PHASE DIAGRAMS

Group 10

Guided by

Prof. BP Kashyap

Introduction-Topics to be covered

Phase Diagrams Gibbs Phase Rule Binary phase diagrams Equilibrium Solidification Non-equilibrium solidification Fe-C Alloy Phase diagrams Intermediate Phases and reactions

Introduction-Topics to be covered

Development of Microstructure Cu-Zn Phase Diagram Stability of Hume-Rothery Phases in Cu-

Zn alloys Isothermal Transformation Diagram Transformation Rate diagram

Phase Diagrams A phase diagram is a type of chart used to show

conditions at which thermodynamic distinct phases can occur at equilibrium.

It conveniently and concisely displays the control of phase structure of a particular system.

The three controllable parameters that will affect phase structure are temperature, pressure, and composition.

Temperature-Composition phase diagram

Pressure-Temperaturephase diagram

Classification• Phase diagrams are classified based on the number of

components in the system Single component systems have unary diagrams. Two-component systems have binary diagrams and so on..

• To show phases are present at different compositions and temperatures under slow cooling (equilibrium) conditions.

• To indicate equilibrium solid solubility of one element/compound in another.

• To indicate temperature at which an alloy starts to solidify and the range of solidification.

• To indicate the temperature at which different phases start to melt.

• Amount of each phase in a two-phase mixture can be obtained.

Significance

Cu-Ni phase diagramTemperature- CompositionBinary isomorphous diagram

CO2 phase diagramPressure-TemperatureUnary diagram

Examples

Gibbs Phase Rule

A thermodynamic law which governs the conditions for phase equilibrium.

Useful in interpreting Phase Diagrams.

P+F=C+N P is the number of phases present F is termed the number of degrees of freedom C in represents the number of components in

the system N is the number of non compositional

variables, which do not depend on the composition of system.

Example

Binary Phase Diagrams A common phase diagram in which

pressure is held constant whereas temperature and composition are kept variable parameters.

They represent the relationships between temperatures and compositions of phases at the equilibrium, which influence the microstructure of the alloy.

Many microstructures develop from phase transformations either it be transition of one phase into another or appearance or disappearance of the phase which do occur on altering the temperature.

A common example for a binary isomorphous alloy is Cu-Ni system.

A study of this diagram would enable us to predict the transformations and equilibrium or nonequilibrium microstructures.

Equilibrium solidification

Non-equilibrium solidification

In practical situations diffusion is not as slow to allow the readjustments in diffusion while maintaining the equilibrium.

How it differs from equilibrium cooling?● Diffusion slow in α phase ● Composition changes with radial

composition and a greater proportion of liquid present

● Shift of solidus line to higher Ni contents to average composition of α

● Slower the cooling rate and more rapid the diffusion process smaller the displacement

● Solidification complete at 1478 F instead of 1493 F with 35% average Ni content

Its consequences?● Segregation occurs where the

concentration gradients are established across grains

● Formation of core-structure with center rich in high melting element whereas increase in concentration of low melting element on going towards grain boundary

● Melting below the equilibrium melting temperature of alloy may happen

● Loss in mechanical integrity due to thin liquid film that separates the grains.

Intermediate Phases & Reactions

Intermediate Phases : ● Pure iron on heating undergoes two changes in the crystal

structure before melting.● At room temperature it exists in α-ferrite upto 1674 F, which has

a BCC crystal structure.● At 1185 F it get polymorphically transformed into FCC γ-austenite

and it persists till 2541 F when it reverts back to BCC δ-ferrite.● At 6.7% carbon concentration Cementite(Fe3C) is formed.

● Iron melts at 2800 F.

Intermediate Reactions :● Eutectic :

• Eutectoid :

Development of MS in Fe-Fe3C system

Case 1 : At Eutectoid composition

>The above is the governing eqn.

> The phase changes from a to b.

> The pearlite structure forms due to the difference b/w parent & product phases.

> This is called Pearlite because of its look of a pearl.

727.C

)

Diffusion through minimal distance by

C-atoms leads to layered structure.

Case 2 : At Hypoeutectoid

Composition • Composition left to the eutectoid,

between 0.022 and 0.76 wt% C, is called hypoeutectoid alloy.

• At c, the microstructure consists entirely of γ-austenite grains.

• On cooling, at d, small α particles form along γ-grain boundaries.

• The composition becomes richer in carbon and α particles grow larger on cooling till 727.C.

• Once the temperature is lowered below 727.C all the γ-phase gets transformed into pearlite.

• α-phase is present as a continuous matrix phase surrounding the isolated pearlite colonies.

Case 3: At Hyper Eutectoid

Composition • Composition right to the eutectoid,

between 0.76 and 2.14 wt% C, is called hyper eutectoid alloy.

• At g, the microstructure consists entirely of γ-austenite grains.

