Presentation on theme: "Chapter 11: Phase Transformations"— Presentation transcript:
1 Chapter 11: Phase Transformations ISSUES TO ADDRESS...• Transforming one phase into another takes time.Feg(Austenite)EutectoidtransformationCFCCFe3C(cementite)a(ferrite)+(BCC)• How does the rate of transformation depend ontime and temperature ?• Is it possible to slow down transformations so that non-equilibrium structures are formed?• Are the mechanical properties of non-equilibriumstructures more desirable than equilibrium ones?
2 Phase Transformations Nucleationnuclei (seeds) act as templates on which crystals growfor nucleus to form rate of addition of atoms to nucleus must be faster than rate of lossonce nucleated, growth proceeds until equilibrium is attainedDriving force to nucleate increases as we increase Tsupercooling (eutectic, eutectoid)superheating (peritectic)Small supercooling slow nucleation rate - few nuclei - large crystalsLarge supercooling rapid nucleation rate - many nuclei - small crystals
3 Solidification: Nucleation Types Homogeneous nucleationnuclei form in the bulk of liquid metalrequires considerable supercooling (typically °C)Heterogeneous nucleationmuch easier since stable “nucleating surface” is already present — e.g., mold wall, impurities in liquid phaseonly very slight supercooling (0.1-10ºC)
4 Homogeneous Nucleation & Energy Effects Surface Free Energy- destabilizesthe nuclei (it takes energy to makean interface)g = surface tensionDGT = Total Free Energy= DGS + DGVVolume (Bulk) Free Energy –stabilizes the nuclei (releases energy)r* = critical nucleus: for r < r* nuclei shrink; for r >r* nuclei grow (to reduce energy)Adapted from Fig.11.2(b), Callister & Rethwisch 3e.
5 Solidification Note: Hf and are weakly dependent on T Hf = latent heat of solidificationTm = melting temperatureg = surface free energyDT = Tm - T = supercoolingr* = critical radiusNote: Hf and are weakly dependent on T r* decreases as T increasesFor typical T r* ~ 10 nm
6 Rate of Phase Transformations Kinetics - study of reaction rates of phase transformationsTo determine reaction rate – measure degree of transformation as function of time (while holding temp constant)How is degree of transformation measured?X-ray diffraction – many specimens requiredelectrical conductivity measurements – on single specimenmeasure propagation of sound waves – on single specimen
7 Rate of Phase Transformation transformation completeFixed TFraction transformed, y0.5maximum rate reached – now amountunconverted decreases so rate slowsrate increases as surface area increases & nuclei growt0.5log tAdapted from Fig ,Callister & Rethwisch 3e.Avrami equation => y = 1- exp (-kt n)k & n are transformation specific parametersS.A. = surface areafraction transformedtimeBy convention rate = 1 / t0.5
8 Temperature Dependence of Transformation Rate 110102104Adapted from Fig , Callister & Rethwisch 3e.(Fig adapted from B.F. Decker and D. Harker, "Recrystallization in Rolled Copper", Trans AIME, 188, 1950, p. 888.)For the recrystallization of Cu, since rate = 1/t0.5 rate increases with increasing temperatureRate often so slow that attainment of equilibrium state not possible!
9 Transformations & Undercooling •Eutectoid transf. (Fe-Fe3C system):gÞa+Fe3C•For transf. to occur, mustcool to below 727°C(i.e., must “undercool”)0.76 wt% C6.7 wt% C0.022 wt% CFe3C (cementite)16001400120010008006004001234566.7Lg(austenite)+L+Fe3CaL+Fe3Cd(Fe)C, wt%C1148°CT(°C)ferrite727°CEutectoid:Equil. Cooling: Ttransf. = 727ºCDTUndercooling by Ttransf. < 727C0.760.022Adapted from Fig ,Callister & Rethwisch 3e. (Fig adapted from Binary Alloy Phase Diagrams, 2nd ed., Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International, Materials Park, OH, 1990.)
