Solidification of pure metal and alloysSolidification of pure metal and alloys

solidification is the most important phase solidification is the most important phase transformationtransformation

Coz almost metal/alloy most undergo this Coz almost metal/alloy most undergo this transformation before becoming useful objects.transformation before becoming useful objects.

Solidification involve solid-liquid phase transformationSolidification involve solid-liquid phase transformation–E.g casting processE.g casting process–Solidification in welding processSolidification in welding process

solidification steps solidification steps – nucleation-formation stable nuclei in meltnucleation-formation stable nuclei in melt–Growth of nuclei into crystal Growth of nuclei into crystal

–Formation of grain structureFormation of grain structure

a) a) Nucleation of crystals, Nucleation of crystals, b)b) crystal growth, crystal growth, c)c) irregular grains form as irregular grains form as crystals grow together, crystals grow together, d)d) grain boundaries as seen in a microscope. grain boundaries as seen in a microscope.

Solidification in cast metalSolidification in cast metal

3 zone structure of an ingot3 zone structure of an ingot chill zonechill zone

– structure consists of many structure consists of many small roughly equiaxed grainssmall roughly equiaxed grains

– nucleation occurs from vary nucleation occurs from vary many site along mold wallmany site along mold wall fine grain structurefine grain structure

Columnar ZoneColumnar Zone– consist dendrites structureconsist dendrites structure– Some of the dendrites Some of the dendrites

oriented perpendicular to oriented perpendicular to wall/dendrite axes at an angle wall/dendrite axes at an angle to the moldto the mold

Equiaxed zoneEquiaxed zone

– Within the liquid at the center there are Within the liquid at the center there are generally many small equiaxed grains generally many small equiaxed grains suspended throughout suspended throughout

– As freezing continues, these small grains As freezing continues, these small grains begin crowd together and finally block begin crowd together and finally block the inward motion of columnar grainsthe inward motion of columnar grains

Metallic solid solution Metallic solid solution most metal are combined to form alloy in order most metal are combined to form alloy in order

to impart specific characteristic to impart specific characteristic

The addition of impurity atoms to a metal will The addition of impurity atoms to a metal will result in the formation of a solid solutionresult in the formation of a solid solution

A A solid solutionsolid solution is a is a solidsolid--statestate solutionsolution of one or of one or more more solutessolutes in a in a solventsolvent. .

Such a Such a mixturemixture is considered a solution rather is considered a solution rather than a than a compoundcompound when the when the crystal structurecrystal structure of of the solvent remains unchanged by addition of the the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single solutes, and when the mixture remains in a single homogeneoushomogeneous phasephase. .

Two terms in solid solutionTwo terms in solid solution SoluteSolute

– Used to denote an element/compound present in a minor Used to denote an element/compound present in a minor concentrationconcentration

SolventSolvent– Element / compound that is present in the greatest Element / compound that is present in the greatest

amountamount– a.k.a host atomsa.k.a host atoms

Characteristic of solid solutionCharacteristic of solid solution

1.1. Form when solute atoms are added to the host material.Form when solute atoms are added to the host material.2.2. Crystal structure is maintainedCrystal structure is maintained3.3. No new structure formedNo new structure formed4.4. Compositionally homogeneousCompositionally homogeneous

Types of solid solutionTypes of solid solution ( based from impurity point defects)( based from impurity point defects)

– SubstitutionalSubstitutional– InterstitialInterstitial

1.1. SubstitutionalSubstitutional– Host atoms are replaced/ substitute Host atoms are replaced/ substitute

with solute/ impurity atoms.with solute/ impurity atoms.– Factors to consider in substitutional s.sFactors to consider in substitutional s.s

Atomic size factors – The difference in Atomic size factors – The difference in atomic radii between two atom is less than atomic radii between two atom is less than about ± 15%about ± 15%

Crystal structure – same crystal structuresCrystal structure – same crystal structures

