Phase Diagrams 1-Heat Treatment
Transcript of 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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Onecomponentsystem:purewater
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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Onecomponentsystem:purewater
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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Onecomponentsystem:purewater
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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Onecomponentsystem:purewater
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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