10 The Fe-Fe3C Eutectoid Transformation • Transformation of austenite to pearlite:Adapted from Fig , Callister & Rethwisch 3e.gapearlitegrowthdirectionAustenite (g)grainboundarycementite (Fe3C)Ferrite (a)Diffusion of Cduring transformationagCarbon diffusion• For this transformation,rate increases with[Teutectoid – T ] (i.e., DT).Adapted from Fig , Callister & Rethwisch 3e.675°C(DT smaller)50y (% pearlite)600°C(DT larger)650°C100Coarse pearlite formed at higher temperatures – relatively softFine pearlite formed at lower temperatures – relatively hard
11 Generation of Isothermal Transformation Diagrams Consider:• The Fe-Fe3C system, for Co = 0.76 wt% C• A transformation temperature of 675°C.100T = 675°Cy,% transformed502411010time (s)40050060070011023450%pearlite100%50%Austenite (stable)TE (727C)Austenite(unstable)PearliteT(°C)time (s)isothermal transformation at 675°CAdapted from Fig ,Callister & Rethwisch 3e. (Fig adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1977, p. 369.)
12 Austenite-to-Pearlite Isothermal Transformation • Eutectoid composition, C0 = 0.76 wt% C• Begin at T > 727°C• Rapidly cool to 625°C• Hold T (625°C) constant (isothermal treatment)4005006007000%pearlite100%50%Austenite (stable)TE (727C)Austenite(unstable)PearliteT(°C)1102345time (s)Adapted from Fig ,Callister & Rethwisch 3e. (Fig adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.)gg
14 Bainite: Another Fe-Fe3C Transformation Product -- elongated Fe3C particles in a-ferrite matrix-- diffusion controlled• Isothermal Transf. Diagram, C0 = 0.76 wt% CFe3C(cementite)a(ferrite)1035time (s)-1400600800T(°C)Austenite (stable)200PBTE0%100%50%A100% pearlite100% bainite5 mmAdapted from Fig , Callister & Rethwisch 3e. (Fig from Metals Handbook, 8th ed., Vol. 8, Metallography, Structures, and Phase Diagrams, American Society for Metals, Materials Park, OH, 1973.)Adapted from Fig , Callister & Rethwisch 3e.
15 Spheroidite: Another Microstructure for the Fe-Fe3C System Adapted from Fig , Callister & Rethwisch 3e. (Fig copyright United States Steel Corporation, 1971.)60 ma(ferrite)(cementite)Fe3C• Spheroidite:-- Fe3C particles within an a-ferrite matrix-- formation requires diffusion-- heat bainite or pearlite at temperature just below eutectoid for long times-- driving force – reductionof a-ferrite/Fe3C interfacial area
16 Martensite: A Nonequilibrium Transformation Product -- g(FCC) to Martensite (BCT)Martensite needlesAustenite60 mxpotentialC atom sitesFe atomsitesAdapted from Fig , Callister & Rethwisch 3e.Adapted from Fig , Callister & Rethwisch 3e.• Isothermal Transf. Diagram• g to martensite (M) transformation..-- is rapid! (diffusionless)-- % transf. depends only on T to which rapidly cooled1035time (s)-1400600800T(°C)Austenite (stable)200PBTE0%100%50%AM + A90%Adapted from Fig , Callister & Rethwisch 3e. (Fig courtesy United States Steel Corporation.)
17 Martensite Formation (FCC) (BCC) + Fe3C slow cooling quench M (BCT)temperingMartensite (M) – single phase– has body centered tetragonal (BCT) crystal structureDiffusionless transformation BCT if C0 > 0.15 wt% CBCT few slip planes hard, brittle
18 Phase Transformations of Alloys Effect of adding other elementsChange transition temp.Cr, Ni, Mo, Si, Mnretard + Fe3Creaction (and formation of pearlite, bainite)Adapted from Fig , Callister & Rethwisch 3e.
