iron-iron carbide phase diagrams

Phase Diagrams

Contents• Definitions and basic concepts:

Component System Phases Solubility limit Microstructure Phase Equilibrium

• Phase Diagram• Interpretation of phase diagram

Contents• Lever’s Rule• Eutectic Reactions• Eutectoid Reactions• Peritectic Reactions• Cu-Ni Phase Diagram• Pb-Sn Phase Diagram• Al-Si Phase Diagram• Iron-Iron Carbide Diagram

Component and System• A component is defined as pure metals of

which an alloy is composed.• A component is chemically recognizable, e.g.

Fe and C are the components in carbon steel.• A binary alloy contains two components, and a

ternary alloys contain three.• A system may relate to the series of possible

alloys consisting of same components, but without regard to alloy composition.

Phase• A phase is defined as a homogenous portion

of the system having uniform physical and chemical characteristics.

• Every pure material is considered to be a single phase.

• Each phase is separated by phase boundaries.• A phase may contain one or two component.• A single phase system is called as homogenous

and systems with two or more phases are heterogeneous systems.

Solubility Limit• A maximum amount of solute that can be

dissolved in the solvent to form a solid solution is termed as solubility limit.

• For example, alcohol has unlimited solubility in water, sugar has limited solubility, and oil is insoluble in water.

• Cu and Ni are mutually soluble in any amount, while C has limited solubility in Fe.

• The addition of solute in excess of this limit results in the formation of two phase solution.

Microstructure• Material physical properties and mechanical

behavior depend on microstructure.• The microstructure is specified by the number

of phases, their proportions, and the manner in which they are distributed.

• The microstructure of an alloy depends on a. Alloying elementsb. Their concentrations andc. The grain size (controlled by heat-treatment

process)

Microstructure

Phase Equilibrium• A system is at equilibrium if at constant

temperature, pressure and composition the system is stable, not changing with time.

Phase Diagram• A phase diagram is a graphical representation

of the combinations of temperature, pressure, composition, or other variables for which specific phases exist at equilibrium.

• We will discuss phase diagrams for binary alloys only and will assume pressure to be constant at one atmosphere.

• The mechanical properties of engineering materials depend strongly upon microstructure.

Phase Diagram• The purpose of phase diagram is to develop an

understanding of the phase transformations, which occur under conditions of slow cooling

• Using phase diagrams, we can easily predict the effects of compositional changes.

• Consider two components A and B, showing complete solid solubility both in liquid as well as solid state.

Phase Diagram

Phase Diagram• Three phase region can be identified on the

phase diagram; 1. Liquid (L)2. solid + liquid (α +L)3. solid (α )

• Liquidus line separates liquid region from (liquid + solid) region, above this line there lies only liquid solution.

• Solidus line separates solid region from (liquid + solid) region, below this line only solid solution is present.

Phase Diagram

Interpretation of Phase DiagramFor a given temperature and composition we

can use phase diagrams to determine:1) The phases that are present2) Composition of the each phase3) The relative fractions of the phases

1) The phases that are present

Point A:At 1100°C, the alloy

composition is 60% Ni & 40% Cu(only α-phase)

Point B:At 1250°C, 35% Ni &

65% Cu, system contains two phases (α +L)

2) Composition of each PhasePoint B:At 1250°C, two phases

(α +L) are present. Composition of each phase can be found by drawing a Tie-Line.

CL 31.5% Ni & 68.5 % Cu

Co 35% Ni

Cα 42.5% Ni &57.5% Cu

3) The relative fractions of the phases• Lever rule is employed to find the relative

mass fractions of the phases present in the alloy system.

• The lever rule is a mechanical analogy to the mass balance calculation.

• The tie line in the two-phase region is analogous to a lever balanced on a fulcrum.

3) The relative fractions of the phasesMass fractions:Co = 35 wt. %, CL = 31.5 wt. %, Cα = 42.5 wt. %

WL = S / (R+S) = (Cα - Co) / (Cα- CL)

Wα = R / (R+S) = (Co - CL) / (Cα- CL)

WL = 0.68Wα = 0.32

Eutectic Reactions• Eutectic reaction is transition between liquid

and mixture of two solid phases, α + β at eutectic concentration CE.

• Eutectic is a Greek word meaning easy to melt

Eutectic Reaction

Eutectoid Reactions• The eutectoid (eutectic-like in Greek) reaction

is similar to the eutectic reaction but occurs from one solid phase to two new solid phases.

• Upon cooling, a solid phase transforms into two other solid phases (γ ↔ α + β)

Eutectic and Eutectoid Reactions

Peritectic Reactions• A Peritectic reaction occurs when a solid and

liquid phase will together form a second solid phase at a particular temperature and composition upon cooling as,

L + α ↔ β• Peritectic reactions are not as common as

eutectics and eutectoids, but they do occur in some alloy systems.

• There is one in the Fe-C system

Peritectic Reactions

Cu-Ni Alloy Phase Diagram• Cu-Ni alloy system presents one of the simplest

cases in which both components are completely soluble in each other in solid as well as in liquid state.

• The reasons of complete solubility are:1. Both have same crystal structure (FCC)2. Similar radii3. Electro negativity4. Valency

• Cu-Ni alloy is an example of Substitutional Solid Solution.

