Test 2. Chapter 10 Flashcards
nucleation
- 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
driving force to nucleate increases as we increase temperature T
- ___________(eutectic, eutectoid) More common
- _________(peritectic)
- supercooling (eutectic, eutectoid)
- superheating (peritectic
small supercooling –>
few nuclei-large crystals
large supercooling –>
rapid nucleation-many nuclei small crystals
what are two solidification processes?
homogeneous nucleation, heterogeneous nucleation
homogeneous nucleation
- nuclei form in the bulk of liquid metal
- requires supercooling (typically 80-300C max)
heterogeneous nucleation
- 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
by convention rate of transformation
r=1/t
reaction rate is a result of ___________ and _________ of crystal
nucleation and growth of crystal
Martensite:
y(FCC) to Martensite (BCT)
y to M transformation?
- is very rapid
- % tranfs. depends on T only
M=_____________ is body centered tetragonal (BCT)
martensite
tempering martensite
- reduces brittleness of martensite
- reduces internal stress cause by quenching
- produces extremely small Fe3C PARTICLES surrounded by alpha
- decreases TS, YS but increases %RA