Electronegativity- similar e/negativity/ smaller diff.Electronegativity- similar e/negativity/ smaller diff. Valances- similar valance electronValances- similar valance electron

e.ge.g system Ag-Au system Ag-Au

system Cu-Nisystem Cu-Ni

system Cu-Znsystem Cu-Zn

systemsystem Crystal Crystal structurestructure

ValencesValences Atomic Atomic radius, Åradius, Å

e/negativitye/negativity Type alloyType alloy solubilitysolubility

Ag-AuAg-Au Ag FCCAg FCC

Au FCCAu FCC11

111.441.44

1.441.441.91.9

2.42.4substitionalsubstitional completecomplete

Cu-NiCu-Ni Cu FCCCu FCC

Ni FCCNi FCC22

221.281.28

1.251.251.91.9

1.81.8substitionalsubstitional completecomplete

Cu-ZnCu-Zn Cu FCCCu FCC

Zn HCPZn HCP22

221.281.28

1.381.381.91.9

1.61.6substitionalsubstitional Not Not

completecomplete

2.2. Interstitial solid solutionInterstitial solid solution

Impurity atoms fill the voids in the solvent atom latticeImpurity atoms fill the voids in the solvent atom lattice It interstices among the host atomsIt interstices among the host atoms Atomic diameter of an interstitial impurity must be Atomic diameter of an interstitial impurity must be

smaller than host atomssmaller than host atoms Normal max. allowable concentration of interstitial Normal max. allowable concentration of interstitial

impurity atom is low (<10%)impurity atom is low (<10%)

SolutionSolutionwhen 2 components combine to form a single phasewhen 2 components combine to form a single phase

SolubilitySolubility Degree to which the two components mixDegree to which the two components mix

Solubility limitSolubility limit The max. concentration of solute that may be added w/o The max. concentration of solute that may be added w/o

forming a new phaseforming a new phase

PhasePhase– A homogeneous portion of a system that has A homogeneous portion of a system that has

uniform physical and chemical characteristicuniform physical and chemical characteristic

– single-phase systemsingle-phase system homogeneous system homogeneous system

– Systems composed of two /more phasesSystems composed of two /more phases mixture/heterogeneous systemmixture/heterogeneous system

(darker phase)

(lighter phase)

Aluminum-CopperAlloy

Phase diagramsPhase diagrams A graphical representation of the relationship A graphical representation of the relationship

between environmental constraints (e.g between environmental constraints (e.g temperature and sometimes pressure), temperature and sometimes pressure), composition and regions of phase stability, composition and regions of phase stability, ordinarily under condition equilibriumordinarily under condition equilibrium

Example of Phase D. (binary)Example of Phase D. (binary)

3 types of Phase D.3 types of Phase D.– Unary Unary – BinaryBinary– TernaryTernary

Binary Phase D.Binary Phase D.– Consists two elements or components in an alloy. Consists two elements or components in an alloy.

3 types of binary Phase D.3 types of binary Phase D.– Complete solid solutionComplete solid solution– No solid solutionNo solid solution– Limited solid solutionLimited solid solution

e.g. Cu and Ni are completely solublee.g. Cu and Ni are completely soluble Zinc has limited solubility in copperZinc has limited solubility in copper Sn has limited solubility in PbSn has limited solubility in Pb Pb insoluble in copperPb insoluble in copper

Binary isomorphous system (complete Solid Binary isomorphous system (complete Solid solution) solution)

• 2 phases: L (liquid) (FCC solid solution)

• 3 phase fields: L L +

wt% Ni20 40 60 80 10001000

1100

1200

1300

1400

1500

1600T(°C)

L (liquid)

(FCC solid solution)

L + liq

uidus

solid

us

• Phase Diagram for Cu-Ni system

Isomorphous coz;Isomorphous coz;– Complete liquid & solid solubilityComplete liquid & solid solubility– Only one solid phase formsOnly one solid phase forms– Same crystal structure.Same crystal structure.