• On cooling, at h, small cementite particles starts forming along γ-grain boundaries.

• Cementite composition remains constant as the temperature changes until 727.C.

• On lowering the temperature below 727.C all the γ-phase gets transformed into pearlite.

• The resulting microstructure consists of pearlite and proeutectoid cementite as microconstituents.

Cu-Zn phase diagram

Stability of Hume-Rothery Phases in Cu-Zn alloys

● Cu-Zn system displays a sequence of phases along the alloy composition called Hume-Rothery phases.

● The criterion for the stability of these phases as per Hume-Rothery concept is a contact of the Brillouin zone (BZ) plane with the Fermi surface (FS) where FS is considered to be a sphere within the nearly

free electron approximation.● Interaction between the BZ boundary & FS opens a

pseudo-gap and reduces the electronic energy, leading to the lowering of the overall structure energy.

● The relative stability of the α,β, and γ-structures is possibly connected with the relative contribution of the Hume-Rothery effect.

● The close-packed α and β-structures begin to transform to new high-pressure phases, and the vacant γ-structure is shown to be stable up to at least 50 GPa.

● The γ-phase shows an anomalous behaviour of some physical properties, such as resistivity, magnetic susceptibility, Hall effect, magnetoresistance and thermoelectric power, which was accounted for by the band-structure effect associated

with the BZ–FS interaction.

.● All these observations supports the

idea of the enhancement of the Hume-Rothery mechanism on compression.

The influence of other alloying elements on Fe-Fe3C system

● Depending on the particular alloying element and its concentration the positions of phase boundaries and the shapes of the phase fields do alter.

● One of the important changes is the shift in position of the eutectoid with respect to temperature and to carbon concentration.

● Steels are normally alloyed for other reasons,however—usually either to improve their corrosion resistance or to render them amenable to heat treatment.

ISOTHERMAL TRANSFORMATION DIAGRAM

Iron-Iron carbide eutectoid reaction:

Temperature plays an important role in the rate of the austenite-to-pearlite transformation . The temperature dependence for an iron–carbon alloy of eutectoid composition is indicated in Figure.which plots S-shaped curves of the percentagetransformation versus the logarithm of time at three different temperatures.For each curve, data were collected after rapidly cooling a specimen composed of100% austenite to the temperature indicated; that temperature was maintained constantthroughout the course of the reaction.

ISOTHERMAL TRANSFORMATION DIAGRAMS

A more convenient way of representing both the time and temperature dependence of this transformation is in the bottom portion of Figure.

The dashed curve corresponds to 50% of transformation completion.

In interpreting this diagram, note first that the eutectoid temperature (1341F)is indicated by a horizontal line; at temperatures above the eutectoid.

The austenite-to-pearlite transformation will occur only if an alloy is super cooled to below the eutectoid; as indicated by the curves, the time necessary for the transformation to begin and then end depends on temperature.

ISOTHERMAL TRANSFORMATION DIAGRAMS

The transformation rate increases with decreasing temperature such that at

( 1000 F) only about 3 s is required for the reaction to go to 50% completion.

ISOTHERMAL TRANSFORMATION DIAGRAMS

In previous graph Very rapid cooling of austenite to a temperature is indicated by the near-vertical line AB, and the isothermal treatment at this temperature is represented by the horizontal segment BCD.

The transformation of austenite to pearlite begins at the intersection, point C (after approximately 3.5 s), and has reached completion by about 15 s, corresponding to point D. Figure 10.14 also shows schematic microstructures at various times during the progression of the reaction.

The thickness ratio of the ferrite and cementite layers in pearlite is approximately 8 to 1. However, the absolute layer thickness depends on the temperature at which the isothermal transformation is allowed to occur. At temperatures just below the eutectoid, relatively thick layers of both the -ferrite and Fe3C phases are produced; this microstructure is called coarse pearlite (shown in next slide) and the region at which it forms is indicated to the right of the completion curve on Figure 10.14

ISOTHERMAL TRANSFORMATION DIAGRAMS

The thin-layered structure produced in the vicinity 540C of is termed fine pearlite; is the dependence of mechanical properties on lamellar thickness. Photomicrographs of coarse and fine pearlite for a eutectoid composition are shown in Figure .

For iron–carbon alloys of other compositions, a proeutectoid phase (either ferrite or cementite) will coexist with pearlite, Thus additional curves corresponding to a proeutectoid transformation also must be included on the isothermal transformation diagram. A portion of one such diagram for a 1.13 wt% C alloy

Summary

Acknowledgements

Materials Science and Engineering-William Callister

Mechanical metallurgy-Dieter Wikipedia Google Search Engine Stability of Hume-Rothery phases in Cu–

Zn alloys at pressures up to 50 GPa by V F Degtyareva1, O Degtyareva

THANK YOU!