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