Why study Phase Diagrams

Slides prepared by: Drs. Caner Durucan & Mert Efe & Caner Şimşir @ METU, based on Course Material of Callister, 6th Ed. 2003.

PHASE DIAGRAMS HELPS us in designing and controlling of heat treatmentprocedures; because most of the material properties are functions of microstructure and consequently of their thermal histories.

Why study Phase Diagrams:

A phase diagram shows what phases exist at equilibrium and what phase transformations we can expect when we change one of the parameters of the system. (Pressure, Temp., Composition)

General Definition:


A phase diagram - graphical representation of thecombinations of temperature, pressure, composition, or other variables for which specific phases exist at equilibrium.

For H2O, a typical diagram shows thetemperature andpressure at which ice (solid),water (liquid) and steam (gas)exist.

FOR A SINGLE COMPONENTSYSTEM: H2O ONLY

Three variables: Pressure, Temperature, Composition


Three Variables: Pressure, Temperature, Composition

A phase diagram showswhat phases exist at equilibrium and what phase transformations we can expect when We change one of the parameters of the system (P, T,composition).

TWO COMPONENTSYSTEM.H2O and SUGAR

In this course we will use phase diagrams for binary systemsi.e. two-component only and will assume P=constant=1 atm


Max concentration forwhich only a solutionoccurs.

• Phase Diagram:Water-Sugar System

Question: What is the solubility limit at 20C?Answer: 65wt% sugar.

If Co< 65wt% sugar: syrupIf Co> 65wt% sugar: syrup + sugar.

• Solubility limit increases with T:e.g., if T = 100C, solubility limit = 80wt% sugar.

Pu

re

Su

ga

r

Te

mp

era

ture

(°C

)

0 20 40 60 80 100Co=Composition (wt% sugar)

L (liquid solution

i.e., syrup)

Solubility Limit L

(liquid)

+ S

(solid sugar)

65

20

40

60

80

100

Pu

re

Wa

ter

Solubility Limit


Changing temperature can change # of phases: path A to B.Changing composition can change # of phases: path B to D.

• water-sugarsystem

Effect of Temperature and Composition

70 80 1006040200

Tem

pe

ratu

re (

°C)

Co=Composition (wt% sugar)

L(liquid solution

i.e., syrup)

A2 phases

B1 phase

20

100

D2 phases

40

60

80

0

L (liquid)

+ S

(solid sugar)


Isomorphous systems:Complete solid solubility of the two components (both in the liquid and solid phases).

Three phase region can be identified on the phase diagram:Liquid (L), solid+liquid ( +L), solid ()

Liquidus line separates liquid from liquid + solidSolidus line separates solid from liquid + solid


Isomorphous systems:In one-component system melting occurs at a well-defined melting temperature.In multi-component systems melting occurs over the range of temperatures, between the solidus and liquidus lines. Solid and liquid phases are in equilibrium in this temperature range.


• When we combine two elements...what equilibrium state do we get?

• In particular, if we specify...--a composition (e.g., wt%Cu - wt%Ni), and--a temperature (T)

then...How many phases do we get?What is the composition of each phase?How much of each phase do we get?

USE of an PHASE DIAGRAM for an ALLOY: TWO COMPONENT SYSTEM


Copper-Nickel Phase Diagram


Interpretation of Phase DiagramsFor a given temperature and composition we can use phase diagram to determine:

1) The phases that are present2) Compositions of the phases3) The relative fractions of the phases

Finding the composition in a two phase region:1. Locate composition and temperature in diagram2. In two phase region draw the tie line or isotherm3. Note intersection with phase boundaries. Readcompositions at the intersections.The liquid and solid phases have these compositions.


•Solidification in the solid + liquid phase occurs gradually upon cooling from the liquidus line.

•The composition of the solid and the liquid change gradually during cooling (as can be determined by the tie-line method.)

•Nuclei of the solid phase form and they grow to consume all the liquid at the solidus line.

Development of microstructure in isomorphous alloys: Equilibrium (very slow) cooling


Finding the amounts of phases in a two phase region:The Lever Rule1. Locate composition and temperature in diagram2. In two phase region draw the tie line or isotherm3. Fraction of a phase is determined by taking the

length of the tie line to the phase boundary for theother phase, and dividing by the total length of tieline.

The lever rule is a mechanicalanalogy to the mass balancecalculation. The tie line in thetwo-phase region is analogous to a lever balanced on a fulcrum.


Mechanical properties of isomorphous alloys: Solid solution strengthening


Alloys with Limited or No Solubility


Binary Eutectic Systems(alloys with limited solubility)

Three single phase regions : solid solution of Ag in Cu matrix, : solid solution of Cu in Ag matrix, L :liquid

Three two-phase regions +L, +L, +

Solvus line separates one solid solution from a mixture ofsolid solutions. Solvus line shows limit of solubility


Eutectic or invariant point - Liquid and two solid phasesco-exist in equilibrium at the eutectic composition CE andthe eutectic temperature TE.Eutectic isotherm - the horizontal solidus line at TE.