19 Continuous Cooling Transformation Diagrams Cooling curveConversion of isothermal transformation diagram to continuous cooling transformation diagramActual processes involves cooling – not isothermalCan’t cool at infinite speedAdapted from Fig ,Callister & Rethwisch 3e.
20 Isothermal Heat Treatment Example Problems On the isothermal transformation diagram for a 0.45 wt% C, Fe-C alloy, sketch and label the time-temperature paths to produce the following microstructures:42% proeutectoid ferrite and 58% coarse pearlite50% fine pearlite and 50% bainite100% martensite50% martensite and 50% austenite
21 Solution to Part (a) of Example Problem 42% proeutectoid ferrite and 58% coarse pearliteA + BA + PA + aABP50%2004006008000.110103105time (s)M (start)M (50%)M (90%)Adapted from Fig ,Callister 5e.Fe-Fe3C phase diagram, for C0 = 0.45 wt% CIsothermally treat at ~ 680°C-- all austenite transformsto proeutectoid a andcoarse pearlite.T (°C)
22 Solution to Part (b) of Example Problem 50% fine pearlite and 50% bainiteT (°C)A + BA + PA + aABP50%2004006008000.110103105time (s)M (start)M (50%)M (90%)Adapted from Fig ,Callister 5e.Fe-Fe3C phase diagram, for C0 = 0.45 wt% CIsothermally treat at ~ 590°C– 50% of austenite transformsto fine pearlite.Then isothermally treatat ~ 470°C– all remaining austenitetransforms to bainite.
23 Solutions to Parts (c) & (d) of Example Problem 100% martensite – rapidly quench to room temperatureT (°C)A + BA + PA + aABP50%2004006008000.110103105time (s)M (start)M (50%)M (90%)Adapted from Fig ,Callister 5e.Fe-Fe3C phase diagram, for C0 = 0.45 wt% C50% martensite& 50% austenite-- rapidly quench to ~ 290°C, hold at this Tc)d)
24 Mechanical Props: Influence of C Content Adapted from Fig , Callister & Rethwisch 3e.C0 > 0.76 wt% CHypereutectoidPearlite (med)Cementite(hard)Pearlite (med)ferrite (soft)C0 < 0.76 wt% CAdapted from Fig , Callister & Rethwisch 3e.HypoeutectoidAdapted from Fig , Callister & Rethwisch 3e. (Fig based on data from Metals Handbook: Heat Treating, Vol. 4, 9th ed., V. Masseria (Managing Ed.), American Society for Metals, 1981, p. 9.)3005007009001100YS(MPa)TS(MPa)wt% C0.51hardness0.76HypoHyperwt% C0.5150100%ELImpact energy (Izod, ft-lb)40800.76HypoHyper• Increase C content: TS and YS increase, %EL decreases
25 Mechanical Props: Fine Pearlite vs. Coarse Pearlite vs. Spheroidite 80160240320wt%C0.51Brinell hardnessfinepearlitecoarsespheroiditeHypoHyper306090wt%CDuctility (%RA)finepearlitecoarsespheroiditeHypoHyper0.51Adapted from Fig , Callister & Rethwisch 3e. (Fig based on data from Metals Handbook: Heat Treating, Vol. 4, 9th ed., V. Masseria (Managing Ed.), American Society for Metals, 1981, pp. 9 and 17.)• Hardness:fine > coarse > spheroidite• %RA:fine < coarse < spheroidite
26 Mechanical Props: Fine Pearlite vs. Martensite 200wt% C0.51400600Brinell hardnessmartensitefine pearliteHypoHyperAdapted from Fig , Callister & Rethwisch 3e. (Fig adapted from Edgar C. Bain, Functions of the Alloying Elements in Steel, American Society for Metals, 1939, p. 36; and R.A. Grange, C.R. Hribal, and L.F. Porter, Metall. Trans. A, Vol. 8A, p )• Hardness: fine pearlite << martensite.
