Chapter 8: Phase diagrams

Question 1

Phase diagrams

Answer 1

Diagrams/maps that depict the different phases of materials.

Question 2

Phase diagram influences (3)

Answer 2

Temperature, pressure, composition

Question 3

Equilibrium in a phase diagram

Answer 3

Phases in the system have the same properties everywhere and there are no concentration gradients.

Question 4

Phase

Answer 4

Region of matter that has homogeneous physical and chemical properties. A single-phase material has the same composition and properties at every point throughout the material.

Question 5

Poly-phase materials

Answer 5

Materials that contain 2+ coexisting phases, and their properties depend upon: the number the phases, composition, structure, properties, size and spatial distribution.

Question 6

Components

Answer 6

The elements (atoms), molecules, or compounds which are initially mixed.

Question 7

Solute

Answer 7

The component of a solution present in minor concentration

Question 8

Solvent

Answer 8

The component of a solution present in major concentration

Question 9

Solubility limit

Answer 9

The maximum concentration of solute that may be added without forming a new phase

Question 10

Metastable

Answer 10

A non-equilibrium state that may persist for a very long time. When given extra energy to stimulate the transformation (EX: heat), they transform to the stable equilibrium phase.

Question 11

Isomorphous

Answer 11

Having the same crystal structure in the solid phase and complete solid solubility for all compositions.

Question 12

Solidus line

Answer 12

Line on a phase diagram where solidification is complete upon equilibrium cooling, or where melting begins upon heating. Below this line only solid is present, while above some liquid is present.

Question 13

Liquidus line

Answer 13

Line on a phase diagram above which melting of a solid is complete when heated, or where solidification begins upon cooling. Above this line only liquid is present, while below this line some solid is present.

Question 14

System composition (Co)

Answer 14

How much of each element is present in total. If more than one phase is present, they will have usually different compositions than the overall composition.

Question 15

Tie Line

Answer 15

Horizontal line constructed across a two phase region of a binary phase diagram. Its intersections with the phase boundaries on either end of represents the equilibrium compositions of the respective phases.

Question 16

Lever rule equations

Answer 16

Sum of weight fractions:
W1 + W2 = 2
Conservation of mass:
Co = W1C1 + W2C2

Question 17

Weight fraction equations

Answer 17

W1 = (C2 - Co)/(C2 - C1)
W2 = (Co - C1)/(C2 - C1)

Question 18

Eutectic phase diagrams

Answer 18

Have an invariant point where there is an equilibrium between a two phase solid region and a liquid.

Question 19

Eutectic reaction

Answer 19

L(Ce) <–> α(Cαe) + β(Cβe)

Question 20

Hypoeutectic composition

Answer 20

Range of compositions at the eutectic temperature between the α solid solubility limit and the eutectic composition.

Question 21

Hypereutectic compositions

Answer 21

Range of compositions at the eutectic temperature between the eutectic composition and the β solid solubility limit.

Question 22

Proeutectic/primary phase

Answer 22

The first phase to start solidifying upon cooling. Forms at temperatures above the eutectic temperature (Te) but below the liquidus.

Question 23

Eutectoid reaction

Answer 23

Similar to a eutectic reaction except the high T liquid phase is replaced by another solid phase so the whole reaction is in the solid state.

Question 24

Austenite (γ)

Answer 24

High temperature solid form of FCC iron that dissolves up to 2 wt% C.

Question 25

α-Ferrite (α)

Answer 25

Soft and ductile BCC iron that exists at 700ºC or higher that dissolves up to 0.022 wt% C.

Question 26

Cementite (Fe3C)

Answer 26

Hard, brittle intermetallic strengthening agent in most steels if it is finely dispersed, but embrittling when present in a continuous network. It has a fixed composition, steels have 0-1% C and are up to 1000ºC.

Question 27

Iron-carbon eutectoid transformation

Answer 27

At 0.77% C and 727ºC, steel goes from austenite to ferrite and cementite, all in the solid phase. γ → α + Fe3C

Question 28

Pearlite

Answer 28

2 phase (α-ferrite and Fe3C) lamellar structure formed by the eutectoid iron-carbon transformation. It is a eutectoid microconstituent that looks like a fingerprint under a microscope.

Question 29

Eutectoid grain transformation and cooling rate

Answer 29

The faster we transform to pearlite, the finer the lamellar spacing and the harder the steel. If we cool more quickly through the eutectoid temperature, then the eutectoid microconstituent will get finer and finer.

Question 30

AISI/SAE steel designation (YYXX)

Answer 30

First two digits (YY) are the alloy number, (10 for “plain carbon steel”), second two digits (XX) give the C concentration in wt%.

Question 31

Pure iron wt% C

Answer 31

<0.008

Question 32

Steel wt% C

Answer 32

0.008 – 1.0

Question 33

Cast iron wt% C

Answer 33

2.14 - 4.5