Phase Transformation In Metals - KSU

Phase Transformation in MetalsPhase transformation goes through two stages:Stage 1: Nucleation (formation of very small particles of the newphase, which are capable of growing.Stage 2: Growth ( nuclei increase in size on the expense of theparent phase.

FRACTION OF TRANSFORMATION Fraction transformed depends on time.Avrami Eqn.n kty 1 efractiontransformed1y0.5timeK, n are time independent constants.0 Transformation rate depends on T.activation energyr 1t0.5 Ae Q /RTFixedTt0.5log (t)R: gas constantT: Absolute temp.Q: Activation energy fora particular reactionA: ConstantRate of transformation is defined as the reciprocal of the timerequired for the transformation to proceed half way to completion.

TRANSFORMATIONS & UNDERCOOLINGEutectoid reaction: at 0.76wt% C and temp.: 727 Cγ ( 0.76wt%C) Æ α (0.022 wt%C) Fe3C (6.7 wt%C)Pearilite Growth of pearlite from austenite: Reaction rateincreases with T (more undercooling results inmore nucleation rate)γγpearlitegrowthdirection500α0650 C50675 C( T smaller)1γα100600 C( T larger)α10010 102 103time (s)% austeniteααγ ααααcementite (Fe3C)ferrite (α)y (% pearlite)Austenite (γ)grainboundaryDiffusive flowof C needed

NUCLEATION AND GROWTH Reaction rate is a result of nucleation and growthof crystals.100% Pearlite50 Nucleationregime0Growthregimet50Log (time) Examples:γpearlitecolonyT just below TENucleation rate lowGrowth rate highγT moderately below TENucleation rate med .Growth rate med.γT way below TENucleation rate highGrowth rate low

Isothermal Transformation Diagrams (TTT Diagram, Time –Temp.-Transformation)NOTE: Just below the eutectoidtemp. (small T smalldegree of under cooling)long times of the order 105s needed for 50%transformation andtherefore the reaction rateis very slow. For large degree of undercooling the time for 50 %transformation is short andthus the reaction rate ishigh. This plot is valid only foreutectoid composition.

EX: COOLING HISTORY Fe-C SYSTEM Eutectoid composition, Co 0.77wt%C Begin at T 727C Rapidly cool to 625C and hold isothermally.T( C)Austenite (stable)TE (727 C)700Adapted from Fig.10.5,Callister 6e.(Fig. 10.5 adapted fromH. Boyer (Ed.) Atlas ofPearlite600γ γγ γγ0%10%50itearlpe0%500IsothermalTransformation andCooling TransformationDiagrams, American110γγSociety for Metals,1997, p. 28.)102103104105time (s)

At temperatures just below the eutectoid temperature, relativelythick layers of α and fe3c phases are produced; thismicrostructure is termed coarse pearlite. At temperatures around 540 (540-600) C thin layered structureis produced termed fine pearlite Pearlite forms above the nose of the curve; in the temperaturerange of 540C to 727 C- Smaller T:colonies arelargerCoarse pearlite- Larger T:colonies aresmallerfine pearlite

Bainite Forms in the range of temperatures from 215 C to 540 C.The microstructure of bainite consist of ferrite and cementite800Austenite (stable)T( C)ATEP600100% pearlitepearlite/bainite boun100% bainite400BA1030%101010-1%50Elongated fe3c particles inneedles of ferrite0%200105time (s)N.B. Once some portion of the alloy transformed to pearlite or bainite,other transformations is not possible with out reheating to form austenite.

OTHER PRODUCTS: Fe-CSpheroiditeIf a steel alloy having either pearlitic or bainitic microstructures isheated to and left at a temperature below the eutectoid temp (727C) for long period of time for example at 700 C for 18-24 hours themicrostructure achieved is shperoidite. Fe3c phase appears assphere – like particles embedded in a continuous α phase matrix.α(ferrite)Fe3C(cementite)60 µm

Martensite:OTHER PRODUCTS: Fe-CMartensite--γ(FCC) to Martensite (BCT) Martensite is formed when austenitized iron-carbon alloys arerapidly cooled or quenched to relatively low temperatures. Martensite is a non-equilibrium single phase structure thatresults from diffusionless transformation of austenite. Martensite occurs when the quenching rate is high enough toprevent diffusion of carbon, so carbon atoms are trapped asinterstitial impurities in body centered tetragonal (BCT)martensite.xFe atompotentialxxsitesC atom sitexxx