3 diff. phase fields/ regions3 diff. phase fields/ regions– Liquid phase Liquid phase homogeneous liq. Solution (Cu + homogeneous liq. Solution (Cu +

Ni)Ni)– Two phaseTwo phase αα + L + L – αα phase phase substitutional s. solution (consists both Cu- substitutional s. solution (consists both Cu-

Ni) Ni)

PHASE DIAGRAMS: # and types of PHASE DIAGRAMS: # and types of phasesphases

Rule 1: If we know T and Co, then we know: --the # and types of phases present.

wt% Ni20 40 60 80 10001000

1100

1200

1300

1400

1500

1600T(°C)

L (liquid)

(FCC solid solution)

A(1100,60)

B(1

250,3

5)

Cu-Niphase

diagram

PHASE DIAGRAMS: composition of phasesPHASE DIAGRAMS: composition of phases• Rule 2: If we know T and Co, then we know: --the composition of each phase.

• Determination of phase compositions1. located the temperature

2. if one phase present, the composition = overall composition (Co) of alloy 3. if two phase present, use tie line

At TA: Only Liquid (L) CL = Co ( = 35wt% Ni)

At TB: Both and L CL = Cliquidus ( = 32wt% Ni here) C = Csolidus ( = 43wt% Ni here)

At TD: Only Solid () C = Co ( = 35wt% Ni)

Co = 35wt%Ni

wt% Ni20

1200

1300

T(°C)

L (liquid)

(solid)

30 40 50

TAA

DTD

TBB

tie line

433532CoCL C

PHASE DIAGRAMS: weight fractions of PHASE DIAGRAMS: weight fractions of phases / relative amount phasephases / relative amount phase

• Rule 3: If we know T and Co, then we know:--the amount of each phase (given in wt%).

wt% Ni

20

1200

1300

T(°C)

L (liquid)

(solid)

30 40 50

TAA

DTD

TBB

tie line

433532CoCL C

R S At TB: Both and L

At TA: Only Liquid (L) WL = 100wt%, W = 0

At TD: Only Solid () WL = 0, W = 100wt%

Co = 35wt%Ni

WL SR S

W RR S

43 3543 32

73wt%

= 27wt%

Binary eutectic for limited solids solution / Binary eutectic for limited solids solution / partially solid solutionpartially solid solution

Characteristic:Characteristic: Solid are partly soluble in one another rather than Solid are partly soluble in one another rather than

be either completely soluble or completely be either completely soluble or completely insolubleinsoluble

α , β = solid solution

ae, be = liquidus

ac, cd, bd = solidus

cf, dg = solvus

Liq. Solution (L)

a

L + α

α + β

A B

e

b

L +β

α

βc

d

hypereutectichypoeutectic

f g

Solvus cf denotes the solubility limit of B in ASolvus cf denotes the solubility limit of B in A

Solvus dg shows the solubility limit of A in BSolvus dg shows the solubility limit of A in B

Determination of phase and phase composition:Determination of phase and phase composition:

Same as in Same as in Binary isomorphous systemBinary isomorphous system and eutectic no and eutectic no solid solutionsolid solution

Determination of weight fractionDetermination of weight fraction

Liq. Solution (L)a

L + α

α + β

A B

e

b

L +β

α βc

d

hypereutectichypoeutectic

f g

Weight fraction of β,

Wβ = R/(R+Q)

Weight fraction of Liquid,

WL= Q/(R+Q)

QR

Binary Eutectic Phase Diagram Binary Eutectic Phase Diagram (1. No solid Solution) (1. No solid Solution)

Characteristic :Characteristic : system 2 components e. g A & B system 2 components e. g A & B completely soluble in one another in liquid statecompletely soluble in one another in liquid state but completely insoluble in one another but completely insoluble in one another

e

Temp Region above line ced=liq. Solution

Line ce and ed = liquidus

Region below horizontal line feg = mixture of solid A & B

Line cfegd= solidus

.

