Materials- Metals: Phase Diagrams

Question 1

What is an alloy?

Answer 1

A metallic substance composed of two or more elements

Question 2

What is a solid solution?

Answer 2

A homogeneous crystalline phase that contains two or more chemical species. The solute atoms occupy either the substitutional or interstitial positions in the solvent lattice and the crystal structure of the solvent is maintained.

Question 3

Define solute

Answer 3

One component or element of a solution present in a minor concentration dissolved in the solvent

Question 4

Define solvent

Answer 4

The component of a solution present in the greatest amount. It dissolves the solute

Question 5

Define solubility limit for an alloy system

Answer 5

The maximum concentration of solute atoms that may dissolve in the solvent to form a solid solution at some specific temperature.

Question 6

What happens if you add solute atoms in excess of the solubility limit?

Answer 6

It results in the formation of another solid solution or compound that has a distinctly different composition

Question 7

What is a phase?

Answer 7

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

Question 8

Define homogeneous and heterogeneous

Answer 8

Homogenous: a single phase system
Heterogeneous: a system composed of two or more phases (mixture)

Question 9

How does the formation of a solid solution depend on atomic size factor, crystal structure and electronegativity?

Answer 9

ASF- the difference in atomic radii between the two atoms must be less than about +/-15% otherwise the solute atoms will create substantial lattice distortions and a new phase will form.
CS- crystals structures for metals of both atom types must be the same
E- greater difference in electronegativity means greater likelihood that an intermetallic compound forms instead of substitutional solid solution

Question 10

What is a multiphase alloy?

Answer 10

An alloy that consists of two or more solid crystal types and/or composition due to more solute atoms being added than can be in solid solution with the solvent

Question 11

How is an alloy made for industrial use?

Answer 11

Melt and mix elements in proportions desired. Cast the mixture to form an ingot. Then there is homogenisation (heat treatment) by hot and cold rolling. This forms a plate or sheet of the alloy.

Question 12

Describe the boundary separating phases in a multi phase system

Answer 12

Represents a discontinuous and abrupt change in physical and/or chemical characteristics

Question 13

What is a binary phase diagram?

Answer 13

A map that represents the relationships between temperature and the compositions and quantities of phases at equilibrium, which influence the micro structure of an alloy. Pressure is held constant (normally 1atm).

Question 14

What is ferrite?

Answer 14

α iron. BCC crystal structure. Forms below 910°C

Question 15

What is austenite?

Answer 15

γ iron. FCC crystal structure. Forms above 910°C up to 1400°C.

Question 16

Describe the binary phase diagram for the simplest alloy system

Answer 16

Y axis of temperature. X axis of composition (in % of one component). Has a liquidus line above which the material is completely liquid. Has a solidus line below which the substance is completely solid. Between the two lines (which meet at 0 and 100%) is the liquid+solid solid field where the alloy exists as a mixture of liquid and solid.

Question 17

What do lower case Greek letters represent on a phase diagram?

Answer 17

Different solid solutions

Question 18

For a single phase region of the binary phase diagram, what is composition of the phase?

Answer 18

The same composition as the alloy at that temperature

Question 19

How to find the compositions of the two phases of a two-phase region of a binary phase diagram at a specified temperature and initial alloy composition, C0

Answer 19

Draw a tie line (isotherm) across the two-phase region at the temperature of the alloy. Note the intersections of the tie line and the phase boundaries on either side. Perpendiculars a dropped from these intersections to the horizontal composition axis from which the composition of each of the respective phases is read.

Question 20

How to find the relative phase amounts in a two-phase region of a binary phase diagram for a known temperature and initial alloy composition (C0)

Answer 20

Construct a tie line across the two-phase region at the temperature of the alloy. The overall alloy composition is located on this tie line. The fraction of one phase is computed by taking the length of the tie line from the overall alloy composition (C0) to the phase boundary for the other phase and dividing by the total tie line length. If the composition axis is scaled in weight %, the answer will be a mass fraction. This is the lever rule

Question 21

What is different about a binary eutectic phase diagram?

Answer 21

There is more than one type of solid solution phase possible in the alloy. The solvus line is the solubility limit line separating two different solid solution phases.

Question 22

What is an invariant point on a binary eutectic phase diagram?

Answer 22

The point through which if the temperature changes (with the specific composition at that point), a eutectic reaction occurs. That is, upon cooling, a liquid phase is transformed into two or more solid phases. The opposite occurs upon heating.

Question 23

Example of an equation representing a eutectic reaction

Answer 23

L(CE)α(CαE)+β(CβE)
All letters after C are subscript 
CE represents the eutectic composition 
L represents liquid
α and β represent different solid phases
CαE and CβE are respective compositions of α and β at the eutectic temperature (written as 61.9wt% Sn for example)

Question 24

What happens if you cool a eutectic alloy so it crosses the solvus line?

Answer 24

The solubility limit for one solid phase is exceeded, resulting in the formation of small particles of a different solid phase. With continued cooling, these particles grow in size.

Question 25

What happens when you cool a eutectic alloy so it passes through an invariant point?

Answer 25

Upon crossing the eutectic temperature at the eutectic composition, the liquid transforms to the two solid phases simultaneously. During this transformation there is a redistribution of the two components (because the α and β phases have different compositions to each other and the liquid). This is accomplished by atomic diffusion just ahead of the eutectic-liquid interface. The microstructure of the solid formed consists of alternating layers (lamellae) of the α and β phases.

Question 26

What is a eutectic structure?

Answer 26

A two-phase microstructure resulting from solidification of a liquid having the eutectic composition. The phases exist as lamellae that alternate with one another.

Question 27

What is an intermetallic compound?

Answer 27

A compound of two metals that has a distinct chemical formula. On a phase diagram it appears as an intermediate phase that exists over a very narrow range of compositions

Question 28

What is a eutectoid reaction?

Answer 28

A reaction wherein, upon cooling, one solid phase transforms isothermally and reversibly into two new solid phases that are intimately mixed

Question 29

What is a peritectic reaction?

Answer 29

A reaction wherein upon cooling, a solid and a liquid phase transform isothermally and reversibly to a solid phase having a different composition