Lecture #17: Fracture & Phase Diagrams - … on phase diagram Sometimes, discrete intermediate...
Transcript of Lecture #17: Fracture & Phase Diagrams - … on phase diagram Sometimes, discrete intermediate...
LECTURE #16: PHASE DIAGRAMS &
PHASE TRANSFORMATIONS
ENGR 151: Materials of Engineering
EUTECTIC MICROSTRUCTURE
If conditions of equilibrium are not maintained
while passing through α+L :
Primary constituent will not have uniform
distribution of solute across grains
Fraction of eutectic microconstituent will be greater
than for the equilibrium situation
INTERMEDIATE PHASES
We have only seen terminal solid solutions: α &
β exist at extremities of phase diagram
Intermediate solid solutions: phases found at
locations other than the extremes
Copper-zinc system: six different solid solutions
Dashed lines indicate indetermination of data
(temperatures too low and reaction times too long)
Each region is still limited to two phases
Extension of simpler case – we deal with each line
as a boundary separating two phase states
INTERMEDIATE PHASES
INTERMEDIATE COMPOUNDS
Intermetallic compounds: compounds
identified on phase diagram
Sometimes, discrete intermediate compounds
rather than solid solutions may be formed
Mg2Pb: only exists at 19 wt% Mg-81 wt% Pb,
represented by vertical line on diagram
Large solubility of one metal in another metal is
signified by a large composition span of solid phase
INTERMEDIATE COMPOUNDS
EUTECTOID REACTIONS
Other invariant points exist involving three phases (Cu-
Zn system, 74 wt% Zn-26 wt% Cu)
Eutectoid (eutectic-like) reaction
One solid phase forms into two other solid phases
Point E is the eutectoid point, the corresponding tie line is the
eutectoid isotherm
cooling
heating
EUTECTOID REACTIONS
PERITECTIC REACTIONS
Upon heating, one solid phase transforms into a liquid
phase and another solid phase:
78.6 wt% Zn-21.4 wt% Cu
cooling
heatingL
CONGRUENT PHASE TRANSFORMATIONS
Congruent transformation: when there is no
change in the composition for phases involved
(Mg2Pb melts congruently)
Incongruent transformation: change occurs in
phase composition during transformation
(peritectic reaction)
CONGRUENT PHASE TRANSFORMATIONS
CERAMIC AND TERNARY PHASE DIAGRAMS
Phase diagrams exist for ceramic systems, not
just for metal-metal systems
Phase diagrams exist for three-component
systems
3-D model required
Complex systems
IRON-CARBON SYSTEM
IRON-CARBON SYSTEM
Most important binary alloy system
Steels and cast irons (contain Fe and C)
Pure iron
At room temperature, stable form of iron is ferrite (α
iron, BCC)
At 912° Celsius, austenite forms (γ iron, FCC)
At 1394° Celsius, δ ferrite forms, BCC
IRON-CARBON SYSTEM
α-ferrite Austenite
THE IRON-CARBON SYSTEM
At 6.7 wt% C, cementite (Fe3C) is present (also
known as iron carbide)
Two phase diagrams
< 6.7 wt% C (iron-rich)
> 6.7 wt% C (100 wt% C = pure graphite)
All steels and cast irons have carbon contents
less than 6.7 wt% C
Focus is on Fe-Fe3C phase diagram only
IRON-CARBON SYSTEM
Carbon is an interstitial impurity
Max solubility in α ferrite is 0.022 wt% at 727°
Celsius
The shape and size of BCC interstitial positions limit
the amount of carbon impurities
Ferrite is relatively soft, can be made magnetic
IRON-CARBON SYSTEM
Max solubility of carbon in austenite is 2.14
wt% at 1147° Celsius
Not stable below 727° C
FCC allows for more carbon atom interstitials
Austenite is nonmagnetic
δ ferrite identical to α ferrite in structure
Only stable at really high temperatures, non-
important
IRON-CARBON SYSTEM
Cementite Forms when the solubility of carbon in α
ferrite is exceeded below 727° C
Hard, brittle, strength of steel is enhanced by presence of cementite
Pearlite Eutectoid steel cooled through eutectoid
temperature
Look like shiny pearl under microscope
Intermediate properties (between soft, ductile ferrite and hard, brittle cementite.
PHASE TRANSFORMATIONS
Alteration of microstructure
Three classifications:
Diffusion-dependent transformations (no change in
either number or composition of the phases
present)
Some alteration in phase compositions and number
of phases present
Diffusionless
KINETICS OF SOLID-STATE REACTIONS
At least one new phase is formed
Different physical/chemical properties and/or
different structure than parent phase
Transformations do not occur instantaneously
(obstacles impede course of reaction)
Diffusion is a time-dependent phenomenon
Increase in energy associated with phase
boundaries created between parent and
product phases
PHASE TRANSFORMATION
Stage 1, Nucleation: formation of very small
particles of new phase (capable of growing)
Likely to grow in imperfection sites
Stage 2, Growth: particles increase in size
(volume of parent phase disappears)
KINETICS OF SOLID-STATE REACTIONS
Time dependence of transformation rate
(kinetics)
Important for heat treatment of materials
Investigation of kinetics:
Temperature maintained as constant
Fraction of reaction is measured as function of time
KINETICS OF PHASE TRANSFORMATION
S-shaped curve (Avrami equation): Y = 1 – exp(-ktn)
k, n = time-independent constants for the reaction
Heating time (t) vs. Fraction of transformation (y)
Rate of transformation r = 1/t0.5
Reciprocal of time required for the transformation to proceed halfway to
completion
MULTIPHASE TRANSFORMATIONS
Rate of approach for solid systems is slow that
true equilibrium structures are rarely achieved
Transformations are shifted to lower
temperature as a result
(supercooling/superheating)
HOMEWORK
HW (Due Monday, April 17th)
9.21, 9.27, 9.34, 9.37, 9.44
HOMEWORK
HW (Due Monday, April 24th)
10.6, 10.8, 10.10, 11.28, 11.29