Liq. Solution (L)

c

d

L + solid A

L + solid B

solid A + solid B

f g

A B

e

Point e = eutectic pointPoint e = eutectic point the lowest temp. at the lowest temp. at which a liq. solution can exist.which a liq. solution can exist.

Determination of phase and phase composition:Determination of phase and phase composition:

Same as in Same as in Binary isomorphous systemBinary isomorphous system and eutectic and eutectic limited solid solutionlimited solid solution

Determination of weight fraction of phaseDetermination of weight fraction of phase

Liq. Solution (L)

c

d

L + solid B

solid A + solid B

f g

A B

P

QR

Weight fraction of solid A,

WA = Q/(R+Q)

Weight fraction of Liquid,

WL= R/(R+Q)

e

Gibbs Phase rule Gibbs Phase rule express the r/ship between no. of phases present and express the r/ship between no. of phases present and

the no of externally controllable variablesthe no of externally controllable variables

Applies only at equilibrium systemApplies only at equilibrium system

P + f = c + nP + f = c + n

P = no of phase presentP = no of phase presentf = no. of degrees of freedom (state variables : temp, f = no. of degrees of freedom (state variables : temp,

pressure & composition)pressure & composition)c = no. of componentsc = no. of componentsn= no. of noncompositional variablesn= no. of noncompositional variables

For condensate systemFor condensate system

P + f = c + 2P + f = c + 2

For alloy/ ceramic system For alloy/ ceramic system

P + f = c + 1P + f = c + 1

g

Temp

sl

Pressure

A

Ex: point A

Phase : 1

Component : 1

F = 2

how about point B and C?

Binary system?

B

C

Cooling curves & phase diagramCooling curves & phase diagram

different composition different composition will give diff. cooling will give diff. cooling curvescurves

For pure metal., the For pure metal., the cooling curves show cooling curves show horizontal thermal horizontal thermal arrest at their freezes arrest at their freezes pointspoints

The slope changes at The slope changes at L1-S1 etc, are L1-S1 etc, are corespond to the corespond to the liquidus and solidus.liquidus and solidus.

L1

S1Freezing zone

L1

S1

Cooling curves for eutectic binary Cooling curves for eutectic binary diagramdiagram

a)a) Cooling curve at eutectic alloyCooling curve at eutectic alloy

b)b) Cooling curve at hypo/hypereutectic Cooling curve at hypo/hypereutectic alloyalloy

A

B

alloy transformation at invariant alloy transformation at invariant equilibrium equilibrium

Invariant equilibrium involve:Invariant equilibrium involve:

1.1. 3 phases co-exist in 3 phases co-exist in 2.2. Exist only at one temperature / fixed temp.Exist only at one temperature / fixed temp.3.3. Composition for 3 phases co-exist is fixed at Composition for 3 phases co-exist is fixed at

the point.the point.

Common example of invariant equilibrium.Common example of invariant equilibrium.1.1. eutectic eutectic 2.2. EutectoidEutectoid3.3. PeritecticPeritectic4.4. monotecticmonotectic

Eutectic transformationEutectic transformation

ll αα + + ββ

(liq.) (s.s) (s.s)(liq.) (s.s) (s.s)

System- Ag-Cu, Pb-SnSystem- Ag-Cu, Pb-Sn

Eutectiod transformationEutectiod transformation

γγ αα + + ββ

(s.s) (s.s) (s.s)(s.s) (s.s) (s.s)

– System Fe-C, Al-CSystem Fe-C, Al-C

Peritectic reaction transformationPeritectic reaction transformation

ll + + αα ββ

(Liq.) (s.s) (s.s)(Liq.) (s.s) (s.s)

– System: Cu-ZnSystem: Cu-Zn

Peritectiod reaction transformationPeritectiod reaction transformation

ββ + + αα γγ(s.s) (s.s) (s.s)(s.s) (s.s) (s.s)

System: Al-Ni, Cu-ZnSystem: Al-Ni, Cu-Zn

Metatectic transformationMetatectic transformation

αα ll + + ββ(s.s) (liq) (s.s)(s.s) (liq) (s.s)

