PY2M20 Material Properties and Phase Diagrams
Transcript of PY2M20 Material Properties and Phase Diagrams
PY2M20PY2M20Material Properties and Phase DiagramsL t 8Lecture 8
P. Stamenov, PhDS h l f Ph i TCDSchool of Physics, TCD
PY2M20-8
Phase transformations
1600
L
1400
1200
L
+LL+Fe C1148°C
T(°C)
tite)
1200
1000
(austenite)
F C
L+Fe3C1148°C
cem
ent
800
+ Fe3C
727°C
Fe3C
(c
600 + Fe3C F
4000 1 2 3 4 5 6 6.7
C wt% CCo, wt% C
Eutectic, Eutectoid and Peritectic R tiReactionsEutectic ReactionEutectic Reaction
)2()1( phaseSolidphaseSolidLiquid
Eutectoid reaction
)3()2()1( phaseSolidphaseSolidphaseSolid
Peritectic Reaction
)3()2()1( phaseSolidphaseSolidphaseLiquid )()()( pppq
Phase Transformations
NucleationNucleation nuclei act as seed points to grow crystals for nucleus to form for nucleus to form
rate of addition of atoms > rate of loss once nucleated, growth equilibrium, g q
Driving force to nucleate increases as we increase Tg supercooling (eutectic, eutectoid) superheating (peritectic)
Small supercooling few nuclei - large crystalsLarge supercooling rapid nucleation - many nuclei,
small crystals
Solidification: Nucleation PProcesses
H l i Homogeneous nucleation nuclei form in the bulk of liquid metal requires supercooling (typically 80-300 °C max)
Heterogeneous nucleation Heterogeneous nucleation much easier since stable “nucleus” is already present
Could be wall of mold or impurities in the liquid Cou d be a o o d o pu t es t e qu dphase
Thi k f h ’d b k d dThink of why you’d been asked never to put a used spoon back into a honey jar…
allows solidification with only 0.1-10 ºC supercooling
concentration drive is also possible and often used!
Rate of Phase Transformation
All out of material –y1.0
All out of material transformation complete
orm
ed, Fixed T
maximum rate reached now amount0 5
n tr
ansf
o maximum rate reached – now amount unconverted decreases so rate slows
0.5
rate increases as surface area increases & l i
log tFrac
tion t0.5 & nuclei grow
gF
Avrami (JMAK) rate equation: y = 1- exp(-k t n) k & n are constants for specific system – n is roughlyk & n are constants for specific system n is roughly
related to the growth front dimensionality + 1
The rate is: r = 1 / t0.5 JMAK – Johnson, Mehl, Avrami, Kolmogorov
Rate of Phase Transformations
Adapted from Fig. 10 11 Callister 7e
135C 119C 113C 102C 88C 43C
10.11, Callister 7e.(Fig. 10.11 adapted
from B.F. Decker and D. Harker,
"Recrystallization in
1 10 102 1041021
Recrystallization in Rolled Copper", Trans AIME, 188, 1950, p.
888.)
T
1 10 102 10410 102
1041
In general, rate increases as T
r = 1/t0 5 = A e -Q/RT0.5
R = gas constant T = temperature (K) T = temperature (K) A = pre-exponential factor Q = activation energy
• r often small: equilibrium not possible!
Eutectoid Transformation Rate
Diffusive flow of C needed
• Growth of pearlite from austenite:
Austenite ()grain boundary
cementite (Fe3C)Ferrite ()
pearlite growth
boundary
gdirection
• Eutectoid ) 100transformationrate increases
675°C50pe
arlit
e 600°C (T larger)
650°C
with T.675 C (T smaller)
0y(%
Course pearlite formed at higher T - softerFine pearlite formed at low T - harder
Nucleation and Growth
• Reaction rate is a result of nucleation and growth of crystals.
% Pearlite100
G thNucleation rate increases with T
% Pearlite50 Nucleation
regime
Growth regime
t
Growth rate increases with T
• Examples:0 log(time)t0.5
p
pearlite colony
T j st belo T T moderatel belo T T a belo TT just below TENucleation rate lowGrowth rate high
T moderately below TENucleation rate med Growth rate med.
Nucleation rate highT way below TE
Growth rate low
Transformations & Supercooling
• Can make it occur at:• Eutectoid transf. (Fe-C System): + Fe3C
0 76 wt% C 6 7 t% C• Can make it occur at: ...727ºC (cool it slowly)...below 727ºC (“supercool” it!)
