1 / 32

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.

tara
Download Presentation

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

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related