System U-MnSystem U-Mn

Monotectic transformationMonotectic transformation

ll1 1 αα ll22

(liq.) (s.s) (liq)(liq.) (s.s) (liq)

– System Cu-PbSystem Cu-Pb

Syntectic reactionSyntectic reaction

ll1 1 + + ll2 2 αα

(liq.) (liq) (s.s)(liq.) (liq) (s.s)

System: K-Zn, Na-ZnSystem: K-Zn, Na-Zn

Chapter 3Chapter 3Iron-iron carbide phase diagramIron-iron carbide phase diagram

Microstructure/microconstituentMicrostructure/microconstituentCementite (FeCementite (Fe33C)C) FormForm

1.1. Below 727Below 727ooC, when C, when solubility limit of solubility limit of carbon in carbon in αα ferrite ferrite is exceeded is exceeded

2.2. Between 1147Between 1147ooC and C and 727oC when solubility727oC when solubility limit of carbon in limit of carbon in γγ is exceededis exceeded

Crystal structure: orthorhombicCrystal structure: orthorhombic Characteristics: hard and brittle, tensile strength , Characteristics: hard and brittle, tensile strength ,

compressive strength compressive strength

Metastable phase- remain as a compound at TMetastable phase- remain as a compound at Trr

– if Feif Fe33C C γγ + C ( T) + C ( T)

– If FeIf Fe33C C αα + C ( 650 -700oC for several year+ C ( 650 -700oC for several year

– The addition of third element e.g Si, Co, Al promotes The addition of third element e.g Si, Co, Al promotes graphitizationgraphitization

– FeFe33C has a greater solubility in C has a greater solubility in γγ and and αα than has than has graphitegraphite

– Reaction : Reaction : γγ αα + + FeFe33C C

L L γγ + + FeFe33C C

Austenite (Austenite (γγ phase of iron) phase of iron)

Unstable below 727oCUnstable below 727oC

Maximum solubility of carbon Maximum solubility of carbon in austenite, 2.14wt% at in austenite, 2.14wt% at 1147oC1147oC

Carbon solubility higher than Carbon solubility higher than in ferrite in ferrite

FCC structure FCC structure

FCC structure –interstitial position larger than BCC.FCC structure –interstitial position larger than BCC.

Interstitial larger , lower strains imposed on Interstitial larger , lower strains imposed on surrounding iron atoms , higher carbon solubilitysurrounding iron atoms , higher carbon solubility

Nonmagnetic Nonmagnetic Properties: highest tensile strength, moderate elongation Properties: highest tensile strength, moderate elongation

and hardness properties. and hardness properties.

Ferrite, Ferrite, αα ironiron

crystal structure: BCCcrystal structure: BCC

Stable at room temperatureStable at room temperature

has magnetic propertieshas magnetic properties

At above 768At above 768ooC, the ferromagnetic properties disappear.C, the ferromagnetic properties disappear.

Nonmagnetic prop’s at 768Nonmagnetic prop’s at 768ooC to 912C to 912ooCC

tensile strength , elongation , hardness , toughness lowtensile strength , elongation , hardness , toughness low

δδ ferrite ferrite virtually = as virtually = as αα ferrite except, stable ferrite except, stable

at relatively high T.at relatively high T. T range between 1394oC until Tm for T range between 1394oC until Tm for

pure ironpure iron Lower solubility of carbon in ironLower solubility of carbon in iron

Pearlite (Pearlite (αα + Fe+ Fe33C)C)

two-phase microstructuretwo-phase microstructure

Resulted from transformation of Resulted from transformation of austenite of eutectoid compositionaustenite of eutectoid composition

Consists of alternating layer Consists of alternating layer (lamellae)(lamellae)

intermediate mechanical properties intermediate mechanical properties between between αα & Fe & Fe33CC

8

10m

- Smaller T: colonies are larger

- Larger T: colonies are smaller

• Ttransf just below TE --Larger T: diffusion is faster --Pearlite is coarser.

