

Two dimensional maps with state variables on the axes are known as what?

A

Phase diagrams.

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17
Q

What does a phase diagram show?

A

The equilibrium position at a given composition and temperature.

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18
Q

For a pure substance what is the melting temperature at a given pressure?

A

The temperature at which the solid and liquid phases coexist.

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19
Q

What is the name of the phase boundary that limits the bottom of the liquid field?

A

The liquidus.

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20
Q

What is the name of the upper limit of the solid field?

A

The solidus.

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21
Q

What does isomorphous mean?

A

Single-structured.

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22
Q

What is the name of boundaries between single and two phase regions?

A

The solvus boundaries.

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23
Q

On a phase diagram what is true at the point of intersection of the solidus and rising solvus boundary?

A

It represents the point of maximum solubility of the single phase region.

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24
Q

A region on a phase diagram is known as what?

A

A field.

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25
Q

What is the special temperature that two solid phases and liquid of that composition can exist?

A

The eutectic point.

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26
Q

Definition of eutectic point.

A

The lower limit of the single phase liquid field formed by the intersection of two liquidus lines.

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27
Q

The state variables (temperature and composition) define a point on the phase diagram called what?

A

The constitution point.

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28
Q

What is the first thing to establish at a constitution point?

A

The number of phases present.

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29
Q

What is a tie-line?

A

A horizontal line across a two phase region that intersects the single phase boundaries through the constitution point.

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30
Q

What is the composition of an alloy at a constitution point in a single phase region?

A

The composition is itself and is found by a vertical line down to the composition axis.

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31
Q

What is the composition of an alloy at a constitution point in a two phase region?

A

The composition is that of the two two boundaries to single phase regions. It is given by the values of the boundaries at each end of the tie line.

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32
Q

In a two phase region, the composition of the phases is fixed by what?

A

The saturation limits.

33
Q

Different compositions at the same temperature in a two phase region will have different what?

A

Different proportions of each phase.

34
Q

How to find the proportions of each phase in a two phase region:

A

Use lever rule.

35
Q

What is chemical spinodal?

A

Where phase separation occurs when d2G/dx^2 <0

36
Q

What two types of binary polymer mixtures can form?

A

Polymer chains in a small molecule solvent
or
A blend of two polymers in a single phase of two phases.

37
Q

Why do large polymers not dissolve well in each other?

A

-T∆S term small so little influence.

38
Q

Under what conditions does clustering occur?

A

If the bond enthalpy of like neighbours is less than unlike neighbours?

39
Q

Under what conditions does ordering occur?

A

If the bond enthalpy of unlike neighbours is less than like neighbours.

40
Q

Peritectic reaction equation.

A

L+S1->S2

41
Q

Monotectic reaction equation.

A

L1->L2+S

42
Q

Eutectoid reaction equation.

A

S1->S2+S3

43
Q

Peritectoid reaction equation.

A

S1+S2->S3

44
Q

Eutectic reaction equation.

A

L->S1+S2

45
Q

What is a miscibility gap?

A

Where phase separation takes place due to lower free energy (on free energy curve)
Appears on phase diagram as solvus.

46
Q

Can peritectic reactions ever be equilibrium processes?

A

No

47
Q

Sketch a peritectic microstructure.

A

Check

48
Q

What is the speed of a peritectoid reaction like?

A

Very slow.

49
Q

Describe moneutectic reactions.

A

Liquid separates into two liquids of varying composition, then at monotectoid point liquid transforms to solid and another liquid.

50
Q

What is an intermediate phase?

A

A single phase region on the phases diagram that exists between other two phase regions.

51
Q

What is a line compound?

A

A compound with a very narrow free energy curve leading to a phase with a very limited composition range.

52
Q

Derive the expression for ∆G including Latent heat when cooling.

A

Check derivation.

53
Q

Derive the expression for work of homogenous nucleation including critical radius.

A

Check derivation.

54
Q

Give an expression for the rate of homogenous nucleation.