Binary Eutectic Systems


Eutectic reaction – transition between liquid and mixtureof two solid phases, + at eutectic concentration CE.The melting point of the eutectic alloy is lower than that ofthe components (eutectic = easy to melt in Greek).At most two phases can be in equilibrium within a phasefield. Three phases (L, , ) may be in equilibrium onlyonly at a few points along the eutectic isotherm. Singlephaseregions are separated by 2-phase regions.





Development of microstructure in eutectic alloys (I)Several different types of microstructure can be formed in slow cooling of different compositions.Let’s consider cooling of liquid lead – tin system atdifferent compositions.

In this case of lead-richalloy (0-2 wt. % of tin)solidification proceeds inthe same manner as forisomorphous alloys (e.g.Cu-Ni) that we discussedearlier.

L +L


M d F b M tE 228

At compositions between the room temperature solubility limit and the maximum solid solubility at the eutectic temperature, phase nucleates as the a solid solubility isexceeded upon crossing the solvus line.

Development of microstructure in eutectic alloys (II)


No changes above the eutectic temperature TE. At TE theliquid transforms to and phases (eutectic reaction).

Development of microstructure in eutectic alloys (III): Solidification at the eutectic composition


Primary phase is formed in the + L region, and the eutectic structure that includes layers of and phases (called eutectic and eutectic phases) is formed uponcrossing the eutectic isotherm.

Development of microstructure in eutectic alloys (IV)Composition other than eutectic but within the range of the eutectic isotherm


Microconstituent – element of the microstructure having a distinctive structure. In the case described in the previous page, microstructure consists of two microconstituents, primary a phase and the eutectic structure.

Primary Eutectic structure

Development of microstructure in eutectic alloys

Although the eutectic structure consists of two phases, it isa microconstituent with distinct lamellar structure andfixed ratio of the two phases.

Microconstituent is a mixture


The Iron – Carbon System(A binary system in solid state)


• Impurity atoms distort the lattice & generate stress.• Stress can produce a barrier to dislocation motion.

Smaller substitutionalimpurity

Larger substitutionalimpurity

Impurity generates local shear at A and B that opposes disl motion to the right.

STRENGTHENING: SOLID SOLUTIONS(Fe:C, steel)

C

D

A

B

C atoms "lock" planes from shearing.


Eutectoid Reactions (5)

contains both an eutecticreaction and its solid-state analog, an eutectoidreaction


In their simplest form, steels are alloys of Iron (Fe) and Carbon (C). The Fe-C phase diagram is a fairly complex one, but we will only consider the steel part of the diagram,up to around 7% Carbon.

The Iron–Iron Carbide (Fe–Fe3C) Phase Diagram

Interested in this reaction


Phases in Fe–Fe3C Phase Diagram

-ferrite - solid solution of C in BCC Fe• Stable form of iron at room temperature.• The maximum solubility of C is 0.022 wt%• Transforms to FCC -austenite at 912 °C



-austenite - solid solution of C in FCC Fe• The maximum solubility of C is 2.14 wt %.• Transforms to BCC -ferrite at 1395 °C• Is not stable below the eutectic temperature(727 ° C) unless cooled rapidly (Chapter 10)


Photomicrographs of (a) -Ferrite, (90x) Austenite, (325x).



Fe3C (iron carbide or cementite)• This intermetallic compound is metastable, itremains as a compound indefinitely at room T, butdecomposes (very slowly, within several years)into -Fe and C (graphite) at 650-700°C

Fe-C liquid solution


Development of Microstructure in Fe-C alloysMicrostructure of eutectoid steel


When alloy of eutectoid composition (0.76 wt % C) is cooled slowly it forms pearlite, a lamellar (layered) structure of two phases: -ferrite and cementite (Fe3C)

The layers of alternating phases in pearlite are formed for the same reason as layered structure of eutectic structures: redistribution C atoms between ferrite (0.022 wt%) and cementite (6.7 wt%) by atomic diffusion.

Mechanically, pearlite has properties intermediate to soft, ductile ferrite and hard, brittle cementite.

Development of Microstructure in Fe-C alloysMicrostructure of eutectoid steel

In the micrograph, the dark areas areFe3C layers, the light phase is -ferrite


Development of Microstructure in Fe-C alloysMicrostructure of hypoeutectoid steel

Compositions to the left of eutectoid (0.022 - 0.76 wt % C) hypoeutectoid (less than eutectoid) alloys.


Hypoeutectoid alloys contain proeutectoid ferrite (formedabove the eutectoid temperature)plus the eutectoid pearlitethat contain eutectoid ferrite and cementite.

Development of Microstructure in Fe-C alloysMicrostructure of hypoeutectoid steel

pearlite


The dependence of eutectoid temperature on alloyconcentration for several alloying elements in steel.

The dependence of eutectoidcomposition (wt% C) on alloy concentration for several alloying elements in steel.

Effect of alloying elements on Fe-Fe3C Phase Diagram


For a 99.65 wt.% Fe - 0.35 wt.% C alloy at a temperature justbelow the eutectoid deteremine the following.

a. The fractions of total ferrite () and cementite (Fe3C) phasesb. The fractions of the pro-eutectoid ferrite and austenite ()c. The fraction of eutectoid ferrite.

Study question for Fe-Fe3C phase diagram:

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Why study Phase Diagrams