0.76 wt% C0.022 wt% C
6.7 wt% C
1600
T(°C)
1400
1200
L
+L
men
tite)1200
1000
(austenite)
+Fe C
L+Fe3C1148°C
C (c
em
800
+Fe3C
+Fe C
ferrite
727°C
Eutectoid:Equil. Cooling: Ttransf. = 727ºCT
Fe3C600
4000 1 2 3 4 5 6 6 7
+Fe3CTSupercooling by Ttransf. < 727C
0.76
0.02
2
0 1 2 3 4 5 6 6.7(Fe) Co, wt%C
00
Isothermal Transformation Diagrams
• Fe-C system, Co = 0.76 wt% CT f ti t T 675°C• Transformation at T = 675°C.
100ed 100
50T = 675°C
y,
ansf
orm
01 102 104%
tra
time (s)
700Austenite (stable) TE (727C)
Austenite
T(°C)
600
(unstable)Pearlite
isothermal transformation at 675°C
400
500
1 10 102 103 104 105 time (s)
Effect of Cooling History in Fe-C System
• Eutectoid composition, Co = 0.76 wt% C• Begin at T > 727°C• Begin at T > 727 C• Rapidly cool to 625°C and hold isothermally.
700
Austenite (stable) TE (727C)Austenite
T(°C)
600
700 Austenite (unstable)
Pearlite
500
600 Pearlite
400
500
400
1 10 102 103 104 105
time (s)
Transformations with P t t id M t i lProeutectoid Materials
CO = 1.13 wt% CT(°C)900 1600
T(°C)
TE (727°C)A
A
900
800 te)
1600
1400 L
+L
TE (727 C)A
+C
P
700
+P em
enti1200
1000
(austenite)
+L
+Fe3C
L+Fe3C
P
500
600 A+
Fe3C
(c800
600
Fe3C
+Fe3C727°C
T
2
time (s)1 10 102 103 104
F
4000 1 2 3 4 5 6 6.7(Fe) Co, wt%C
0.76
0.02
2
1.13
Hypereutectoid composition proeutectoid cementite
o,
Hypereutectoid composition – proeutectoid cementite
Non-Equilibrium Transformation P d t F CProducts: Fe-C• Bainite: (Davenport & Bain)
t i ith l- strips with longneedles of Fe3C
- diffusion controlled.
Fe3C(cementite)
(f )diffusion controlled.• Isothermal Transf. Diagram (ferrite)
800 A t it ( t bl )
5 m600
800
T(°C)Austenite (stable)
PTEA
100% pearlite
400
600
B
pearlite/bainite boundary
A
100% bainite
00% pea e
400
200
A
10 103 105
ti ( )10-1
200
120 mtime (s) 120 m
cf: Pearlite
Spheroidite: Fe-C System
- + grains with sphericalFe3C (ferrite)
- diffusion dependent.Fe3C- heat bainite or pearlite
for long times (e g 18 h at 700°C)
(cementite)
Fe3C
(e.g. 18 h at 700 C)
- reduces interfacial area (driving force) 60 m
Martensite: Fe-C System
• Martensite: (Martens)(FCC) t M t it (BCT)- (FCC) to Martensite (BCT)
mx potentialFe atom
(involves single atom jumps)
60
xx x
xx
potential C atom sites
Fe atom sites
• Isothermal Transf. DiagramMartensite needlesA t it
800 Austenite (stable)T Austenite
600
T(°C)( )
PTEA
• to M transformation..is rapid!
