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Bruce Mayer, PE Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege

Engineering 45. Metal Phase Transforms (1). Bruce Mayer, PE Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege.edu. Fe. C. Fe. 3. g. Eutectoid. (cementite). transformation. +. (Austenite). a. C. (BCC). FCC. (ferrite). Learning Goals.1 – Phase Xforms.

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Bruce Mayer, PE Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege

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  1. Engineering 45 Metal PhaseTransforms (1) Bruce Mayer, PE Licensed Electrical & Mechanical EngineerBMayer@ChabotCollege.edu

  2. Fe C Fe 3 g Eutectoid (cementite) transformation + (Austenite) a C (BCC) FCC (ferrite) Learning Goals.1 – Phase Xforms • Transforming one phase into another is a Function of Time: • Understand How time & TEMPERATURE (t & T) Affect the Transformation Rate • Learn how to Adjust the Transformation RATE to Engineer NONequilibrium Structures

  3. Fe C Fe 3 g Eutectoid (cementite) transformation + (Austenite) a C (BCC) FCC (ferrite) Learning Goals.1 – PhaseX2 • Transforming one phase into another is a Function of Time: • Understand the Desirable mechanical properties of NONequilibrium-phase structures

  4. Classes of Phase XForms • Diffusion Dependent – Single Phase • No Change in Either The Number or Composition of Phases • e.g.: Allotropic Transforms, Grain-Growth • Diffusion Dependent – MultiPhase • Two-Phase Structure; e.g. α + Mg2Pb in Mg-Pb alloy system • DiffusionLess – MetaStable Phase • NonEquil Structure “Frozen” in Place

  5. Phase Xform → Nucleation • Nuclei (seeds) act as the template to grow crystals • For a nucleus to form the rate of addition of atoms to the nucleus must be greater than rate of loss • Once nucleated, the new “structure” grows until reaching equilibrium

  6. Nucleation Driving Force • Driving force to nucleate increases as we increase ΔT • SuperCooling → Temp Below the eutectic or, eutectoid • SuperHeating → Temp Above the peritectic • Small Super Cooling → Few & Large Nuclei • Large Super Cooling → Rapid nucleation - many nuclei, small crystals

  7. Solid-State Reaction Kinetics • “Kinetic” → Time Dependent • Phase Xforms Often Require Changes in Atom Position to Affect • Crystal Structure • Local Chemical Composition • Atom Movement Requires DIFFUSION • Diffusion is a TIME DEPENDENT Physical Process

  8. Solidification by Nucleation • Homogeneous nucleation • Nuclei form in the bulk of liquid metal • Requires supercooling (typically 80-300°C) • Heterogeneous nucleation • Much easier since stable “nucleus” is already present at “defect” sites • Could be wall of a casting-mold or impurities in the liquid phase • Allows solidification with only 0.1-10ºC supercooling

  9. Surface Free Energy-destabilizes the nuclei (it takes energy to make an interface) g = surface tension DGT = Total Free Energy = DGS + DGV Volume (Bulk) Free Energy – stabilizes the nuclei (releases energy) Homogeneous Nucleation & Energy Effects r* = critical nucleus: nuclei < r* shrink; nuclei>r* grow (to reduce energy) Adapted from Fig.10.2(b), Callister 7e.

  10.  r* decreases asT increases For typicalTr* ca. 100Å Solidification Quantified r* = critical radius g = surface free energy Tm = melting temperature HS = latent heat of solidification DT = Tm - T = supercooling • Note:HS = strong function of T  =weak function ofT

  11. Incubation Phase Xform Processes • Phase Transforms Typically Entail Two significant Time-Regions • Nucleation  Formation of Very Small New-Phase “Starting” Particles • Distribution is Usually Random, but can be assisted by “defects” in the Solid State • Also Called the “Incubation” phase T = const

  12. Phase Xform Processes cont. • Growth  New-Phase expands from the Nucleation “Starting” Particles to eventually Consume the Old-Phase • If “Allowed” toProceed TheEquilibrium Phase-Fractions WillEventually Emerge • This Stage of theXform ischaracterized by theTransformation Fraction, y T = const

  13. The Avrami Eqn Describes the Kinetics of Phase Transformation 1 y 0.5 Fixed T 0 t log (t) 0.5 Avrami Phase Xform Kinetics • Where • y  New-Phase Fraction (0-1, 0-100%) • t  Time (s) • k, n  Time-Independent Constants (S-n, unitless) • RATE of Xform r • Where • t0.5 Time Needed for 50% New-PhaseFormation