A

Check, Ahrrenius type

55
Q

What is growth like in most metals?

A

Attachment is easy and can occur anywhere on the interface leading to non-faceted growth.

56
Q

What is growth like in most non-metals?

A

Attachment is difficult and only occurs at steps leading to faceted growth.

57
Q

What is the speed of non-faceted growth like?

A

Rapid and fast in all directions even with small undercoolings.

58
Q

What is the speed of growth like in faceted materials like?

A

Slow and requires significant undercoolings are required to be maintained.

59
Q

On cooling, what happens if there is a positive temperature gradient ahead of the solid-liquid interface?

A

Latent heat is removed through the solid and the faceted or non-faceted growth is planar, parallel to the isotherms.

60
Q

On cooling, what happens if there is a negative temperature gradient ahead of the solid-liquid interface?

A

Latent heat is removed through the liquid, any bump that grows on the solid-liquid interface is undercooled and therefore grows faster. Planar interface becomes unstable giving rise to dendritic growth.

61
Q

Differences between real and equilibrium cooling curves.

A

Equilibrium cooling-release of latent heat causes a change in the gradient of the cooling curve.
Real cooling-undercooling until nucleation takes place, recalescence because of latent heat evolution, then falls again when solidification is complete.

62
Q

What is the partition coefficient?

A

Under equilibrium solidification, it is the ratio between liquid and solid composition. Given letter k.

63
Q

Does k vary with T?

A

No as liquidus and solidus are assumed to be straight lines meaning k is independent of both temperature and composition.

64
Q

What values can k take?

A

k<1 or k>1, less than for hypoeutectics and greater than for hypereutectics.

65
Q

Describe equilibrium cooling for k<1.

A

Solidification beginning with composition X0.
First solid forms with composition kX0
Solute rejected from solid as k<0
Constant diffusion between solid and liquid with temperature falling to keep L and S in equilibrium
At solidus, solidification complete with last solid forming from the liquid of composition X0/k.
All solid has composition X0 as equilibrium cooling and no segregation results.

66
Q

Derive the Scheil equation.

A

Check derivation.

67
Q

What assumptions are made to derive the Scheil equation?

A

Homogeneous mixing of liquid.
The A-B mixture of composition X0 at T in equilibrium freezing range is a mix of solid and liquid phases.
Mole fractions of L and S phases at equilibrium follow the lever rule.

68
Q

What three distinct regions make up a cast microstructure?

A

The chill zone.
The columnar zone.
The equiaxed zone.

69
Q

How can microstructure be controlled in cast metals?

A

Controlled by cooling rate.

70
Q

Why does a chill zone form at the surface of cast metal?

A

The temperature difference between the metal and mould causes rapid nucleation at the outer surface.

71
Q

Why does the columnar zone have the morphology that it does?

A

The large temperature gradient between the chill zone and liquid usually causes dendritic growth inwards.

72
Q

What is the morphology of the equiaxed zone?

A

Grain structures roughly the same size in all directions due to no strong direction of heat flow in the centre of the casting.

73
Q

Can the eutectic reaction be reached if the starting composition does not cross the eutectic line?

A

Yes, with non-equilibrium cooling and the Scheil equation, the eutectic can be reached.

74
Q

Why does solubility limit increase with increasing temperature?

A

Average KE increases overcoming the driving force to phase separate.

75
Q

When a species is dissolved in a matrix, how does its concentration relate to its activity?

A

a»conc

76
Q

When a species is in two phases in equilibrium, what can be said about its activity?

A

Its activity is the same in both so it is constant over a composition range.

77
Q

Derive an expression for the driving force of nucleation from a supersaturated solid solution?

A

Check derivation

78
Q

Typically, nucleation in a solid solution will happen via what to reach equilibrium?

A

Via a series of metastable intermediate precipitate phases.

79
Q

Metastable precipitates nucleate before the equilibrium precipitate, why?

A

The free energy of the metastable state is higher, but the activation barrier is lower.

MS3 (5 decks)

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