1. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 1 Bruce Mayer, PE Engineering-45: Materials of Engineering Bruce Mayer, PE Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege.edu Engineering 45 Metal Phase Transforms (1)

2. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 2 Bruce Mayer, PE Engineering-45: Materials of Engineering 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. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 3 Bruce Mayer, PE Engineering-45: Materials of Engineering Learning Goals.1 – PhaseX2  Understand the Desirable mechanical properties of NONequilibrium-phase structures  Transforming one phase into another is a Function of Time:

4. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 4 Bruce Mayer, PE Engineering-45: Materials of Engineering Classes of Phase XForms 1.Diffusion Dependent – Single Phase No Change in Either The Number or Composition of Phases e.g.: Allotropic Transforms, Grain-Growth 2.Diffusion Dependent – MultiPhase Two-Phase Structure; e.g. α + Mg 2 Pb in Mg-Pb alloy system 3.DiffusionLess – MetaStable Phase NonEquil Structure “Frozen” in Place

5. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 5 Bruce Mayer, PE Engineering-45: Materials of Engineering 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. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 6 Bruce Mayer, PE Engineering-45: Materials of Engineering 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. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 7 Bruce Mayer, PE Engineering-45: Materials of Engineering 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. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 8 Bruce Mayer, PE Engineering-45: Materials of Engineering 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. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 9 Bruce Mayer, PE Engineering-45: Materials of Engineering r* = critical nucleus: nuclei r* grow (to reduce energy) Adapted from Fig.10.2(b), Callister 7e. Homogeneous Nucleation & Energy Effects  G T = Total Free Energy =  G S +  G V Surface Free Energy- destabilizes the nuclei (it takes energy to make an interface)  = surface tension Volume (Bulk) Free Energy – stabilizes the nuclei (releases energy)

10. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 10 Bruce Mayer, PE Engineering-45: Materials of Engineering Solidification Quantified Note:  H S = strong function of  T  = weak function of  T  r* decreases as  T increases For typical  T r* ca. 100Å  H S = latent heat of solidification T m = melting temperature  = surface free energy  T = T m - T = supercooling r* = critical radius

11. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 11 Bruce Mayer, PE Engineering-45: Materials of Engineering Phase Xform Processes  Phase Transforms Typically Entail Two significant Time-Regions 1.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 Incubation

12. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 12 Bruce Mayer, PE Engineering-45: Materials of Engineering Phase Xform Processes cont. 2.Growth  New-Phase expands from the Nucleation “Starting” Particles to eventually Consume the Old-Phase If “Allowed” to Proceed The Equilibrium Phase- Fractions Will Eventually Emerge This Stage of the Xform is characterized by the Transformation Fraction, y T = const

13. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 13 Bruce Mayer, PE Engineering-45: Materials of Engineering Avrami Phase Xform Kinetics  The Avrami Eqn Describes the Kinetics of Phase Transformation y log (t) Fixed T 0 0.5 1 t Where –y  New-Phase Fraction (0-1, 0-100%) –t  Time (s) –k, n  Time- Independent Constants (S -n, unitless) Where –t 0.5  Time Needed for 50% New-Phase Formation  RATE of Xform r

14. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 14 Bruce Mayer, PE Engineering-45: Materials of Engineering Rcn Rate, r, as Fcn of T  Temperature is a Controlling Variable in the Heat Treating Process thru an Arrhenius Rln: 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  C119  C113  C102  C88  C43  C 11010 2 10 4

15. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 15 Bruce Mayer, PE Engineering-45: Materials of Engineering 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. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 16 Bruce Mayer, PE Engineering-45: Materials of Engineering Recall Fe-C Eutectoid Xform  The Austenite to Ferrite+Cemtite Eutectoid Rcn Requires Large Redistribution of Carbon  Fe 3 C 0.77wt%C 0.022wt%C 6.7wt%C  Forms Pearlite Can Equilibrium Cool: 727.5C → 726.5C; and SLOWLY Or Can UNDERCool by Amount  T; say 727C → 600C; and QUICKLY

17. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 17 Bruce Mayer, PE Engineering-45: Materials of Engineering Eutectoid Xform Rate ~  T  Recall the Growth of Pearlite from Cooled Austenite        pearlite growth direction Austenite (  ) grain boundary cementite (Fe 3 C) ferrite (  )  Diffusive flow of C needed       The  →Pearlite Rcn Rate Increases with the Degree of UnderCooling (larger  T) 675°C (  T smaller) 1 10 2 3 time (s) 0 50 100 y (% pearlite) 0 50 100 600°C (  T larger) 650°C % austenite

18. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 18 Bruce Mayer, PE Engineering-45: Materials of Engineering Eutectoid Xform Rate ~  T cont.1  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 675°C (  T smaller) 1 10 2 3 time (s) 0 50 100 y (% pearlite) 0 50 100 600°C (  T larger) 650°C % austenite

19. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 19 Bruce Mayer, PE Engineering-45: Materials of Engineering Eutectoid Xform Rate ~  T cont.2  More RAPID Xform at LOWER Temps Seems to Contradict Arrhenius 675°C (  T smaller) 1 10 2 3 time (s) 0 50 100 y (% pearlite) 0 50 100 600°C (  T larger) 650°C % austenite  Lower Rcn Rate is Countered by Higher NUCLEATION rates for SuperCooled Conditions Competing Process max

20. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 20 Bruce Mayer, PE Engineering-45: Materials of Engineering Nucleation and Growth  Transformation Rate Results from the Combination of Nucleation AND Growth % Pearlite 0 50 100 Nucleation regime Growth regime log (time) t 50 Nucleation Rate INcreases With SuperCooling (  T↑) Grown Rate DEcreases with Super Cooling (  T↑)  Examples T just below T E Nucleation rate low Growth rate high  pearlite colony T moderately below T E  Nucleation rate med Growth rate med Nucleation rate high T way below T E  Growth rate low

21. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 21 Bruce Mayer, PE Engineering-45: Materials of Engineering IsoThermal Xform Diagrams  a.k.a. TIME-TEMP- TRANSFORM (T-T-T) diagram Example = Fe-C at Eutectiod; C 0 = 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

22. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 22 Bruce Mayer, PE Engineering-45: Materials of Engineering IsoThermal Xform Dia. cont  Notice Xform Lines make Asymptotic approach to T E –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  T; perhaps  727 − 550 C

23. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 23 Bruce Mayer, PE Engineering-45: Materials of Engineering Rapid Cooling of Fe-C from   Eutectoid Composition; C 0 = 0.77 wt%  Cool Rapidly: ~740C → 625C 110 2 3 4 5 time (s) 500 6 7 T(°C)       Austenite (stable) Pearlite 0%pearlite 100% 5 0% T E (727°C)  Persists for about 3S Prior to Pearlite Nucleation To 50% Pearlite at about 6S –r = 1/6S Transformation Complete at about 15S

24. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 24 Bruce Mayer, PE Engineering-45: Materials of Engineering Pearlite vs  T - Morphology 10 µm  T Xform Just Below T E Higher T → C-Diffusion is Faster (can go Further) Pearlite is Coarser  T Xform WELL Below T E Lower T → C-Diffusion is Slower (Shorter Diff-Dist) Pearlite is Finer

25. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 25 Bruce Mayer, PE Engineering-45: Materials of Engineering Fe-C NonEquil Xform Products  Bainite Ferrite, , lathes (strips) with long rods of Fe 3 C  Fe 3 C (cementite) 5  m  (ferrite)  Diffusion Controlled Formation Bainite & Pearlite Compete –Bainite Forms Below The Boundary at About 540 °C 10 3 5 time (s) 10 400 6 8 T(°C) Austenite (stable) 200 P B T E 0% 100% 5 0% 100% bainite pearlite/bainite boundary 100% pearlite A A

26. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 26 Bruce Mayer, PE Engineering-45: Materials of Engineering Fe-C NonEquil Xform  Spherodite Ferrite, , Xtal-Matrix with spherical Fe 3 C “Globules” diffusion dependent heat bainite or pearlite for LONG times –T-T-T Diagram → ~10 4 seconds reduces  -Fe 3 C Phase Boundary (driving force) 60  m  (ferrite) Fe 3 C (cementite) 10 3 5 time (s) 10 400 6 8 T(°C) Austenite (stable) 200 P B T E 0% 100% 5 0% A A Spheroidite 100% spheroidite

27. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 27 Bruce Mayer, PE Engineering-45: Materials of Engineering Fe-C NonEquil Xform Products  Martensite A Diffusionless, and Hence Speed-of- Sound Rapid, Xform from FCC  Poorly Understood Single Carbon-Atom Jumps Convert FCC Austenite to a Body Centered Tetragonal (BCT) Form x x x x x x potential C atom sites Fe atom sites

28. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 28 Bruce Mayer, PE Engineering-45: Materials of Engineering Martensite T-T-T Diagram  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  →M is Rapid %-Xformed to M depends ONLY on Temperature –A = Austenite –P = Pearlite –B = Bainite –S = Spherodite –M = Martensite time (s) 10 3 5 400 6 8 T(°C) Austenite (stable) 200 P B T E 0% 100% 5 0% A A S M + A 0% 50% 90%

29. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 29 Bruce Mayer, PE Engineering-45: Materials of Engineering Martensite Formation slow cooling tempering quench M (BCT) M = martensite is body centered tetragonal (BCT) Diffusionless transformation BCT if C > 0.15 wt% BCT  few slip planes  hard, brittle  (BCC) + Fe 3 C  (FCC)

30. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 30 Bruce Mayer, PE Engineering-45: Materials of Engineering 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. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 31 Bruce Mayer, PE Engineering-45: Materials of Engineering 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 single-crystal gamma matrix. The blade is directionally-solidified via a spiral selector, which permits only one crystal to grow into the blade. 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. http://www.msm.cam.ac.uk/phase-trans/2001/slides.IB/photo.html

32. BMayer@ChabotCollege.edu ENGR-45_Lec-23_Metal_Phase_Xforms-1.ppt 32 Bruce Mayer, PE Engineering-45: Materials of Engineering Fe-C Phase Transforms  Eutectoid Xform Pearlite only  Hypo Eutectoid Includes ProEeutectiod α ProE α