  14. Temperature is a Controlling Variable in the Heat Treating Process thru an Arrhenius Rln: Rcn Rate, r, as Fcn of T • Where • R  Gas Constant (8.31 J/mol-K) • T  Absolute Temperature (K) • Q  Activation Energy for the Reaction (J/mol) • A  Temperature-Independent Scalar (1/S) • e.g. Cu Recrystallization • In general, rate increases as T↑ 135C 119C 113C 102C 88C 43C 1 10 102 104

  15. MetaStability • The Previous Eqn. Indicates that Rcn Rates are Thermally Activated • Typical Equilibrium Rcn Rates are Quite Sluggish; Too slow to Be Maintained in a Practical Metal-Production Process • Most Metals are cooled More Rapidly Than Equilibrium Conditions • Most Practical Metals are Thus SuperCooled and do NOT Exist in Equilibrium • They are thus MetaStable • Quite Time-Stable; but Not Strictly in Equilibrium

  16. The Austenite to Ferrite+Cemtite Eutectoid Rcn Requires Large Redistribution of Carbon g Þ a + Fe C 3 0.77wt%C 6.7 wt%C 0.022wt%C T(°C) 1600 L 1400 g +L g 1200 L+Fe C 3 austenite 1000 g a +Fe C 3 Eutectoid: a ferrite + Equil. cooling: T = 727ºC Fe C 800 transf. 3 g 727°C D a T +Fe C cementite 3 6 00 D Undercooling by T : T < 727ºC transf. 0.022 0.77 6.7 4 00 0 1 2 3 4 5 6 C , wt% C (Fe) o Recall Fe-C Eutectoid Xform • Forms Pearlite • Can Equilibrium Cool: 727.5C → 726.5C; and SLOWLY • Or Can UNDERCool by Amount DT; say 727C → 600C; and QUICKLY

  17. Diffusive flow of C needed cementite (Fe3C) Austenite (g) a grain a ferrite ( ) a a boundary g a g g g a 0 10 0 pearlite a a 600°C growth a D ( T larger) 650°C direction a % austenite y (% pearlite) 50 50 675°C D ( T smaller) 10 0 0 2 3 10 10 1 10 time (s) Eutectoid Xform Rate ~ DT • Recall the Growth of Pearlite from Cooled Austenite • The g→Pearlite Rcn Rate Increases with the Degree of UnderCooling (larger DT)

  18. UnderCooling Analogy Liquid Water Can be cooled below 32 °F (SuperCooled or UnderCooled) If any Ice Nucleates the Entire Liq body RAPIDLY Freezes The Greater the SuperCooling, The More Rapid the Phase Transform 0 10 0 600°C D ( T larger) 650°C % austenite y (% pearlite) 50 50 675°C D ( T smaller) 10 0 0 2 3 10 10 1 10 time (s) Eutectoid Xform Rate ~ DT cont.1

  19. More RAPID Xform at LOWER Temps Seems to Contradict Arrhenius 0 10 0 600°C D ( T larger) 650°C % austenite y (% pearlite) 50 50 675°C D ( T smaller) 10 0 0 2 3 10 10 1 10 time (s) Eutectoid Xform Rate ~ DT cont.2 Competing Process • Lower Rcn Rate is Countered by Higher NUCLEATION rates for SuperCooled Conditions max

  20. 100 pearlite Growth colony regime g g % Pearlite g 50 Nucleation regime t 50 0 log (time) T just below TE T moderately below TE T way below TE Nucleation rate low Nucleation rate med Nucleation rate high Growth rate high Growth rate med Growth rate low Nucleation and Growth • Transformation Rate Results from the Combination of Nucleation AND Growth • Nucleation Rate INcreases With SuperCooling (DT↑) • Grown Rate DEcreases with Super Cooling (DT↑) • Examples

  21. a.k.a. TIME-TEMP-TRANSFORM (T-T-T) diagram Example = Fe-C at Eutectiod; C0 = 0.77 Wt%-Carbon At 675C Moving Lt→Rt at 675C notice intersection with 0% line → Incubation Time 50% line → Transformation Rate 100% line → Completion 1 00 T=675°C y, % transformed 5 0 0 2 4 time (s) 1 10 10 T(°C) Austenite (stable) T (727°C) E 7 00 Austenite isothermal Xform at 675°C (unstable) Pearlite 6 00 5 5 00 100% 0% 0%pearlite 400 time (s) 2 3 4 5 1 10 10 10 10 10 IsoThermal Xform Diagrams

  22. Notice Xform Lines make Asymptotic approach to TE LONG Xform Times for Equil Cooling Knee at Left on 0% line Suggests Nucleation Rate reaches a MAXIMUM (i.e.; it saturates at some large DT; perhaps  727−550 C 1 00 T=675°C y, % transformed 5 0 0 2 4 time (s) 1 10 10 T(°C) Austenite (stable) T (727°C) E 7 00 Austenite isothermal Xform at 675°C (unstable) Pearlite 6 00 5 5 00 100% 0% 0%pearlite 400 time (s) 2 3 4 5 1 10 10 10 10 10 IsoThermal Xform Dia. cont

