Test 2. Chapter 10

Question 1

nucleation

Answer 1

• nuclei (seeds ) act as template to grow crystals
• for nucleus to form rate of addition of atoms to nucleus must be faster than rate of loss
• once nucleated, grow until reach equilibrium

Question 2

driving force to nucleate increases as we increase temperature T

• ___________(eutectic, eutectoid) More common
• _________(peritectic)

Answer 2

• supercooling (eutectic, eutectoid)

- superheating (peritectic

Question 3

small supercooling –>

Answer 3

few nuclei-large crystals

Question 4

large supercooling –>

Answer 4

rapid nucleation-many nuclei small crystals

Question 5

what are two solidification processes?

Answer 5

homogeneous nucleation, heterogeneous nucleation

Question 6

homogeneous nucleation

Answer 6

• nuclei form in the bulk of liquid metal

- requires supercooling (typically 80-300C max)

Question 7

heterogeneous nucleation

Answer 7

• much easier since stable “nucleus” is already present
  could be wall of mold or impurities in the liquid phase
  -allow solidification with only .1-10C supercooling

Question 8

by convention rate of transformation

Answer 8

r=1/t

Question 9

reaction rate is a result of ___________ and _________ of crystal

Answer 9

nucleation and growth of crystal

Question 10

Martensite:

Answer 10

y(FCC) to Martensite (BCT)

Question 11

y to M transformation?

Answer 11

• is very rapid

- % tranfs. depends on T only

Question 12

M=_____________ is body centered tetragonal (BCT)

Answer 12

martensite

Question 13

tempering martensite

Answer 13

• reduces brittleness of martensite
• reduces internal stress cause by quenching
• produces extremely small Fe3C PARTICLES surrounded by alpha
• decreases TS, YS but increases %RA