Cu-Ni Alloy Phase Diagram

Cu-Ni Alloy Grain Growth

Pb-Sn Alloy Phase Diagram• Pb-Sn alloy system represents a phase

diagram that shows partial solid solubility.• The α-phase is a solid solution of tin in lead at

the left side of the diagram.• The β-phase is a solid solution of lead in tin at

the right side of the diagram.• At eutectic temperature (183 °C), lead can

hold up to 18.3% tin in a single-phase solution and tin can hold up to 2.2% lead within its structure and still be single phase.

Pb-Sn Alloy Phase Diagram

Pb-Sn Alloy Phase Diagram• There are three single phase regions; α-phase

β-phase and the liquid phase.• Two phase regions are also three; α + L, β +L,

α +β.• Solvus line separates one solid solution from a

mixture of solid solutions. The Solvus line shows limit of solubility

Pb-Sn Alloy Grain Growth

Pb-Sn Alloy Grain Growth

Pb-Sn Alloy Grain Growth

Pb-Sn Alloy Grain Growth

Calculation of relative amounts of micro-constituents

Calculation of relative amounts of micro-constituents (Eutectic & α)

Amount of Eutectic mixture:

We = P / (P+Q)

Amount of α:

Wα = Q / (P+Q)

Calculation of relative amounts of micro-constituents (α & β)

Amount of α:

Wα = (Q+R)/(P+Q+R)

Amount of β :

Wβ = P/(P+Q+R)

Al-Si Alloy Phase Diagram• Al-Si alloys differ from our "standard" phase

diagram in that aluminum has zero solid solubility in silicon at any temperature.

• This means that there is no beta phase and so this phase is "replaced" by pure silicon.

• The eutectic on this phase diagram contains much more alpha than Si and so we expect the eutectic mixture (alpha+Si) to be mainly alpha.

• For hypereutectic, primary Si forms first, depleting the liquid of Si until it reaches the eutectic composition where the remaining solidification follows the eutectic reaction.

Al-Si Alloy Phase Diagram

Fe-Fe3C Phase Diagram

Single Phase Regions in Fe-Fe3C Phase Diagram1. Fe-C liquid solution2. α-ferrite - solid solution of C in BCC Fe

o Stable form of iron at room temperature.o The maximum solubility of C is 0.022 wt%o Transforms to FCC γ-austenite at 912 °C

3. γ-austenite - solid solution of C in FCC Feo The maximum solubility of C is 2.14 wt %.o Transforms to BCC δ-ferrite at 1395 °Co Is not stable below the eutectoid temperature

(727 ° C) unless cooled rapidly

Single Phase Regions in Fe-Fe3C Phase Diagram3. δ-ferrite - solid solution of C in BCC Fe

o The same structure as α-ferriteo Stable only at high T, above 1394 °CoMelts at 1538 °C

4. Fe3C (iron carbide or cementite)This intermetallic compound is

metastable, it remains as a compound indefinitely at room T, but decomposes (very slowly, within several years) into α-Fe and C (graphite) at 650 - 700 °C

Important things to remember• C is an interstitial impurity in Fe. It forms a solid

solution with α, γ, δ phases of iron.• Maximum solubility in BCC α-ferrite is limited

(max 0.022 wt% at 727 °C) - BCC has relatively small interstitial positions.

• Maximum solubility in FCC austenite is 2.14 wt% at 1147°C - FCC has larger interstitial positions.

• Cementite is very hard and brittle – can strengthen steels. Mechanical properties also depend on the microstructure, that is, how ferrite and cementite are mixed.

Important things to rememberThree types of ferrous alloys:1. Iron: less than 0.008 wt % C in α−ferrite at

room temperature.2. Steels: 0.008 - 2.14 wt % C (usually < 1 wt % )

α-ferrite + Fe3C at room temperature.3. Cast iron: 2.14 - 6.7 wt % (usually < 4.5 wt %)

Eutectic and Eutectoid Reactions

Microstructure of Eutectoid Steel• Microstructure depends on composition

(carbon content) and heat treatment. • In the discussion, we consider slow cooling in

which equilibrium is maintained.• When alloy of eutectoid composition (0.76 wt

% C) is cooled down slowly it forms a lamellar or layered structure of two phases: α-ferrite and cementite (Fe3C). This two phase structure is called as Pearlite.

Microstructure of Eutectoid Steel

In the micrograph, the dark areas are Fe3C layers, the light phase is α-ferrite

Microstructure of Hypo-eutectoid Steel

Compositions to the left of eutectoid point, (0.022 - 0.76 wt % C) are termed as hypo-eutectoid (less than eutectoid) Steels.

γ → Proeutectoid α + γ → Proeutectoid α + Pearlite

(Eutectoid α + Fe3C)

Microstructure of Hypo-eutectoid Steel

Microstructure of Hyper-eutectoid Steel

Compositions to the right of eutectoid point, (0.76 – 2.14 wt % C) are termed as hyper-eutectoid (greater than eutectoid) Steels.

γ → Proeutectoid Fe3C + γ→ Proeutectoid Fe3C + Pearlite

(Eutectoid Fe3C + α)

Microstructure of Hyper-eutectoid Steel