Phase Diagrams 1-Heat Treatment

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PhaseDia rams

Phase E uilibria

A branch of chemical thermodynamics o un ers an e s a y an compos on

of various phases in a chemical systems

nvo v ng one or more componen s.Essential to those working with multiphase

structures

Includes all branches of engineering:Materials Engineering Metallurgy, Ceramics,Composites, Chemical Engineering


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PhaseDia rams

Toolsto

estimate

the

stability

of

various

phases

of

the

c em ca systemasa unct ono temperature,pressureandcomposition

Graphicalrepresentation

of

the

equilibrium

state

of

a

system

Provideessentialinformationonanalyzing,

controllin

im rovin

anddevelo in

otential

materialssystems


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PhaseDia rams

One of the most important sources of informationconcern ng e e av or o e emen s, compoun s an

solutions

Provide the knowledge of phase stability as a function of T, P

an

Permit us to study and control important processes such as

Phase separation Solidification

Sintering

Purification Growth and doping of crystals


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Principally, P Ds provide information about systems at

But they can also assist in predicting nonEquilibrium phaserelations. Compositional changes and structures.


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occur during heat treatment processes that involve phasetr n iti n r i it ti n r ti n r t lliz ti n t .

materials can be known

Indicate the degree of control needed for experimental.

In systems invo ving oxi es, t e e ects o c angingTemperature and/or oxygen partial pressure


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,

Any material can exist as gas, liquid or solid depending on

over disruptive thermal forces

Generally, stability of different states of aggregation is a

or examp e. ncrease pressure ncrease t e nteratom cattraction of atoms of gas


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System

The region under consideration, as distinguished from theres o e un verse e env ronmen .

Systems may be separated from environments by boundariesthat prevent the transfer of mass (a closed system), of heat

an a a at c system , or o any energy an so ate system .

Sometimes the word system is also used to refer to all

possible compositions defined by a particular set ofcomponents

Examples: ZrO2 system, PbSn system, MgO

SiO2 system,

etc.


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Phase

ny port on o a system w c s p ys ca y omogeneouswithin itself and bounded by a surface so that it is

Practically speaking, phases are the physical states of mattersuch as solids, liquids, and vapors.

Some phases have crystal structure (solids, includingminerals), others are amorphous (liquids, vapors, glasses).

Some only form with a definite chemical composition (purephases), others can take on a range of compositions

(solutions).


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Phase

Sing ep ase:

Pure water liquid

White gold AuAgNi: FCC metalsmixed at atomic level

Air a mixture of O2, N2, Ar, CO2, SO2, N2O etc. CaoZrO2 solid solution: Ca ion sitting on Zr site

Twophase:

Oil and water

Glass ceramics: vision cookware: Pyroceram: glassy matrixand cr stalline reci itates chan e in atomic order


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Equilibrium

qu r um s a con t on w c represents t e owestenergy state of the system

Properties are invariant with time

Metastabilit : stead state achieved but could be more stable

Ex:Diamond: formed under different conditions and brou ht

under ambient conditions

Metallic glasses: if sufficient activation energy given by heating,crystalline state will be achieved


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Component

A chemical formula that can be used to express thecompositional range available to a system.

It is important to think of components as mere mathematicalconstructions, as basis vectors for expressing composition,and not to confuse them either with phases or species.

A measure of com lexit

,composition, has one component.


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Note

,though they are often given the same name or symbol: e.g.,, 2 ,

two might be interchanged in casual usage (as in"the quartzcom onent" r "a silica hase" .

2can form steam, liquid water, and about ten different

multidimensional component space (e.g., olivine, in which, , , , ,

Ca, etc.).


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Varaiance

The number of degrees of freedom or unconstrained,

e mens ona ty o a reg on n pressuretemperaturecomposition space where a given phase assemblage can exist.


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No. of phases 1 2o. o components

1 Pure water Iceandwater(slush)

aoZr 2 l andwater

3 WhitegoldAuAgNi

LithiumoxideAluminaSilica


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GibbsPhaseRuleEstablishes the number of thermodynamic variables that

mus e exper men a y xe n or er o u y e ne e

properties of an equilibrium chemical system

Mathematically

F= CP + 2

F= number of degrees of freedom or variance

C= number of components

= num er o p ases presen a equ r um

Constant2= noncompositional thermodynamic variables (usually T andP


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GibbsPhaseRuleCaCO3 (s) = CaO (s)+ CO2 (g)

Three different chemical constituents but number ofcomponents are on y ecause any two can comp ete ydefine the system in Eq.

F = CP+2

= 23+2 = 1

Only one variable either T or P can be changed independently


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GibbsCondensedPhaseRuleFor Onecomponent system, max. number of variables are

For Two

component (binary) system, max. THREE (T, P, C)

Solidliquid systems are usually studied @ constant pressure

an so on y var a es nee to e cons ere

Condensed phase rule:

F=C

P

+1


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Onecomponentsystem:purewater


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t any point of P, T water exists as solid, liquid or vapor

Curves represent points at which two phases coexist in Eq.

Triple point: All three phases co

exist


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Phase trans ormations:

Liquidsolid: solidification

qu vapor: evaporat onVapor liquid: condensation

Solidvapor or vaporsolid: sublimation

Phase diagram delineates the boundaries of the phase fields

But does not indicate uantit o an hase


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Negative slopeof liquidsolid phase line

ice @

2o

C when subjected to high pressure,w trans orm to qu 2

Skating not on ice, but on water !!!!!!!!

A characteristic of materials having a higher coordination

For ex. H2O, Bi, Ge, Si

Liquid metals on the other hand will condense under pressureo a so p ase


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Specialty of water is that it contains more open bonding in

solid than in liquid

That means, densit o solid < densit o li uid water

Ice cubes FLOAT in water !!!!

@ 20 mm Hg pressure, water boils @ 20oC


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Phase Diagrams 1-Heat Treatment