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Material Science > MAE 3324 CH9 > Flashcards

MAE 3324 CH9 Flashcards

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1
Q

Why should we study phase diagrams?

A

The development of the microstructure of an alloy, and therefore mechanical properties, are related to the characteristics of the alloy’s phase diagram.

Ref: Pg 252

Extra info: Phase diagrams also provide valuable information about melting, casting, crystallization, and other phenomena.

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2
Q

Define “Component.”

A

Pure metals and/or compounds of which an alloy is composed.

Ref: Pg 253

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3
Q

Define “system” with respect to material science.

A

A system may refer to a specific body of material under consideration (e.g., a ladle of molten steel).

Extra notes: It may also relate to the series of possible alloys consisting of the same components but without regard to alloy composition.

Ref: Pg 253

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4
Q

Define “solid solution.”

A

A solid solution consists of atoms of at least two different types; the solute atoms occupy either substitutional or interstitial positions in the solvent lattice, and the crystal structure of the solvent is maintained.
Ref: Pg 253

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5
Q

Define “Solubility Limit.”

A

For many alloy systems and at some specific temperature, there is a maximum concentration of solute atoms that may dissolve into the solvent to form a solid solution.
Ref: Pg 253

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6
Q

What happens if the solubility limit is exceeded?

A

The addition of solute in excess of this solubility limit results in the formation of another solid solution or compound that has a distinctly different composition.

Extra Notes: An example would be adding too much sugar to a fixed volume of water.

Ref: Pg 253

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7
Q

True or False

The solubility limit for a particular weight percentage will increase if the temperature is also increased.

A

True

Ref: Pg. 253

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8
Q

Define a “Phase.”

A

A homogeneous portion of a system that has uniform physical and chemical characteristics.

Ref: Pg 254

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9
Q

True or False

Every pure metal is considered to be in phase; so also is every solid, liquid, and gaseous solution.

A

True

Ref: Pg 254

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10
Q

True or False
If more than one phase is present in a given system, each will have its own…
1. Distinct properties
2. Boundary across which there will be no change in physical and chemical characteristics.

A 
#1: True
#2: False, the boundary represents the discontinuity where there is an abrupt change in both physical (mechanical) and chemical characteristics.

Ref: Pg. 254

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11
Q

If water and ice are both in the same container, are one or two distinct phases present?

A

Two phases exist.
Explanation: While the chemical makeup is the same; the physical properties are different. One is a liquid while the other is solid.

Ref: Pg. 254

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12
Q

True or False
When a structure contains a polymorphic form (e.g. having both BCC and FCC structures) each of these structures can be considered in the same phase.

A

False
Each structure is in a separate phase because their respective physical characteristics differ.

Ref: Pg. 254

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13
Q

A single-phase system is often referred to as ____.

A

Homogeneous.

Ref: Pg 254

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14
Q

Systems composed of two or more phases are referred to as _____.

A

Mixtures or a heterogeneous system.

Ref: Pg 254

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15
Q

True or False

Most metallic alloys, ceramics, polymeric, and composite systems are homogeneous.

A

False. Most are heterogeneous.

Ref: Pg. 254

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16
Q

In metal alloys, the microstructure is characterized by what three items?

A
  1. Number of phases present.
  2. Phase proportions (how big they are compared to each other).
  3. Manner in which the phases are arranged.

Ref: Pg 254

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17
Q

True or False

Often in a two-phase alloy, the different phases can be distinguished by color.

A

True

Ref: Pg 254

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18
Q

True or False

When only a single-phase or solid solution is present, the texture is uniform, except for grain boundaries.

A

True

Ref. Pg. 254

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19
Q

Equilibrium is best described in terms of what?

A

A thermodynamic concept called “free energy.”

Extra Notes: Also called Gibbs free energy.

Ref: Pg. 254

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20
Q

Describe “free energy.”

A

A thermodynamic quantity that is a function of both the internal energy (enthalpy) and entropy (or randomness) of a system. At equilibrium, the free energy is at a minimum.

Ref: Glossary

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21
Q

When is a system at equilibrium?

A

When its free energy is at a minimum under some specified combination of temperature, pressure, and composition.

Extra Note: For example, the PT diagram for water shows what combinations of pressure and temp will give a given phase of water in equilibrium conditions.

Ref: Pg. 254

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22
Q

True or False

When a system is in equilibrium, its macroscopic characteristics do NOT change with time.

A

True

Extra Note: A “stable system” is a term that is also used when describing equilibrium.

Ref: Pg 254

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23
Q

Define “Phase Equilibrium” with respect to general macroscopic characteristics.

A

When the characteristics of the system do not change over time.

Ref: Pg 254 & 255

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24
Q

True or False

In many metallurgical and material systems of interest, phase equilibrium involves just solid phases.

A

True

Ref: Pg 255

25
Q

What is a “metastable state?”

A

In solid systems, sometimes a state of equilibrium is never completely achieved because the rate of approach to equilibrium is extremely slow. Such a system is said to be in non-equilibrium or a metastable state.

Ref: Pg 255

26
Q

True or False

A phase diagram is the same thing as an equilibrium diagram.

A

True

Ref: Pg 255

27
Q

What are the three externally controlled parameters that affect phase structure?

A
  1. Temperature
  2. Pressure
  3. Composition

Ref: Pg. 255

28
Q

True or False

In a one-component system, the composition is held constant.

A

True

Extra notes: These diagrams are used to show the phase characteristics of pure substances. A unary phase diagram is also called a pressure-temperature (PT) diagram. Most often the pressure axis is scalled logarithmically.

