Chapter 9: Phase Transformations

Question 1

Phase transformations

Answer 1

Transitions of a material to a new arrangement of its phases.

Question 2

Driving force for a phase transformation

Answer 2

Lowering the total free energy (G)

Question 3

Undercooling (ΔT)

Answer 3

The magnitude of the temperature difference between the equilibrium transformation temperature and the temperature of the system being investigated.

Question 4

Diffusional phase transformations

Answer 4

Involves a change in composition and/or number of phases and not a change in composition/number of phases.

Question 5

Diffusional phase transformations examples (4)

Answer 5

eutectic/eutectoid reactions, diet coke and mentos, beer bubbles, ice crystals on a windshield

Question 6

Stages of diffusional phase transformations (2)

Answer 6

1. Nucleation
2. Growth

Question 7

Nucleation stage of diffusional phase transformations

Answer 7

The formation of a group of atoms which constitute a small nucleus of the phase that can achieve stable growth.

Question 8

Growth stage of diffusional phase transformations

Answer 8

The nucleus grows via mass transport of atoms or molecules to and from the growing interface to maintain the composition of the stable phase(s).

Question 9

Diffusionless transformations

Answer 9

Can produce metastable phases (EX: austenite to martensite) and involve a reordering of the atoms. This requires nucleation but no diffusion in order to have a phase transformation.

Question 10

Increasing ductility/decreasing strength of iron-carbon microconstituents (6)

Answer 10

1. Martensite
2. Tempered Martensite
3. Bainite
4. Fine pearlite
5. Coarse pearlite
6. Spheroidite

Question 11

Grain size and stability

Answer 11

Least stable phases have the finest grains.

Question 12

Time Temperature Transformation (TTT) diagrams uses (3)

Answer 12

1. Name(s) of the microconstituent(s) that will be present after isothermal decomposition at a temperature
2. Time at which a transformation will start
3. Phase transformation rate as a function of undercooling.

Question 13

Diffusion on TTT and CCT diagrams

Answer 13

Diffusional phase transformation regions are contained within curved lines. Diffusionless phase transformations occur between the horizontal lines.

Question 14

Normalizing

Answer 14

Heating into the austenite region (austenizing) then air cooling, producing fine pearlite

Question 15

Full annealing

Answer 15

Austenitizing and then furnace cooling, produces coarse pearlite

Question 16

Spheroidizing

Answer 16

Heating any microconstituent for a long time just below the eutectoid to make the cementite into spherical balls. Produces a soft, very ductile steel for forming.

Question 17

Quenching

Answer 17

Rapidly cooling, usually to produce martensite.

Question 18

Hardenability

Answer 18

The depth to which martensite can be formed in an alloy (ease of avoiding the pearlite or bainite and obtaining martensite). An alloy which can form martensite deeper into its center has higher hardenability.