Ch. 9 - Phase Diagrams

Question 1

Component

Answer 1

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

Question 2

Solvent

Answer 2

The base.

Question 3

Solute

Answer 3

The thing that is being dissolved.

Question 4

Solubility limit

Answer 4

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

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

Question 5

Equilibrium

Answer 5

When a system’s free energy is at a minimum under some specified combination of temperature, pressure, and composition. Characteristics of the system do not change with time but persist indefinitely.

Question 6

Free energy

Answer 6

Function of the internal energy and the randomness of the atoms or molecules (entropy).

Question 7

Phase equilibrium

Answer 7

Equilibrium as it applies to systems in which more than one phase may exist. Reflected by a consultancy with time in the phase characteristics of a system.

Question 8

Metastable

Answer 8

A state of true equilibrium is never completely achieved because the rate of approach is extremely slow. The system is in nonequilibrium. This state may persist indefinitely, experiencing only extremely slight and almost imperceptible changes as time progresses.

Question 9

Isomorphous

Answer 9

As in the nickel-copper system. The two elements are nearly completely soluble within one another. This is due to the fact that the two elements have the same crystal structure (FCC), nearly identical atomic radii and electronegativities, and similar valences.

Question 10

Tie line

Answer 10

An isotherm that extends across the two-phase region and terminates at the phase boundary lines on either side. Perpendiculars to the intersections are dropped and the wt% of the solute at the boundary can be determined.

Question 11

Lever rule / inverse lever rule

Answer 11

Tie line is constructed. The overall alloy composition is located on the tie line. The fraction of one phase is computed by taking the length of the tie line from the overall alloy composition to the phase boundary for the OTHER phase and dividing this length by the total tie length.

Question 12

Equilibrium cooling

Answer 12

Extremely slow cooling. Sufficient time must be allowed at each temperature for the appropriate compositional readjustments.

Question 13

Nonequilibrium cooling

Answer 13

In practice, it is impossible to cool slow enough for compositional readjustments to fully realize. When cooling from a liquid to a solid, this type of solidification results in grains that have higher concentrations of the solid at their center. The AVERAGE concentration of the grain will be approximately what is expected based on the intersection of the tie line. The degree of displacement of the new solidus curve from the equilibrium solidus curve depends on the rate of cooling. Less time given to cooling means greater displacement.

Generally results in a cored structure, which gives rise to less than optimal properties. Grain boundaries will melt faster because they are less rich in the higher-melting component. Results in a sudden loss of mechanical integrity

Question 14

Segregation

Answer 14

Concentration gradients are established across the grains as a result of nonequilibrium cooling.

Question 15

Solvus line

Answer 15

Solid solubility limit line separating the A solid and the A solid + B solid phase regions.

Question 16

Solidus line

Answer 16

Boundary between the solid region and the solid + Liquid regions. Basically represents the lowest temperature at which a liquid can exist.

Question 17

Liquidus line

Answer 17

The line between The Liquid region and the Liquid + solid region.

Question 18

Invariant point

Answer 18

The point corresponding to the eutectic concentration and temperature.

Question 19

Eutectic reaction

Answer 19

When cooling, the Liquid turns into two different solids, A and B. This occurs at the lowest temperature at which the liquid will still be liquid. Handy for solder.

Question 20

Eutectic structure

Answer 20

Microstructure that occurs at a temperature just below the eutectic point, and it consists of alternating layers (lamellae) of the two solids, A and B. Pearlite.

Question 21

Two forms of A (alpha) solid

Answer 21

When the hypoeutectic microstructure forms, A grains form within the pearlite. The solid A grains are known as primary A and the A that exists in the pearlite is known as Eutectic A. The eutectic A will have the same weight percentage as it did immediately before it crossed the eutectic isotherm. The pearlite will have the same weight percentage as the liquid.

Question 22

Microconstituent

Answer 22

An element of the microstructure having an identifiable and characteristic structure.

Question 23

Terminal solid solutions

Answer 23

Solid solutions of a phase diagram that exist only at the extreme horizontals of the diagram.

Question 24

Intermediate solid solutions

Answer 24

A solid that exists within a phase diagram somewhere other than at one of the two horizontal extremes.

Question 25

Eutectoid reaction

Answer 25

When cooled, a solid phase turns into two different solid phases.

Question 26

Peritectic reaction

Answer 26

When cooled, a solid + liquid phase turn into a single solid phase.

Question 27

Congruent transformations

Answer 27

Those phase transformations for which there are no compositional alterations.

Eutectic and eutectoid reactions are INcongruent transformations.

Question 28

Ferrite

Answer 28

Part of the iron-iron carbide phase diagram. The stable form that exists at room temperature, also known as alpha iron. Has a BCC crystal structure.

Question 29

Austenite

Answer 29

Part of the iron-iron carbide phase diagram. Occurs when ferrite experiences a polymorphic transformation to an FCC structure at 912°C. Also known as gamma (y) iron.

Question 30

Cementite

Answer 30

Part of the iron-iron carbide phase diagram. Formed at 6.70 wt% C. Fe3C. Prior to this phase, the iron-carbon system is considered to be “iron rich”.

Question 31

Delta ferrite

Answer 31

Part of the iron-iron carbide phase diagram. Occurs when austenite (gamma ferrite) reaches 1394°C and reverts back to a BCC structure.

Question 32

Eutectic of iron-iron carbide system

Answer 32

L –> austenite + cementite

Question 33

Carbon concentrations in iron

Answer 33

Iron: <0.008 wt% C
Steel: 0.008 to 2.14 wt% C
Cast iron: 2.14 to 6.70 wt% C

Question 34

Pearlite

Answer 34

Microstructure of eutectoid steel that is slowly cooled through the eutectoid temperature. Consists of alternating layers or lamellae of the two phases (ferrite and cementite). Carbon atoms diffuse away from the 0.022 wt% ferrite and goes to the 6.70 wt% cementite layers.

Question 35

Hypoeutectoid alloy

Answer 35

When cooling occurs to the left of the eutectoid, between 0.022 and 0.76 wt% C. Consists of grains of pearlite (which form from the austenite) and grains of ferrite (which begin growing in the ferrite + austenite phase region). The ferrite in the pearlite is known as proeutectoid ferrite.

Question 36

Hypereutectoid alloys

Answer 36

Those microstructures that are formed to the right of the eutectoid point, between 0.76 and 2.14 wt% C. Consists of grains of pearlite (which form from the austenite) and grains of cementite (which begin growing in the cementite + austenite phase region). The cementite in the pearlite is known as proeutectoid cementite.