  23. Eutectoid Composition; C0 = 0.77 wt% Cool Rapidly: ~740C → 625C T(°C) Austenite (stable) T (727°C) E 7 00 Pearlite 6 00 g g g g g g g 5 100% 0% 5 00 0%pearlite 2 3 4 5 1 10 10 10 10 10 time (s) Rapid Cooling of Fe-C from g • g Persists for about 3S Prior to Pearlite Nucleation • To 50% Pearlite at about 6S • r = 1/6S • Transformation Complete at about 15S

  24. 10 µm 10 µm Pearlite vs DT - Morphology • TXform Just Below TE • Higher T → C-Diffusion is Faster (can go Further) • Pearlite is Coarser • TXform WELL Below TE • Lower T → C-Diffusion is Slower (Shorter Diff-Dist) • Pearlite is Finer

  25. Bainite Ferrite, a, lathes (strips) with long rods of Fe3C 8 00 Austenite (stable) Fe C 3 T E T(°C) A (cementite) P a (ferrite) 6 00 100% pearlite pearlite/bainite boundary 100% bainite m 5 m B 4 00 A 2 00 5 0% 100% 0% -1 10 3 5 10 10 10 time (s) Fe-C NonEquil Xform Products • Diffusion Controlled Formation • Bainite & Pearlite Compete • Bainite Forms Below The Boundary at About 540 °C

  26. Spherodite Ferrite, a, Xtal-Matrix with spherical Fe3C “Globules” diffusion dependent heat bainite or pearlite for LONG times T-T-T Diagram → ~104 seconds reduces a-Fe3C Phase Boundary (driving force) 8 00 Austenite (stable) T E T(°C) a A P (ferrite) 6 00 100% spheroidite Spheroidite 100% spheroidite Fe C 3 B 4 00 A (cementite) m 60 m 2 00 5 0% 100% 0% -1 10 3 5 time (s) 10 10 10 Fe-C NonEquil Xform

  27. Martensite A Diffusionless, and Hence Speed-of-Sound Rapid, Xform from FCC g Poorly Understood Single Carbon-Atom Jumps Convert FCC Austenite to a Body Centered Tetragonal (BCT) Form x Fe atom potential x x sites C atom sites x x x Fe-C NonEquil Xform Products

  28. Martensite, M, is NOT an Equil. Phase Does NOT Appear on the PHASE Diagram But it DOES Form So Seen on Isothermal Phase Xform Diagram xForm g→M is Rapid %-Xformed to M depends ONLY on Temperature 8 00 Austenite (stable) T E T(°C) A P 6 00 S B 4 00 A 100% 5 0% 0% 0% 2 00 M + A 50% M + A 9 0% M + A -1 10 3 5 10 10 10 time (s) Martensite T-T-T Diagram • A = Austenite • P = Pearlite • B = Bainite • S = Spherodite • M = Martensite

  29. slow cooling quench tempering M (BCT) Martensite Formation  (FCC)  (BCC) + Fe3C • M = martensite is body centered tetragonal (BCT) • Diffusionless transformation BCT if C > 0.15 wt% • BCT  few slip planes  hard, brittle

  30. WhiteBoard Work • None Today • Some Cool Pearlite • So Named Because it Looks Like Mother-of-Pearl Oyster Shell • Under MicroScope with Proper Mag & Lighting

  31. Appendix – 1-Xtal Turbine blds The blades are made out of a nickel-base superalloy with a microstructure containing about 65% of gamma-prime precipitates in a polycrystalline gamma matrix. The creep life of the blades is limited by the grain boundaries which are easy diffusion paths. The blade is made out of a nickel-base superalloy with a microstructure containing about 65% of gamma-prime precipitates in a polycrystalline gamma matrix. It has been directionally-solidified, resulting in a columnar grain structure which mitigates grain-boundary induced creep. The blade is made out of a nickel-base superalloy with a microstructure containing about 65% of gamma-prime precipitates in a polycrystalline gamma matrix. It has been Spiral-solidified, resulting in a single grain structure which eliminates grain-boundary induced creep. The blade is made out of a nickel-base superalloy with a microstructure containing about 65% of gamma-prime precipitates in a single-crystal gamma matrix. The blade is directionally-solidified via a spiral selector, which permits only one crystal to grow into the blade. http://www.msm.cam.ac.uk/phase-trans/2001/slides.IB/photo.html

  32. Eutectoid Xform Pearlite only Hypo Eutectoid Includes ProEeutectiod α Fe-C Phase Transforms ProEα

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