400 BA- is rapid!- % transf. depends on
T only.200 M + A
M + AM A
0%50%90% y
- can be stress-induced10 103 105 t/s10-1
M + A
Martensite Formation
(FCC) (BCC) + Fe3Cslow cooling ( ) ( ) 3
quench
temperingM (BCT)
M = martensite is body centered tetragonal (BCT)
Diff i l f i BCT if C 0 1 %Diffusionless transformation BCT if C > 0.15 wt%BCT few slip planes hard, brittle
Phase Transformations of Alloys
Effect of adding other elementsChange transition
temp.p
Cr Ni Mo Si MnCr, Ni, Mo, Si, Mnretard + Fe3C t f titransformation
Dynamic Phase Transformations
On the isothermal transformation diagramOn the isothermal transformation diagram for 0.45 wt% C Fe-C alloy, sketch and label th ti t t th t d ththe time-temperature paths to produce the following microstructures:a) 42 % proeutectoid ferrite and 58 % coarse
pearlitepb) 50 % fine pearlite and 50 % bainitec) 100 % martensitec) 100 % martensited) 50 % martensite and 50 % austenite
Example Problem for Co = 0.45 wt%
a) 42 % proeutectoid
A + A800
proeutectoidferrite and 58 % coarse pearlite
A + P
A + A
P
p(amounts determined by phase diagram)
T (°C)
A + BBP
A
600phase diagram)
first make A50%400 M (start)
M (50%)
first make ferritethen pearlite
200M (90%)
coarse pearlite
00 1 10 103 105
higher T0.1 10 10 10
time (s)
Example Problem for Co = 0.45 wt%
a. the amount of pearlite and proeutectoid ferrite () t t f lit t f j t b Tnote: amount of pearlite = amount of just above TE
Co = 0.45wt% CCo 0.45wt% CC = 0.022 wt% CCpearlite = C = 0.76 wt% C 1600
%9.57100x
CCo te)
1400
1200
L
+LL+Fe3C1148°C
T(°C)%9.57100x
CC
emen
tit
1200
1000
(austenite)
+ Fe3C
L+Fe3C1148 C
pearlite = 58 % e 3C
(ce
800
600 + Fe C
727°CR S
pearlite 58 %proeutectoid = 42 %
F600
4000 1 2 3 4 5 6 6.7
+ Fe3C
C wt% CCOCC Co, wt% CCO
Example Problem for Co = 0.45 wt%
b) 50 % fine )pearlite and 50% bainite A + A
80050% bainiteT (°C)
A + P
A + A
Pfirst make pearlitethen bainite A + B
BP
A
600
fi lit
A50%400 M (start)
M (50%)fine pearlite
lower T 200M (90%)
00 1 10 103 1050.1 10 10 10
time (s)
Example Problem for Co = 0.45 wt%
c) 100 %
A + A800
c) 100 % martensite – quench =
A + P
A + A
P
quench = rapid cool T (°C)
A + BBP
A
600
A50%400 M (start)
M (50%)
d) 50 % martensiteand 50 % d)
200M (90%)austenite
d)
00 1 10 103 105
c)
0.1 10 10 10time (s)
Mechanical Prop: Fe-C System (1)
• Effect of wt% C Pearlite (med)
Co < 0.76 wt% C
Pearlite (med)ferrite (soft)
Co > 0.76 wt% C
Cementite(hard)
Hypoeutectoid Hypereutectoid
TS(MPa) Hypo Hyper%EL 80
Hypo Hyper
900
1100TS(MPa)
hardness
100%EL
od, f
t-lb)
80
TSIzod
500
700hardness
50
nerg
y (Iz
o
40
EL
300
500YS
16 0 Impa
ct e0
• More wt% C: TS and YS increase, %EL decreases.wt% C0 0.5 1
0.76 wt% C0 0.5 1
0
0.76
Mechanical Prop: Fe-C System (2)
• Fine vs coarse pearlite vs spheroidite
320
Hypo Hyper 90 Hypo Hyper
240
ness
fine pearlite
coarsepearlite
60
y (%
AR
)
spheroidite
160
inel
l har
d pearlitespheroidite
30Duc
tility
coarsepearlite
80
wt%C0 0.5 1
Br
0t%C
fine pearlite
0 0.5 1wt%C wt%C0
• Hardness:• %RA:
fine > coarse > spheroiditefine < coarse < spheroiditee coa se sp e o d te
Mechanical Prop: Fe-C System (3)
• Fine Pearlite vs Martensite:
Hypo Hyper
600
ness martensite
400
inel
l har
d
200
Br
fine pearlite
H d fi lit << t it
0wt% C0 0.5 1
• Hardness: fine pearlite << martensite.
Tempering Martensite
• reduces brittleness of martensite,• reduces internal stress caused by quenching.
MPaMPa
1600
1800
TS
m
1400
1600
YS
9
1000
1200
405060
%RA%RA
800 3040
200 400 600Tempering T (°C)
• produces extremely small Fe3C particles surrounded by
Tempering T (°C)
• decreases TS, YS but increases %RA
Summary: Processing Options
Austenite ()()
slow cool
moderatecool
rapid quench
BainitePearlite Martensite
cool cool quench
Bainite( + Fe3C plates/needles)
Pearlite( + Fe3C layers + a proeutectoid phase)
Martensite(BCT phase diffusionless
t f ti )transformation)
reheatMartensite T M t it
Tempered Martensiteen
gth
ctili
tyT Martensite
bainite fine pearlite Martensite
( + very fine Fe3C particles)
Stre Ducfine pearlite
coarse pearlitespheroidite
General Trends