60 µmMartensite appearnaceMartentite needlesAusteniteRetained austeniteAustenite that did nottransform duringquenchingQuenching rate has to behigh enough to avoid hittingthe nose of the curve

COOLING EX: Fe-C SYSTEM (1) Co Ceutectoid Three histories.Rapid Hold Rapid Hold Rapidcool to: for: cool to: for: cool to:350 C 104s Troom (a)104s Troom250 C 102s Troom 102s Troom(b)650 C 20s 400 C 103s TroomCase I800Austenite (stable)T( C)A600(a)AP(c)SB40010100%A%500%200 M AM AM A1010-1103100%B0%0%50%90%Adaptedfrom Fig.10.15,Callister 6e.100% Bainite105 time (s)

COOLING EX: Fe-C SYSTEM (2) Co Ceutectoid Three histories.Rapid Hold Rapid Hold Rapidcool to: for: cool to: for: cool to:350 C 104s Troom 104s Troom250 C 102s Troom (b)102s TroomCase II800Austenite (stable)T( C)A650 C 20s 400 CP600SAB400100%A%200 M AM AM A1010-11050(b)0%0%0%50%90%Adaptedfrom Fig.10.15,Callister 6e.M trace of A103105 time (s)103s Troom

COOLING EX: Fe-C SYSTEM (3) Co Ceutectoid Three histories.Rapid Hold Rapid Hold Rapidcool to: for: cool to: for: cool to:350 C 104s Troom 104s TroomCase III800Austenite (stable)T( C)A100%A600A(c)50%P, 50%APSB40050%P, 50%B50%P, 50%A%500%200 M AM AM A1010-1250 C 102s Troom 102s Troom650 C 20s 400 C 103s Troom (c)100%0%50%90%Adaptedfrom Fig.10.15,Callister 6e.50103 %P 105 time (s),50%B14

MECHANICAL PROPERTIESPearlite: consist of alternating layers of soft α and hard fe3c. Finepearlite is stonger than coarse pearlite because there is greater phaseboundary area per unit volume and these boundaries serve as barriersto dislocation motion. Coarse pearlite, on the other hand is moreductile than fine pearlite.Spheroidite: The fe3c phase which is considered as the reinforcingphase is coarse and this leads to less phase boundary area. Of all steelalloys, ones containing spheroidite are the softest and the weakest.Bainite: Bainitic steels have fine structures (smaller ferrite andcementite phases) they are generally stronger and harder thanpearlitic steels.

MECHANICAL PROPERTIES Effect of wt%CTS(MPa)1100YS(MPa)Co 0.77wt%CHypoeutectoidHypoHyperCo 407005050000.5100.7700.77300Impact energy (Izod, ft-lb)Pearlite (med)ferrite (soft)Pearlite (med)Cementite(hard)10.50wt%Cwt%C More wt%C: TS and YS increase, %EL decreases.

Fine vs coarse pearlite vs spheroiditeHyperBrinell dite8000.51wt%C90Ductility te3000 Hardness: fine coarse spheroidite %AR:fine coarse spheroiditeHyper0.51wt%C

Martensite: The hardest and the strongest and in addition themost brittle.Its hardness is dependent on the carbon content uptoabout 0.6 wt% C. This strength is attributed to the effectivenessof interstitial trapped carbon atoms in hindering dislocationmotion.Tempered martensite:Ductility and toughness of martensite may beenhanced by a heat treatment called tempering, in which martensitic steelsare heated to temperature in the range 250C to 650C.Martensite (BCT, single phase) Æ Tempered martensite (α fe3c phase)Extremely fine particlesof fe3c in α matrix

AR800200400600Tempering T ( C

DuctilityStrengthMartensiteT Martensitebainitefine pearlitecoarse pearlitespheroiditeGeneral Trends

50%C. pearlite 50% Austenite50% pearlite 25%bainite 25% Austenite50% F. pearlite 50%Austeniteadc50% pearlite 25%bainite 25% martensite100%martensite50%C. pearlite 50% martensite

ef40%bainite100%bainite 60% martensite

g100%martensiteWhenreheatedat 315Cfor 1 hourwe gettemperedmartensiteh100 % F.pearlite

Phase Transformation in Metals Phase transformation goes through two stages: Stage 1: Nucleation (formation of very small particles of the new phase, which are capable of growing. Stage 2: Growth ( nuclei increase in size on the expense of the parent phase. FRACTION OF TRANSFORMATION Fraction transformed depends on time. fraction transformed time y 1 e ktn Avrami Eqn .