CH 10 - Phase Transformations

Question 1

What is nucleation?

Answer 1

• nuclei (seeds) act as templates on which crystals grow

- once nucleated, growth proceeds until equilibrium is attained

Question 2

What is the relationship between supercooling and crystal size?

Answer 2

small supercooling:
-slow nucleation rate
-few nuclei
-large crystals
large supercooling:
-rapid nucleation rate
-many nuclei
-small crystals

Question 3

What is homogenous nucleation?

Answer 3

• nuclei form in the bulk of liquid metal

- requires considerable supercooling

Question 4

What is heterogeneous nucleation?

Answer 4

• “nucleating surface” already present
• much easier than homogeneous
• only slight supercooling required

Question 5

How is pearlite formed?

Answer 5

• when cooled, austenite transforms to a ferrite phase w/much lower carbon content & cementite w/much higher carbon concentration
• austenite to pearlite transform occurs only if an alloy is supercooled below the eutectoid
• on graph, left of transform = only austenite, right of finish curve = only pearlite

Question 6

At what temperatures is coarse pearlite formed?

Answer 6

higher temps

Question 7

At what temperatures is fine pearlite formed?

Answer 7

lower temps

Question 8

What is bainite?

Answer 8

• microstructure

- consists of ferrite & cementite phases

Question 9

How does bainite form?

Answer 9

• transformation occurs at temps below where pearlite forms
• pearlite and bainite compete with each other; once a portion of the alloy has become either bainite or pearlite, transform to the other microstruct. is not possible w/o reheating to form austenite

Question 10

How does shperoidite form?

Answer 10

-steel alloy having either pearlitic or bainitic mictrostruct. is heated to and left at a temp just below the eutectoid for long times

Question 11

How does martensite form?

Answer 11

• austenitized iron-carbon alloys are rapidly cooled to a relatively low temp
• competitive with pearlite and bainite

Question 12

What microstructure forms when austenite is slowly cooled?

Answer 12

pearlite

Question 13

What microstructure forms when austenite is moderately cooled?

Answer 13

bainite

Question 14

What microstructure forms when austenite is rapidly quenched?

Answer 14

martensite

Question 15

What microstructure forms when martensite is reheated?

Answer 15

tempered martensite

Question 16

List all mictrostructures from strongest to weakest

Answer 16

martnesite, tempered martensite, bainite, fine pearlite, coarse pearlite, spheroidite