Ref: Pg 255 & 256

29
Q

What is the “triple point” on a PT diagram?

A

The point on the PT diagram where all three boundaries meet. At this point, all three phases are in equilibrium.

Extra notes: Another name for the triple point is the “invariant point.”

Ref: Pg256

30
Q

Define “isomorphous,” with respect to a binary alloy system.

A

Having the same structure. In the phase diagram sense, isomorphicity means having the same crystal structure or complete solid solubility for all compositions.

Ref: Textbook Glossary

31
Q

For metallic alloys, solid solutions are commonly designated by…

A

Lowercase Greek letters.

Ref: Pg 257

32
Q

On a binary phase diagram, the “liquidus line” represents ____.

A

The liquidus line represents the phase boundary between the liquid (L) and the (alpha + L) phase fields.

Ref: Pg 257

33
Q

On a binary phase diagram, the “solidus line” represents ____.

A

The solidus line represents the phase boundary between the solid solution + liquid (alpha + L) and the solid solution (alpha) phase fields.

Extra note: The alpha phase field is the solid solution portion of the diagram.

Ref: Pg 257

34
Q

What does the (alpha + L) portion of a binary diagram represent?

A

A two-phase field in which the solid solution and liquid phases are in equilibrium.

Ref: Pg. 257

35
Q

The points where the solidus and liquidus lines intersect are referred to as ___.

A

The melting temperatures of the two pure components.

Ref: Pg 258

36
Q

What primary three pieces of information can be extracted from a binary diagram?

A
  1. Phases present.
  2. The compositions of the phases
  3. Percentages or fractions of the phases.

Ref: Pg 259

37
Q

On a binary eutectic phase diagram between copper and silver, what are the values for alpha, and beta?

A

Alpha is the solid solution with silver as the solute component.

Beta is the solid solution with copper as the solute.

Ref: pg 266

38
Q

What is a eutectic reaction?

A

Upon cooling, a liquid phase is transformed into the two solid alpha and beta phases at the temperature Te. The opposite occurs upon heating.

Ref: pg 268

39
Q

Tie lines represent ___.

A

Isotherms in two-phase regions.

Ref: Pg 259

40
Q

Define “Isomorphous.”

A

Having the same structure. In the phase diagram sense, isomorphicity means having the same crystal structure or complete solid solubility for all compositions

Ref: Textbook Glossary

41
Q

Consider an isomorphous alloy undergoing extremely slow cooling. If the temperature starts well above the liquidus line and decreases slowly, where does microstructural change start to occur?

A

Once the liquidus line is reached.

Ref: Pg 263

42
Q

Why is equilibrium cooling not a practical method of manufacturing alloys?

A

Because diffusion that occurs between the liquid and solid phases as cooling takes place occurs extremely slowly.

Ref: Pg264

43
Q

True or False

On a pure substance phase diagram, the pressure axis is usually scaled logarithmically.

A

True

Ref: Pg256

44
Q

What does a Binary Phase diagram show?

A

Binary phase diagrams are maps that represent the relationships between temperature and the composition and qualities of phases at equilibrium which influence the microstructure of an alloy.

Ref: Pg 257

45
Q

Most microstructures in binary alloys result after a ___.

A

Phase Transformation.

Ref: Pg 257

46
Q

Binary phase diagrams are useful in predicting ____.

A

Phase transformations, and the resulting microstructure which may have equilibrium or non-equilibrium character.

Ref: Pg 257

47
Q

True or False

A tie line is the same thing as an isotherm on a binary phase diagram.

A

True

Ref: Pg 259

48
Q

The composition of phases are expressed in ___.

A

In terms of the weight percents of the components.

Ref: Pg 261

49
Q

True or False

For an alloy consisting of a single-phase, the composition of that phase is the same as the total alloy composition.

A

True

Ref: Pg 261

50
Q

True or False

With regard to fractional phase amounts, when a single phase exists, the alloy is completely that phase.

A

True

Ref: Pg 261

51
Q

Describe the speed of diffusion rates with respect to solid solution cooling. How does temperature affect the diffusion rate in this scenario?

A

Diffusion rates are especially low for the solid phase.

Rates decrease even further with diminishing temperature.

Ref: Pg264

52
Q

True or False
Virtually in all practical solidification situations, cooling rates are much too rapid to allow composition readjustment and maintenance of equilibrium through diffusion.

A

True

Ref: Pg 264

53
Q

Due to cooling rates in excess of what normal diffusion processes can keep up with ____.

A

Microstructures outside of the extremely slow cooling model presented in section 9.9 are developed.

Ref: Pg 264

54
Q

On a binary eutectic phase diagram, the line separating the alpha and alpha + beta regions is termed ___.

A

The solvus line.

Ref: Pg 268

55
Q

Describe a Eutectic reaction.

A

When a liquid solution immediately is transformed into a two-phase solid.

Ref: Pg 269

56
Q

What is different between a Eutectic reaction and the solidification of a pure component?

A

In a Eutectic reaction when cooling occurs across the eutectic isotherm, the liquid solution transforms into a two-phase solid solution. For a pure substance, only one phase is present in the solid solution formed.

Ref: Pg 270

57
Q

True or False

For a eutectic system, three phases (alpha, beta, and liquid (L)) can never be in equilibrium.

A

False. They can be in equilibrium but only at points along the eutectic isotherm.

Ref: Pg 269

58
Q

True or False
The lever rule cannot be used at the eutectic isotherm temperature (Te) because the compositions are in equilibrium. However, it can be used at temperatures high and lower than Te.

A

True

Ref: Pg 271

Material Science (15 decks)

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