Two cases:• Ttransf well below TE --Smaller T: diffusion is slower --Pearlite is finer.

PEARLITE MORPHOLOGYPEARLITE MORPHOLOGY

observation under microscope: a white matrix is α phase

Microstructure changeMicrostructure change

occurs either in cooling or heating occurs either in cooling or heating processprocess

factors that influence the factors that influence the microstructure formation : crystal microstructure formation : crystal structure, physical & chemical structure, physical & chemical properties, composition, processing properties, composition, processing (cooling rate, heat treatment)(cooling rate, heat treatment)

Slow cooling of plain carbon steel Slow cooling of plain carbon steel

1.1. At eutectoid plain carbon steelAt eutectoid plain carbon steel

Explanation microstructure changes at Explanation microstructure changes at eutectoideutectoid

1.1. At temperature _____ m/structure of At temperature _____ m/structure of γγ is existis exist

2.2. Further cooling process to the 727oC Further cooling process to the 727oC at eutectoid temperature or just below at eutectoid temperature or just below the Te, it will form a pearlite the Te, it will form a pearlite m/structurem/structure

3.3. only pearlite for at composition only pearlite for at composition eutectoid.eutectoid.

Additional information of pearlite at eutectoid.Additional information of pearlite at eutectoid.

At eutectoid , the relative layer thickness for At eutectoid , the relative layer thickness for αα and Feand Fe33C ( 8:1)C ( 8:1)

Often termed colonies ( coz they form colony Often termed colonies ( coz they form colony layers that oriented in essentially the same layers that oriented in essentially the same direction)direction)

Dif. colony Dif. colony dif. oriented direction dif. oriented direction

pearlite growth direction

Austenite () grain boundary

cementite (Fe3C)

ferrite ()

Diffusive flow of C needed

2. Microstructure changes at hypoeutectiod2. Microstructure changes at hypoeutectiod

Explanation Explanation

1.1. At ____ , point __, the microstructure will consist At ____ , point __, the microstructure will consist entirely of grains of entirely of grains of γγ phases.phases.

2.2. In cooling to point___, at temp __, which is within In cooling to point___, at temp __, which is within αα + + γγ phase region, small phase region, small αα will form along the original will form along the original γγ grain boundaries. (composition changes) grain boundaries. (composition changes)

3.3. At point ___, just above the eutectoid, At point ___, just above the eutectoid, αα particles will particles will have grown larger. (composition)have grown larger. (composition)

4.4. As temp. lower, just below the eutectoid to point____, As temp. lower, just below the eutectoid to point____, all all γγ phase that was present at temperature Te will phase that was present at temperature Te will transform to pearlitetransform to pearlite

5.5. Below Te, m/structure Below Te, m/structure αα phase present as a phase present as a continuous matrix phase surrounding the isolated continuous matrix phase surrounding the isolated pearlite coloniespearlite colonies

6.6. Ferrite present in pearlite (eutectoid ferrite) and Ferrite present in pearlite (eutectoid ferrite) and proeutectoid (pre/before eutectoid) ferrite.proeutectoid (pre/before eutectoid) ferrite.

3. Microstructure changes at hypereutectiod3. Microstructure changes at hypereutectiod

Microstructure changes (an explanation)Microstructure changes (an explanation)

1.1. At point ___& temp___, only At point ___& temp___, only γγ phase will be phase will be present with a composition of Cpresent with a composition of C11

2.2. Further cooling to point ___, at Further cooling to point ___, at γγ + Fe+ Fe33C region, the C region, the cementite phase will begin to form along the initial cementite phase will begin to form along the initial γγ grain boundaries and called as proeutectoid grain boundaries and called as proeutectoid cementite.cementite.

3.3. As temperature is lowered through the eutectoid to As temperature is lowered through the eutectoid to point ____, all remaining austenite of eutectoid point ____, all remaining austenite of eutectoid composition is converted to perlitecomposition is converted to perlite