27 Tempered Martensite • • Heat treat martensite to form tempered martensite• tempered martensite less brittle than martensite• tempering reduces internal stresses caused by quenchingYS(MPa)TS(MPa)8001000120014001600180030405060200400600Tempering T (°C)%RATSYSAdapted from Fig , Callister & Rethwisch 3e. (Fig adapted from Fig. furnished courtesy of Republic Steel Corporation.)Adapted from Fig , Callister & Rethwisch 3e. (Fig copyright by United States Steel Corporation, 1971.)9 mm•tempering produces extremely small Fe3C particles surrounded by a.•tempering decreases TS, YS but increases %RA
28 Summary of Possible Transformations Adapted from Fig , Callister & Rethwisch 3e.Austenite (g)Pearlite(a + Fe3C layers + aproeutectoid phase)slowcoolBainite(a + elong. Fe3C particles)moderatecoolMartensite(BCT phasediffusionlesstransformation)rapidquenchStrengthDuctilityMartensiteT Martensitebainitefine pearlitecoarse pearlitespheroiditeGeneral TrendsTemperedMartensite(a + very fineFe3C particles)reheat
29 Precipitation Hardening • Particles impede dislocation motion.• Ex: Al-Cu system• Procedure:1020304050wt% CuL+Laa+qq+L300400500600700(Al)T(°C)composition rangeavailable for precipitation hardeningCuAl2A-- Pt A: solution heat treat (get a solid solution)BPt B-- Pt B: quench to room temp. (retain a solid solution)C-- Pt C: reheat to nucleate small q particles within a phase.Other alloys that precipitation harden: • Cu-Be • Cu-Sn • Mg-AlAdapted from Fig , Callister & Rethwisch 3e. (Fig adapted from J.L. Murray, International Metals Review 30, p.5, 1985.)Temp.TimePt A (sol’n heat treat)Pt C (precipitate )Adapted from Fig , Callister & Rethwisch 3e.
30 Influence of Precipitation Heat Treatment on TS, %EL • 2014 Al Alloy:• Maxima on TS curves.• Increasing T acceleratesprocess.• Minima on %EL curves.%EL (2 in sample)1020301min1h1day1mo1yr204°C149°Cprecipitation heat treat timeprecipitation heat treat timetensile strength (MPa)2003004001001min1h1day1mo1yr204°Cnon-equil.solid solutionmany smallprecipitates“aged”fewer large“overaged”149°CAdapted from Fig (a) and (b), Callister & Rethwisch 3e. (Fig adapted from Metals Handbook: Properties and Selection: Nonferrous Alloys and Pure Metals, Vol. 2, 9th ed., H. Baker (Managing Ed.), American Society for Metals, p. 41.)
31 Melting & Glass Transition Temps. What factors affect Tm and Tg?Both Tm and Tg increase with increasing chain stiffnessChain stiffness increased by presence ofBulky sidegroupsPolar groups or sidegroupsChain double bonds and aromatic chain groupsRegularity of repeat unit arrangements – affects Tm onlyAdapted from Fig , Callister & Rethwisch 3e.3131
32 Thermoplastics vs. Thermosets Callister,Fig. 16.9TMolecular weightTgTmmobileliquidviscousrubbertoughplasticpartiallycrystallinesolid• Thermoplastics:-- little crosslinking-- ductile-- soften w/heating-- polyethylenepolypropylenepolycarbonatepolystyrene• Thermosets:-- significant crosslinking(10 to 50% of repeat units)-- hard and brittle-- do NOT soften w/heating-- vulcanized rubber, epoxies,polyester resin, phenolic resinAdapted from Fig , Callister & Rethwisch 3e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.)3232
33 Summary• Heat treatments of Fe-C alloys produce microstructures including:-- pearlite, bainite, spheroidite, martensite, tempered martensite• Precipitation hardening--hardening, strengthening due to formation ofprecipitate particles.--Al, Mg alloys precipitation hardenable.• Polymer melting and glass transition temperatures